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Cathodoluminescence and Degradation of Oxide Thin Film and Powder Phosphors

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CATHODOLUMINESCENCE AND DEGRADAT ION OF OXIDE THIN FILM AND POWDER PHOSPHORS By LIZANDRA CLARISSA WILLIAMS A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2004

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Copyright 2004 by Lizandra C. Williams

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This dissertation is dedicated to the Triloge n and the memory of my loving grandfather, Andrew Guy, Sr.

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iv ACKNOWLEDGMENTS I would like to first thank God for his ble ssings and unending presen ce in my life. He is the reason I have been so successful t hus far in life. I must then acknowledge and thank my mother for my life and all the sacr ifices she has made for me. Her help and endless support is another reason I have made it to this point in life. Of course, the rest of my family and friends are next on the list. I would like to thank my advisor, Dr Holloway, and committee (Dr. Abernathy, Dr. Norton, Dr. Hummel, and Dr. Tanner) for th eir interest and support in my academic endeavors. I would like to acknowledge the Major Analytical and Instrumentation Center (MAIC) staff for their assistance with ch aracterization as well as training on the instruments. I would like to like to acknowle dge the assistance rece ived from the staff of Oak Ridge National Laboratory, Dr. Kumar a nd his staff at Nort h Carolina A&T State University, and Mr. Kang and Dr. Summers at Georgia Institute of Technology for their assistance with this work. I truly appreciate Ludies attention to a ll the details that made working in this research group even better. I am grateful for all her help and talks. A big thanks goes to all of the members of Dr. Holloways group, pa st and present for th eir assistance.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES...........................................................................................................viii LIST OF FIGURES...........................................................................................................ix ABSTRACT.....................................................................................................................xi ii CHAPTER 1 INTRODUCTION........................................................................................................1 2 LITERATURE REVIEW.............................................................................................3 2.1 Introduction.............................................................................................................3 2.2 Development of Cathodoluminescent Phosphors...................................................4 2.3 Evolution of the Field Emission Display................................................................7 2.4 Cathodoluminescent Phosphor Materials...............................................................9 2.4.1 Host Material................................................................................................9 2.4.2 Activator-Luminescent Center...................................................................10 2.5 Materials Development.........................................................................................11 2.5.1 Sulfide Phosphors.......................................................................................11 2.5.2 Oxide Phosphors.........................................................................................13 2.5.2.1 Zinc silicate phosphors (Zn2SiO4: Mn)............................................13 2.5.2.2 Zinc germanate phophors (Zn2GeO4: Mn).......................................15 2.6 Processing of Phosphors.......................................................................................17 2.6.1 Powder Phosphors......................................................................................17 2.6.2 Thin Film Phosphors..................................................................................17 2.6.2.1 Pulsed laser deposition.....................................................................18 2.6.2.2 Sputter deposition.............................................................................22 2.7 Evaluation of Phosphor.........................................................................................24 2.7.1 Chromaticity...............................................................................................25 2.7.2 Spectral Distribution...................................................................................25 2.7.3 Degradation Characteristics........................................................................27

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vi 3 CATHODOLUMINESCENCE FROM THIN FILM ZINC GERMANATE DOPED WITH MANGANESE PHOSPHORS........................................................................33 3.1 Introduction...........................................................................................................33 3.2 Experimental Procedures......................................................................................33 3.2.1 Substrates and th eir Preparation.................................................................33 3.2.2 Phosphor Processing...................................................................................34 3.2.3 Thin Film Phosphor Characterization........................................................37 3.2.3.1 Xray diffraction..............................................................................37 3.2.3.2 Relative composition analysis: EDX................................................38 3.2.3.3 Cathodoluminescence characterization............................................40 3.2.3.4 Photoluminescence characterization................................................41 3.3 Results...................................................................................................................4 2 3.3.1 XRay Diffraction Results.........................................................................42 3.3.2 Cathodoluminescent Properties..................................................................45 3.3.3 Zinc to Germanium Ratio in the Films.......................................................48 3.3.4 Photoluminescence Excitation and Emission.............................................49 3.4 Discussion.............................................................................................................50 4 DEVELOPMENT OF ZINC SILICATE DOPED WITH MANGANESE THIN FILM PHOSPHORS...................................................................................................54 4.1 Introduction...........................................................................................................54 4.2 Experimental Procedures......................................................................................54 4.2.1 Substrates and th eir Preparation.................................................................54 4.2.2 Phosphor Processing...................................................................................55 4.2.2.1 Sputter deposition.............................................................................55 4.2.2.2 Pulsed laser deposition.....................................................................56 4.2.2.3 Combustion chemical vapor deposition...........................................57 4.2.3 Thin Film Phosphor Characterization........................................................57 4.2.3.1 Xray diffraction..............................................................................57 4.2.3.2 Scanning electron microscopy.........................................................58 4.2.3.3 Wavelength dispersive Xray spectrometry....................................58 4.2.3.4 Xray photoelectron spectroscopy...................................................59 4.2.3.5 Cathodoluminescence characterization............................................60 4.3 Results from Pulsed Laser Deposited Phosphors.................................................60 4.3.1 Structural Characterization.........................................................................60 4.3.2 Morphological Characterization.................................................................63 4.3.3 Cathodoluminescence Results....................................................................65 4.3.4 Composition of Films.................................................................................67 4.4 Results from Sputter Deposited Phosphors..........................................................68 4.4.1 Target 1.......................................................................................................68 4.4.1.1 Structural characterization................................................................69 4.4.1.2 Cathodoluminescence results...........................................................70 4.4.2 Target 2.......................................................................................................74 4.4.2.1 Structural characterization................................................................75 4.4.2.2 Cathodoluminescence results...........................................................75

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vii 4.5 Results from Combustion Chemi cal Vapor Deposition Phosphors......................77 4.5.1 Cathodoluminescence Results....................................................................77 4.6 Discussion.............................................................................................................78 5 DEGRADATION OF ZINC SILICATE DOPED WITH MANGANESE POWDER AND THIN FILM PHOSPHORS..............................................................................81 5.1 Introduction...........................................................................................................81 5.2 Experimental Procedures......................................................................................81 5.3 Results...................................................................................................................8 3 5.3.1 Thin Film Zn2SiO4: Mn Phosphor..............................................................83 5.3.1.1 24 hour CL degradation....................................................................83 5.3.1.2 Cathodoluminescence recovery........................................................89 5.3.2 Powder Zn2SiO4: Mn Phosphor..................................................................93 5.3.2.1 24 hour CL degradation....................................................................93 5.3.2.2 Cathodoluminescence recovery......................................................101 5.4 Discussion...........................................................................................................105 5.4.1 Thin Film Zn2SiO4: Mn phosphor Degradation.......................................105 5.4.2 Powder Zn2SiO4: Mn phosphor Degradation...........................................108 6 CONCLUSIONS......................................................................................................111 6.1 Cathodoluminescence of Zn2GeO4: Mn Thin Films...........................................111 6.2 Development of Zn2SiO4: Mn Thin Films..........................................................112 6.3 Degradation of Zn2SiO4: Mn Phosphors.............................................................112 LIST OF REFERENCES.................................................................................................115 BIOGRAPHICAL SKETCH...........................................................................................121

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viii LIST OF TABLES Table page 2-1 Color and wavelength emissi on for common dopants in ZnS host..........................12 2-2 Common oxide phosphors and their luminescent properties...................................13 2-3 Summary of Zn2SiO4: Mn properties.......................................................................15 2-4 Summary of Zn2GeO4: Mn properties......................................................................16 2-5 Typical excimer lasers and their operating wavelengths..........................................19 3-1 Raw materials used to make target...........................................................................36 3-2 Zn/Ge Atomic Ratio in Deposited Zn2GeO4: Mn Film vs. Deposition Temperature on Various Substrates.........................................................................49 4-1 Rapid thermal anneal recipe for h eat treatment of thin film samples......................56 4-2 Processing conditions for sputter deposition of Zn2SiO4: Mn thin films using Target 1....................................................................................................................69 4-3 Processing conditions for sputter deposition of Zn2SiO4: Mn thin films using Target 2....................................................................................................................75 5-1 Species monitored during AES analysis..................................................................82

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ix LIST OF FIGURES Figure page 2-1 Cathode Ray Tube......................................................................................................5 2-2 Generation of electron beams in a CRT display........................................................6 2-3 Pixel view beyond shadow mask where A red, green, blue posphors; B shadow mask; C glass of display screen................................................................7 2-4 Illustration of how the red, green, and bl ue hues combine to give a full spectrum of colors for full color displays..................................................................................7 2.5 Pixel view of field emission display..........................................................................8 2-6 Cross section view of phosphor screen for a CRT.....................................................9 2-7 Energy excitation (absorpti on) leads to photon emission........................................11 2-8 PL spectra for as deposited and annealed Zn2Si0.5Ge0.5O4: Mn thin films...............17 2-9 Sketch of basic pulsed laser deposition system........................................................18 2-10 Picture of plume developed during PLD..................................................................21 2-11 Schematic diagram of a typical set up for a sputter system.....................................23 2-12 Planar magnetron sputter source..............................................................................25 2-13 CIE chromaticity chart.............................................................................................26 2-14 Visible light spectrum a nd corresponding wavelengths...........................................26 2-15 Semi logarithmic plot of CL intensity vs. electron dose for a ZnS: Ag phosphor...31 2-16 Linear and logarithmic plot of S A uger peak to peak height (APPH) vs. electron dose for ZnS: Ag phosphor........................................................................31 2-17 Plot of CL intensity of Y2O3: Eu phosphor and selected Auger peak to peak heights vs. electron dose...........................................................................................32 3-1 Flow chart outlining substrate prep aration and phosphor processing method.........35

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x 3-2 Representative sketch of mo saic target where 75% area = Zn2GeO4: Mn and 25% area = ZnO.......................................................................................................36 3-3 Interaction of incident and diffract ed Xrays in an XRD specimen........................38 3-4 Interaction volume in sample where electron beam penetrates and the resulting signals are generated................................................................................................40 3-5 Picture of cathodoluminescence vacuum system.....................................................42 3-6 XRD pattern for Zn2GeO4: Mn films grown on MgO substrate at different deposition temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C where denotes GeO2 impurity phase...................................................................................43 3-7 XRD pattern for films grown on Si substr ate at different depo sition temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C..........................................................44 3-8 XRD pattern for films grown on YS Z substrate at different deposition temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C...................................45 3-9 CL emission spectrum of Zn2GeO4: Mn on MgO substrate at various deposition temperatures.............................................................................................................46 3-10 CL emission spectrum of Zn2GeO4: Mn on a Si substrate for various deposition temperatures.............................................................................................................47 3-11 CL emission spectrum of Zn2GeO4: Mn on YSZ substrate at various deposition temperatures.............................................................................................................48 3-12 Green emission spectrum of Zn2GeO4: Mn excited with 325 nm radiation at 540 nm and excitation spectrum monitored at 540 nm...................................................50 3-13 Red emission spectrum of Zn2GeO4: Mn excited with 325 nm radiation at 625 nm and excitation spectrum monitored at 625 nm.........................................................51 4-1 Powder XRD pattern for Zn2SiO4: Mn. This is consistent with the rhombohedral crystal structur e reported in JCPDS 371485..................................61 4-2 XRD pattern of Zn2SiO4: Mn thin film phosphor (0.85 m thick) before and after annealing. designates peaks from SiO2.........................................................62 4-3 XRD pattern of Zn2SiO4: Mn thin film phosphor (1.40 m thick) before and after annealing. designates peaks from SiO2.........................................................63 4-4 SEM pictures of the surface of the phosphor films after annealing at 1100 C for the A) 0.85 m sample and for B) 1.4 m thick sample....................................64

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xi 4-5 Cathodoluminescent emission spectra from Zn2SiO4: Mn at V= 5keV, i= 8.0 A, with inset of CL spectrum for 1.4 m thick sample...............................66 4-6 CL brightness versus prim ary beam voltage for PLD Zn2SiO4: Mn thin films.......66 4-7 Comparison of composition in the PLD thin film samples......................................68 4-8 XRD pattern of 1100 C annealed spu ttered films at two different sputter conditions.................................................................................................................70 4.9 Cathodoluminescent spectra from spu ttered deposited thin film samples...............71 4-10 Maximum CL brightness for s putter deposited and annealed Zn2SiO4: Mn thin films from 1000 to 5000 eV.....................................................................................72 4-11 Maximum CL intensity at V= 5keV for i= 2.5 to 7 A...........................................73 4-12 Maximum CL brightness fo r PLD vs. sputter deposited Zn2SiO4: Mn thin films from 500 to 5000 eV................................................................................................74 4-13 Xray diffraction pattern from Zn2SiO4: Mn thin film sputter deposited with target 2.............................................................................................................................. ..76 4-14 Cathodoluminescent emission from sputter deposited Zn2SiO4: Mn at electron beam V= 5keV, i= 15 A for sputter powers of 4070W........................................76 4-15 CL spectra from CCVD Zn2SiO4: Mn phosphor films at beam V= 5keV, i= 7.0 A..................................................................................................................78 5-1 Cathodoluminescent degradation of thin film phosphor at V= 2 keV, i= 3.0 A (95.49 A/ cm2)........................................................................................................84 5-2 Degradation within the first 20 minutes for the thin film phosphor at i= 3.0 A....85 5-3 Cathodoluminescent spectrum of thin film phosphor before and after degradation at low current excitation density..........................................................86 5-4 Auger electron spectrum before and after degradation on thin film, i= 3.0 A.......87 5-5 Cathodoluminescent degradation of thin film phosphor at V= 2 keV, i= 13 A.....87 5-6 Cathodoluminescent spectrum of thin film phosphor before and after degradation at high exci tation current density.........................................................88 5-7 Auger spectrum before and after degradation on thin film, i= 13.0 A...................88

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xii 5-8 CL spectra of thin film Zn2SiO4:Mn at V= 2keV, i= 3.0 A for 10 minutes of continuous beam exposure and recovery over 3.67 hours indicating permanent degradation of phosphor...........................................................................................90 5-9 AES spectra from Zn2SiO4: Mn thin film phosphor during and after beam exposure (i= 3.0 A)................................................................................................91 5-10 CL spectra of thin film Zn2SiO4:Mn at V= 2keV, i= 13.0 A for 10 minutes of continuous beam exposure and recovery over 60 minutes indicating permanent degradation of phosphor...........................................................................................92 5-11 AES spectra from Zn2SiO4: Mn thin film phosphor during and after beam exposure (i= 13.0 A)..............................................................................................93 5-12 Cathodoluminescent degradation of powder phosphor at V= 2 keV, i= 3.0 A......94 5-13 Degradation within the first 20 minutes for the powder phosphor at i= 3.06.0 A...........................................................................................................95 5-14 Cathodoluminescent spectrum of powder phosphor before and after 24 hours of degradation at low exc itation current density..........................................................97 5-15 Auger spectrum before and after 24 hours degradation on powder phosphor, i= 3.0 A..................................................................................................................98 5-16 Cathodoluminescent degradation of powder phosphor at V= 2 keV, i= 13 A........99 5-17 Cathodoluminescent spectrum of powder phosphor before and after degradation at high exci tation current density.......................................................100 5-18 Auger spectrum before and after degradation on powder phosphor, i= 13.0 A..101 5-19 CL spectra of powder Zn2SiO4:Mn at V= 2keV, i= 3.0 A for 10 minutes of continuous beam exposure and recovery over 15 hours indicating permanent degradation of phosphor.........................................................................................102 5-20 AES spectra from Zn2SiO4: Mn powder phosphor during and after beam exposure (i= 3.0 A)..............................................................................................103 5-21 CL spectra of powder Zn2SiO4:Mn at V= 2keV, i= 13.0 A for 10 minutes of continuous beam exposure and recovery over 12 hours indicating permanent degradation of phosphor.........................................................................................104 5-22 AES spectra from Zn2SiO4: Mn powder phosphor during and after beam exposure (i= 13.0 A)............................................................................................105

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xiii Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CATHODOLUMINESCENCE AND DEGR ADATION OF OXIDE THIN FILM AN D POWDER PHOSPHORS By Lizandra Clarissa Williams December 2004 Chair: Paul H. Holloway Major Department: Materials Science and Engineering The low voltage cathodolumines cent characteristics of Zn2GeO4: Mn thin film phosphors grown by pulsed laser deposition were i nvestigated. The effects of substrate heating (600-750C) and subs trate type (MgO, Si, and yt tria-stabilized zirconia) on cathodoluminescent properties were studied. A characteristic green emission peak at 540 nm was observed at substrate temperatures of 650, 700, or 750 C. However, the emission was red shifted to 650 nm for a substrate temperature of 600 C. The red shift in emission wavelength from 540 to 650 nm was attri buted to a change in ionic state of the activator from Mn2+ to Mn4+. While the spectral position was independent of substrate type, the relative intensities of the cathodoluminescent emi ssion peak varied with both substrate type and substrate temperatur e. At a substrate temperature of 600 C, the crystal structure of the film was mixed polycrystalli ne and amorphous. However, at substrate temperatures ranging from 650-750 C, the films were only poly crystalline, with varying degrees of crystallinity that correlat ed with the cathodoluminescence intensity.

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xiv Zn2SiO4: Mn thin film phosphors were de veloped by pulsed laser deposition, sputter deposition, and combustion chemical vapor deposition. Films were grown by pulsed laser deposition at an oxygen pressure of 300 mTorr onto (100) Si substrates heated to 700C. The films were polycrystalline after a ra pid thermal anneal (RTA) for five minutes at 1100C in N2 atmosphere. The cathodoluminescence of the films depended upon the Zn/ Si atomic ratio, which was less than stoichiometric. The film with the higher zinc content exhibited the brightest cathodoluminescence although the film was also the thinnest. An increase in the deposition time was correlated with a reduction in zinc content. Sputter deposition of Zn2SiO4: Mn thin films were completed on room temperature quartz substrates in O2/ Ar atmosphere. The films we re polycrystalline and green cathodoluminescence was observed from the samples after a RTA for five minutes at 1100C in N2 atmosphere. The cathodoluminescence from the films was compared for a sputter power range, 4070W. The cathodoluminescence was brightest for films deposited at 6070W and dimmest for film s deposited at 4050 W. The increased brightness from the films deposited at the highe r power was attributed to an increase in the film thickness. The films created by sputter and pulsed la ser deposition were compared. The sputter deposited film was at least 30% brighter than the films made by pulsed laser deposition. The lower response was attributed to the Zn deficiency present in the films made by pulsed laser deposition. The brightest films were made by combus tion chemical vapor deposition. The Zn2SiO4: Mn phosphor films were deposited onto quartz substrates heated to 1200C at atmospheric pressure with ambient room environment supplying the oxygen needed for

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xv this reaction. The cathodoluminescence was compared at two dopant levels: 2 and 4 mol% Mn, with the higher intensity coming from the 4 mol% Mn film. The degradation behavior was examined for Zn2SiO4: Mn thin film and powder phosphors. The degradation was less for bot h thin film and powder phosphors at a low current excitation density. The CL intensity decreased to 86 and 44% of the CL intensity at time = 0 min. after continuous beam elect ron beam exposure for 24 hours for the thin film and powder phosphor, respectively. As the beam current density increased, an increase in degradation was also noted. Th e degradation for the film phosphor was 74% and 24% of initial intensity for the powder after exposure to a c ontinuous electron beam for 24 hours. The increased degradation with increased beam curre nt was correlated to the presence of more severe charging from the phosphor, as observed from the Auger data. At both current levels, the thin f ilm phosphor degraded less than the powder phosphor. No changes in the surface compositi on, other than the removal of adventitious carbon, were observed through Auger electr on spectroscopy during the periods of electron beam exposure. The degradation of the phosphors is attributed to development of internal electric fields a nd charging of phosphor surface.

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1 CHAPTER 1 INTRODUCTION Cathodoluminescent phosphors may have applications in many emissive technologies, such as field emission flat pa nel displays, where the phosphor is responsible for production of the screen image. There is a need by consumers for portable displays that are more durable and have better resolu tion. Developing an ad equate display that meets these requirements depends on developmen t of the screen material as a critical component. Performance of potential phosphors must be studied to determine which materials would be most adequate. Performance para meters, such as brightness and chemical stability of the phosphor, which affect the reli ability and lifetime of the device must be investigated and understood for successful adva ncements to be made toward identifying potential phosphors for display technologies. Chapter 2 will provide a survey of recent research developments for cathodoluminescent phosphors, with emphasis on oxide phosphors. A discussion of the experimental parameters and characterization methods will be presented in Chapter 3 for Zn2GeO4:Mn thin films grown by pulsed lase r deposition. The results from the development of thin film Zn2SiO4: Mn by sputter deposition, pulsed laser deposition, and combustion chemical vapor deposition are given in Chapter 4. Insight is given into the critical processing parameters n eeded to successfully grow Zn2SiO4: Mn. Chapter 5 provides an analysis of cathodoluminescent de gradation of both powder and thin film Zn2SiO4: Mn phosphors at high and low beam cu rrent densitites, with an aim of

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2 determining the mechanism for oxide degrada tion. Chapter 6 provides a summary of the conclusions from the experimental results for these oxide phosphors.

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3 CHAPTER 2 LITERATURE REVIEW 2.1 Introduction Cathode ray tube (CRT) monitors and tele visions are the conventional display for image and visible information transfer. The CRT television is still the most popular type, [1] although there is competition from other di splay technologies. Flat panel display technologies such as the active matrix liquid crystal display and the plasma display panel are the newest options in the television market [1]. The CRT display has several advantages over its competitors. It offers the best cont rast, best resolution, widest viewing angle, and lowest cost [1]. With a lower price tag than its competitors, the CRT display offers the best value for your money. However, a major disadvantage of the CRT monitor is the size and bulk a ssociated with this type of display. These CRTs are the heaviest of all types of televisions [1]. Many competitive technologies have been developed to respond to this inconvenience. Demand is growing for displays that are lighter and thinner, offering the convenience of easy portability. One technology that meets the demand for a more compact and portable display is liquid crystal display (LCD). The LCD t echnology was developed in the 1970s.[1] These LCDs are used in front projectors, rear-projection TVs, and flat panel displays. The main advantage of LCDs is that they ar e very marketable to consumers who need a compact and portable display. An LCD offers a thinner, lighter, and sleeker alternative to the CRT. However, the customer can exp ect to pay more for this convenience. Disadvantages of using an LCD include limited viewing angle and contrast ratio,

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4 resulting in displays with a lower resolution than the conventional choice.[1] There are markets where small displays would be desirabl e, but an LCD is not the optimal choice. Developing displays that are brighter and more rugged; and that have an extended operating temperature range, and less sensi tivity to constant movement would be beneficial to applications involving militar y, medicine, and transportation displays.[2] 2.2 Development of Cathodoluminescent Phosphors Cathodoluminescent phosphors have been researched for over 100 years. Improvements are still being made in this area, as new applications for cathodoluminescent phosphors have emerged. Braun invented the cathode ray tube oscilloscope in 1897 [3,4], which is noted as th e first practical applic ation of displaying an image by bombardment of electrons. This le d to more investigations on the behavior of phosphors. Phosphors are materials that em it light when stimulated by an incident energy source [5]. In this example of cathodoluminescence (CL), the electrons emitted from the cathode were the source of energy. An example of a cathode ray tube is shown in Figure 2.1. The cathode ray tube is a gla ss tube sealed to maintain a vacuum, where most of the air has been removed. Inside the vacuum tube, a piece of metal coated with a phosphor is placed horizontally between the cathode and anode. This coating on the metal allows the observer to visibly observe the path of the elect rons. The electrons originate from the cathode, which is the nega tive electrode. The electr ode is connected to a power source by the alligator clip. The anode directs the path of the electrons due to the attraction of the electrons to the positive electrode. The result is a visible luminescent line between the negative and positive electrodes indicating the path of the electrons. In 1907, Boris Rosing made a significant developm ent with the cathode ray tube when he demonstrated that it was possible to transmit an image. Since then, many inventions and

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5 discoveries have been made. Presently, cat hode ray tubes are used in many commercial display devices such as televisions, com puter monitors, modern oscilloscopes, and electron microscopes. Alligator Clip Cathode Stand Glass Tube Anode Photon Emission Figure 2-1. Cathode Ray Tube [6]. Initially, black and white televisions relied on only two phosphors for its display image. The phosphors combined to produce a bl uishwhite emission color [7]. The best phosphors for this applica tion were ZnS:Ag and Zn0.5Cd0.5S:Ag or Zn0.9Cd0.1S:Cu, Al [7]. With the development of full color displays the number of elect ron guns increased to three. Each electron beam emitted from the gun is directed to one of three phosphors emitting red, green, or blue. An example of a full color CRT with an illustration on the operation of the electron beams is shown in Figur e 2.2. The display screen is divided into subdivisions called pixels. Each pixel has a red, green, and blue phosphor in its sub-pixel matrix. The number of pixels on a display screen determines the resolution that is

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6 available for the display. As the number of pi xels increase, the color detail for the images also increase. The pixel matrix is depicted in Figure 2.3. Additive mixing of red, green, and blue (RGB) colors result in all of the colo rs needed for a full color display [7-9]. The illustration in Figure2.4 shows the range of colors produced by the RGB phosphors as they are mixed together. Th e best phosphors for full color CRT displays are ZnS: Ag (blue); ZnS: Cu, Al (green); and Y2O2S: Eu (red) [7]. The choice of phosphors will change as the thick CRT evol ves into a thinner model. Figure 2-2. Generation of electr on beams in a CRT display [10].

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7 Figure 2-3. Pixel view beyond shadow mask where A red, green, blue posphors; B shadow mask; C glass of display screen [10] Figure 2-4. Illustration of how the red, gr een, and blue hues combine to give a full spectrum of colors for full color displays [9]. 2.3 Evolution of the Fi eld Emission Display Field emission displays (FEDs) are flat panel displays that are similar to the conventional cathode ray tube (CRT) displays [11]. Both technologies produce an image on the display screen by using an electron sour ce. The main components of the FED that have been the attention of many researcher s are the phosphor and emitter. The emitter is the component that produces the electrons. The phosphor has the responsibility of producing the image on the device screen. The filed-emitters for an FED are analogous to the electron guns found in CRTs. A CRT uses only three electron guns, whose focu sed beam of electrons are scanned across the screen to produce an image; whereas, FEDs use an array of thousands of micron scale

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8 cold cathode emitters to direct electrons at each pixel as shown in Figure 2.5. These small emitters allow FEDs to be scaled to millimeters in width, allowing for the feasibility of a thin display. However, the narrow thickness requires device operation at lower voltages to prevent vacuum breakdown [12]. In order to obtain an acceptable brightness at low voltages, the current de nsity requirement must be increased. A thermionic cathode using high accelerating voltages produces electrons in CRT. The voltages may range from 2030 kV for a CRT [13,14]. In FED, the electrons are produced via a cold cathode by electron tunne ling at high fields. The target operating voltage for an FED ranges from 18 kV [1 3]. The lower voltage results in a higher current to maintain necessary power to yiel d adequate brightness from the phosphor. The amount of light that is emitted from the phos phor depends upon beam power, i.e. primary beam voltage times current [13]. The phosphor s used for CRTs are usually coated by a thin aluminum layer (0.20.5 m), as shown in Figure 2.6 [13]. [13]. The phosphors used for CRTs are usually coated by a thin aluminum layer (0.20.5 m), as shown in Figure 2.5. Pixel view of field emission display.

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9 Figure 2.6 [13]. The cross section shows that the aluminum layer may protect the phosphor from the reaction with residual vacuum gases. However, for the FED, the aluminum layer is not always applied. This leaves the phosphor e xposed to the vacuum environment. Consequently, the effect of the residual vacuum gases on the phosphor are a critical reliability concern for the devi ce. Thus, conditions such as charging and outgassing are huge concerns for this applicati on [2,13]. Therefore, it is important to understand what happens to the phosphor duri ng electron beam exposure in different ambients. Anticipated advantages of FEDs over LCDs include: better viewing angle, better response time, wider operating te mperature range, better price & power consumption [13]. Figure 2-6. Cross section view of phosphor screen for a CRT [13]. 2.4 Cathodoluminescent Phosphor Materials 2.4.1 Host Material The host material is responsible for th e electrical and optical properties of the phosphor. The inert host lattice is transpar ent to the excitation radiation [13]. Nevertheless, the surroundings of the activator in the host material dictate the optical behavior of the activator.

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10 2.4.2 Activator-Luminescent Center The activators control the emission spectra of luminescent materials [15]. An activator, or luminescent center, is an impur ity added to the host material in small quantities. It is known as the light-emitti ng center of a phosphor and is responsible for the optical properties of a phos phor. Activators provide dis tinct energy levels in the energy gap between the conduction and valence bands of the host material [15]. The interaction of the incident energy source with a luminescent material is critical for the production of light. In the case of electronhole pair exci tation by the primary electrons for CL, the energy levels associated with the activa tors determines the energy of the photon emission. A phosphor absorbs the incident energy, which leads to electron excitation from the valence band to the conduction band. It is the behavior of these electrons, which determines the luminescence of a phosphor. Th e electrons are excited either to the conduction band or to a trap as s hown in the first step illustra ted in Figure 2.7. The traps are the distinct energy levels formed by the ac tivator. These energy levels are referred to as traps since electrons and/ or holes may become held in them for long times at the temperature of operation. In the second step, shown in Figure 2.7, the electrons may be excited from one trap to anothe r or into the conduction band. The elemctrons can then be captured by upper empty activator levels, and subsequently emit photons when they drop further down to lower excited state or ground stat e activator levels, as shown in step 3 of Figure 2.7. The energy of the photon emitted, E, is associated with a specific wavelength of light, as given by [16]: E hv (2-1)

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11 and with substitution of the appropriate phosphors, results in: ) ( 2398 1 ) (eV E m (2-2) Traps Band Gap Energy, Eg Excitation Activator level Emission 1 2 3 Activator level donor state Activator level acceptor state Emission 3 Valence band Hole Conduction band Electron Figure 2-7. Energy excitation (absor ption) leads to photon emission. 2.5 Materials Development Phosphors are often characterized as either a sulfide or oxide phosphor. Conventional cathodoluminescen t phosphors are sulfide based, where sulfur is an element present in the composition. 2.5.1 Sulfide Phosphors A lot of research has been devoted to su lfide phosphors since they are used in many display applications. Zinc sulfide, ZnS, is a common host material for cathodoluminescent and electroluminescent phosphors. It has been noted for its excellent electrical properties. The most efficien t materials for host lattice excitation have relatively small values for the band gap energy, Eg. The band gap energy of ZnS = 3.75

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12 eV. Thus, zinc sulfide has been noted for its excellent cathodoluminescent efficiency of 2025% [3]. Depending upon the dopant, this phosphor can exhib it a specific color luminescence. Many elements have been incorp orated in the ZnS host lattice. Table 2.1 shows some of the dopants and colors that may be emitted from the ZnS phosphor. Table 2-1 Color and wavelength emission for common dopants in ZnS host [2,3,7]. DOPANT COLOR Mn Yellow Mn (filtered) Red Cl Blue Ag, Cl Blue Cu, Cl (or Al) Green Tb or TbOF Green Tm Blue Sm, Cl Red Sulfide phosphors have also been considered for use in FEDs [17]. However, these phosphors are not very efficient at low volta ges. In addition, conventional sulfide phosphors such as ZnS:Cu,Al degrade signifi cantly and decompose under electron beam bombardment at the operating conditions requir ed for field emission displays [18-20]. The products of degradation may potentia lly contaminate the cathode components, reducing the performance of the emitters [20,21].

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13 2.5.2 Oxide Phosphors Oxide phosphors have also received attenti on as potential phos phors in low voltage display applications, such as the FED. Ox ide materials are generally more chemically and thermodynamically stable than sulf ide phosphors [22,23], so they are now investigated as the potential phosphor type for low voltage ap plications. There are a host of oxides that have been i nvestigated and a summary of these phosphors are given in the following table. Table 2-2 Common oxide phosphors and th eir luminescent properties [24]. PHOSPHOR COLOR Zn2SiO4: Mn Green ZnO: Zn Green ZnGa2O4: Mn Green Y3Al5O12: Ce Green ZrO2: Mn, Cl [25] Red Y2O3: Eu Red YVO4: Eu Red Y2SiO5: Ce Blue ZnGa2O4 Blue CaWO4 Blue 2.5.2.1 Zinc silicate phosphors (Zn2SiO4: Mn) Zinc silicate is already known as a phosphor for CRT and plasma display applications. Several research groups have investigated its luminescent properties in powder and thin film form [26-33]. Its f easibility as a phosphor for field emission

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14 displays, electroluminescent devices, medi cal imaging detectors for lowvoltage radiography and fluoroscopy is also being studied. Sun et al. observed a corr elation between the film cr ystallinity and morphology for as deposited and annealed films. The films were grown on heated 300 C substrates by pulsed laser deposition and were annealed at 1000 C, before Zn2SiO4 crystallized [30] indicating that a high temperatur e treatment is needed for a pol ycrystalline material. The morphology, observed by scanning electron micr oscopy, indicated that the finer grains and rougher surface correlates with an increase in the photoluminescence intensity due to disruption of light guiding in high index thin films [30,34]. The high crystallizati on temperature of Zn2SiO4 is a definite disa dvantage. Cho et al. demonstrated that temp eratures as high as 1400 C may be necessary to fully crystallize Zn2SiO4 when the powder is processed in a solid state reaction. The authors also developed a novel solution reaction met hod to lower the processing temperature. However, the temperature was reduced by only 200 C [29]. One group has tried to overcome the temperature hurdle, by processing zinc silicate by combustion synthesis. Nonetheless, after in itial processing at 500 C, the powder still re quired heating to 900 C for 1 hour in a reducing atmosphere, where th e powder then gained the characteristic white appearance [28]. The authors also in dicated that the sec ond thermal treatment improved the luminescence of the Zn2SiO4: Mn phosphor. Zinc silicate is a promising phosphor for CL applications. A brief summary of the relevant properties for zinc silicate is s hown in Table 2.3. Cathodoluminescent emission has been observed with peak emission exhibi ted around 525 nm [33]. Its high efficiency makes it a possible phosphor in low voltage appl ications. However, the need for a high

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15 processing temperature is a de finite disadvantage. Anothe r approach for getting beyond the temperature hurdle has been to alter the composition of the material as will be discussed in the next section. Table 2-3 Summary of Zn2SiO4: Mn properties [30]. Property Value Bandgap 5.5 eV Index of Refraction 1.69 Crystal structure Rhombohedral 2.5.2.2 Zinc germanate phophors (Zn2GeO4: Mn) Zinc germanate has been evaluated as a potential phosphor in alternating current thin film electroluminescent displays (ACTF EL) [22,35-37]. In this phosphor, Ge atoms substitute for the Si atoms from Zn2SiO4. The resulting composition, Zn2GeO4, leads to a lower crystallization temperature and a smaller bandgap. Crystalliza tion temperatures as low as 650 C have been reported. Some resear chers found that a slight raise in processing temperature to 810 C improved the luminance of the electroluminescent phosphor. A survey of the l iterature indi cates that Zn2GeO4: Mn thin films have been fabricated mostly by sput ter deposition [22,35-37]. A brief summary of known zinc germanate properties are shown in Table 2.4. The electroluminescent emission peak of Zn2GeO4: Mn has been observed to be between 535540 nm [22,35,38]. Zn2GeO4: Mn has been characterized as having a short decay time (100 microseconds), resulting from the formation of a perfect structure and the low coordination number (4) of manganese in host lattice [38]. The cathodoluminescent

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16 efficiency of Zn2GeO4: Mn at 2 keV has been reported as 2.4 lm/W for the thin film phosphor [23]. Table 2-4 Summary of Zn2GeO4: Mn properties. Property Value Bandgap 4.68 eV [35] Index of Refraction 1.80 [22] Dielectric constant 6 [22] Crystal structure Rhombohedral Zinc silicategermanate (Zn2Si0.5Ge0.5O4: Mn) thin film phosphors have also been created by sputter deposition [ 39,40]. This phosphor was developed from incorporating Ge into the silicate (SiO4) lattice of Zn2 SiO4. The Ge substitutes for the Si resulting in a lower annealing temperature. Temperatures as low as 700 C have been reported for annealing conditions, resulting in a polycrystalline film [39,40]. The photoluminescent and electroluminescent emi ssion peak of annealed Zn2Si0.5Ge0.5O4: Mn is at ~ 530 nm [39,40], a slightly shorter wavelength than pure Zn2GeO4: Mn. It is interesting, however, that the asdeposited amorphous phosphor fi lm show PL emission at two separate wavelengths, which is not near the emissi on at 531nm, observed only in the annealed case [40]. This PL emission behavior is show n in Figure 2.8. No further explanation of the emission results was given by the authors.

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17 Figure 2-8. PL spectra for as deposited and annealed Zn2Si0.5Ge0.5O4: Mn thin films [40]. 2.6 Processing of Phosphors 2.6.1 Powder Phosphors Powder phosphors are usually made by a solid state reaction, although there are various other methods that are employed as well. In this process, the raw materials are first combined through chosen synthesis method. The powder is then fired with a flux and the activator [7]. After the product is sieved, the flux is removed. The powder is then milled with care to obtain th e desired particle size [7]. 2.6.2 Thin Film Phosphors Thin film cathodoluminescent phosphors have been grown by a variety of growth techniques. Pulsed laser depos ition, sputter deposition, elec tron beam evaporation, and metalorganic chemical vapor deposition ar e some of the common processes used to develop thin film phosphors [30,32,41-44]. In this work, pulsed laser deposition and rf magnetron sputter deposition were used to creat e the thin film phosphors. Accordingly, further details of these methods are given below.

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18 2.6.2.1 Pulsed laser deposition Pulsed laser deposition is one of the most simplistic methods to deposit thin films. It allows for the precise arri val rates of atoms for compound f ilms. This has been shown to be favorable for obtaining stoichiometric films of multi-compone nt materials with a high energy of dissociation [45]. This method has received considerab le attention for its ability to deposit the comple x oxides needed to produce su perconducting thin films [4549]. It also has the ability to operate in high pressure reactiv e gases, unlike other deposition methods [45]. A deposition system usually consists of an excimer laser and optical elements to maneuver and focus the laser beam. Some of the optical elements that are used in the set up are focusing lens, apertures, mirrors, beam splitters and laser windows. A schematic of a basic deposition system that uses oxygen as its reactive gas is shown in Figure 2.9. Target Substrate Laser Vacuum Pump Oxygen Gas Plume Figure 2-9. Sketch of basic pulse d laser deposition system [45].

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19 Excimer lasers with wavelengths between 200 and 400nm are most often used for pulsed laser deposition [45]. A list of co mmon excimer lasers and their operating wavelengths are given in Table 2.3. Excimer lasers below 200nm are not typically used for PLD due to the possibility of absorpti on by the SchumannRunge bands of molecular oxygen. As shown in Figure 2.9, the laser source is located external to the vacuum chamber. The external energy source allows the film growth proce ss to take place in a reactive environment with any type and amount of gas. The external source gives the added advantage of the laser being availa ble for more than one deposition system. Table 2-5 Typical excimer lasers a nd their operating wavelengths [45]. Excimer Wavelength (nm) F2 157 ArF 193 KrCl 222 KrF 248 XeCl 308 XeF 351 Once the laser is focused into the chambe r, the target absorbs the energy from the laser. The ultraviolet (UV) radiation is converted to elec tronic excitation. This is converted into thermal, chemical, and m echanical energy, lead ing to ablation and evaporation of the target. The evaporants fo rm a mixture of energetic species including atoms, molecules, electrons, ions, and micron si zed particulates. This mixture is the often referred to as a plume. An example what a plume looks like during the film growth process is shown in Figure 2.10. The plum e quickly expands in the vacuum from the

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20 target to form a “nozzle jet” [45]. As th e plume reaches the substrate (which may be heated), film nucleation commences. The quality of the thin films produced by pulsed laser deposition is dependent on several variables. Laser power and spot size have a significant effect on particulate size and density. As the laser fluence is increased beyond a threshold, the number of particulates that are formed also increases [45]. Laser fluence is defined as the laser energy per unit area and thus, may be adjusted by varying the laser power or laser spot size. Background gases may change growth pa rameters such as the deposition rate and the kinetic energy distribution of the depositing species [45] For instance, an oxidizing environment can help oxides to form and stabilize the desired crystal phase at the deposition temperature [45]. Substrate temper ature has an effect on the stoichiometry of the film as well as the film structure [45]. Film structure has also been influenced by the deposition rate. Wu et al. found that an increas e in the deposition rate led to a decrease in the crystallinity of YBa2Cu3O7thin films. At the higher deposition rates, the arrival rate exceeds the diffusion rate. Equilibrium cond itions are not maintained and structural defects are formed [47].

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21 Figure 2-10. Picture of plum e developed during PLD [45]. In summary, the advantages of this growth method include [45]: Flexibility to use energy source with more than deposition chamber Easy process control Ability to use high reactive gas pressures Decreased contamination from outside sources Control of film stoichiometry. The short laser pulses result in congruent evaporants. Congr uent evaporation aids in stoichiometry control of the thin films during mass transfer from target to substrate. One obvious disadvantage is the pres ence of micron sized particulates. Also scale up to large area deposition is not easily completed. Pulsed laser deposition has been shown as an appropriate method for growing phosphor films, specifically oxide phosphors. Yttria and silicate phos phors are some of the oxide materials that have b een grown by this method [30,34].

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22 2.6.2.2 Sputter deposition Sputter deposition is another thin film growth method that allows the use of a solid target based on the same composition that is expected in the resu lting film [50,51]. Sputter deposition is performed by extracting ions (usually Ar) from a plasma that strike a target consisting of the material to be de posited. The plasma is formed by partially ionizing an inert background gas that is flowcontrolled into the system. The energetic Ar ions produce a continuous fl ux of sputtered atoms that deposit on a nearby substrate. The plasma is sustained by a DC voltage, radio frequency (rf) power, or a magnetron operating at milliTorr pressures. A typical s puttering system is composed of a stainless steel vacuum chamber, pumping system, intern al sputter source, and a biased substrate holder. A schematic of a typical sputter de position chamber is given in Figure 2.11. Radio frequency (rf) magnetron sputtering is often used for dielectric thin film growth [52-54]. The higher de gree of ionization associated with this sputtering method makes it a popular choice for those material s which would otherwise be affected by charging [51,54,55]. With this method, the ta rget material self biases to a negative potential and is charged as the cathode [53] In this case, the background gas is first ionized by primary electrons. These positive ions may be accelerated to energies adequate for sputtering the negatively powered cathode, and upon bombardment, emit secondary electrons and atoms from target su rface. Magnetron sputter sources have a magnetic field of 50500 gauss parallel to th e target surface [52]. The placement of the magnets relative to the target is shown in Fi gure 2.12. In combination with an electric field, the magnetic field causes the secondary elec trons to drift in a cl osed circuit in front of the target surface. Consequently, the magnetic field controls the motion of the

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23 electrons since a magnetic field can exert a force on a charged particle in motion as dictated by Lorentz’s law [56]. Lorentz’s law is given by: B v q F (2-3) where F is the force exerted on a particle with charge, q, and velocity, v, from an incident magnetic field B. Figure 2-11. Schematic diagram of a typi cal set up for a s putter system [52]. The magnetic field also dictates that the particles move in a helical path with a radius, r, determined by [56]: qB mv r (2-4)

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24 where m is the mass of the charged particle, q is the charge of the particle, and v is the component of the particle’s velocity normal to the applie d magnetic field. Due to electrostatic attraction, the Ar ions move with the electrons keeping the plasma neutral. Under the best conditions, the plasma discharg e is kept close to the cathode surface, and bombardment of the growing film by electrons and ions is minimized. However, it is possible to get “negative ion resputtering”, resu lting in sputtering of film as well [57]. Ion resputtering is characterized by damage to film, resulting in amorphous structure [57]. Condensation of the at oms from the target onto th e substrate initiates film nucleation [54]. Sputter deposition is used in many comm ercial applications. Compact discs, integrated circuits, magneto optical storage media, window coatings, and wearresistant coatings are some of the industries that have found sputter deposition applicable to their respective processing methods [52,53]. Some of the advantages of sputter deposition that have been realized are [52]: Control of thickness Good film adhesion High deposition rates Good film uniformity over large area. The disadvantages of sputter deposition include the inevitable waste of target material. Only about 2030% of the target materi al is used in magne tron sputtering [52]. 2.7 Evaluation of Phosphor There are several experiments that ar e completed to evaluate the overall performance of phosphors. This includes chroma ticity, spectral dist ribution, and lifetime of luminescence.

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25 Figure 2-12. Planar magnetron sputter source [56]. 2.7.1 Chromaticity A quantitative method has been establis hed that relates the color produced on display screens to a standard value. The 1931 Commission Internationale de l’Eclairage (CIE) established a standard that defines ch romaticity by x,y, and z coordinates. The chromaticity values are plotted on a twodime nsional graph, shown in Figure 2.13, using the x and y coordinates. The th ird value, z, is found by knowing x+y+z=1. Thus, when mentioning the chromaticity of a phosphor, usually the x and y coordinates are only given. The CIE diagram also shows how re d, green, and blue blend to give all hues needed for image production on a display screen. 2.7.2 Spectral Distribution The spectral distribution gives information about the characteristic emission of a phosphor. The visible light emission of phosphors is within 400700 nm This range of wavelengths is divided into six major divi sions for the following colors: red, orange, yellow, green, blue, and violet. An example of this range with its corresponding colors is

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26 shown in Figure 2.14. The color that each phosphor emits corresponds to a wavelength within the visible light spectrum. Figure 2-13. CIE chromaticity chart. Figure 2-14. Visible light spectrum and corresponding wavelengths [58].

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27 2.7.3 Degradation Characteristics In this section, vacuum ambient eff ects on phosphor performance and electron stimulated surface chemical reac tions will be discussed. A phosphor is said to experience degradation when there is a loss in the CL intensity over time during exposure to an electron beam. Several research groups ha ve studied degradation of sulfide phosphors extensively [18,19,59-61]. Several theories have been developed to explain the degradation of a phosphor. Pfahnl has studied the rate of degradation of several phosphors. (T his will be expandedneed to get book) Surface chemical reactions have also been investigated as a culprit for the observed degradation in cathodoluminescent phosphors. The electron stimulated surface chemical reaction model was developed to explain the loss of cathodoluminescence for these phosphors through the development of a “dead laye r”. The dead layer is the surface layer that inhibits or reduces the CL intensity. A mathematical model for describing electron stimulated surface chemical r eactions (ESSCR) has been developed by Holloway et al. This model explains that the degradation is dependent upon the type and concentration of gases present in the vacuum, energy of electron beam, as well the beam current and time of exposure to electron beam. A descrip tion of this mathematical model follows. This model focuses on the surface intera ctions, since the low beam energy would involve the surface of the phosphor rather th an the bulk [18]. Additionally, the model uses the ZnS phosphor for its description of the material system. In terms of cathodoluminescent degradation for the ZnS system, the loss in sulfur (S) has been correlated with a loss in CL intensity. This concentration of S on the surface was modeled by the following standard chemical reaction rate equation:

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28 n as s sC kC dt dC (2-5) where Cs is the concentration of S on the surface, k is the chemical rate constant, Cas is the concentration of the adsorbed atomic species that will react with ZnS, and n=1 where a first order reaction is assumed [18]. The model also assumes that the chemical reaction takes place on the surface, not in the gas phase, such that Cas can be expressed as: as m ma asJ C Z C (2-6) where Z is the number of reactive atomic species produced from the parent molecule, ma is the dissociation cross section of the molecule to atoms, Cm is the surface concentration of the molecular species, J is the current density of the electron beam causing the dissociation, and as is the lifetime of the reactive sp ecies lifetime [18]. It should be noted that the dissociation cross-section is a function of the electron beam energy. This expression dictates that the reaction rate is limited by th e rate of production of the adsorbed species. In other words, the rate of production of the adsorbed atomic species that will react with ZnS is controlled by the surface concentra tion of the molecular species. This adsorbed molecule concentration, Cm, may be expressed by Henry’s isotherm as: 2 / 12 exp mkT P kT Q Cm o m (2-7) where is the molecular sticking coefficient (ass umed to be independent of coverage), o is the mean time between attempts by the physisorbed molecule to escape from the surface, Q is the energy required to desorb from the surface, k is Boltzman’s constant, T is the absolute temperature, and Pm is the partial pressure of the molecular gas in the vacuum. The first term in brackets in the preceding equation colle ctively describes the

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29 molecular mean stay time on the surface, wher e the second term is an expression for the molecular flux onto the surface. Inserting e quation 2-7 into 2-6 and then equation 2-6 into 2-5 results in the following rate expression: 2 / 12 exp mkT P kT Q J Z C k dt dCm o as ma s s (2-8) This equation may be adjusted to dt JP K C dCm s s' (2-9) where 2 / 12 exp mkT kT Q Z k Ko as ma (2-10) Integrating equation 2-7 with respect to time and using the boundary condition, Cs = Co for t = 0, results in: Jt P K C Cm o s s' exp (2-11) where Jt is the coulombic dose [Coulombs/ Area]. The resulting model predicts that the concentration of sulfur will decrease expone ntially with the coulombic dose and the cathodoluminescence loss rate increases with a higher pressure of th e molecular gas in the system. These predictions have been observed e xperimentally [18] and are shown in Figures 2.15 and 2.16. Figure 2.15 shows th e loss in CL intensity for a ZnS: Ag phosphor irradiated by a 2 keV electron beam at varying vacuum pressures. This graph shows experimentally that the CL degradati on rate is dependent upon the gas pressure in

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30 the vacuum system as predicted by the mathema tical model. The rate of degradation is higher at higher gas pressures. Figure 2.16 shows that the prediction from the model for an exponential loss in sulfur surface concentration with electron dose can be observed experimentally. The degradation of one oxide phosphor, Y2O3: Eu, has been investigated [62]. The degradation behavior of this powder phosphor was evaluated as function of the oxygen pressure in the vacuum. Y2O3: Eu was exposed to electron beam energy of 2 keV and a high current density of 88.5 mA/ cm2 in a vacuum atmosphere of 1x 10-7 Torr oxygen. After an electron dosage of 3500 C/ cm2, the CL intensity was reduced to 45% of its original intensity as shown in Figure 2.17 [62]. The yttrium, carbon, and oxygen peaks were also monitored by Auger electron spect roscopy (Figure 2.17) during electron beam exposure. However, no change in the si gnal from these elements was detected. Therefore, even though it was clear that the presence of oxygen affects the rate of degradation, the mechanism that resulted in the reduced CL intensity could not be determined [62].

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31 Figure 2-15. Semi logarithmic plot of CL intensity vs. electron dose for a ZnS: Ag phosphor [18]. Figure 2-16. Linear and loga rithmic plot of S Auger peak to peak height (APPH) vs. electron dose for ZnS: Ag phosphor [18].

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32 Figure 2-17. Plot of CL intensity of Y2O3: Eu phosphor and selected Auger peak to peak heights vs. electron dose [62].

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33 CHAPTER 3 CATHODOLUMINESCENCE FROM THIN FILM ZINC GERMANATE DOPED WITH MANGANESE PHOSPHORS 3.1 Introduction In this study, the low volta ge cathodoluminescent propert ies of thin film zinc germanate doped with manganese (Zn2GeO4: Mn) are examined. Pulsed laser deposition was used to grow Zn2GeO4: Mn on magnesium oxide (MgO), yttria stabilized zirconia (YSZ), or silicon (Si) substrates. Th e structural properties as well as the cathodoluminescence (CL) and photoluminescence (PL) spectra of the films are reported. The variation of the CL emission spectra with film deposition temperature and crystalline quality of film is also studied. The first section in this chapter outlines the experimental procedures used to develop and characterize the thin film samples. The second section reports on the results from the characterization and experiments. Finally, the last section will provide insight into the influence of deposition temperature and type of substrate on the luminescent qualities of the Zn2GeO4: Mn phosphor. 3.2 Experimental Procedures The preparation of the substrates and the processing steps for the film development in outlined in Figure 3.1. The details of each step are described in this section. 3.2.1 Substrates and their Preparation Single crystal (100) MgO, ( 100) yttria stabilized zirc onia (YSZ), and (100) Si substrates were chosen as the substates for th is experiment. The substrates were cleaned

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34 by a solvent wash in a sonicator prior to deposition. The solvents that were used were trichloroethylene, acetone and methanol. Th e substrates were cleaned in 3 sequential solvent washes, with each wash lasting five mi nutes. The substrates were dried by air. 3.2.2 Phosphor Processing A Zn2GeO4 ablation target doped with 1.5 at % Mn was prepared by mixing and then ball milling ZnO, GeO2, and MnO2 powders for 60 minutes. The mass of each powder used for the mixture is given in Table 3.1. The mixture was then calcined in air at 1000 C for 8 hr in covered alumina crucible s that were placed in a conventional furnace. The powder mixture was milled again for 60 minutes and was then checked for photoluminescence. A hand held ultraviolet (UV) lamp was used for the check. Visible green luminescence from the powder was obser ved with human eye. Next, the powder mixture was pressed into 2.5 cm diameter target s. The targets were sintered in air at 1250 C for 36 hours in a conventional furnace. In addition, ZnO powder was milled, pressed into targets, and sintered. A Zn2GeO4: Mn/ ZnO mosaic target was formed to control the cation ratio of Zn/Ge in the films. The area ratio of the target was 75 % Zn2GeO4: Mn and 25 % ZnO. Previous research has shown th at a zinc rich target can compensate for Zn loss at elevated temperatures during de position, which is due to the high vapor pressure of zinc [63,64]. The target was ro tated during the depositi on cycles to ensure that the laser ablated the Zn2GeO4: Mn and ZnO portions of the target.

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35 Figure 3-1. Flow chart outlining substrate preparation and phosphor processing method. Select & cut substrate s : (100 ) Si, (100) MgO, and YSZ Sonicator wash in each for 5 minutes : A) Trichloroe t hy lene B) Acetone C) Methanol All 3 s ubstrate s attached to heater stage with silver paint Heater stage heated to desired temperature (600, 650, 700, or 750 C) Pho sphor deposited onto substrates Cool substrates t o room temperature inside deposition chamber

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36 Figure 3-2 Representative sketch of mosaic target where 75% area = Zn2GeO4: Mn and 25% area = ZnO. Table 3-1 Raw materials used to make target. Material Mass (g) ZnO 12.1758 GeO2 7.8242 MnO2 0.0975 The films were grown using an excimer KrF laser with a wavelength of 248 nm. The energy of the laser was 130 mJ/pulse. The target was preablated before each deposition cycle. Four different depo sition temperatures (600, 650, 700, and 750 C) were maintained by a substrate heater and a th ermocouple attached to the substrate stage. The substrates were allowed to reach the desired temperature before film growth commenced. The number of pulses and pul se frequency during deposition was 10,000 pulses at 10 Hz, immediately followed by a second cycle of 20,000 pulses at 20 Hz. The oxygen partial pressure in the system was maintained at 100 mTorr during all film growth.

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37 3.2.3 Thin Film Phosphor Characterization 3.2.3.1 Xray diffraction Xray diffraction (XRD) is a useful method for identifying the crystalline phases present in a sample, as well as for measuring the structural properties of these phases. For instance, it can also be used to determ ine the preferred orientation of the phases present in a crystalline material [65]. Xray diffraction results from in cident Xrays that are scattered by the atomic planes present in a crystal. When there is constructive interference from these Xrays, a diffracti on peak is observed [65]. The condition for constructive interference is dict ated by Bragg’s law such that hkl hkld sin 2 (3-1) where is the wavelength of the incident Xray, dhkl is the dspacing between (hkl) planes, and hkl is the angle between the atomic plan es and the incident Xray beam. These terms are illustrated in Figure 3.3 whic h shows the interaction of the Xray with the specimen. The diffraction angle, 2 is the angle between the inci dent and diffracted Xrays (Figure 3.3). The XRD experiment yields da ta for the diffracted intensity vs. diffraction angle. For polycrystalline thin films, diffraction occurs from any crystallite or small crystalline region that satisfies the diffracti on conditions [65]. If the distribution of orientations is random, diffraction peaks will re sult from more than one plane, similar to a powder diffraction pattern. As a result, multiple peaks will be present in the pattern representing the different orientations of th e crystallites. A textured film will possess a preferred orientation of the crystallites, wh ere most of the crystallites have parallel planes.

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38 Figure 3-3 Interaction of in cident and diffracted Xrays in an XRD specimen [65]. For this study, the crystalline quality of the films was investigated using a Philips APD 3720 Xray diffractometer. Xray diffraction (XRD) was completed with Cu K radiation (0.15406 nm wavele ngth) generated by a 40 keV and 20 mA electron beam. The scan range for the samples was from 10 to 80 2theta degrees, while the scan rate was 0.080 2/ sec in continuous scan mode. The re sulting Xray diffraction patterns were then indexed with a collection of patt erns from the Joint Committee on Powder Diffraction Standards (JCPDS) catalog. 3.2.3.2 Relative composition analysis: EDX Electrons may interact with the sample to produce multiple signals. These signals include X-rays, UV and visible emission, and au ger electrons, as depicted in Figure 3.4. The signals result from the electron interac tions within the sample volume. Electron probe microanalysis (EPMA) is a compos itional analysis method that uses the characteristic Xray emission generated by an electron beam to provide a quantitative analysis of the elements pres ent in a specimen. This met hod relies on an electron probe,

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39 thus resulting in fine spatial resolution as low as 100 nm [65]. The accuracy of the quantitative analysis is dependent upon the st andards used for the analysis. To reduce error, the sample and standards must be measured under identical conditions of beam energy and spectrometer parameters and should be normalized to the same electron dose. Failure to replicate conditions will in crease the error in the measurements. Energy dispersive Xray (EDX) spectrosc opy is one type of EPMA that detects elements with an atomic number higher than beryllium, Z= 4. For EDX, the incident electron beam ionizes and ejects inner shell electrons from the atoms within the sample. The atoms are returned to ground state when an electron from a higher energy shell moves to fill the inner shell vacancy. Duri ng the transition, the electron releases the amount of energy equal to energy difference be tween the two shells. This excess energy, unique for each atomic transition, may be rel eased in the form of an Xray photon [65]. This X-ray is referred to as the characteristic Xray for that atom and is detected by the spectrometer. The minimum detection limit for an EDX spectrometer attached to a scanning electron microscope is about 0.1 wt % [65]. Energy dispersive Xray fluorescence (EDX) with a JEOL JSM 6400 scanning electron microscope (SEM) at a primary beam energy of 5 keV was used to measure the relative concentrations of the elements in th e film. The absolute values of the atomic content for these films are considered to be s ubject to large errors because the sensitivity factors for the Zn and Ge L signals used were from l ookup tables using different transitions and higher primary beam energies Neither factor could be accurately corrected for by the quantitativ e calculation program.

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40 Figure 3-4. Interaction volume in sample where electron beam penetrates and the resulting signals are generated [66]. 3.2.3.3 Cathodoluminescence characterization The theory of cathodoluminescence was introduced in Chapter 2. Cathodoluminescence (CL) from the phosphors wa s measured in an ultra high vacuum stainless steel chamber depicted in Figure 3. 5. The ultra high vacuum was maintained by a Perkin Elmer Ultek DI ion pump, wh ich maintained a system pressure of approximately 1 X 10 –8 Torr. An Edwards RV3 turbo molecular pump was used for initial pumping of the system to about 1 X 10-6 Torr, after which the system was crossed over to the ion pump. The system also has a load-lock chamber. The chamber allows introduction of new samples into the vacuum system without disturbing the vacuum. The CL system recovers to its normal pressure within thirty minutes Cathodoluminescence

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41 was stimulated by an electron beam generate d by a Kimball Physics EFG7 electron gun operated at 4 keV and 8.5 A. An optical fiber connected to an Ocean Optics S2000 optical spectrometer was used to detect phot oemission from the samples. The detected spectral range was from 200 to 850 nm. 3.2.3.4 Photoluminescence characterization Photoluminescence (PL) is the excitation of photons by absorption of light. It can be used to provide a qualitative analysis of a sample. The wavelength of the emitted light is longer than the incident light [65]. The spectral properties can be analyzed to provide information such as the optical, electrical, and structural properties of the material [65]. A photoluminescence excitation (PLE) spectrum can be used to find out information about the excitation process th at leads to the photon being emitted. The intensity and spectral properties of a PLE spectrum is de pendent upon the absorption of the incident light and the initial and relaxed excited states that take part in emission [65]. For PLE, a set emission wavelength is monitored and the wavelength of the incident light is scanned through a desired range. The photoluminescence measurements were taken in a dark room at room temperature. A xenon lamp (Oriel Instru ments, model 66902) was selected as the excitation energy source for photoluminescen ce. The excitation wavelength could be tuned to the desired level with a monochro mator (Oriel Instruments, Cornerstone 74100 spectrometer) placed in between the lamp and sample. Any wavelength between 200 and 1200 nm could be selected with the excita tion monochromator. The emitted light was focused to a monochromator (Oriel Instru ments, MS257) and photomultiplier tube

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42 (Oriel Instruments, 77265) that has the ab ility to detect photoemission from 300 to 800 nm. Electron Gun Cathodoluminescent Phosphor Electron Gun Cathodoluminescent Phosphor Figure 3-5 Picture of cathodoluminescence vacuum system. 3.3 Results 3.3.1 XRay Diffraction Results The Xray diffraction (XRD) data in Figur e 3.5 show the effect of the deposition temperature on the structural properties for film grow n on MgO substrates. At 600 C

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43 (Fig. 3.5d), (223) and (550) weak diffraction peaks for Zn2GeO4 are evident indicating that some structural order exists. Howeve r, the weak peak intensity and the broad maximum at 2of ~ 18 suggest that the majority of the film was amorphous with respect to Xray diffraction. The presence of additional Zn2GeO4 diffraction peaks in the XRD pattern shown in Figure 3.5 (ac) indica te that the films grown at 650, 700, and 750 C are polycrystalline and exhibi t better long-range order. Additional diffraction peaks at 2of ~ 28 and 36 for films grown at 750 and 700 C are attributed to an impurity GeO2 phase. The shoulder diffraction peak at 2of ~ 41, detectable only in film grown at 750 and 700 C, also results from the GeO2 impurity phase. 102030405060702 (degrees)Intensity (a.u.)(a) (b) (c) (d) MgO(100)(220) (113) (410) (223) (333) 2 (deg) 102030405060702 (degrees)Intensity (a.u.)(a) (b) (c) (d) MgO(100)(220) (113) (410) (223) (333) 102030405060702 (degrees)Intensity (a.u.)(a) (b) (c) (d) MgO(100)(220) (113) (410) (223) (333) 102030405060702 (degrees)Intensity (a.u.)(a) (b) (c) (d) MgO(100)(220) (113) (410) (223) (333) 2 (deg)* * * Figure 3-6 XRD pattern for Zn2GeO4: Mn films grown on MgO substrate at different deposition temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C where denotes GeO2 impurity phase.

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44 XRD patterns from films grown on Si are shown in Figure 3.6, which are comparable to the results for films grown on MgO. At 600 C, the presence of weak (223) and (550) Zn2GeO4 peaks and a broad peak at 2of ~ 18 in the diffraction pattern indicate that the film structure is mixed am orphous and polycrystalli ne. Again, the XRD patterns shown in Figure 3.6 (ac) indicate th at the films grown at temperatures from 650750 C are polycrystalline with be tter long range order. Figure 3-7. XRD pattern for films grown on Si substrate at different deposition temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C. Figure 3.7 presents the XRD pattern for films deposited on the third substrate, YSZ, at different deposition temperatures. For films deposited at 600 C (Fig. 3.7d), only a weak (550) diffraction peak is present, which indicates that Zn2GeO4 is again mixed amorphous and polycrystalline. Ad ditional diffraction peaks from Zn2GeO4 were 1020304050607080Intensity (a.u.)2 (deg)(a) (b) (c) (d)Si substrate(220) (113) (223) (333) (550) Si substrate

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45 observed from films deposited at higher temperatures. Polycrystalline Zn2GeO4 films were formed at 650 and 700 C. The diffraction pattern for the film grown at 750 C (Fig. 3.7a) reveals a strongly preferred (110) texture. Figure 3-8. XRD pattern for films grown on YSZ substrate at different deposition temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C. 3.3.2 Cathodoluminescent Properties With respect to luminescent properties, the characteristic PL and EL emission wavelength of Zn2GeO4: Mn is a broad peak with a maximum at 540 nm [22,35,37] as discussed in Chapter 2. Th e CL emission spectra for Zn2GeO4: Mn on MgO deposited at various temperatures are shown in Figure 3. 8. The CL emission peak intensity was observed to be at 540 nm, as well. The highest 540 nm CL intensity came from films that 10203040506070Intensity (a.u.)2 (deg)(550) (220) (110) (630)(a) (b) (c) (d)(410)YSZ (200) ( 1 4 0 ) ( 3 3 0 ) ( 4 4 0 ) 10203040506070Intensity (a.u.)2 (deg)(550) (220) (110) (630)(a) (b) (c) (d)(410)YSZ (200) 10203040506070Intensity (a.u.)2 (deg)(550) (220) (110) (630)(a) (b) (c) (d)(410)YSZ (200) ( 1 4 0 ) ( 3 3 0 ) ( 4 4 0 )

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46 were grown at 650 and 700 C. For 600 C sample, no CL emission at 540 nm is noted. Rather, the emission peak has shifted to 650 nm. 0 200 400 600 800 1000 1200 1400 1600 1800 2000 200300400500600700800900Wavelength (nm)Intensity (a.u.) 650 C 700 C 600 C 750 C 0 200 400 600 800 1000 1200 1400 1600 1800 2000 200300400500600700800900Wavelength (nm)Intensity (a.u.) 650 C 700 C 600 C 750 C Figure 3-9. CL emission spectrum of Zn2GeO4: Mn on MgO substrate at various deposition temperatures. The CL emission spectra from film deposit ed onto Si substrates are shown in Figure 3.9. CL emission at 540 nm result ed only from the film grown at 750 and 700 C. Films grown at temperatures of 600 and 650 C did not show any detectable CL emission at 540 nm, but did show emission at 650 nm.

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47 0 200 400 600 800 1000 1200 1400 1600 200300400500600700800900Wavelength (nm)Intensity (a.u.) 700 C 600 C 650 C 750 C 0 200 400 600 800 1000 1200 1400 1600 200300400500600700800900Wavelength (nm)Intensity (a.u.) 700 C 600 C 650 C 750 C Figure 3-10. CL emission spectrum of Zn2GeO4: Mn on a Si substrate for various deposition temperatures. The CL emission spectra from film depos ited onto YSZ are presented in Figure 3.10. The highest CL intensity, in this case, results from the film grown at 750 C. In contrast to the results for films grown on MgO and Si, the intensities for CL emission from the films deposited at 700 and 650 C on YSZ are less than that from the film grown at 750 C. There was no CL response at 540 nm from the film grown at 600 C, but again the emission peak was sh ifted and broadened to 650 nm.

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48 0 500 1000 1500 2000 2500 3000 3500 200300400500600700800900Wavelength (nm)Intensity (a.u.) 750 C 700 C 650 C 600 C 0 500 1000 1500 2000 2500 3000 3500 200300400500600700800900Wavelength (nm)Intensity (a.u.) 750 C 700 C 650 C 600 C Figure 3-11. CL emission spectrum of Zn2GeO4: Mn on YSZ substrate at various deposition temperatures. 3.3.3 Zinc to Germanium Ratio in the Films The Zn to Ge atomic percent ratio was measured using energy dispersive Xray fluorescence (EDX). These ratios, given in Table 3.2, were measured for films deposited at T 650 C. All of the deposited films exhibited a low Zn/Ge ratio, ranging from 0.31 to 0.89, suggesting a Zn deficiency. As disc ussed earlier, the absolute values of these ratios are subject to error. Nonetheless, the relative cha nges in the ratios correlate well with the changes in crystalline quality and CL intensities, suggesting that the ratios present valid trends. The fact that the XRD patterns indicate that Zn2GeO4 with varying degrees of crystallinities was present is consis tent with these ratios having large absolute errors, but accurate trends. As shown in Table 3.2, films deposited on MgO and Si

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49 substrates at 650 and 700 C have a higher Zn/Ge ratio than films deposited at 750 C. Because of the low Zn/Ge ratio for films deposited at 750 C on MgO or Si substrates, the crystalline quality d ecreases. The Zn/Ge ratio is high est (0.89) for the (110) textured film deposited at 750 C on YSZ, and this film has the best diffraction pattern, indicating the highest crystalline quality. Table 3-2 Zn/Ge Atomic Ratio in Deposited Zn2GeO4: Mn Film vs. Deposition Temperature on Various Substrates 750 C 700 C 650 C MgO 0.31 0.60 0.36 Si 0.42 0.47 0.57 YSZ 0.89 0.65 0.44 3.3.4 Photoluminescence Excitation and Emission The photoluminescence (PL) emission a nd the excitation (PLE) spectra for a Zn2GeO4 sample that emits green light are shown in Figure 3.11. The emission excited with 325 nm radiation showed a peak at 540 nm. The excita tion spectrum, monitored at 540 nm, exhibited an excitation peak at 310 nm with a small shoulder at 265 nm. The results are similar to what has been noted for green emission from Zn2GeO4: Mn [35]. The PL emission and excitation spectra for Zn2GeO4 samples that have longer wavelength emission are shown in Figure 3.12. The PL emission for these samples, when excited with 325 nm radiation, results in a broad peak at 625 nm with a smaller peak showing at 535 nm. The excitation sp ectrum, monitored at 625 nm, exhibited a peak at 270 nm with a small shoulder at 330 nm.

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50 100.00200.00300.00400.00500.00600.00700.00800.00Wavelength (nm)Intensity (a.u.)Excitation Emission310 nm 265 nm 540 nm 100.00200.00300.00400.00500.00600.00700.00800.00Wavelength (nm)Intensity (a.u.)Excitation Emission310 nm 265 nm 540 nm Figure 3-12. Green emission spectrum of Zn2GeO4: Mn excited with 325 nm radiation at 540 nm and excitation spectrum monitored at 540 nm. 3.4 Discussion In summary, Zn2GeO4: Mn was successfully pulsed laser deposited onto MgO, Si, and YSZ substrates. The Xray diffraction da ta show improved crystallinity in films deposited at T 650 C. For the films deposited at 600 C, on all substrates, the structure of films was mixed amorphous and polycrystallin e. The quality of the diffraction peaks was good for films deposited at 650 and 700 C on MgO and Si substrates. The best diffraction pattern was for the te xtured film deposited at 750 C on YSZ substrate. A relationship between the CL emission ma ximum intensity and the film crystal quality and stoichiometry is obvious. The film with the best CL emission intensity at 540

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51 nm was the (110) textured film grown on the YSZ substrate at 750 C. The Zn/Ge ratio is highest (0.89) for this film and this film has the best diffraction pattern, indicating the highest crystalline quality. 100.00200.00300.00400.00500.00600.00700.00800.00Wavelength (nm)Intensity (a.u.)Excitation Emission270 nm 325 nm 535 nm 640 nm 100.00200.00300.00400.00500.00600.00700.00800.00Wavelength (nm)Intensity (a.u.)Excitation Emission270 nm 325 nm 535 nm 640 nm Figure 3-13. Red emission spectrum of Zn2GeO4: Mn excited with 325 nm radiation at 625 nm and excitation spectrum monitored at 625 nm. The films deposited at 700 and 650 C on MgO, which were determined to have good crystal quality and Zn/Ge ratios ~0.5 also exhibited the highest CL emission intensities at 540 nm for film s deposited on this substrate. For films deposited on a Si substrate, the CL emission was also highest for the films with the best crystal quality, which again correlated with high er Zn/Ge ratios. The fact that higher Zn/Ge ratios lead to brighter CL emission is consistent with observation of better crystallinity for the Zn2GeO4 crystal structure.

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52 The highly textured film grow n on the YSZ substrate at 750 C had the highest crystallinity, highest Zn /Ge ratio, and the best CL propertie s. It has been suggested that grain boundaries may limit the luminescent performance of a phosphor [63], which is consistent with emission from more randomly oriented polycrystalline film being lower when compared to the textured film on YSZ. A red shift in wavelength of emission from green to red has also been noted in Mndoped ZnGa2O4 phosphors [67]. It was reported that Mn4+ ions in the octahedral sites led to red emission, while Mn2+ ions that occupied the tetrahed ral sites in the spinel structure resulted in green emission [67]. At 600 C, where the Zn2GeO4: Mn films exhibit mixed short and long range order, the dominant CL emission was at 650 nm. It is postulated that the Mn4+ ions are responsible for red emissi on at 650 nm. The rhombohedral crystal structure of Zn2GeO4 has only tetrahedral sites [68]. Th erefore, a change in substitution site must occur to accommodate th e change in valency of Mn in Zn2GeO4: Mn. The Mn4+ ions may substitute for Ge4+ at the lower temperature. This substitution is possible since the ionic radii of both ions are 0.39 [69]. At higher deposition temperatures (650, 700, & 750 C), the films develop a polycry stalline structure and the Mn2+ ion gains enough energy to move into the expected tetrahedral site and substitute for Zn2+. This is again possible since the ionic radii of thes e atoms are close, 0.60 versus 0.66 for Zn2+ and Mn2+, respectively [69]. The Mn2+ activator ions resulte d in emission at 540 nm, as expected. The difference in photoluminescence excita tion spectra for green (540 nm) and red (650 nm) luminescence indicates that excitati on of different states are responsible for each type of emission. This correlation betw een the excitation of di fferent ionic states

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53 and the PL excitation spectra has al so been observed in for theZnGa2O4 host [67]. The excitation spectra are consistent with the Mn ion exhibiting differen t valence states and substituting for either Zn or Ge, dependi ng upon the temperature during deposition.

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54 CHAPTER 4 DEVELOPMENT OF ZINC SILICATE DO PED WITH MANGANESE THIN FILM PHOSPHORS 4.1 Introduction This chapter describes the development of zinc silicate doped with manganese (Zn2SiO4: Mn) thin film phosphors. The overall effects of the processing conditions on the resulting films will be evaluated. In the fi rst section, the experimental procedures that were used to grow the films will be desc ribed. The films were produced by sputter deposition, pulsed laser depos ition, or combustion chemical vapor deposition. The luminescent and structural properties of these films have been evaluated and are discussed in the results section. The resul ting composition of the films will be discussed as it relates to the film growth method a nd postprocessing conditions. These growth methods are excellent choices for growing th in film phosphors, however it was found that the zinc loss must be controlled to obtain the best performance from the phosphor film. The thin film phosphors were also studied for cathodoluminescence degradation, as will be discussed in Chapter 5. 4.2 Experimental Procedures 4.2.1 Substrates and their Preparation Clear fused quartz and (100) Si were chosen as the substrates for film growth. These substrates were chosen since they c ould withstand the high annealing temperatures required to crystallize zinc silicate. The s ubstrates were cleaned by a solvent wash in trichloroethylene, acetone, and then in metha nol. The substrates were placed in a beaker

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55 with each solvent for fifteen minutes inside a sonicator and then removed. The substrates were then placed in the next solvent, and the wash was repeated. The substrates were then blown dry by compressed air. 4.2.2 Phosphor Processing 4.2.2.1 Sputter deposition The Zn2SiO4: Mn sputter targets were made from raw material produced at Shanghai Yuelong New Materials Co. Ltd. in Shanghai, China, and sold primarily for plasma display panel applications. The 2” s putter target was made by Plasmaterials, Inc. by hot pressing and sintering. The target was also bonded to a 0.125” thick Cu backing plate using metallic bonding by Plasmaterials, Inc. The density of the resulting target was estimated to be ~90% of theoretical. The sputter deposition was completed with two different targets, however the starting material was the same in both cases. The initial target was changed after cracks propagated through the target, and films deposited from it had an uncharacteristic brown tint, which was determined by EDX analysis to be carbon contaminati on. The source of the carbon contamination was th e bonding material for adhering the target to the copper backing plate. The films that were carbon c ontaminated are not included in this study. A second target was used in a sputter gun that did not require a copper backing plate. This adjustment eliminated the bonding material as a source of carbon contamination. The deposited thin films were treated to a postdeposition thermal anneal. The rapid thermal anneal (RTA) was completed in an AG Associates Heatpulse Model 4100 furnace with a highpurity nitrogen atmosphere The anneal recipe used for the heat treatments is given below in Table 4-1. The annealing temperature for the samples was chosen as 1100 C, since it was high enough to cause crystallization of Zn2SiO4

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56 [26,27,29]. The duration of th e anneal was limited to five minutes due to the operating capabilities of the furnace. Upon completion of the anneal, the heat source was turned off and an increase in N2 flow helped to cool the sa mples before removal from the furnace. Table 4-1 Rapid thermal anneal recipe for heat treatment of thin film samples. Segment Type Time Temp. (C) N2 Flow (slpm) 1 Steady 10 sec 0 10 2 Ramp 125 C/ sec 1100 2.5 3 Steady 300 sec 1100 2.5 4 Steady 60 sec 0 10 4.2.2.2 Pulsed laser deposition Thin film samples were also made by pulsed laser deposition (PLD). The PLD target was made from the commercial Zn2SiO4: Mn phosphor powder obtained from Shanghai Yuelong New Materials Co. Ltd. This target was pressed using a 1” stainless steel die. The thin film processing wa s completed at North Carolina A & T State University. The pulsed laser deposited phosphor films we re all grown at the same processing conditions on (100) Si substrates. The oxyge n pressure during growth was at 300 mTorr and the substrates were heated to and mainta ined at 700 C during the deposition cycles. After deposition, the thin film PLD samples were heat treated with a rapid thermal anneal in N2 atmosphere for five minutes at 1100 C. The same apparatus and program described in section 4.2.2.1 was used for the annealing of these samples.

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57 4.2.2.3 Combustion chemical vapor deposition Zn2SiO4: Mn thin film phosphors were depos ited onto quartz substrates by combustion chemical vapor deposition (CCVD) at Georgia Institute of Technology. Combustion chemical vapor deposition is a novel growth method that produces crystallized films at high temperatures, elim inating the need for a postdeposition anneal [70]. In CCVD, the precursors are sprayed near or in a flame that causes the precursors to chemically react leading to the vapor depos ition of a film onto a substrate [71]. The substrates are heated during the deposition through exposure to the open flame and the temperature is controlled by the substrate di stance from the flame end. The films are grown at atmospheric pressure using ambien t air for the reaction, making this method suitable for oxide materials [70]. The precursor solution consisted of zinc nitrate, tetraethyl orthosilicate (TEOS), and manganese nitrate [70]. 4.2.3 Thin Film Phosphor Characterization 4.2.3.1 Xray diffraction The crystalline quality of the films was investigated using a Philips APD 3720 Xray diffractometer. Xray diffraction (see se ction 3.2.3.1) was completed with Cu K radiation (0.15406 nm wavele ngth) generated by a 40 keV and 20 mA electron beam. The 2scan range for the samples was from 10 to 80 degrees, while the scan rate was 0.080 / sec in a continuous scan mode. The resulting Xray diffraction patterns were indexed using a collection of patterns from the Joint Committee on Powder Diffraction Standards (JCPDS) catalog, where the rhombohedral crystal structure was found to correlate with the Zn2SiO4 developed in this study.

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58 A high resolution Xray diffractometer wa s also used to measure the crystal structure and quality of the th inner films (less than 1000 ). This method was useful for the thin films that were too thin fo r the powder diffraction method. The 2scans were taken from 20 to 40 degrees at a glancing angle of 0.7 A Philips MRD X’ Pert X-ray diffraction system was used for the measurements. 4.2.3.2 Scanning electron microscopy Scanning electron microscopy (SEM) is us eful for characterizing the sample morphology. A scanning electron microscope provides a magnified image of the surface. The magnification for this type of micr oscopy may go as high as 300,000 [65]. A JEOL scanning electron microscope m odel 6400 was used for obtaining the micrographs. The SEM was operated at 15 keV. Because of the high accelerating voltage and the low conductivity of the sample s, the samples were coated with carbon to reduce the effects of charging. The car bon layer was deposited onto the samples by electron beam evaporation. 4.2.3.3 Wavelength dispersive Xray spectrometry Wavelength dispersive Xray (WDX) sp ectroscopy is an elemental analysis technique that is used to provide informati on about the composition of a sample. It can be used to detect elements from beryllium to the actinides. This method relies on the generation of characteristic Xrays by a fo cused electron beam, as in energy dispersive spectroscopy (EDX). However, WDX (versu s EDX) relies for energy analysis by the constructive diffraction of Xrays by a crystal [65] for elemental analysis. Several crystals with various dspacings are us ed to incorporate a broad energy range. Correction factors for Xray intensity are us ually applied in the qua ntitative analysis.

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59 The ZAF [65] correction takes into accounts the effects relate d to the atomic number of an element (Z), Xray absorption (A), and secondary fluorescence (F). 4.2.3.4 Xray photoelectron spectroscopy Xray photoelectron spectros copy (XPS) is a very benefi cial analytical method for examining the surface chemistry of a sample [65,72]. Energetic X-rays, greater than 1000 eV [72], bombard a sample causing charact eristic photoelectrons to be ejected. Chemical identification is made by analyz ing the kinetic energies of the ejected photoelectrons. Electron binding energies are se nsitive to the chemical state of the atom. With sensitivity to small changes in kinetic energy, information about the chemical state of the identified element can be determined This method is good for detecting most elements, except for hydrogen and helium. The kinetic energy of the photoelectron is dependent upon the impinging Xray photon as dictated by Einste in photoelectric law: BE h KE (4-1) where KE is the kinetic energy, h is the energy of the impinging photon, and BE is the binding energy of the electron in the atom. Since the energy of the photon is known and the kinetic energy is measured, the binding energy can be determined. An XPS spectra is usually given as the binding energy versus peak intensity. The peak intensities in an XPS plot are dependent upon the phot oionization crosssection. The photoionization cross – section is dependent upon the probability fo r photoejection, which is different for each quantum orbital [65]. The probability de pends upon the orbital of ejection for each element and on the energy of the X-ray. A quantitative analysis can be made if the photoionization crosssection is known, al ong with other experimental parameters.

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60 4.2.3.5 Cathodoluminescence characterization The cathodoluminescence was measured in the ultrahigh vacuum chamber described in Chapter 3. A Kimball Physic s model EFG7 electron gun was used as the energy source to stimulate cathodoluminescence. The accelerating voltage was varied for these experiments from 5005000 eV. The cu rrent level also varied from 2.5 to 8 A typically in an area of 0.0314 cm2, corresponding to curren t densities of 80 to 255 A/ cm2. A fiber optic inputted Ocean Optics S 2000 spectrometer was used to detect the visible light emission from the samples. 4.3 Results from Pulsed Laser Deposited Phosphors The thickness of the films was measur ed by secondary ion mass spectrometry (SIMS) profiling followed by profilometer measur ements of the sputter crater and depth. The films were grown to tw o thicknesses: 0.85 and 1.4 m. The increased thickness was a result of a longer deposition time. The deposition rate for both growth conditions was about 500 / min. The samples were char acterized for structure and morphology, composition, and cathodoluminescence emission spectrum and peak intensity. 4.3.1 Structural Characterization The asdeposited samples and annealed samples were examined by Xray diffraction. The XRD diffraction patterns for the thin film samples were compared with the Zn2SiO4: Mn powder that was used to make the ablation target. This pattern for the powder has been indexed with its corresponding planes, and is shown in Figure 4.1. The powder pattern matches closely to the JCPDS file for rhombohedral zinc silicate. All of the peaks from the powder sample can be attributed to Zn2SiO4. Figure 4.2 compares the diffraction pattern for the 0.85 m thick sample for asdeposited at 700 C and annealed

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61 at 1100 C for 5 minutes. The asdeposited film had no discerna ble diffraction peaks from Zn2SiO4. However, diffractions peaks for th e (100) Si substrat e are observed along with a peak at 44 2 degrees. This peak has been identified as an impurity SiO2 phase and is denoted by ‘*’ in Figure 4.2. The XRD pattern for the annealed film indicates that Zn2SiO4 is present and has a polycrystalline struct ure. Figure 4.3 shows the improvement in structural quality for the thicker film, 1.4 m, after the anneal and the corresponding zinc silicate diffraction peaks are denoted. The pattern fo r the asdeposited at 700 C film only shows diffraction peaks resulting from the Si substrate. Impurity SiO2 phases are also present in this film, as indicate d by the diffraction peaks at 22, 27, and 44 degrees and is denoted in the diffrac tion patterns by an asterisk ‘*’. Figure 4-1. Powder XRD pattern for Zn2SiO4: Mn. This is consistent with the rhombohedral crystal structur e reported in JCPDS 371485. 0 20 40 60 80 100 10203040506070 2 theta (degrees)Intensity (a.u.)(603) (523) (710) (006) (630) (713) (550) (633) (110) (300) (223) (600) (520) (333) (214) (410) (131) (241) (113) (122) (220) 0 20 40 60 80 100 10203040506070 2 theta (degrees)Intensity (a.u.)(603) (523) (710) (006) (630) (713) (550) (633) (110) (300) (223) (600) (520) (333) (214) (410) (131) (241) (113) (122) (220) (603) (523) (710) (006) (630) (713) (550) (633) (110) (300) (223) (600) (520) (333) (214) (410) (131) (241) (113) (122) (220) (110) (300) (223) (600) (520) (333) (214) (410) (131) (241) (113) (122) (220)

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62 1020304050607080 2 theta (degrees) Before Anneal After Anneal(110) (300) (333) (220) (223) (410) (113)Si Substrate * 1020304050607080 2 theta (degrees) Before Anneal After Anneal(110) (300) (333) (220) (223) (410) (113)Si Substrate Si Substrate * Figure 4-2 XRD pattern of Zn2SiO4: Mn thin film phosphor (0.85 m thick) before and after annealing. designates peaks from SiO2.

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63 101520253035404550556065707580 2 theta (degrees) Before Anneal After AnnealSi Substrate (110)*(410) (113) (220)* 101520253035404550556065707580 2 theta (degrees) Before Anneal After AnnealSi Substrate (110)*(410) (113) (220)* Figure 4-3. XRD pattern of Zn2SiO4: Mn thin film phosphor (1.40 m thick) before and after annealing. designates peaks from SiO2. 4.3.2 Morphological Characterization All films were specular reflec ting before the anneal at 1100 C. After the anneal in N2, the thinner sample retained its reflectiv ity, while the thicker sample had a dull appearance. The morphologies were inve stigated by scanning electron microscopy (SEM) to better understand the differences in the samples, and the micrographs are shown in Figure 4.4. The micrographs indica te that different surface morphologies exist for the two different conditions. Figur e 4.4 (a) shows that the thinner Zn2SiO4: Mn film was produced with particulates on the surface. It is

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64 A) B) Figure 4-4. SEM pictures of the surface of the phosphor films after annealing at 1100 C for the A) 0.85 m sample and for B) 1.4 m thick sample.

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65 common for particulates to form on the surf ace during pulsed laser deposition [45]. The thicker film was investigated further to determine what the clusters on the surface were. It was found to compose of a siliconoxyge n rich matrix, much like the remaining surface. A small amount of zinc (1 3 atomic%) was also present. 4.3.3 Cathodoluminescence Results The CL emission spectra for annealed Zn2SiO4: Mn for each thickness are shown in Figure 4.5. The primary beam voltage and current for these spectra was 5 keV and 8.0 The spectra have the characteristic Gaussian profile. The maximum peak intensity was observed at a wavelength of 530 nm. The thinner sample is significantly brighter than the thicker sample. The cathodolumines cent brightness is 9 times greater for the 0.85 m film, compared to the 1.4 m sample. The maximum peak intensity was also measured for each film from 5005000eV as is shown in Figure 4.6. The trend indi cates that the thinner film has a higher CL intensity than the thick film at all voltage le vels. Previous resear chers have observed the opposite trend [3,34], where th e CL intensity is higher as the thickness of the films increase. CL intensity increases with an increasing accelerating vo ltage for the thinner sample. However, the slope of change in CL intensity for the thicker sample with beam voltage is very small.

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66 0 500 1000 1500 2000 2500 400500600700 Wavelength (nm)Intensity (arb. units) 0.85 um 1.4 um Figure 4-5. Cathodoluminescen t emission spectra from Zn2SiO4: Mn at V= 5keV, i= 8.0 A, with inset of CL spectrum for 1.4 m thick sample. Figure 4-6 CL brightness versus primary beam voltage for PLD Zn2SiO4: Mn thin films. 0 500 1000 1500 2000 2500 3000 0100020003000400050006000 Accelerating Voltage (eV)Intensity (arb. units) 0.85 um PLD (530 nm) 1.4 um PLD (530 nm)

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67 4.3.4 Composition of Films The composition of the films was meas ured by electron probe microanalysis (EPMA). The atomic % of each element for the two samples is shown in Figure 4.7. It can clearly be seen that the thin sample has a higher zinc content (12 at %) than the thicker sample (3 at %). The Zn/Si ratios were 0.58 and 0.11 for the 0.85 and 1.40 m thick films, respectively. Both samples exhibit a lower than expected Zn/Si ratio, suggesting a Zn deficiency. The XR D diffraction patterns indicate that Zn2SiO4 is present, but to varying degrees. The 1.40 m thick film had a diffraction pattern that included several peaks resulting from SiO2 and Zn2SiO4. This correlates to the trend of the composition data suggesting that less Zn is present in this sample versus the 0.85 m thick film. These data suggest that the tre nds from the EPMA data are correct although the absolute values may not be accurate.

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68 Figure 4-7 Comparison of composition in the PLD thin film samples. 4.4 Results from Sputter Deposited Phosphors 4.4.1 Target 1 Two samples were grown with Target 1unde r different sputter power (80 versus 100W) while all other variables remained the same. The films were deposited onto quartz substrates. The growth parameters ar e defined in Table 4.2. A rapid thermal anneal at 1100 C was necessary fo r detectable cathodoluminescence. 0 10 20 30 40 50 60 70 Atomic % ZnSiO 0.85 um thick 1.40 um thick

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69 Table 4-2 Processing conditions for sputter deposition of Zn2SiO4: Mn thin films using Target 1. Processing Parameter Selected Conditions Sputter power 80100 W System pressure 18 mTorr O2 partial pressure 3 mTorr Substrate temperature room temperatureno heating RTA temperature 1100 C The thickness of a step in the thin films was measured by a profilometer. The sample that was sputtered at 80W had a de position rate of 16 / min, resulting in a thickness of 1925 . The sample sputtered at 100 W had a meas ured thickness of 2262 , corresponding to a deposition rate of 18.8 / min. 4.4.1.1 Structural characterization Xray diffraction data from the films was collected before an d after the 1100 C anneal for 5 minutes. Both asdeposite d films were amorphous with no discernable diffraction peaks. The Xray diffraction patterns fo r two samples that were annealed at 1100 C are shown in Figure 4.8. All of th e diffraction peaks were indexed to zinc silicate, indicating that a polyc rystalline solid exists for bot h growth conditions after the postdeposition anneal.

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70 2025303540 2 theta (degrees) 80 W 100 W(300) (220) (113) (410) (223) 2025303540 2 theta (degrees) 80 W 100 W(300) (220) (113) (410) (223) Figure 4-8. XRD pattern of 1100 C annealed sputtered films at tw o different sputter conditions. 4.4.1.2 Cathodoluminescence results The cathodoluminescent (CL) spectra are compared in Figure 4.9 for the two different growth conditions. The emissi on spectra resulted from an electron beam excitation source with an energy of 5000 eV and 7.0 A of beam current. There were no observable differences in the two spectra. The maximum emission peak for these samples was at 526 nm consistent with visible green emission. The film deposited at the higher power (100 W) was brighter than the film deposited at 80 W. In the case of sputter deposited films, the thicker film resulted in brighter luminescence. As expected, the CL brightness increased with accelerati ng voltage (Figure 4.10). The CL emission was evaluated for accelerating voltages from 10005000 eV at a beam current of 2.5 A. However, at the low voltages there is little difference in the maximum emission intensity for the two samples. In contrast, at 5000 eV the difference in the maximum intensity is 1.3 times as shown in Figure 4.11.

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71 The cathodoluminescence from 5005000 eV and a beam current of 8.0 A from films grown by sputter (0.1925 m) versus pulsed laser deposition (0.85 m) is compared in Figure 4.12. The thinner pulsed laser deposited sample was chosen since it had the best brightness for that growth method. It is compared with th e less bright sample created by sputter deposition. Th e sputter sample is still more than 30% brighter than the thin film phosphor sample made by pulsed laser deposition, even though it is a factor of four times thinner. Figure 4.9. Cathodoluminescent spectra from sputtered deposited thin film samples. 0 0.2 0.4 0.6 0.8 1 400450500550600650700 wavelength (nm)normalized intensity 100 W 80 W

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72 Figure 4-10. Maximum CL brightness fo r sputter deposited and annealed Zn2SiO4: Mn thin films from 1000 to 5000 eV. 0 200 400 600 800 1000 1200 1400 0123456 Voltage (keV)Max. Intensity (a.u.) 100W 80W Li(100W) Pl(80W)

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73 Figure 4-11. Maximum CL intensit y at V= 5keV for i= 2.5 to 7 A. 0 500 1000 1500 2000 2500 2345678 Current (uA)Max. Intensity (a.u.) 100 W 80 W

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74 Figure 4-12. Maximum CL brightne ss for PLD vs. sputter deposited Zn2SiO4: Mn thin films from 500 to 5000 eV. 4.4.2 Target 2 As discussed earlier, a sec ond target was used after it was noticed that the first target had become a source of carbon contam ination due to the cracking and exposure of the carbon based bonding material A different sputter gun was used with target 2, which eliminated the need for bonding the target to a backing plate w ith a carbon based material. A different deposition chamber was also used for these film growth cycles. The growth parameters are outlined in Ta ble 4.2. The films were deposited onto quartz substrates. Each deposition cycle last ed 60 minutes. A rapid thermal anneal at 1100 C for 5 minutes was necessary to detect cathodoluminescence. 0 0.2 0.4 0.6 0.8 1 0100020003000400050006000 Beam voltage (eV)Normalized Intensity 0.85 um PLD (530 nm) 0.1925 um SpD (525 nm)

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75 Table 4-3 Processing conditions for sputter deposition of Zn2SiO4: Mn thin films using Target 2. Processing Parameter Selected Conditions Sputter power 4070W Sputter pressure 13 mTorr O2 partial pressure 2.5 mTorr Substrate temperature room temperatureno heating RTA temperature 1100 C 4.4.2.1 Structural characterization The asdeposited samples were all amor phous as indicated by their respective diffraction patterns. A represen tative Xray diffraction patter n for the annealed at 1100 C phosphors is shown in Figure 4.13. All of the diffraction peaks are indexed to zinc silicate, indicating that a polycrystalline solid ex ists after the anneal. 4.4.2.2 Cathodoluminescence results The cathodoluminescence spectra from the ann ealed sputter deposited films are shown in Figure 4.14. The maximum emission peak fo r these samples was at 526 nm consistent with visible green emission. The two bright est phosphors were from sputter conditions at the higher sputter powers, 70 and 60 W. As th e sputter power decrea sed, a decrease in the cathodoluminescence is also observed. The composition was measured by Xray photoelectron spectrosco py (XPS) to further investigate the drop in luminescence at the lower sputter power. However, the results di d not accurately correlate to the trends for cathodoluminescence.

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76 Figure 4-13 Xray diffr action pattern from Zn2SiO4: Mn thin film sputter deposited with target 2. Figure 4-14 Cathodoluminescent emi ssion from sputter deposited Zn2SiO4: Mn at electron beam V= 5keV, i= 15A for sputter powers of 4070W. 0 200 400 600 800 1000 1200 1400 1600 400500600700 Wavelength (nm)Intensity (a.u.) 40 W 50 W 60 W 70 W 2025303540 2 theta (degrees)(300) (220) (113) (223) (410) 2025303540 2 theta (degrees)(300) (220) (113) (223) (410)

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77 The composition was measured by XPS which has a detection depth of about 3 nm, while the CL data result from the electron be am voltage of 5 keV penetrating more than 100 nm into the film. The XPS results indicate that a zinc deficiency exists at the surface for all films, but may not be repr esentative of the bulk composition. 4.5 Results from Combustion Chemic al Vapor Deposition Phosphors The phosphor films grown on quartz substr ates by combustion chemical vapor deposition (CCVD) contain Mn at two concen tration levels, 2 and 4 mol %. The asdeposited films were investigated with Xray diffraction, and their structure was determined to be polycrystalline Zn2SiO4 [70]. Therefore, no post deposition heat treatment was completed for these samples. 4.5.1 Cathodoluminescence Results The cathodoluminescence spectra for the CCVD phosphor films are shown in Figure 4.15, and their peak maximum at 525 nm is consistent with the spectra from PLD and sputter deposited Zn2SiO4: Mn. The emission from the 4 mol% Mn concentration phosphor is 5% brighter than the emission from the 2 mol% Mn phosphor. Additionally, the CL intensity was compared with the brightest sputter deposited film in Figure 4.13. The films produced by CCVD were at least 40% brighter.

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78 Figure 4-15 CL spect ra from CCVD Zn2SiO4: Mn phosphor films at beam V= 5keV, i= 7.0 A. 4.6 Discussion Zn2SiO4: Mn thin film phosphors were successfully made by pulsed laser deposition, sputter deposition, and combustion chemical vapor deposition. The Xray diffraction data indicated that the asdeposit ed films for PLD and s putter deposition were amorphous, even though the PLD films were de posited onto substrates at 700 C. The thin films were polycrystalline after a rapid thermal anneal at 1100 C in N2 atmosphere. The CL performance of the PLD films wa s compared for two thicknesses. The thinner film was actually 94% brighter than the thicker film. This indicates that the brightness of the films does not depend solely on the thickness, since thicker films are expected to result in brighter luminescence. The low CL performance of the thick film 0 500 1000 1500 2000 2500 3000 3500 4000 4500 400500600700Wavelength (nm)Intensity (a.u.) CCVD 4% Mn Sputter Dep from commercial phosphor CCVD 2% Mn

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79 was correlated to a low Zn to Si ratio, compared to the brighter film which had a higher Zn to Si ratio. Additionally, the impurity SiO2 phase is strongly represented in the diffraction pattern for the thicker film indi cating that the film is a mixture of nonluminescent SiO2 and luminescent Zn2SiO4. The Mn activator is expected to substitute for the Zn atoms in the lattice structure. However, there is less Zn present in this film. The reduction in cathodoluminescence is attri buted to a reduction in the luminescent center density. This is a reasonable assu mption since the CL intensity changes only slightly as the excitation volume increases. The cathodoluminescence was also characte rized for films that were sputtered deposited. The results of th e sputter deposited films were divided according to the sputter target and deposition chamber used for film growth. The films that used target 1 were sputter deposited at two different powers and were compared based upon growth condition and film thickness. The 20% higher s putter power resulted in a film that was 15% thicker. Green cathodoluminescence wa s observed from the samples after the anneal was completed, with the thicker sample being 25% brighter than the thin sample. The sputter deposited films from target 2 were grown at a slightly lower pressure and a lower sputter power range, 4070W. The cathodoluminescence was brightest for films deposited at 6070W and dimmest for those films deposited at 4050W sputter power. The increase brightness is due to increased th ickness of the phosphor films as the sputter power is increased. The films created by sputter and pulsed lase r deposition were also compared. The cathodoluminescence was at least 30% brighter for the films made by sputter deposition

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80 versus pulsed laser depositi on. The pulsed laser depos ited samples has an SiO2 impurity phase, which limits the activator density. The films grown by combustion chemi cal vapor deposition developed a polycrystalline structure duri ng growth at 1200 C at atmos pheric conditions. Therefore, no postgrowth heat treatment was completed for these films. The films were made at two dopant levels where the 4 mol% Mn had a brighter cathodoluminescent emission than the film doped with 2 mol% Mn. The CCVD films were bright er than the films made by sputter deposition. The brightest films from this study were those made by CCVD and were chosen to study the degradation of Zn2SiO4: Mn thin films, as discussed in the following chapter.

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81 CHAPTER 5 DEGRADATION OF ZINC SILICATE DOP ED WITH MANGANESE POWDER AND THIN FILM PHOSPHORS 5.1 Introduction The degradation behavior of thin film and powder Zn2SiO4: Mn phosphors is evaluated and discussed in th is chapter. The degradation was observed under low (95 A/ cm2) and high (460 A/ cm2) current excitation densities for a period up to 24 hours. Continuous exposure to the electron source over the 24-hour period has been shown to affect the brightness of sulfide phosphors [19,73]. The first section in this chapter discusse s the experimental procedures used to monitor the degradation of th e phosphors. The changes in the surface chemistry of the phosphors have been observed by Auger el ectron spectroscopy. The decrease in cathodoluminescence observed during electron beam exposure will also be discussed. The differences between the degradation behavi ors of the thin film phosphors versus the powder phosphors are discussed. It was found th at the excitation density also affects the rate of degradation for bo th types of phosphors. 5.2 Experimental Procedures The thin film phosphors were grown by a novel combustion chemical vapor deposition (CCVD) method [70]. The Zn2SiO4: Mn phosphor was introduced and evaluated for its cathodoluminescent emission in Chapter 4. It was determined to be the brightest thin film phosphor of all phosphors developed in Chapter 4, and therefore deserves further evaluation. The thin f ilm phosphor on a quartz substrate was mounted

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82 onto a stainless steel holder. The powder phosphor is a commercial plasma display panel (PDP) phosphor obtained from Shanghai Yuelon g New Materials Co., Ltd. The powder samples were cold pressed into 4 mm deep 6 mm diameter holes in a stainless steel holder. The degradation experiments took place in an ultrahigh vacu um stainless steel chamber with a base pressure of 35 X10-9 Torr. The phosphors were exposed continuously to an electron beam for 24 hours. The electron source was from a coaxial electron gun in a PHI model 545 scanning Auger electron spec trometer. During electron beam exposure, the cathodoluminescence was me asured every minute to monitor changes in luminescence leading to degradation. An Oriel model 77400 multispectrometer was used to detect the emission spectra and peak intensities. Additionally, the surface chemistry was simultaneously observed thr ough Auger electron spect roscopy (AES). A cylindrical mirror analyzer (CMA) model 15110 was used as the Auger electron detector. The spectra were taken every 5 minutes during the continuous beam exposure for 24 hours. The program created for AES an alysis scanned the kinetic energy range of Zn, Si, O, and C for the range as specified in Table 5-1. Consecutive scans were taken for each of the species during 5minute intervals of data collection. Table 5-1 Species monitored during AES analysis. SPECIES ENERGY RANGE SCANNED (eV) C 200400 O 400600 Si 70100

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83 Table 5-1. Continued SPECIES ENERGY RANGE SCANNED (eV) Zn1 9001050 Zn2 4080 Survey of All Energies 301100 5.3 Results 5.3.1 Thin Film Zn2SiO4: Mn Phosphor 5.3.1.1 24 hour CL degradation Thin film Zn2SiO4: Mn was exposed to a 2000 eV, 3.0 A electron beam for 24 hours. Over this time period, the thin film phosphor decreased by 14% from its original cathodoluminescent intensity at th e low current density of 95.5 A/ cm2 as shown in Figure 5.1. The spikes in CL intensity are due to system fluctuations, which have been correlated to instabilities with the Peltier cooler system th at helps to reduce electrical noise. Figure 5.2 shows the CL intensity duri ng the first 20 minutes of degradation. The phosphor actually decreased to its final in tensity in less than ten minutes. The cathodoluminescent spectra before and after degradation were compared in Figure 5.3 to determine any difference in the spectral distribution. No shifts in the spectral distribution or changes in peak shape were noted. Changes in the surface chemistry were also observed during the degradati on through monitoring of the A uger electron spectra. The before and after spectra are shown in Figure 5.4, indicati ng composition changes of the phosphor that evolved during beam exposure. A strong emission from carbon was noted initially, but decreased significantly by th e end of the experiment. The carbon Auger electron signal results from adventitious surf ace contamination. It has been shown that

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84 electron beam bombardment in oxidizing gases may remove this carbon during the initial beam exposure [18,19]. The zinc and oxyge n signals are stronge r at the end of degradation. This increase is due to the removal of the surface layer of carbon, where now the signal comes exclusively from Zn2SiO4. There is no evidence of a chemical reaction or change within the phosphor. Ch arging of the phosphor during electron beam exposure is evident from the before and af ter AES spectra as illustrated by the charging feature at the beginning of both spectra and the shift in the individual Auger signals. cl degradation on thin film (i= 3.03.5 uA) 0 0.2 0.4 0.6 0.8 1 1.2 0120240360480600720840960108012001320144015601680 time (min.)Normlized Intensity (a.u.)system fluctuations cl degradation on thin film (i= 3.03.5 uA) 0 0.2 0.4 0.6 0.8 1 1.2 0120240360480600720840960108012001320144015601680 time (min.)Normlized Intensity (a.u.)system fluctuations Figure 5-1. Cathodoluminescent de gradation of thin film phos phor at V= 2 keV, i= 3.0 A (95.49 A/ cm2). A second CL degradation experiment was completed on the thin film phosphor at the same beam voltage (2000 eV), but with a higher current density of 461 48 A/ cm2 (electron beam current of 14.5 1.5 A, spot size of 2 mm). The phosphor decreased to 26% of its original cathodoluminescent intensit y at the higher current density as indicated in Figure 5.5. In this case it took a longer period of tim e (~ 14 hours) for the phosphor

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85 brightness to decrease to its final in tensity level. The before and after cathodoluminescent spectra are shown in Figur e 5.6, and no changes in the spectral distribution or peak shape is observed. The composition of the phosphor was measured by AES during exposure to the electron beam. The electron beam assisted in the removal of carbon from the surface, as indicated by th e decrease in the carbon signal in the AES after spectra shown in Figure 5.7. There is no Auger signal from carbon at the end of the degradation experiment. There is a slight increa se in the Auger signals from oxygen and zinc. Again, this increase is expected based on the nearly complete removal of the adventitious carbon from the surface which will reduce Auger electron scattering from Zn2SiO4 by C. The charging features at the lo w energies and the shift in the Auger signals indicate that the surface is ch arging during electron beam exposure. cl degradation on thin film (i= 3.03.5 uA)1.00 0.90 0.90 0.89 0.89 0.87 0.85 0.85 0.86 0.85 0.86 0.86 0.85 0.84 0.85 0.83 0.84 0.83 0.84 0.850 0.2 0.4 0.6 0.8 1 1.2 02468101214161820 time (min.)normalized intensity Figure 5-2. Degradation within the first 20 mi nutes for the thin film phosphor at i= 3.0 A.

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86 Zn2SiO4: Mn (thin film phosphor) CL Spectrum V= 2keV, i= 3.0uA, P= 3.2 E -9 Torr 0 500 1000 1500 2000 2500 3000 400450500550600650700 Wavelength (nm)Intensity (a.u.) after degradation before degradation Figure 5-3. Cathodoluminescent spectrum of thin film phosphor before and after degradation at low current excitation density.

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87 -600 -500 -400 -300 -200 -100 0 100 200 300 502504506508501050 Kinetic Energy (eV)dN(E) before deg after degO C Zn Figure 5-4. Auger electron spectrum before a nd after degradation on thin film, i= 3.0 A. cl degradation on thin film (i= 1316.2 uA) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0120240360480600720840960108012001320144015601680 Time (min.)Normalized Intensitysystem fluctuations cl degradation on thin film (i= 1316.2 uA) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0120240360480600720840960108012001320144015601680 Time (min.)Normalized Intensitysystem fluctuations Figure 5-5. Cathodoluminescent de gradation of thin film pho sphor at V= 2 keV, i= 13 A.

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88 Figure 5-6. Cathodoluminescent spectrum of thin film phosphor before and after degradation at high exci tation current density. -700 -600 -500 -400 -300 -200 -100 0 100 200 300 502504506508501050 Kinetic Energy (eV)dN(E) before deg after degZn O C Figure 5.7. Auger spectrum before and af ter degradation on thin film, i= 13.0 A. Zn2SiO4: Mn (thin film phosphor) CL Spectrum V= 2keV, i= 13.016.2uA, P= 3.6 E -9 Torr 0 1000 2000 3000 4000 5000 6000 400450500550600650700 Wavelength (nm)Intensity (a.u.) before after

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89 5.3.1.2 Cathodoluminescence recovery The same phosphor was examined to dete rmine if the loss in cathodoluminescence could be recovered over time. For this experiment, the phosphors were exposed continuously to an electron beam for 10 mi nutes (E= 2000 eV) and then the beam was turned off. For the first 30 minutes after beam exposure, the phosphor was exposed to the electron beam for ~5 seconds every minut e to measure the CL response from the phosphor. At the end of 30 minutes, a CL m easurement was taken followed immediately by an Auger survey over the next 1.5 minutes. Another CL scan was taken upon completion of the Auger survey. This experiment was completed at low and high current densities and the result s are discussed below. As reported, thin film Zn2SiO4: Mn degraded to 80% of its original intensity within 10 minutes of continuous beam expo sure at low beam current = 3.0 A (J= 95 A/ cm2). The CL spectra for the degraded phosphor ar e compared for several time intervals in Figure 5.8 to determine if the phosphor could recover to its origin al intensity without continuous beam exposure. The maximum recove ry was to 89% of th e original intensity after 30 minutes of intermittent exposure to the electron beam. Auger scans were taken from 200300 eV during the 10 minutes of c ontinuous beam exposure and the C signal shifted with time as indicated in Figure 5.9. The surface charging indicated by the shift in C kinetic energy with time and the presen ce of charging features within the Auger spectra can be correlated with the decr ease and nonrecovery of cathodoluminescent intensity. A similar response from the phosphor was observed as the current density was increased to 414 A/ cm2 (i=13 A). The phosphor degraded to 92% of its original

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90 intensity during the 10 minutes of continuous beam exposure. The maximum recovery was to 95% of original inte nsity after intermittent beam exposure for 30 minutes as shown in Figure 5.10. Figure 5.11 shows th e AES spectra which were collected during and after beam exposure. Charging is presen t after beam exposure and continues to be evident even after 60 minutes of limited beam exposure. Figure 5-8. CL spectra of thin film Zn2SiO4:Mn at V= 2keV, i= 3.0 A for 10 minutes of continuous beam exposure and recovery over 3.67 hours indicating permanent degradation of phosphor. Zn2SiO4: Mn (thin film) CL Spectrum V= 2keV, i= 3.0uA, P= 2.7 E -9 Torr 0 500 1000 1500 2000 2500 3000 400450500550600650700 Wavelength (nm)Intensity (a.u.) beam on 10 min. beam on 30 min. beam off 30 min. beam off, auger 3.67 hours later 3.67 hours later, auger maximum recovery ~89%

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91 Figure 5-9. AES spectra from Zn2SiO4: Mn thin film phosphor during and after beam exposure (i= 3.0 A). -1200 -1000 -800 -600 -400 -200 0 200 400 600 800200220240260280300Kinetic Energy (eV)dN(E) beam on 10 min. beam on 30 min. after beam off 3.67 hrs after beam off charging features C signal

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92 Figure 5-10. CL spectra of thin film Zn2SiO4:Mn at V= 2keV, i= 13.0 A for 10 minutes of continuous beam exposure and re covery over 60 minutes indicating permanent degradation of phosphor. Zn2SiO4: Mn (thin film) CL Spectrum V= 2keV, i= 13.0uA, P= 3.5 E -9 Torr 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 400450500550600650700 Wavelength (nm)Intensity (a.u.) beam on 10 min.beam on 30 min. beam off 30 min. beam off, Auger 60 min. beam off 60 min. beam off, Auger maximum recovery ~95%

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93 Figure 5-11. AES spectra from Zn2SiO4: Mn thin film phosphor during and after beam exposure (i= 13.0 A). 5.3.2 Powder Zn2SiO4: Mn Phosphor 5.3.2.1 24 hour CL degradation The cathodoluminescent degrada tion was also observed for Zn2SiO4: Mn powder phosphors. Conditions similar to those for the thin film experiments were chosen for the experiments for the powder phosphor. The CL degradation trend for a beam voltage= 2000 eV and a low beam current, i= 3.5 0.5 A, is given in Figure 5.12. The current density was measured as 111 15.0 A/ cm2. The cathodoluminescence decreased by 56% during beam exposure. The biggest decrease in cathodolu minescent intensity occurred during the first twenty minutes of b eam exposure, as seen in Figure 5.13. The phosphor degraded to 70% of its original intensity in le ss than 20 minutes. The CL -500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 200220240260280300 Kinetic Energy (eV)dN(E) beam on 10 min. beam on 30 min. after beam off 60 min. after beam off charging features C signal

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94 spectra from before and after degradation ar e shown in Figure 5.14. Again, the electron beam exposure does not affect the shape or position of the CL emission spectra. Figure 5-12. Cathodoluminescent degradation of powder phosphor at V= 2 keV, i= 3.0 A. cl degradation on powder (i= 3.06.0 uA) 0 0.2 0.4 0.6 0.8 1 1.2 0120240360480600720840960108012001320144015601680 Time (min.)Norm Intensity (a.u.)system fluctations

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95 Figure 5-13. Degradation within the first 20 minutes for th e powder phosphor at i= 3.06.0 A. Auger spectra of the powder phosphor befo re and after degrad ation are shown in Figure 5.15. The carbon signal is present befo re and after degrada tion and has shifted due to charging. Nevertheless, the intens ity of the C signal has decreased indicating some removal of adventitious carbon from the phosphor surface. A ll of the carbon is not removed during beam exposure due to the p acking of particles with 3 dimensional surfaces exposed to the electron beam. The zinc signal is low, but detectable before and after degradation. The oxygen signal is slightly stronger af ter degradation, presumably because of C removal from the phosphor. Ther e are also Auger signals from iron before and after degradation. This signal comes fr om the stainless stee l sample holder during electron beam exposure to the phosphor. The powders were also observed under an increased beam current, i= 15.0 1.9 A. The decrease in CL intensity with time is shown in Figure 5. 16, at a high current cl degradation on powder (i= 3.06.0 uA)1.00 0.88 0.85 0.82 0.80 0.79 0.77 0.76 0.74 0.73 0.73 0.72 0.71 0.71 0.70 0.70 0.70 0.69 0.69 0.750 0.2 0.4 0.6 0.8 1 1.2 02468101214161820 Time (min.)Norm Intensity (a.u.)

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96 density corresponding to a current density of 477 60.5 A/ cm2. In this experiment, the cathodoluminescent intensity decrea sed to 24 % of its original intensity in 24 hours. The greatest decrease took place in the first 60 minutes of degradation during which the phosphor brightness decreased to 40% of its orig inal intensity. By 3 hours of continuous beam exposure, the phosphor has already degrad ed to 35 % of its original intensity. The before and after spectra are shown in Fi gure 5.17 where no changes in the spectral distribution or peak shape for the phosphor are noted. AES da ta were collected from this phosphor during electron beam exposure. The changes in surface composition before and after degradation are shown by the Auger elect ron spectra in Figure 5.18. The spectra also give an indication of the charging th e sample experiences based on the artifacts indicated in the spectra.

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97 Figure 5-14. Cathodoluminescent spectrum of powder phosphor before and after 24 hours of degradation at low ex citation current density. Zn2SiO4: Mn (powder) CL Spectrum V= 2keV, i= 3.06.0 uA, P= 4.2 E -9 Torr 0 1000 2000 3000 4000 5000 6000 7000 400450500550600650700 Wavelength (nm)Intensity (a.u.) before deg after deg

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98 -600 -500 -400 -300 -200 -100 0 100 200 300 502504506508501050 Kinetic Energy (eV)dN(E) before deg after degC O Fe Zn Figure 5-15 Auger spectrum before and afte r 24 hours degradation on powder phosphor, i= 3.0 A.

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99 Figure 5-16. Cathodoluminescent degradati on of powder phosphor at V= 2 keV, i= 13A. cl degradation of powder (i= 13uA) 0 0.2 0.4 0.6 0.8 1 1.2 0120240360480600720840960108012001320144015601680 Time (min.)Normalized Intensitysystem fluctuations cl degradation of powder (i= 13uA) 0 0.2 0.4 0.6 0.8 1 1.2 0120240360480600720840960108012001320144015601680 Time (min.)Normalized Intensitysystem fluctuations

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100 Figure 5-17. Cathodoluminescent spectrum of powder phosphor before and after degradation at high exci tation current density. Zn2SiO4: Mn (powder) CL Spectrum V= 2keV, i= 13.016.8 uA, P= 5.8 E -9 Torr 0 2000 4000 6000 8000 10000 12000 400450500550600650 Wavelength (nm)Intensity (a.u.) before after

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101 Figure 5-18. Auger spectrum before and after degradation on powder phosphor, i= 13.0 A. 5.3.2.2 Cathodoluminescence recovery The experiment described in section 5.3.1.2. measuring any recovery of cathodoluminescence with the primary beam off was also performed for the powder phosphors. The phosphor was exposed to th e electron beam for 10 minutes and the recovery of the phosphor was monitored ove r the next 30 minutes with limited (5 seconds) intermittent beam exposures. Th e powder phosphor was examined at low and high current density. The Zn2SiO4: Mn phosphor powder degraded to 74% of its initial intensity for low current density = 115 A/ cm2 (i= 3.6 A) after 10 minutes of continuous electron beam exposure. The CL emission spectra for severa l time intervals are shown in Figure 5.19. -5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 502504506508501050 Kinetic Energy (eV)dN(E) before deg after degcharging Si O C

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102 The CL intensity increased from the degraded CL intensity after 5 sec. intermittent exposure to the electron beam in 1minute intervals for a span of 30 minutes. The maximum recovery during intermittent beam exposure over 30 minutes was from 74% to 85% of initial intensity. The CL was eval uated 15 hours later for further CL recovery. After 15 hours of no beam exposure, the phos phor only recovered to the CL intensity measured after 30 min. of intermittent exposure to the beam. AES spectra were collected during observation of the cathodoluminescence an d are shown in Figure 5.20. The shift in the carbon Auger signal over time indicate s that the phosphor is charging, which can be correlated with the loss in cathodoluminescent intensity. Zn2SiO4: Mn (powder) CL Spectrum V= 2keV, i= 3.6uA, P= 1.7 E-8 Torr 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 400450500550600650700 Wavelength (nm)Intensity (a.u.) beam on 10 min. beam on 30 min. beam off 30 min. beam off, Auger 15 hours later 15 hours later, Auger maximum recovery ~85% Figure 5-19 CL spectra of powder Zn2SiO4:Mn at V= 2keV, i= 3.0 A for 10 minutes of continuous beam exposure and recovery over 15 hours indicating permanent degradation of phosphor.

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103 Figure 5-20. AES spectra from Zn2SiO4: Mn powder phosphor during and after beam exposure (i= 3.0 A). The powder phosphor was also examined fo r CL recovery at a higher current density of 414 A/ cm2 (i= 13 A). The Zn2SiO4: Mn degraded to 34% of its initial intensity within 10 minutes of beam exposure. The maximum recovery from degradation was to 70% of the initial intensity after 30 minutes of intermittent exposure to the electron beam, as shown in Figure 5.21. The CL intensity was monitored for 12 hours after the 10 minute degradation experiment to determine if further recovery of the CL was possible, however no further recovery of intensity was detected. The loss of CL can be correlated to the Auger spectra shown in Figure 5.22. The shift in the Auger energy signal for carbon indicates that the surface is charging, even after 12 hours of limited electron beam exposure. -600 -500 -400 -300 -200 -100 0 100 200 300 200220240260280300 Kinetic Energy (eV)dN(E) beam on 10 min. beam on 30 min. after beam off 60 min. after beam off 15 hours after beam off C signal

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104 Figure 5-21. CL spectra of powder Zn2SiO4:Mn at V= 2keV, i= 13.0 A for 10 minutes of continuous beam exposure and recovery over 12 hours indicating permanent degradation of phosphor. Zn2SiO4: Mn (powder) CL Spectrum V= 2keV, i= 13.0uA, P= 7.7 E-9 Torr 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 400450500550600650 Wavelength (nm)Intensity (a.u.) beam on 10 min. beam on 30 min. beam off 4.5 hrs beam off 4.5 hrs beam off, Auger 12 hours later maximum recovery ~ 70%

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105 Figure 5-22. AES spectra from Zn2SiO4: Mn powder phosphor during and after beam exposure (i= 13.0 A). 5.4 Discussion 5.4.1 Thin Film Zn2SiO4: Mn phosphor Degradation The thin film phosphor degraded to 86% of its original intens ity under low current excitation density, J= 95 A/ cm2. This decrease in cat hodoluminescence occurred within the first ten minutes of beam exposure, where the CL intensity has already reached 86% of the initial intensity. The fast d ecrease in cathodoluminescence results from the phosphor charging on the surface as well the devel opment of an internal electric field, as will be discussed further. Charging of the phosphor was evident for both low and high current densities. Charging was observed based on nonA uger features at lower energies ( 200 eV) in the -500 -400 -300 -200 -100 0 100 200 300 200220240260280300 Kinetic Energy (eV)dN(E) beam on 10 min. beam on 30 min. after beam off 60 min. after beam off 4.5 hours after beam off 12 hours after beam off C signal

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106 secondary electron spectra and from a shift in Auger electr on peaks. The decrease in cathodoluminescence is correlated to the char ging observed during these times. The degradation that resulte d during early beam exposure was examined further to determine if recovery of the CL was possi ble and to investigate the mechanism which leads to the rapid CL loss. A shorter de gradation experiment was completed with continuous beam exposure for 10 minutes. After exposure to th e electron beam (V= 2keV, i= 3A), thin film Zn2SiO4: Mn degraded to 80% of its initial intensity. At the end 10 minutes of beam exposure, a shift in the C Auger peak due to charging is also noted. The CL improved and recovered to 89% of its original intensity after 30 min. of intermittent beam exposure for ~5 sec. every minut e. It is also reasonable to assume that an internal electric field has b een generated. Bang et al. has shown that an electric field generated within seconds can quench cathodoluminescence in La2O2S and ZnS phosphors. The induced internal electric fi eld results from charge separation at the surface [74]. A negative space charge present ne ar the end of the range of electron beam penetration and a positive charge near the in sulator surface would indu ce an electric field effect in insulators as su ggested by Cazaux [75] and in phosphors as postulated by Bang [76]. The data also suggests that the induced internal electric fiel d leads polarization of the surface. However, it has been shown th at the phosphor crystals may retain their polarization after the electric field is re moved until it is depolarized by high energy radiation such as high energy electrons [77]. The intermittent beam exposure depolarizes the phosphor where now an increase in lumi nescence is observed after 30 minutes of intermittent beam exposure. The intermittent exposure helped to increase the CL by depolarizing the phosphor.

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107 Conversely, the CL quickly decreased to the final degraded intensity (80%) during the time (1.25 min.) required to complete an Auger survey of the surface which can be correlated with charging, as i ndicated by a shift of the C Auge r signal to higher energies. No further recovery of cathodoluminescence was observed for the phosphor over 3.6 hours of observation. Neverthe less, charging of the phosphor wa s still evident, which is considered to contribute to the degradation. Surface charging occurs when there is net charge accumulation on the surface. The accu mulation is dependent upon the ability of the electron beam current to flow through th e phosphor to an electrical ground. The amount of current travel is reduced for wide band gap phosphors, such as Zn2SiO4. The shift of the Auger signa l to higher energies indicates a net negative surface charge is present relative to ground. Chargi ng has been shown to degrade the cathodoluminescence in ZnS, SrGa2S4, ZnO, and Y2O3 phosphors [78]. The charging reduces cathodoluminescence by enhancing nonradiative effects at the surface due to recombination between electrons and holes. An increase in nonradiative effects decreases the radiant efficiency of a phosphor, which is observed as decreased luminescence and efficiency of a phosphor. The magnitude of degradation for the cathodoluminescence brightness increased as the excitation density was also increased. The phosphor degrad ed to 74% of its original intensity when the current density was increased to 461 A/ cm2 over 24 hours. There was no shift or change in the spectral dist ribution for the CL emission spectrum for the thin films at either current density after exposure to the electron beam during this time period.

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108 A shorter degradation study was also completed on this phosphor. With an increased current, i= 13 A, the phosphor degraded to 92% of its original intensity, with maximum recovery to 95% of original intensit y. The maximum recovery occurred within the same time, 30 minutes, as the phosphor degrad ed at the lower current density. In this case, charging was evident as well and is c onsidered as the cause for the loss in CL intensity. At high and low current dens ities, the AES spectra indi cated that the carbon peak was decreased by electron beam exposure. At the end of the degrada tion experiments, an increase in the oxygen and zinc signal was obser ved. This is correla ted with the removal of carbon from the surface. No other cha nges in the surface chemistry were observed through the Auger analysis. Electron stim ulated surface chemical reactions (ESSCR) does not contribute to the degradation of Zn2SiO4: Mn during electron beam exposure. 5.4.2 Powder Zn2SiO4: Mn phosphor Degradation The powder phosphor also degraded less at the lower current excitation density. The phosphor decreased to 44% of its original CL intensity at a beam current density, J ~ 111.4 A/ cm2. The biggest decrease occurred with in 20 minutes to about 70% of its initial intensity. The degradation of the Zn2SiO4: Mn phosphor is dependent upon the excitation density. The magnitude of CL degradation increased to 24% of initial intensity when the beam current density was increased to ~ 477.4 A/ cm2. The CL emission spectra indicated no shift or change in the spectral distribution for the for the powder phosphors at either current density after expos ure to the electron beam for 24 hours. Recovery and the rapid loss of the cat hodoluminescence was investigated for the powder phosphors. The CL intensity for the Zn2SiO4: Mn powder was reduced to 74% of

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109 its initial intensity within 10 minutes of electron beam (V= 2keV) exposure at low current, i= 3.6 A. The maximum recovery was to 85% of initial intensity, which was obtained after 30 minutes of intermittent electron beam exposure. Again, it is postulated that intermittent electron beam exposure improves the CL by depolarization of the phosphor, allowing radiative photon emission to occur. As shown for the thin film phosphors, the emission was reduced to its 10 minute degraded CL in tensity level after only 1.25 minutes of electron beam exposure. Th e Auger spectra indicate that charging is still present in the phosphor. After 15 hours of very limited exposure to the electron beam, the CL recovers to same intensity level as seen after 30 minutes of intermittent beam exposure. A reduction in the charging is also evident from a positive shift of the C Auger signal towards the initial C Auger signal. The CL intensity also degraded to 34% of its initial intensity after 10 minutes of continuous beam exposure at high current density. As the current level was increased to 13 A, the phosphor increased its level of degradation to 34% of its initial CL inte nsity with 10 minutes of continuous beam exposure. The maximum CL recovery to 70% of its initial intensity again occurred after 30 minutes of intermittent beam exposure. In this case, the CL intensity recovered (68% of initial intensity) with minimum beam exposur e within 4.5 hours and remained at that level 12 hours after the initial beam exposur e. This change can be expected after examining the Auger spectra, where there are no shifts in the Auger signal at 4.5 and 12 hours. An increase in charging is expected as th e excitation density in creases. The Auger data correlates to this expected behavior. The degradation of the CL intensity is the result

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110 of charging, as with the th in film phosphors. As the charging increases and becomes more evident, the phosphor experiences a higher magnitude of degradation. The phosphors were permanently degraded and were limited in their recovery. Charging was observed for the phosphor through the Auger spectra and was correlated with the degradation of cathodoluminescence. The powder and thin film phosphors experi enced the highest degradation at the higher excitation current density for the same experimental conditi ons. The degradation was more severe for the powder phosphor. The lower level of degradation for the thin film phosphor may be attributed to the ability of the thin film to possess an uninterrupted electrical path to ground due to its conti nuous nature, thus reducing the surface charge leading to degradation.

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111 CHAPTER 6 CONCLUSIONS 6.1 Cathodoluminescence of Zn2GeO4: Mn Thin Films Zn2GeO4: Mn was successfully pulsed laser deposited onto MgO, Si, and YSZ substrates and the cathodoluminescent and phot oluminescent properties of the film were characterized. The characteristic green emission peak for the Zn2GeO4: Mn phosphor film was observed at 540nm for films deposited at T> 650 C. The intensity of the cathodoluminescence varied with the depositi on temperature, being high from films deposited at 650700 C for MgO and Si substrate samples, and being highest for films deposited at 750 C for YSZ substrate samples. The film deposited at 750 C onto YSZ had a strongly preferred (110) texture and the largest Zn/GE atomic ratio of 0.89. All other films that resulted in the characteristic green CL emission at 540 nm were more randomly polycrystalline. The numerical value of the ratio of Zn/Ge ratio was discussed and concluded to have large errors, but to accurately represent trends in the change in concentrations of Zn and Ge with deposition parameters. Higher ratios were shown to be consistent with improvement in crystallinity and CL intensity. A shift to red emission at 650 nm wa s noted for all films grown at 600 C. This shift in emission is attributed to change in the valence state of the activator from Mn2+ to Mn4+. A distinct difference was noted in th e PL excitation spectra, indicating that different excited states are responsible for gr een and red emission. The change in valence state was suggested to result from a change in the substitution site for the Mn ion. The

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112 Mn2+ ion substitutes for the Zn2+ in Zn2GeO4: Mn, resulting in green emission. However, when the Mn4+ ion substitutes for the Ge4+ ion, the emission is red-shifted to 650 nm. 6.2 Development of Zn2SiO4: Mn Thin Films Thin film Zn2SiO4: Mn phosphors were successfully grown by pulsed laser deposition, sputter deposition, and combustion chemical va por deposition. The films were made on silicon for PLD and quartz su bstrates for sputter and CCVD. There was no luminescence from the asdeposited films, even with an elev ated film growth temperature for those made by PLD and spu tter deposition. The film did have the characteristic green emission and spectral pr operties after a rapid th ermal anneal to 1100 C in N2 atmosphere, indicating that an elevated temperature is necessary to improve the crystallinity of the phosphors. The asdeposited CCVD films were polycrystalline. The films were characterized in terms of struct ural properties and luminescent quality. The composition of the films is critic al for developing the good luminescent phosphors. The luminescence of the phosphors in creased as the zinc to silicon ratio increased. As the crystallinity and thickne ss of the films increase, an increase in the phosphors CL intensity was also observed. 6.3 Degradation of Zn2SiO4: Mn Phosphors The degradation behavior was characterized for thin film and powder Zn2SiO4: Mn phosphors. An electron beam voltage of 2 keV was used for all the experiments in this section. The thin film phosphor degraded to 86% of its origin al intensity under low current excitation density (~95 A/ cm2). The decrease in cathodoluminescence occurred within the first ten minutes of beam exposur e. The magnitude of degradation for the cathodoluminescence brightness increased as th e excitation density was also increased (~460 A/ cm2). The phosphor degraded to 74% of its original intensity under high

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113 current excitation density. In both cases, the AES spectra indicated that carbon was removed during the film surface during electr on beam exposure. At the end of the degradation experiments, an increase in the oxygen and zinc signal could be observed. This is the result of the carbon removal from the surface. No detectable changes in the surface composition that may have been stim ulated by electron beam exposure were observed through the Auger analysis. The decrea se in luminescence is attributed to the development of an internal electric fi eld and surface charging observed during degradation. The powder phosphor also degraded less at the lower current excitation density. The phosphor initially decreased to 74 % of its original CL inte nsity at a beam current, i= 3.0 A. The degradation of the Zn2SiO4: Mn phosphor is dependent upon the excitation density. The magnitude of degradation increase s to 24% of its original intensity as the beam current density also increases to 13 A. An increase in charging is expected as the excitation density increases. The Auger data correlates to this expected behavior. The degradation of the CL intensity can be relate d to the development of an internal electric field and charging, as with the thin film phosphors. As the charging increases and becomes more evident in the data, the phosphor experiences a higher magnitude of degradation. The powder and thin film phosphors experi enced the highest degradation at the higher excitation density. The degradati on was more severe for the powder phosphor. The lower degradation for the thin film phosphor may be attributed to the ability of the thin film to continuous electrical path to gr ound. The electron beam did not affect the

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114 chemistry of the phosphor surfaces and no ch anges in the composition of the phosphors were noted for all conditions. The Zn2SiO4: Mn phosphor has been shown to maintain chemical stability during electron beam exposure. This attribute sa tisfies one of the critical components for application in low voltage field emission display devices. Continued work towards improvement in the brightness of this phos phor would help to make this phosphor a feasible candidate for use in FED technology.

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117 31 Z. Ji, S. Yongliang, and Y. Zhizhen, Journal of Crystal Growth 255, 35356 (2003). 32 R. Selomulya, S. Ski, K. Pita, C. H. Kam, Q. Y. Zhang, and S. Buddhudu, Materials Science & Engineering B B100, 13641 (2003). 33 V. Bondar, M. Vasyliv, Ya. Vasyltziv, M. Grytsiv, and Yu. Dubov, presented at the 4th International C onference on the Science a nd Technology of Display Phosphors & 9th Intern ational Workshop on Inorganic and Organic Electroluminescence, Bend, Oregon, 1998. 34 Sean Liam Jones, PhD., University of Florida, Gainesville, 1997. 35 John S. Lewis and Paul H. Holloway, Journal of the Electrochemical Society 147 (8), 3148 50 (2000). 36 Tao Feng, Ph.D., University of Florida, Gainesville, 2001. 37 John South Lewis III, Ph.D., University of Florida, Gainesville, 2000. 38 V. D. Bondar, Yu G. Dubov, and M. R. Pamasiuk, presented at the 12th International Workshop on Inorganic and Organic Electroluminescence & 2004 International Conference on the Science and Technology of Emissive Displays and Lighting, Toronto, Canada, 2004. 39 G. Stuyven, P. De Visschere, K. Neyts, and A. Kitai, Journal of Applied Physics 93 (8), 462227 (2003). 40 C. Baker, J. Heikenfeld, and A. J. St eckl, IEEE Journal of Selected Topics in Quantum Electronics 8 (6), 142026 (2002). 41 D. A. Trivedi, N. M. Kalkhoran, W. D. Ha lverson, G. D. Vakerlis, and C. A. Del Gado, SID 97 Digest, 61922 (1997). 42 S. Suh, D. M. Hoffman, L. M. Atagi, and D. C. Smith, Chemical Vapor Deposition 7 (2), 8184 (2001). 43 X. Ouyang, A. H. Kitai, and T. Xiao, J. Appl. Phys. 79 (6), 322934 (1996). 44 V. Bondar, M. Vasyliv, M. Grytsiv, Y. Dbov, S. Popovich, L. Akselrud, V. Davydov, and I. Kucharsky. 45 Douglas B. Chrisey a nd Graham K. Hubler, Pulsed Laser Deposition of Thin Films. (John Wiley & Sons, Inc., New York, 1994).

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118 46 C. C. Chang, X. D. Wu, R. Ramesh, X. X. Xi, T. S. Ravi, T. Venkatesan, D. M. Hwang, R. E. Muenchausen, S. Foltyn, and N. S. Nogar, Appl. Phys. Lett. 57 (17), 181416 (1990). 47 X. D. Wu, R. E. Muenchausen, S. Foltyn, R. C. Estler, R. C. Dye, C. Flamme, N. S. Nogar, A. R. Garcia, J. Martin, and J. Tesmer, Appl. Phys. Lett. 56 (15), 148183 (1990). 48 J. Narayan, N. Biunno, R. Singh, O. W. Holland, and O. Auciello, Appl. Phys. Lett. 51 (22), 184547 (1987). 49 G. Koren, A. Gupta, R. J. Baseman, M. I. Lutwyche, and R. B. Laibowitz, Appl. Phys. Lett. 55 (23), 245052 (1989). 50 Angus Rockett, Sputter Deposition of Thin Films. (American Vacuum Society, 2002). 51 R. K. Waits, J. Vac. Sci. Technol. 15 (2), 17987 (1978). 52 John L. Vossen and Werner Kern, Thin Film Processes II. (Academic Press, Inc., New York, 1991). 53 Milton Ohring, The Materials Science of Thin Films. (Academic Press, New York, 1992). 54 Yves Pauleau, Chemical Physics of Thin Film Deposition Processes for Microand NanoTechnologies. (Kluwer Academic Publishers, Norwell, MA, 2002). 55 J. A. Thornton, J. Vac. Sci. Technol. 15 (2), 17177 (1978). 56 Aicha A. R. ElshabiniRiad and III Fred D. Barlow, Thin Film Technology Handbook. (McGraw-Hill Companies, Inc., New York, 1997). 57 M. R. Davidson, B. Pathangey, P.. H. Holloway, P. D. Rack, S.S. Sun, and C. N. King, Journal of Electronic Materials 26 (11), 135560 (1997). 58 Light Tour: Electromagnetic Spectrum, http://cse.ssl.berkeley.edu (Regents of the University of California, 1996). 59 J. S. Sebastian, H. C. Swart, T. A. Tro ttier, S. L. Jones, and P. H. Holloway, J. Vac. Sci. Technol. A 15 (4), 234953 (1997). 60 H. C. Swart, T. A. Trottier, J. S. Sebastian, S. L. Jones, and P. H. Holloway, Journal of Applied Physics 83 (9), 457883 (1998).

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119 61 J. M. FitzGerald, T. A. Trottier, R. K. Singh, and P. H. Holloway, Appl. Phys. Lett. 72 (15), 183839 (1998). 62 O. M. Ntwaeaborwa, K. T. Hillie, and H. C. Swart, phys. stat. sol. (c) (2004). 63 Y. E. Lee, D. P. Norton, and J. D. Budai, Appl. Phys. Lett. 74 (21), 315557 (1999). 64 Y.E. Lee, D. P. Norton, J.D. Budai, P.D. Rack, J.Peterson, and M.Potter, in MRS Electron Emissive Materials, Vacuum Mi croelectronics, and Flat Panel Display (MRS, San Francisco, 2000). 65 C. Richard Brundle, Charles A. Evans Jr., and Shaun Wilson, Encyclopedia of Materials Characterization. (Butterworth Heinemann, Boston, 1992). 66 B. G. Yacobi and D. B. Holt, Cathodoluminescence Micr oscopy of Inorganic Solids. (Plenum Press, New York, 1990). 67 C.F.Yu and P.Lin, J. Appl. Phys. 79 (9), 719197 (1996). 68 D. T. Palumbo and Jr. J. J. Brow n, J. Electrochem. Soc.: SOLID STATE SCIENCE 117 (9), 118488 (1970). 69 D. R. Lide, CRC Handbook of Chemistry & Physics, 78th ed. (CRC Press, Boca Raton, FL, 1997). 70 Z. T. Kang, Y. Liu, B. K. Wagner, R. Gilstrap, M. Liu, and C. J. Summers, presented at the 12th Internationa l Workshop on Inorganic and Organic Electroluminescence & 2004 Internati onal Conference on the Science and Technology of Emissive Displays an d Lighting, Toronto, Canada, 2004. 71 A. T. Hunt, W. B. Carter, and J. K. Cochran, Appl. Phys. Lett. 63 (2), 26668 (1993). 72 John B. Hudson, Surface Science: An Introduction. (John Wiley & Sons, Inc., New York, 1992). 73 Billie Lynn Abrams, University of Florida, Gainesville, 2001. 74 J. Bang, B. Abrams, and Paul H. Holloway, J. Appl. Phys. 91 (11), 7091100 (2003). 75 J. Cazaux, J. Appl. Phys. 85 (2), 113747 (1999). 76 Jungsik Bang, Ph.D., University of Florida, Gainesville, 2004.

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120 77 L. Ozawa, Appl. Phys. Lett. 43 (11), 107374 (1983). 78 C. H. Seager, W. L. Warren, and D. R. Tallant, J. Appl. Phys. 81 (12), 79948001 (1997).

PAGE 136

121 BIOGRAPHICAL SKETCH Lizandra Clarissa Williams was born in Mo rgan City, Louisiana. She completed her high school studies at the Louisiana School for Math, Science, and the Arts with a concentration in Math Science and Humanities. Upon graduation, she went to Florida A & M University and obtained a B.S. magna cum laude degree in mechanical engineering. After working for a year fo r 3M in Minnesota, she began her graduate studies in Materials Science and Engineering at the University of Florida. She obtained her M.S. in 2002 and her Ph.D. in 2004.


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Title: Cathodoluminescence and Degradation of Oxide Thin Film and Powder Phosphors
Physical Description: Mixed Material
Copyright Date: 2008

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CATHODOLUMINESCENCE AND DEGRADATION OF OXIDE THIN FILM AND
POWDER PHOSPHORS















By

LIZANDRA CLARISSA WILLIAMS


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2004

































Copyright 2004

by

Lizandra C. Williams

































This dissertation is dedicated to the Trilogen and the memory of my loving grandfather,
Andrew Guy, Sr.















ACKNOWLEDGMENTS

I would like to first thank God for his blessings and unending presence in my life.

He is the reason I have been so successful thus far in life. I must then acknowledge and

thank my mother for my life and all the sacrifices she has made for me. Her help and

endless support is another reason I have made it to this point in life. Of course, the rest of

my family and friends are next on the list.

I would like to thank my advisor, Dr. Holloway, and committee (Dr. Abernathy,

Dr. Norton, Dr. Hummel, and Dr. Tanner) for their interest and support in my academic

endeavors. I would like to acknowledge the Major Analytical and Instrumentation Center

(MAIC) staff for their assistance with characterization as well as training on the

instruments. I would like to like to acknowledge the assistance received from the staff of

Oak Ridge National Laboratory, Dr. Kumar and his staff at North Carolina A&T State

University, and Mr. Kang and Dr. Summers at Georgia Institute of Technology for their

assistance with this work.

I truly appreciate Ludie's attention to all the details that made working in this

research group even better. I am grateful for all her help and talks. A big thanks goes to

all of the members of Dr. Holloway's group, past and present for their assistance.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES .......................................... viii

LIST OF FIGURES ....................................... ........... ............................ ix

A B S T R A C T .................................................................................................... ........ .. x iii

CHAPTER

1 IN T R O D U C T IO N ................................................. .............................................. .

2 LITER A TU R E R EV IEW .................................................................... ...............3...

2 .1 Introdu action .................................................................................................... .. .3
2.2 Development of Cathodoluminescent Phosphors.............................................4...
2.3 Evolution of the Field Emission Display..........................................................7...
2.4 Cathodoluminescent Phosphor Materials ............... ....................................9...
2 .4 .1 H o st M material ....................................................................................... 9
2.4.2 Activator-Luminescent Center ..................................................10
2 .5 M materials D evelopm ent......................................... ........................ ............... 11
2.5.1 Sulfi de Phosphors ...................................... .. .......... ............ .. ........ .... 11
2 .5 .2 O x id e P h o sp h ors ........... ... .. ..... ......... ...... ...................... .................... 13
2.5.2.1 Zinc silicate phosphors (Zn2SiO4: Mn) ......................................13
2.5.2.2 Zinc germanate phophors (Zn2GeO4: Mn).................................15
2 .6 P processing of P hosphors ........................................ ....................... ............... 17
2 .6.1 P ow der P hosphors ....................................... ....................... ............... 17
2 .6.2 T hin F ilm P hosphors ............................................................. ............... 17
2.6.2.1 Pulsed laser deposition ................................................ ............... 18
2.6.2.2 Sputter deposition.................. .................................................. 22
2.7 E valuation of P hosphor......................................... ........................ ................ 24
2.7.1 C hrom aticity ............. .. ............... .............................................. 25
2 .7.2 Spectral D distribution .............................................................. ................ 25
2.7.3 D egradation C haracteristics................................................... ................ 27






v









3 CATHODOLUMINESCENCE FROM THIN FILM ZINC GERMANATE DOPED
WITH MANGANESE PHOSPHORS ......................... .........................................33

3 .1 In tro d u ctio n ........................................................................................................... 3 3
3.2 E xperim ental Procedures ....................................... ....................... ................ 33
3.2.1 Substrates and their Preparation ..................................................33
3.2 .2 P hosphor P processing .............................................................. ................ 34
3.2.3 Thin Film Phosphor Characterization ...................................................37
3.2 .3.1 X ray diffraction ........................................ ................. ................ 37
3.2.3.2 Relative composition analysis: EDX...........................................38
3.2.3.3 Cathodoluminescence characterization .......................................40
3.2.3.4 Photoluminescence characterization ...........................................41
3 .3 R e su lts..................... ... .................................................................................. .. 4 2
3.3.1 X R ay D iffraction R results .................................................... ................ 42
3.3.2 Cathodolum inescent Properties ............................................. ................ 45
3.3.3 Zinc to Germanium Ratio in the Films..................................................48
3.3.4 Photoluminescence Excitation and Emission.......................................49
3 .4 D iscu ssio n ............................................................................................................. 5 0

4 DEVELOPMENT OF ZINC SILICATE DOPED WITH MANGANESE THIN
F IL M P H O SP H O R S ... ........................................................................... ............... 54

4 .1 In tro d u ctio n ........................................................................................................... 5 4
4.2 E xperim ental Procedures ....................................... ....................... ................ 54
4.2.1 Substrates and their Preparation ..................................................54
4 .2 .2 P hosphor P processing .............................................................. ................ 55
4.2.2.1 Sputter deposition .................. .................................................. 55
4.2.2.2 Pulsed laser deposition ................................................ ................ 56
4.2.2.3 Combustion chemical vapor deposition......................................57
4.2.3 Thin Film Phosphor Characterization ..................................................57
4 .2 .3.1 X ray diffraction ......................................................... ................ 57
4.2.3.2 Scanning electron m icroscopy .............................. ..................... 58
4.2.3.3 Wavelength dispersive X- ray spectrometry ...............................58
4.2.3.4 X- ray photoelectron spectroscopy..............................................59
4.2.3.5 Cathodoluminescence characterization ......................................60
4.3 Results from Pulsed Laser Deposited Phosphors ...........................................60
4.3.1 Structural C haracterization .................................................... ................ 60
4.3.2 M orphological Characterization............................................ ................ 63
4.3.3 Cathodolum inescence R results ............................................... ................ 65
4.3.4 C om position of Film s ............................................................ ................ 67
4.4 Results from Sputter Deposited Phosphors .............. ....................................68
4 .4 .1 T arg et 1 ............................................. ................. ............. ... ........... 6 8
4.4.1.1 Structural characterization........................................... ................ 69
4.4.1.2 Cathodoluminescence results ......................................................70
4.4.2 Target 2 ................. ... .... ...................... 74
4.4.2.1 Structural characterization........................................... ................ 75
4.4.2.2 Cathodoluminescence results ......................................................75









4.5 Results from Combustion Chemical Vapor Deposition Phosphors...................77
4.5.1 Cathodolum inescence R results ............................................... ................ 77
4 .6 D iscu ssio n ............................................................................................................. 7 8

5 DEGRADATION OF ZINC SILICATE DOPED WITH MANGANESE POWDER
A N D TH IN FILM PH O SPH O R S ............................................................ ............... 81

5 .1 In tro d u ctio n ........................................................................................................... 8 1
5.2 E xperim ental Procedures ....................................... ....................... ................ 81
5 .3 R e su lts................................................ ........................................................... 8 3
5.3.1 Thin Film Zn2SiO 4: M n Phosphor......................................... ................ 83
5.3.1.1 24 hour CL degradation............................................... ................ 83
5.3.1.2 Cathodolum inescence recovery................................... ................ 89
5.3.2 Pow der Zn2SiO 4: M n Phosphor............................................. ................ 93
5.3.2.1 24 hour CL degradation............................................... ................ 93
5.3.2.2 Cathodoluminescence recovery...... .................. .................. 101
5.4 D discussion ........... ...... ... .. .... ......................................... 105
5.4.1 Thin Film Zn2SiO4: Mn phosphor Degradation ..................................105
5.4.2 Powder Zn2SiO4: Mn phosphor Degradation ................ ...................108

6 CON CLU SION S ............... ...................................... .. ... 111

6.1 Cathodoluminescence of Zn2GeO4: Mn Thin Films........................................111
6.2 Development of Zn2SiO4: Mn Thin Films...... .... ................................... 112
6.3 Degradation of Zn2SiO4: Mn Phosphors...... .... ..................................... 112

LIST O F REFEREN CE S .. .................................................................... ............... 115

BIOGRAPH ICAL SKETCH .................. .............................................................. 121















LIST OF TABLES


Table page

2-1 Color and wavelength emission for common dopants in ZnS host.......................12

2-2 Common oxide phosphors and their luminescent properties ...............................13

2-3 Sum m ary of Zn2SiO 4: M n properties.................................................. ............... 15

2-4 Sum m ary of Zn2GeO4: M n properties................................................. ................ 16

2-5 Typical excimer lasers and their operating wavelengths.....................................19

3-1 R aw m materials used to m ake target...................................................... ................ 36

3-2 Zn/Ge Atomic Ratio in Deposited Zn2GeO4: Mn Film vs. Deposition
Tem perature on V various Substrates .................................................... ................ 49

4-1 Rapid thermal anneal recipe for heat treatment of thin film samples ................... 56

4-2 Processing conditions for sputter deposition ofZn2SiO4: Mn thin films using
T a rg e t 1 ................................................................................................................ .. 6 9

4-3 Processing conditions for sputter deposition ofZn2SiO4: Mn thin films using
T a rg e t 2 ............................................................................................................... .. 7 5

5-1 Species m monitored during AES analysis ............................................. ................ 82















LIST OF FIGURES


Figure page

2-1 Cathode Ray Tube .................... ............. .............................. 5

2-2 Generation of electron beams in a CRT display ................................... ...............6...

2-3 Pixel view beyond shadow mask where A -- red, green, blue posphors; B --
shadow m ask; C --glass of display screen ........................................... ...............7...

2-4 Illustration of how the red, green, and blue hues combine to give a full spectrum
of colors for full color display s ............................................................. ...............7...

2.5 Pixel view of field em mission display ..................................................... ...............8...

2-6 Cross section view of phosphor screen for a CRT ................................ ...............9...

2-7 Energy excitation (absorption) leads to photon emission. ..................................11

2-8 PL spectra for as deposited and annealed Zn2Si0o.5Ge0o.504: Mn thin films ............17

2-9 Sketch of basic pulsed laser deposition system ................................... ................ 18

2-10 Picture of plum e developed during PLD ............................................. ................ 21

2-11 Schematic diagram of a typical set up for a sputter system ................................23

2-12 Planar m agnetron sputter source ......................................................... ................ 25

2-13 C IE chrom aticity chart .................................................................. ................ 26

2-14 Visible light spectrum and corresponding wavelengths......................................26

2-15 Semi logarithmic plot of CL intensity vs. electron dose for a ZnS: Ag phosphor...31

2-16 Linear and logarithmic plot of S Auger peak to peak height (APPH) vs.
electron dose for ZnS: A g phosphor ................................................... ................ 31

2-17 Plot of CL intensity of Y203: Eu phosphor and selected Auger peak to peak
heights v s. electron dose ................................................ ...................... .... ........... 32

3-1 Flow chart outlining substrate preparation and phosphor processing method ........35









3-2 Representative sketch of mosaic target where 75% area = Zn2GeO4: Mn and
2 5% area = Z nO ............. ................................................ .................. .. 36

3-3 Interaction of incident and diffracted X- rays in an XRD specimen..................... 38

3-4 Interaction volume in sample where electron beam penetrates and the resulting
signals are generated .............. .... .............. ................................................ 40

3-5 Picture of cathodoluminescence vacuum system ...............................................42

3-6 XRD pattern for Zn2GeO4: Mn films grown on MgO substrate at different
deposition temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C where *
denotes G eO 2 im purity phase ............................................................. ................ 43

3-7 XRD pattern for films grown on Si substrate at different deposition temperatures:
(a) 750 C, (b) 700 C, (c)650 C, (d)600 C ..................................... ................ 44

3-8 XRD pattern for films grown on YSZ substrate at different deposition
temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C...............................45

3-9 CL emission spectrum of Zn2GeO4: Mn on MgO substrate at various deposition
tem p eratu re s. ............................................................................................................ 4 6

3-10 CL emission spectrum of Zn2GeO4: Mn on a Si substrate for various deposition
tem p eratu re s. ............................................................................................................ 4 7

3-11 CL emission spectrum of Zn2GeO4: Mn on YSZ substrate at various deposition
tem p eratu re s. ............................................................................................................ 4 8

3-12 Green emission spectrum of Zn2GeO4: Mn excited with 325 nm radiation at 540
nm and excitation spectrum monitored at 540 nm. .............................................50

3-13 Red emission spectrum of Zn2GeO4: Mn excited with 325 nm radiation at 625 nm
and excitation spectrum m monitored at 625 nm ................................... ................ 51

4-1 Powder XRD pattern for Zn2SiO4: Mn. This is consistent with the
rhombohedral crystal structure reported in JCPDS 37- 1485. ................................61

4-2 XRD pattern of Zn2SiO4: Mn thin film phosphor (0.85 |tm thick) before and
after annealing. designates peaks from SiO2. ................................... ................ 62

4-3 XRD pattern of Zn2SiO4: Mn thin film phosphor (1.40 |tm thick) before and
after annealing. designates peaks from SiO2. ................................... ................ 63

4-4 SEM pictures of the surface of the phosphor films after annealing at 1100 C
for the A) 0.85 |tm sample and for B) 1.4 |tm thick sample. ..............................64









4-5 Cathodoluminescent emission spectra from Zn2SiO4: Mn at V= 5keV,
i= 8.0 pA, with inset of CL spectrum for 1.4 |tm thick sample ...............66

4-6 CL brightness versus primary beam voltage for PLD Zn2SiO4: Mn thin films. ......66

4-7 Comparison of composition in the PLD thin film samples.................................68

4-8 XRD pattern of 1100 C annealed sputtered films at two different sputter
c o n d itio n s. ............................................................................................................... 7 0

4.9 Cathodoluminescent spectra from sputtered deposited thin film samples .............71

4-10 Maximum CL brightness for sputter deposited and annealed Zn2SiO4: Mn thin
fi lm s from 1000 to 5000 eV ....................................... ...................... ................ 72

4-11 Maximum CL intensity at V= 5keV for i= 2.5 to 7 A. .......................................... 73

4-12 Maximum CL brightness for PLD vs. sputter deposited Zn2SiO4: Mn thin films
from 500 to 5000 eV ........................... ............................................. 74

4-13 X- ray diffraction pattern from Zn2SiO4: Mn thin film sputter deposited with target
2 ............................................................................................................ ......... 76

4-14 Cathodoluminescent emission from sputter deposited Zn2SiO4: Mn at electron
beam V= 5keV, i= 15[tA for sputter powers of 40- 70W. ................................... 76

4-15 CL spectra from CCVD Zn2SiO4: Mn phosphor films at beam V= 5keV,
i= 7 .0 p A ............................................................................................................. .. 7 8

5-1 Cathodoluminescent degradation of thin film phosphor at V= 2 keV, i= 3.0 pA
(95.49 [pA / cm 2) ......................................................................... ............................. 84

5-2 Degradation within the first 20 minutes for the thin film phosphor at i= 3.0 pA. ...85

5-3 Cathodoluminescent spectrum of thin film phosphor before and after
degradation at low current excitation density. .................................... ................ 86

5-4 Auger electron spectrum before and after degradation on thin film, i= 3.0 pA.......87

5-5 Cathodoluminescent degradation of thin film phosphor at V= 2 keV, i= 13 ptA.....87

5-6 Cathodoluminescent spectrum of thin film phosphor before and after
degradation at high excitation current density. ................................... ................ 88

5-7 Auger spectrum before and after degradation on thin film, i= 13.0 A ................... 88









5-8 CL spectra of thin film Zn2SiO4:Mn at V= 2keV, i= 3.0 ptA for 10 minutes of
continuous beam exposure and recovery over 3.67 hours indicating permanent
degradation of phosphor ......................................... ......................... ................ 90

5-9 AES spectra from Zn2SiO4: Mn thin film phosphor during and after beam
exposure (i= 3.0 tA ). ..................... .. ........................... ................. 91

5-10 CL spectra of thin film Zn2SiO4:Mn at V= 2keV, i= 13.0 [tA for 10 minutes of
continuous beam exposure and recovery over 60 minutes indicating permanent
degradation of phosphor ......................................... ......................... ................ 92

5-11 AES spectra from Zn2SiO4: Mn thin film phosphor during and after beam
exposure (i= 13.0 ptA ). ............. ................ ............................................... 93

5-12 Cathodoluminescent degradation of powder phosphor at V= 2 keV, i= 3.0 pA......94

5-13 Degradation within the first 20 minutes for the powder phosphor at
i= 3.0- 6.0 [tA ........................................................................... .............................. 95

5-14 Cathodoluminescent spectrum of powder phosphor before and after 24 hours of
degradation at low excitation current density. .................................... ................ 97

5-15 Auger spectrum before and after 24 hours degradation on powder phosphor,
i= 3 .0 tA ............................................................................................................. .. 9 8

5-16 Cathodoluminescent degradation of powder phosphor at V= 2 keV, i= 13tA ........ 99

5-17 Cathodoluminescent spectrum of powder phosphor before and after
degradation at high excitation current density. ..................................100

5-18 Auger spectrum before and after degradation on powder phosphor, i= 13.0 [tA..101

5-19 CL spectra of powder Zn2SiO4:Mn at V= 2keV, i= 3.0 ptA for 10 minutes of
continuous beam exposure and recovery over 15 hours indicating permanent
degradation of phosphor ..................................... ........................ ............... 102

5-20 AES spectra from Zn2SiO4: Mn powder phosphor during and after beam
exposure (i= 3.0 tA). ............................. ............. ......................... 103

5-21 CL spectra of powder Zn2SiO4:Mn at V= 2keV, i= 13.0 ptA for 10 minutes of
continuous beam exposure and recovery over 12 hours indicating permanent
degradation of phosphor ..................................... ........................ ............... 104

5-22 AES spectra from Zn2SiO4: Mn powder phosphor during and after beam
exposure (i= 13.0 [tA ). ............. ................ ............................................... 105















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

CATHODOLUMINESCENCE AND DEGRADATION OF
OXIDE THIN FILM AND POWDER PHOSPHORS

By

Lizandra Clarissa Williams

December 2004

Chair: Paul H. Holloway
Major Department: Materials Science and Engineering

The low voltage cathodoluminescent characteristics of Zn2GeO4: Mn thin film

phosphors grown by pulsed laser deposition were investigated. The effects of substrate

heating (600-750C) and substrate type (MgO, Si, and yttria-stabilized zirconia) on

cathodoluminescent properties were studied. A characteristic green emission peak at 540

nm was observed at substrate temperatures of 650, 700, or 7500C. However, the

emission was red shifted to 650 nm for a substrate temperature of 6000C. The red shift in

emission wavelength from 540 to 650 nm was attributed to a change in ionic state of the

activator from Mn2 to Mn4 While the spectral position was independent of substrate

type, the relative intensities of the cathodoluminescent emission peak varied with both

substrate type and substrate temperature. At a substrate temperature of 6000C, the crystal

structure of the film was mixed polycrystalline and amorphous. However, at substrate

temperatures ranging from 650-7500C, the films were only polycrystalline, with varying

degrees of crystallinity that correlated with the cathodoluminescence intensity.









Zn2SiO4: Mn thin film phosphors were developed by pulsed laser deposition,

sputter deposition, and combustion chemical vapor deposition. Films were grown by

pulsed laser deposition at an oxygen pressure of 300 mTorr onto (100) Si substrates

heated to 700C. The films were polycrystalline after a rapid thermal anneal (RTA) for

five minutes at 1100C in N2 atmosphere. The cathodoluminescence of the films

depended upon the Zn/ Si atomic ratio, which was less than stoichiometric. The film

with the higher zinc content exhibited the brightest cathodoluminescence although the

film was also the thinnest. An increase in the deposition time was correlated with a

reduction in zinc content.

Sputter deposition of Zn2SiO4: Mn thin films were completed on room temperature

quartz substrates in 02/ Ar atmosphere. The films were polycrystalline and green

cathodoluminescence was observed from the samples after a RTA for five minutes at

1100C in N2 atmosphere. The cathodoluminescence from the films was compared for a

sputter power range, 40- 70W. The cathodoluminescence was brightest for films

deposited at 60- 70W and dimmest for films deposited at 40- 50 W. The increased

brightness from the films deposited at the higher power was attributed to an increase in

the film thickness. The films created by sputter and pulsed laser deposition were

compared. The sputter deposited film was at least 30% brighter than the films made by

pulsed laser deposition. The lower response was attributed to the Zn deficiency present

in the films made by pulsed laser deposition.

The brightest films were made by combustion chemical vapor deposition. The

Zn2SiO4: Mn phosphor films were deposited onto quartz substrates heated to 12000C at

atmospheric pressure with ambient room environment supplying the oxygen needed for









this reaction. The cathodoluminescence was compared at two dopant levels: 2 and 4

mol% Mn, with the higher intensity coming from the 4 mol% Mn film.

The degradation behavior was examined for Zn2SiO4: Mn thin film and powder

phosphors. The degradation was less for both thin film and powder phosphors at a low

current excitation density. The CL intensity decreased to 86 and 44% of the CL intensity

at time = 0 min. after continuous beam electron beam exposure for 24 hours for the thin

film and powder phosphor, respectively. As the beam current density increased, an

increase in degradation was also noted. The degradation for the film phosphor was 74%

and 24% of initial intensity for the powder after exposure to a continuous electron beam

for 24 hours. The increased degradation with increased beam current was correlated to

the presence of more severe charging from the phosphor, as observed from the Auger

data. At both current levels, the thin film phosphor degraded less than the powder

phosphor. No changes in the surface composition, other than the removal of adventitious

carbon, were observed through Auger electron spectroscopy during the periods of

electron beam exposure. The degradation of the phosphors is attributed to development

of internal electric fields and charging of phosphor surface.














CHAPTER 1
INTRODUCTION

Cathodoluminescent phosphors may have applications in many emissive

technologies, such as field emission flat panel displays, where the phosphor is responsible

for production of the screen image. There is a need by consumers for portable displays

that are more durable and have better resolution. Developing an adequate display that

meets these requirements depends on development of the screen material as a critical

component.

Performance of potential phosphors must be studied to determine which materials

would be most adequate. Performance parameters, such as brightness and chemical

stability of the phosphor, which affect the reliability and lifetime of the device must be

investigated and understood for successful advancements to be made toward identifying

potential phosphors for display technologies.

Chapter 2 will provide a survey of recent research developments for

cathodoluminescent phosphors, with emphasis on oxide phosphors. A discussion of the

experimental parameters and characterization methods will be presented in Chapter 3 for

Zn2GeO4:Mn thin films grown by pulsed laser deposition. The results from the

development of thin film Zn2SiO4: Mn by sputter deposition, pulsed laser deposition, and

combustion chemical vapor deposition are given in Chapter 4. Insight is given into the

critical processing parameters needed to successfully grow Zn2SiO4: Mn. Chapter 5

provides an analysis of cathodoluminescent degradation of both powder and thin film

Zn2SiO4: Mn phosphors at high and low beam current densitites, with an aim of






2


determining the mechanism for oxide degradation. Chapter 6 provides a summary of the

conclusions from the experimental results for these oxide phosphors.














CHAPTER 2
LITERATURE REVIEW

2.1 Introduction

Cathode ray tube (CRT) monitors and televisions are the conventional display for

image and visible information transfer. The CRT television is still the most popular type,

[1] although there is competition from other display technologies. Flat panel display

technologies such as the active matrix liquid crystal display and the plasma display panel

are the newest options in the television market [1]. The CRT display has several

advantages over its competitors. It offers the best contrast, best resolution, widest

viewing angle, and lowest cost [1]. With a lower price tag than its competitors, the CRT

display offers the best value for your money. However, a major disadvantage of the CRT

monitor is the size and bulk associated with this type of display. These CRTs are the

heaviest of all types of televisions [1]. Many competitive technologies have been

developed to respond to this inconvenience. Demand is growing for displays that are

lighter and thinner, offering the convenience of easy portability.

One technology that meets the demand for a more compact and portable display is

liquid crystal display (LCD). The LCD technology was developed in the 1970s.[1]

These LCDs are used in front projectors, rear-projection TVs, and flat panel displays.

The main advantage of LCDs is that they are very marketable to consumers who need a

compact and portable display. An LCD offers a thinner, lighter, and sleeker alternative to

the CRT. However, the customer can expect to pay more for this convenience.

Disadvantages of using an LCD include limited viewing angle and contrast ratio,









resulting in displays with a lower resolution than the conventional choice.[1] There are

markets where small displays would be desirable, but an LCD is not the optimal choice.

Developing displays that are brighter and more rugged; and that have an extended

operating temperature range, and less sensitivity to constant movement would be

beneficial to applications involving military, medicine, and transportation displays. [2]

2.2 Development of Cathodoluminescent Phosphors

Cathodoluminescent phosphors have been researched for over 100 years.

Improvements are still being made in this area, as new applications for

cathodoluminescent phosphors have emerged. Braun invented the cathode ray tube

oscilloscope in 1897 [3,4], which is noted as the first practical application of displaying

an image by bombardment of electrons. This led to more investigations on the behavior

of phosphors. Phosphors are materials that emit light when stimulated by an incident

energy source [5]. In this example of cathodoluminescence (CL), the electrons emitted

from the cathode were the source of energy. An example of a cathode ray tube is shown

in Figure 2.1. The cathode ray tube is a glass tube sealed to maintain a vacuum, where

most of the air has been removed. Inside the vacuum tube, a piece of metal coated with a

phosphor is placed horizontally between the cathode and anode. This coating on the

metal allows the observer to visibly observe the path of the electrons. The electrons

originate from the cathode, which is the negative electrode. The electrode is connected to

a power source by the alligator clip. The anode directs the path of the electrons due to

the attraction of the electrons to the positive electrode. The result is a visible luminescent

line between the negative and positive electrodes indicating the path of the electrons. In

1907, Boris Rosing made a significant development with the cathode ray tube when he

demonstrated that it was possible to transmit an image. Since then, many inventions and









discoveries have been made. Presently, cathode ray tubes are used in many commercial

display devices such as televisions, computer monitors, modern oscilloscopes, and

electron microscopes.





Cathode Photon Emission

Alligator Clip Anode










Stand Glass Tube





Figure 2-1. Cathode Ray Tube [6].

Initially, black and white televisions relied on only two phosphors for its display

image. The phosphors combined to produce a bluish- white emission color [7]. The best

phosphors for this application were ZnS:Ag and Zno.5Cdo.5S:Ag or Zn0.9Cdo.iS:Cu, Al [7].

With the development of full color displays, the number of electron guns increased to

three. Each electron beam emitted from the gun is directed to one of three phosphors

emitting red, green, or blue. An example of a full color CRT with an illustration on the

operation of the electron beams is shown in Figure 2.2. The display screen is divided into

subdivisions called pixels. Each pixel has a red, green, and blue phosphor in its sub-pixel

matrix. The number of pixels on a display screen determines the resolution that is








available for the display. As the number of pixels increase, the color detail for the images

also increase. The pixel matrix is depicted in Figure 2.3. Additive mixing of red, green,

and blue (RGB) colors result in all of the colors needed for a full color display [7-9]. The

illustration in Figure2.4 shows the range of colors produced by the RGB phosphors as

they are mixed together. The best phosphors for full color CRT displays are ZnS: Ag

(blue); ZnS: Cu, Al (green); and Y202S: Eu (red) [7]. The choice of phosphors will

change as the thick CRT evolves into a thinner model.


Q Cathode
Conductive coating
@Anode


D Phosphor-coated screen
0 Electron beams
( Shadow mask


Figure 2-2. Generation of electron beams in a CRT display [10].


















I1
02000 ow ShbiffWork$

Figure 2-3. Pixel view beyond shadow mask where A -- red, green, blue posphors; B --
shadow mask; C --glass of display screen [10]



GREEN YELLOW







Figure 2-4. Illustration of how the red, green, and blue hues combine to give a full
spectrum of colors for full color displays [9].

2.3 Evolution of the Field Emission Display

Field emission displays (FEDs) are flat panel displays that are similar to the

conventional cathode ray tube (CRT) displays [11]. Both technologies produce an image

on the display screen by using an electron source. The main components of the FED that

have been the attention of many researchers are the phosphor and emitter. The emitter is

the component that produces the electrons. The phosphor has the responsibility of

producing the image on the device screen.

The filed-emitters for an FED are analogous to the electron guns found in CRTs. A

CRT uses only three electron guns, whose focused beam of electrons are scanned across

the screen to produce an image; whereas, FEDs use an array of thousands of micron scale









cold cathode emitters to direct electrons at each pixel as shown in Figure 2.5. These

small emitters allow FEDs to be scaled to millimeters in width, allowing for the

feasibility of a thin display. However, the narrow thickness requires device operation at

lower voltages to prevent vacuum breakdown [12]. In order to obtain an acceptable

brightness at low voltages, the current density requirement must be increased.

A thermionic cathode using high accelerating voltages produces electrons in CRT.

The voltages may range from 20- 30 kV for a CRT [13,14]. In FED, the electrons are

produced via a cold cathode by electron tunneling at high fields. The target operating

voltage for an FED ranges from 1- 8 kV [13]. The lower voltage results in a higher

current to maintain necessary power to yield adequate brightness from the phosphor. The

amount of light that is emitted from the phosphor depends upon beam power, i.e. primary

beam voltage times current [13]. The phosphors used for CRTs are usually coated by a

thin aluminum layer (0.2- 0.5 [tm), as shown in Figure 2.6 [13]. [13]. The phosphors

used for CRTs are usually coated by a thin aluminum layer (0.2- 0.5 [tm), as shown in


Phosphors






Electrons
"- Gate
Microtips


Figure 2.5. Pixel view of field emission display.









Figure 2.6 [13]. The cross section shows that the aluminum layer may protect the

phosphor from the reaction with residual vacuum gases. However, for the FED, the

aluminum layer is not always applied. This leaves the phosphor exposed to the vacuum

environment. Consequently, the effect of the residual vacuum gases on the phosphor are

a critical reliability concern for the device. Thus, conditions such as charging and

outgassing are huge concerns for this application [2,13]. Therefore, it is important to

understand what happens to the phosphor during electron beam exposure in different

ambients. Anticipated advantages of FEDs over LCDs include: better viewing angle,

better response time, wider operating temperature range, better price & power

consumption [13].





Electron beam


u- S u Alfilm
/ Phosphor
Substrate


hv

Figure 2-6. Cross section view of phosphor screen for a CRT [13].

2.4 Cathodoluminescent Phosphor Materials

2.4.1 Host Material

The host material is responsible for the electrical and optical properties of the

phosphor. The inert host lattice is transparent to the excitation radiation [13].

Nevertheless, the surroundings of the activator in the host material dictate the optical

behavior of the activator.









2.4.2 Activator-Luminescent Center

The activators control the emission spectra of luminescent materials [15]. An

activator, or luminescent center, is an impurity added to the host material in small

quantities. It is known as the light-emitting center of a phosphor and is responsible for

the optical properties of a phosphor. Activators provide distinct energy levels in the

energy gap between the conduction and valence bands of the host material [15]. The

interaction of the incident energy source with a luminescent material is critical for the

production of light.

In the case of electron- hole pair excitation by the primary electrons for CL, the

energy levels associated with the activators determines the energy of the photon

emission. A phosphor absorbs the incident energy, which leads to electron excitation

from the valence band to the conduction band. It is the behavior of these electrons, which

determines the luminescence of a phosphor. The electrons are excited either to the

conduction band or to a trap as shown in the first step illustrated in Figure 2.7. The traps

are the distinct energy levels formed by the activator. These energy levels are referred to

as traps since electrons and/ or holes may become held in them for long times at the

temperature of operation. In the second step, shown in Figure 2.7, the electrons may be

excited from one trap to another or into the conduction band. The elemctrons can then be

captured by upper empty activator levels, and subsequently emit photons when they drop

further down to lower excited state or ground state activator levels, as shown in step 3 of

Figure 2.7. The energy of the photon emitted, E, is associated with a specific wavelength

of light, A, as given by [16]:

hv
A hv (2-1)
E









and with substitution of the appropriate phosphors, results in:

1.2398
A(/im) -- (2-2)
E(eV)



Electro Traps Conduction band














Hole Valence band



Figure 2-7. Energy excitation (absorption) leads to photon emission.

2.5 Materials Development

Phosphors are often characterized as either a sulfide or oxide phosphor.

Conventional cathodoluminescent phosphors are sulfide based, where sulfur is an

element present in the composition.

2.5.1 Sulfide Phosphors

A lot of research has been devoted to sulfide phosphors since they are used in many

display applications. Zinc sulfide, ZnS, is a common host material for

cathodoluminescent and electroluminescent phosphors. It has been noted for its excellent

electrical properties. The most efficient materials for host lattice excitation have

relatively small values for the band gap energy, Eg. The band gap energy of ZnS = 3.75









eV. Thus, zinc sulfide has been noted for its excellent cathodoluminescent efficiency of

20- 25% [3]. Depending upon the dopant, this phosphor can exhibit a specific color

luminescence. Many elements have been incorporated in the ZnS host lattice. Table 2.1

shows some of the dopants and colors that may be emitted from the ZnS phosphor.

Table 2-1 Color and wavelength emission for common dopants in ZnS host [2,3,7].

DOPANT COLOR

Mn Yellow

Mn (filtered) Red

Cl Blue

Ag, Cl Blue

Cu, Cl (or Al) Green

Tb or TbOF Green

Tm Blue

Sm, Cl Red



Sulfide phosphors have also been considered for use in FEDs [17]. However, these

phosphors are not very efficient at low voltages. In addition, conventional sulfide

phosphors such as ZnS:Cu,Al degrade significantly and decompose under electron beam

bombardment at the operating conditions required for field emission displays [18-20].

The products of degradation may potentially contaminate the cathode components,

reducing the performance of the emitters [20,21].









2.5.2 Oxide Phosphors

Oxide phosphors have also received attention as potential phosphors in low voltage

display applications, such as the FED. Oxide materials are generally more chemically

and thermodynamically stable than sulfide phosphors [22,23], so they are now

investigated as the potential phosphor type for low voltage applications. There are a host

of oxides that have been investigated and a summary of these phosphors are given in the

following table.

Table 2-2 Common oxide phosphors and their luminescent properties [24].
PHOSPHOR COLOR

Zn2SiO4: Mn Green

ZnO: Zn Green

ZnGa204: Mn Green

Y3A15012: Ce Green

ZrO2: Mn, Cl [25] Red

Y203: Eu Red

YVO4: Eu Red

Y2SiO5: Ce Blue

ZnGa204 Blue

CaWO4 Blue


2.5.2.1 Zinc silicate phosphors (Zn2SiO4: Mn)

Zinc silicate is already known as a phosphor for CRT and plasma display

applications. Several research groups have investigated its luminescent properties in

powder and thin film form [26-33]. Its feasibility as a phosphor for field emission









displays, electroluminescent devices, medical imaging detectors for low- voltage

radiography and fluoroscopy is also being studied.

Sun et al. observed a correlation between the film crystallinity and morphology for

as deposited and annealed films. The films were grown on heated 300 C substrates by

pulsed laser deposition and were annealed at 1000 C, before Zn2SiO4 crystallized [30]

indicating that a high temperature treatment is needed for a polycrystalline material. The

morphology, observed by scanning electron microscopy, indicated that the finer grains

and rougher surface correlates with an increase in the photoluminescence intensity due to

disruption of light guiding in high index thin films [30,34].

The high crystallization temperature of Zn2SiO4 is a definite disadvantage. Cho et

al. demonstrated that temperatures as high as 1400 C may be necessary to fully

crystallize Zn2SiO4 when the powder is processed in a solid state reaction. The authors

also developed a novel solution reaction method to lower the processing temperature.

However, the temperature was reduced by only 200 C [29]. One group has tried to

overcome the temperature hurdle, by processing zinc silicate by combustion synthesis.

Nonetheless, after initial processing at 500 C, the powder still required heating to 900 C

for 1 hour in a reducing atmosphere, where the powder then gained the characteristic

white appearance [28]. The authors also indicated that the second thermal treatment

improved the luminescence of the Zn2SiO4: Mn phosphor.

Zinc silicate is a promising phosphor for CL applications. A brief summary of the

relevant properties for zinc silicate is shown in Table 2.3. Cathodoluminescent emission

has been observed with peak emission exhibited around 525 nm [33]. Its high efficiency

makes it a possible phosphor in low voltage applications. However, the need for a high









processing temperature is a definite disadvantage. Another approach for getting beyond

the temperature hurdle has been to alter the composition of the material as will be

discussed in the next section.

Table 2-3 Summary of Zn2SiO4: Mn properties [30].
Property Value

Bandgap 5.5 eV

Index of Refraction 1.69

Crystal structure Rhombohedral


2.5.2.2 Zinc germanate phophors (Zn2GeO4: Mn)

Zinc germanate has been evaluated as a potential phosphor in alternating current

thin film electroluminescent displays (ACTFEL) [22,35-37]. In this phosphor, Ge atoms

substitute for the Si atoms from Zn2SiO4. The resulting composition, Zn2GeO4, leads to a

lower crystallization temperature and a smaller bandgap. Crystallization temperatures as

low as 650 C have been reported. Some researchers found that a slight raise in

processing temperature to 810 C improved the luminance of the electroluminescent

phosphor. A survey of the literature indicates that Zn2GeO4: Mn thin films have been

fabricated mostly by sputter deposition [22,35-37].

A brief summary of known zinc germanate properties are shown in Table 2.4. The

electroluminescent emission peak of Zn2GeO4: Mn has been observed to be between 535-

540 nm [22,35,38]. Zn2GeO4: Mn has been characterized as having a short decay time

(100 microseconds), resulting from the formation of a perfect structure and the low

coordination number (4) of manganese in host lattice [38]. The cathodoluminescent









efficiency ofZn2GeO4: Mn at 2 keV has been reported as 2.4 lm/W for the thin film

phosphor [23].

Table 2-4 Summary of Zn2GeO4: Mn properties.
Property Value

Bandgap 4.68 eV [35]

Index of Refraction 1.80 [22]

Dielectric constant 6 [22]

Crystal structure Rhombohedral


Zinc silicate- germanate (Zn2Sio.5Geo.504: Mn) thin film phosphors have also been

created by sputter deposition [39,40]. This phosphor was developed from incorporating

Ge into the silicate (Si04) lattice ofZn2 Si04. The Ge substitutes for the Si resulting in a

lower annealing temperature. Temperatures as low as 700 C have been reported for

annealing conditions, resulting in a polycrystalline film [39,40]. The photoluminescent

and electroluminescent emission peak of annealed Zn2Sio.5Geo.504: Mn is at ~ 530 nm

[39,40], a slightly shorter wavelength than pure Zn2GeO4: Mn. It is interesting, however,

that the as- deposited amorphous phosphor film show PL emission at two separate

wavelengths, which is not near the emission at 53 Inm, observed only in the annealed

case [40]. This PL emission behavior is shown in Figure 2.8. No further explanation of

the emission results was given by the authors.













M 800 C Anneal



S700C Anneal



0-
0 _-------, --- ...
400 500 600 700 800
? (nm)




Figure 2-8. PL spectra for as deposited and annealed Zn2Sio.5Geo.504: Mn thin films [40].

2.6 Processing of Phosphors

2.6.1 Powder Phosphors

Powder phosphors are usually made by a solid state reaction, although there are

various other methods that are employed as well. In this process, the raw materials are

first combined through chosen synthesis method. The powder is then fired with a flux

and the activator [7]. After the product is sieved, the flux is removed. The powder is

then milled with care to obtain the desired particle size [7].

2.6.2 Thin Film Phosphors

Thin film cathodoluminescent phosphors have been grown by a variety of growth

techniques. Pulsed laser deposition, sputter deposition, electron beam evaporation, and

metal- organic chemical vapor deposition are some of the common processes used to

develop thin film phosphors [30,32,41-44]. In this work, pulsed laser deposition and rf

magnetron sputter deposition were used to create the thin film phosphors. Accordingly,

further details of these methods are given below.









2.6.2.1 Pulsed laser deposition

Pulsed laser deposition is one of the most simplistic methods to deposit thin films.

It allows for the precise arrival rates of atoms for compound films. This has been shown

to be favorable for obtaining stoichiometric films of multi-component materials with a

high energy of dissociation [45]. This method has received considerable attention for its

ability to deposit the complex oxides needed to produce superconducting thin films [45-

49]. It also has the ability to operate in high pressure reactive gases, unlike other

deposition methods [45].

A deposition system usually consists of an excimer laser and optical elements to

maneuver and focus the laser beam. Some of the optical elements that are used in the set

up are focusing lens, apertures, mirrors, beam splitters and laser windows. A schematic

of a basic deposition system that uses oxygen as its reactive gas is shown in Figure 2.9.


Target

Plume




lt'


Substrate





Vacuum Pumrn


Oxygen Gas

Figure 2-9. Sketch of basic pulsed laser deposition system [45].









Excimer lasers with wavelengths between 200 and 400nm are most often used for

pulsed laser deposition [45]. A list of common excimer lasers and their operating

wavelengths are given in Table 2.3. Excimer lasers below 200nm are not typically used

for PLD due to the possibility of absorption by the Schumann- Runge bands of molecular

oxygen. As shown in Figure 2.9, the laser source is located external to the vacuum

chamber. The external energy source allows the film growth process to take place in a

reactive environment with any type and amount of gas. The external source gives the

added advantage of the laser being available for more than one deposition system.

Table 2-5 Typical excimer lasers and their operating wavelengths [45].
Excimer Wavelength (nm)

F2 157

ArF 193

KrCl 222

KrF 248

XeCl 308

XeF 351


Once the laser is focused into the chamber, the target absorbs the energy from the

laser. The ultra- violet (UV) radiation is converted to electronic excitation. This is

converted into thermal, chemical, and mechanical energy, leading to ablation and

evaporation of the target. The evaporants form a mixture of energetic species including

atoms, molecules, electrons, ions, and micron sized particulates. This mixture is the often

referred to as a plume. An example what a plume looks like during the film growth

process is shown in Figure 2.10. The plume quickly expands in the vacuum from the









target to form a "nozzle jet" [45]. As the plume reaches the substrate (which may be

heated), film nucleation commences.

The quality of the thin films produced by pulsed laser deposition is dependent on

several variables. Laser power and spot size have a significant effect on particulate size

and density. As the laser fluence is increased beyond a threshold, the number of

particulates that are formed also increases [45]. Laser fluence is defined as the laser

energy per unit area and thus, may be adjusted by varying the laser power or laser spot

size. Background gases may change growth parameters such as the deposition rate and

the kinetic energy distribution of the depositing species [45]. For instance, an oxidizing

environment can help oxides to form and stabilize the desired crystal phase at the

deposition temperature [45]. Substrate temperature has an effect on the stoichiometry of

the film as well as the film structure [45]. Film structure has also been influenced by the

deposition rate. Wu et al. found that an increase in the deposition rate led to a decrease in

the crystallinity of YBa2Cu307- thin films. At the higher deposition rates, the arrival rate

exceeds the diffusion rate. Equilibrium conditions are not maintained and structural

defects are formed [47].
































Figure 2-10. Picture of plume developed during PLD [45].

In summary, the advantages of this growth method include [45]:

* Flexibility to use energy source with more than deposition chamber
* Easy process control
* Ability to use high reactive gas pressures
* Decreased contamination from outside sources
* Control of film stoichiometry.


The short laser pulses result in congruent evaporants. Congruent evaporation aids in

stoichiometry control of the thin films during mass transfer from target to substrate. One

obvious disadvantage is the presence of micron sized particulates. Also, scale up to large

area deposition is not easily completed.

Pulsed laser deposition has been shown as an appropriate method for growing

phosphor films, specifically oxide phosphors. Yttria and silicate phosphors are some of

the oxide materials that have been grown by this method [30,34].









2.6.2.2 Sputter deposition

Sputter deposition is another thin film growth method that allows the use of a solid

target based on the same composition that is expected in the resulting film [50,51].

Sputter deposition is performed by extracting ions (usually Ar) from a plasma that strike

a target consisting of the material to be deposited. The plasma is formed by partially

ionizing an inert background gas that is flow- controlled into the system. The energetic

Ar ions produce a continuous flux of sputtered atoms that deposit on a nearby substrate.

The plasma is sustained by a DC voltage, radio frequency (rf) power, or a magnetron

operating at milliTorr pressures. A typical sputtering system is composed of a stainless

steel vacuum chamber, pumping system, internal sputter source, and a biased substrate

holder. A schematic of a typical sputter deposition chamber is given in Figure 2.11.

Radio frequency (rf) magnetron sputtering is often used for dielectric thin film

growth [52-54]. The higher degree of ionization associated with this sputtering method

makes it a popular choice for those materials which would otherwise be affected by

charging [51,54,55]. With this method, the target material self biases to a negative

potential and is charged as the cathode [53]. In this case, the background gas is first

ionized by primary electrons. These positive ions may be accelerated to energies

adequate for sputtering the negatively powered cathode, and upon bombardment, emit

secondary electrons and atoms from target surface. Magnetron sputter sources have a

magnetic field of 50- 500 gauss parallel to the target surface [52]. The placement of the

magnets relative to the target is shown in Figure 2.12. In combination with an electric

field, the magnetic field causes the secondary electrons to drift in a closed circuit in front

of the target surface. Consequently, the magnetic field controls the motion of the










electrons since a magnetic field can exert a force on a charged particle in motion as

dictated by Lorentz's law [56]. Lorentz's law is given by:


F = qf x B


(2-3)


where F is the force exerted on a particle with charge, q, and velocity, v, from an incident

magnetic field B.


LEAK VALVE


SHUTTER


SPUTTERED ATOM

PLASMA

SPUTTER SOURCE




CRYPUMP FR H2


CRYOPUMP FOR H 2


HIGH VACUUM PUMP


Figure 2-11. Schematic diagram of a typical set up for a sputter system [52].

The magnetic field also dictates that the particles move in a helical path with a radius, r,

determined by [56]:

my
r = (2-4)
qB









where m is the mass of the charged particle, q is the charge of the particle, and v1 is the

component of the particle's velocity normal to the applied magnetic field. Due to

electrostatic attraction, the Ar ions move with the electrons keeping the plasma neutral.

Under the best conditions, the plasma discharge is kept close to the cathode surface, and

bombardment of the growing film by electrons and ions is minimized. However, it is

possible to get "negative ion resputtering", resulting in sputtering of film as well [57].

Ion resputtering is characterized by damage to film, resulting in amorphous structure

[57]. Condensation of the atoms from the target onto the substrate initiates film

nucleation [54].

Sputter deposition is used in many commercial applications. Compact discs,

integrated circuits, magneto optical storage media, window coatings, and wear- resistant

coatings are some of the industries that have found sputter deposition applicable to their

respective processing methods [52,53]. Some of the advantages of sputter deposition that

have been realized are [52]:

* Control of thickness
* Good film adhesion
* High deposition rates
* Good film uniformity over large area.

The disadvantages of sputter deposition include the inevitable waste of target material.

Only about 20- 30% of the target material is used in magnetron sputtering [52].

2.7 Evaluation of Phosphor

There are several experiments that are completed to evaluate the overall

performance of phosphors. This includes chromaticity, spectral distribution, and lifetime

of luminescence.












Target
(the material to be deposited) Grounded shield



Internal magnets











Figure 2-12. Planar magnetron sputter source [56].

2.7.1 Chromaticity

A quantitative method has been established that relates the color produced on

display screens to a standard value. The 1931 Commission Internationale de l'Eclairage

(CIE) established a standard that defines chromaticity by x,y, and z coordinates. The

chromaticity values are plotted on a two- dimensional graph, shown in Figure 2.13, using

the x and y coordinates. The third value, z, is found by knowing x+y+z 1. Thus, when

mentioning the chromaticity of a phosphor, usually the x and y coordinates are only

given. The CIE diagram also shows how red, green, and blue blend to give all hues

needed for image production on a display screen.

2.7.2 Spectral Distribution

The spectral distribution gives information about the characteristic emission of a

phosphor. The visible light emission of phosphors is within 400- 700 nm. This range of

wavelengths is divided into six major divisions for the following colors: red, orange,

yellow, green, blue, and violet. An example of this range with its corresponding colors is









shown in Figure 2.14. The color that each phosphor emits corresponds to a wavelength

within the visible light spectrum.


Figure 2-13. CIE chromaticity chart.


Figure 2-14. Visible light spectrum and corresponding wavelengths [58].









2.7.3 Degradation Characteristics

In this section, vacuum ambient effects on phosphor performance and electron

stimulated surface chemical reactions will be discussed. A phosphor is said to experience

degradation when there is a loss in the CL intensity over time during exposure to an

electron beam. Several research groups have studied degradation of sulfide phosphors

extensively [18,19,59-61].

Several theories have been developed to explain the degradation of a phosphor.

Pfahnl has studied the rate of degradation of several phosphors. (This will be expanded-

need to get book)

Surface chemical reactions have also been investigated as a culprit for the observed

degradation in cathodoluminescent phosphors. The electron stimulated surface chemical

reaction model was developed to explain the loss of cathodoluminescence for these

phosphors through the development of a "dead layer". The dead layer is the surface layer

that inhibits or reduces the CL intensity. A mathematical model for describing electron

stimulated surface chemical reactions (ESSCR) has been developed by Holloway et al.

This model explains that the degradation is dependent upon the type and concentration of

gases present in the vacuum, energy of electron beam, as well the beam current and time

of exposure to electron beam. A description of this mathematical model follows.

This model focuses on the surface interactions, since the low beam energy would

involve the surface of the phosphor rather than the bulk [18]. Additionally, the model

uses the ZnS phosphor for its description of the material system. In terms of

cathodoluminescent degradation for the ZnS system, the loss in sulfur (S) has been

correlated with a loss in CL intensity. This concentration of S on the surface was

modeled by the following standard chemical reaction rate equation:









dC,
c= -kCC (2-5)
dt

where Cs is the concentration of S on the surface, k is the chemical rate constant, Cas is

the concentration of the adsorbed atomic species that will react with ZnS, and n 1 where

a first order reaction is assumed [18]. The model also assumes that the chemical reaction

takes place on the surface, not in the gas phase, such that Cas can be expressed as:

Cs = Z(D.aCJrs (2-6)

where Z is the number of reactive atomic species produced from the parent molecule, 0ma

is the dissociation cross section of the molecule to atoms, Cm is the surface concentration

of the molecular species, Jis the current density of the electron beam causing the

dissociation, and zrs is the lifetime of the reactive species lifetime [18]. It should be

noted that the dissociation cross-section is a function of the electron beam energy. This

expression dictates that the reaction rate is limited by the rate of production of the

adsorbed species. In other words, the rate of production of the adsorbed atomic species

that will react with ZnS is controlled by the surface concentration of the molecular

species. This adsorbed molecule concentration, Cm, may be expressed by Henry's

isotherm as:


C, = fro exp LQ [ 2 (2-7)
UkT [2lmkT]

where ois the molecular sticking coefficient (assumed to be independent of coverage), To

is the mean time between attempts by the physisorbed molecule to escape from the

surface, Q is the energy required to desorb from the surface, k is Boltzman's constant, T

is the absolute temperature, and Pm is the partial pressure of the molecular gas in the

vacuum. The first term in brackets in the preceding equation collectively describes the








molecular mean stay time on the surface, where the second term is an expression for the

molecular flux onto the surface. Inserting equation 2-7 into 2-6 and then equation 2-6

into 2-5 results in the following rate expression:


d = -koC,Z(i maJrzajo exp mT /2 (2-8)
dt kT [2(2-8)T

This equation may be adjusted to

dC,
= K' JPmdt (2-9)


where


K exp
K'= koZOnzras /2 (2-10)
[2HrinkT] 12


Integrating equation 2-7 with respect to time and using the boundary condition, Cs = Co

for t = 0, results in:

C, = C' exp[- K'PmJt] (2-11)

where Jt is the coulombic dose [Coulombs/ Area]. The resulting model predicts that the

concentration of sulfur will decrease exponentially with the coulombic dose and the

cathodoluminescence loss rate increases with a higher pressure of the molecular gas in

the system.

These predictions have been observed experimentally [18] and are shown in

Figures 2.15 and 2.16. Figure 2.15 shows the loss in CL intensity for a ZnS: Ag

phosphor irradiated by a 2 keV electron beam at varying vacuum pressures. This graph

shows experimentally that the CL degradation rate is dependent upon the gas pressure in









the vacuum system as predicted by the mathematical model. The rate of degradation is

higher at higher gas pressures. Figure 2.16 shows that the prediction from the model for

an exponential loss in sulfur surface concentration with electron dose can be observed

experimentally.

The degradation of one oxide phosphor, Y203: Eu, has been investigated [62]. The

degradation behavior of this powder phosphor was evaluated as function of the oxygen

pressure in the vacuum. Y203: Eu was exposed to electron beam energy of 2 keV and a

high current density of 88.5 mA/ cm2 in a vacuum atmosphere of lx 107 Torr oxygen.

After an electron dosage of 3500 C/ cm2, the CL intensity was reduced to 45% of its

original intensity as shown in Figure 2.17 [62]. The yttrium, carbon, and oxygen peaks

were also monitored by Auger electron spectroscopy (Figure 2.17) during electron beam

exposure. However, no change in the signal from these elements was detected.

Therefore, even though it was clear that the presence of oxygen affects the rate of

degradation, the mechanism that resulted in the reduced CL intensity could not be

determined [62].
























60 80 1
EkeiRo DoSe (CicM2)


Figure 2-15. Semi logarithmic plot of CL intensity vs. electron dose for a ZnS: Ag
phosphor [18].


Eke~tiai Doie (CkM4n) E~atrm os e (ICkuJ)


Figure 2-16. Linear and logarithmic plot of S Auger peak to peak height (APPH) vs.
electron dose for ZnS: Ag phosphor [18].


















2-

200
L CLIntenrsily C

K 180

160
Stt riu m -1

Carbon 14D
0
D 1 1D 2000 300U 400D
Electron Dose (CJcm2


Figure 2-17. Plot of CL intensity of Y203: Eu phosphor and selected Auger peak to peak
heights vs. electron dose [62].














CHAPTER 3
CATHODOLUMINESCENCE FROM THIN FILM ZINC GERMANATE DOPED
WITH MANGANESE PHOSPHORS

3.1 Introduction

In this study, the low voltage cathodoluminescent properties of thin film zinc

germanate doped with manganese (Zn2GeO4: Mn) are examined. Pulsed laser deposition

was used to grow Zn2GeO4: Mn on magnesium oxide (MgO), yttria stabilized zirconia

(YSZ), or silicon (Si) substrates. The structural properties as well as the

cathodoluminescence (CL) and photoluminescence (PL) spectra of the films are reported.

The variation of the CL emission spectra with film deposition temperature and crystalline

quality of film is also studied.

The first section in this chapter outlines the experimental procedures used to

develop and characterize the thin film samples. The second section reports on the results

from the characterization and experiments. Finally, the last section will provide insight

into the influence of deposition temperature and type of substrate on the luminescent

qualities of the Zn2GeO4: Mn phosphor.

3.2 Experimental Procedures

The preparation of the substrates and the processing steps for the film development

in outlined in Figure 3.1. The details of each step are described in this section.

3.2.1 Substrates and their Preparation

Single crystal (100) MgO, (100) yttria stabilized zirconia (YSZ), and (100) Si

substrates were chosen as the substates for this experiment. The substrates were cleaned









by a solvent wash in a sonicator prior to deposition. The solvents that were used were

trichloroethylene, acetone and methanol. The substrates were cleaned in 3 sequential

solvent washes, with each wash lasting five minutes. The substrates were dried by air.

3.2.2 Phosphor Processing

A Zn2GeO4 ablation target doped with 1.5 at% Mn was prepared by mixing and

then ball milling ZnO, GeO2, and MnO2 powders for 60 minutes. The mass of each

powder used for the mixture is given in Table 3.1. The mixture was then calcined in air

at 1000 C for 8 hr in covered alumina crucibles that were placed in a conventional

furnace. The powder mixture was milled again for 60 minutes and was then checked for

photoluminescence. A hand held ultra- violet (UV) lamp was used for the check. Visible

green luminescence from the powder was observed with human eye. Next, the powder

mixture was pressed into 2.5 cm diameter targets. The targets were sintered in air at 1250

C for 36 hours in a conventional furnace. In addition, ZnO powder was milled, pressed

into targets, and sintered. A Zn2GeO4: Mn/ ZnO mosaic target was formed to control the

cation ratio of Zn/Ge in the films. The area ratio of the target was 75 % Zn2GeO4: Mn

and 25 % ZnO. Previous research has shown that a zinc rich target can compensate for

Zn loss at elevated temperatures during deposition, which is due to the high vapor

pressure of zinc [63,64]. The target was rotated during the deposition cycles to ensure

that the laser ablated the Zn2GeO4: Mn and ZnO portions of the target.









Select & cut substrates: (100) Si, (100)
MgO, and YSZ


All 3 substrates attached to heater
stage with silver paint


Heater stage heated to desired
temperature (600, 650, 700, or 750 C)


Phosphor deposited onto substrates


Figure 3-1. Flow chart outlining substrate preparation and phosphor processing method.


Sonicator wash in each for 5 minutes:
A) Trichloroethylene
B) Acetone
C) Methanol


Cool substrates to room temperature
inside deposition chamber





















Figure 3-2 Representative sketch of mosaic target where 75% area = Zn2GeO4: Mn and
25% area = ZnO.

Table 3-1 Raw materials used to make target.
Material Mass (g)


ZnO 12.1758


GeO2 7.8242


MnO2 0.0975


The films were grown using an excimer KrF laser with a wavelength of 248 nm.

The energy of the laser was 130 mJ/pulse. The target was pre- ablated before each

deposition cycle. Four different deposition temperatures (600, 650, 700, and 750 C)

were maintained by a substrate heater and a thermocouple attached to the substrate stage.

The substrates were allowed to reach the desired temperature before film growth

commenced. The number of pulses and pulse frequency during deposition was 10,000

pulses at 10 Hz, immediately followed by a second cycle of 20,000 pulses at 20 Hz. The

oxygen partial pressure in the system was maintained at 100 mTorr during all film

growth.









3.2.3 Thin Film Phosphor Characterization

3.2.3.1 X- ray diffraction

X- ray diffraction (XRD) is a useful method for identifying the crystalline phases

present in a sample, as well as for measuring the structural properties of these phases.

For instance, it can also be used to determine the preferred orientation of the phases

present in a crystalline material [65]. X- ray diffraction results from incident X- rays that

are scattered by the atomic planes present in a crystal. When there is constructive

interference from these X- rays, a diffraction peak is observed [65]. The condition for

constructive interference is dictated by Bragg's law such that

A = 2dhki sin Ohki (3-1)

where A is the wavelength of the incident X- ray, dhkl is the d- spacing between (hkl)

planes, and Ohki is the angle between the atomic planes and the incident X- ray beam.

These terms are illustrated in Figure 3.3 which shows the interaction of the X- ray with

the specimen.

The diffraction angle, 20, is the angle between the incident and diffracted X- rays

(Figure 3.3). The XRD experiment yields data for the diffracted intensity vs. diffraction

angle. For polycrystalline thin films, diffraction occurs from any crystallite or small

crystalline region that satisfies the diffraction conditions [65]. If the distribution of

orientations is random, diffraction peaks will result from more than one plane, similar to

a powder diffraction pattern. As a result, multiple peaks will be present in the pattern

representing the different orientations of the crystallites. A textured film will possess a

preferred orientation of the crystallites, where most of the crystallites have parallel

planes.



























Figure 3-3 Interaction of incident and diffracted X- rays in an XRD specimen [65].

For this study, the crystalline quality of the films was investigated using a Philips

APD 3720 X- ray diffractometer. X- ray diffraction (XRD) was completed with Cu Kuc

radiation (0.15406 nm wavelength) generated by a 40 keV and 20 mA electron beam.

The scan range for the samples was from 10 to 80 2theta degrees, while the scan rate was

0.080 0 20 / sec in continuous scan mode. The resulting X- ray diffraction patterns were

then indexed with a collection of patterns from the Joint Committee on Powder

Diffraction Standards (JCPDS) catalog.

3.2.3.2 Relative composition analysis: EDX

Electrons may interact with the sample to produce multiple signals. These signals

include X-rays, UV and visible emission, and auger electrons, as depicted in Figure 3.4.

The signals result from the electron interactions within the sample volume. Electron

probe microanalysis (EPMA) is a compositional analysis method that uses the

characteristic X- ray emission generated by an electron beam to provide a quantitative

analysis of the elements present in a specimen. This method relies on an electron probe,









thus resulting in fine spatial resolution as low as 100 nm [65]. The accuracy of the

quantitative analysis is dependent upon the standards used for the analysis. To reduce

error, the sample and standards must be measured under identical conditions of beam

energy and spectrometer parameters and should be normalized to the same electron dose.

Failure to replicate conditions will increase the error in the measurements.

Energy dispersive X- ray (EDX) spectroscopy is one type of EPMA that detects

elements with an atomic number higher than beryllium, Z= 4. For EDX, the incident

electron beam ionizes and ejects inner shell electrons from the atoms within the sample.

The atoms are returned to ground state when an electron from a higher energy shell

moves to fill the inner shell vacancy. During the transition, the electron releases the

amount of energy equal to energy difference between the two shells. This excess energy,

unique for each atomic transition, may be released in the form of an X- ray photon [65].

This X-ray is referred to as the characteristic X- ray for that atom and is detected by the

spectrometer. The minimum detection limit for an EDX spectrometer attached to a

scanning electron microscope is about 0.1 wt % [65].

Energy dispersive X- ray fluorescence (EDX) with a JEOL JSM 6400 scanning

electron microscope (SEM) at a primary beam energy of 5 keV was used to measure the

relative concentrations of the elements in the film. The absolute values of the atomic

content for these films are considered to be subject to large errors because the sensitivity

factors for the Zn and Ge Lca signals used were from look- up tables using different

transitions and higher primary beam energies. Neither factor could be accurately

corrected for by the quantitative calculation program.















d Auger
Samrnple Surface Electrons


E Ilcrons

Backscattered
=EO" Electrons

E E. Characteristic

X-Rays-






X-Ray Resolution



Figure 3-4. Interaction volume in sample where electron beam penetrates and the
resulting signals are generated [66].

3.2.3.3 Cathodoluminescence characterization

The theory of cathodoluminescence was introduced in Chapter 2.

Cathodoluminescence (CL) from the phosphors was measured in an ultra high vacuum

stainless steel chamber depicted in Figure 3.5. The ultra high vacuum was maintained by

a Perkin Elmer Ultek D- I ion pump, which maintained a system pressure of

approximately 1 X 10 -8 Torr. An Edwards RV3 turbo molecular pump was used for

initial pumping of the system to about 1 X 10-6 Torr, after which the system was crossed

over to the ion pump. The system also has a load-lock chamber. The chamber allows

introduction of new samples into the vacuum system without disturbing the vacuum. The

CL system recovers to its normal pressure within thirty minutes. Cathodoluminescence









was stimulated by an electron beam generated by a Kimball Physics EFG- 7 electron gun

operated at 4 keV and 8.5 ptA. An optical fiber connected to an Ocean Optics S2000

optical spectrometer was used to detect photoemission from the samples. The detected

spectral range was from 200 to 850 nm.

3.2.3.4 Photoluminescence characterization

Photoluminescence (PL) is the excitation of photons by absorption of light. It can

be used to provide a qualitative analysis of a sample. The wavelength of the emitted light

is longer than the incident light [65]. The spectral properties can be analyzed to provide

information such as the optical, electrical, and structural properties of the material [65].

A photoluminescence excitation (PLE) spectrum can be used to find out information

about the excitation process that leads to the photon being emitted. The intensity and

spectral properties of a PLE spectrum is dependent upon the absorption of the incident

light and the initial and relaxed excited states that take part in emission [65]. For PLE, a

set emission wavelength is monitored and the wavelength of the incident light is scanned

through a desired range.

The photoluminescence measurements were taken in a dark room at room

temperature. A xenon lamp (Oriel Instruments, model 66902) was selected as the

excitation energy source for photoluminescence. The excitation wavelength could be

tuned to the desired level with a monochromator (Oriel Instruments, Cornerstone 74100

spectrometer) placed in between the lamp and sample. Any wavelength between 200 and

1200 nm could be selected with the excitation monochromator. The emitted light was

focused to a monochromator (Oriel Instruments, MS257) and photo- multiplier tube









(Oriel Instruments, 77265) that has the ability to detect photoemission from 300 to 800

nm.

















Electron Gun







Cathodoluminescent
Phosphor










Figure 3-5 Picture of cathodoluminescence vacuum system.

3.3 Results

3.3.1 X- Ray Diffraction Results

The X- ray diffraction (XRD) data in Figure 3.5 show the effect of the deposition

temperature on the structural properties for film grown on MgO substrates. At 600 C






43


(Fig. 3.5d), (223) and (550) weak diffraction peaks for Zn2GeO4 are evident indicating

that some structural order exists. However, the weak peak intensity and the broad

maximum at 20 of- 180 suggest that the majority of the film was amorphous with

respect to X- ray diffraction. The presence of additional Zn2GeO4 diffraction peaks in the

XRD pattern shown in Figure 3.5 (a- c) indicate that the films grown at 650, 700, and 750

C are polycrystalline and exhibit better long-range order. Additional diffraction peaks at

20 of ~ 28 and 36 for films grown at 750 and 700 C are attributed to an impurity

GeO2 phase. The shoulder diffraction peak at 20 of ~ 410, detectable only in film grown

at 750 and 700 C, also results from the GeO2 impurity phase.


20 30 40 50 60
20 (deg)


Figure 3-6 XRD pattern for Zn2GeO4: Mn films grown on MgO substrate at different
deposition temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C where
denotes GeO2 impurity phase.






44


XRD patterns from films grown on Si are shown in Figure 3.6, which are

comparable to the results for films grown on MgO. At 600 C, the presence of weak

(223) and (550) Zn2GeO4 peaks and a broad peak at 20 of- 180 in the diffraction pattern

indicate that the film structure is mixed amorphous and polycrystalline. Again, the XRD

patterns shown in Figure 3.6 (a- c) indicate that the films grown at temperatures from

650- 750 C are polycrystalline with better long range order.


20 30 40 50 60 70
20 (deg)


Figure 3-7. XRD pattern for films grown on Si substrate at different deposition
temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C.

Figure 3.7 presents the XRD pattern for films deposited on the third substrate,

YSZ, at different deposition temperatures. For films deposited at 600 OC (Fig. 3.7d), only

a weak (550) diffraction peak is present, which indicates that Zn2GeO4 is again mixed

amorphous and polycrystalline. Additional diffraction peaks from Zn2GeO4 were









observed from films deposited at higher temperatures. Polycrystalline Zn2GeO4 films

were formed at 650 and 700 C. The diffraction pattern for the film grown at 750 C

(Fig. 3.7a) reveals a strongly preferred (110) texture.


20 30 40 50 60
20 (deg)


Figure 3-8. XRD pattern for films grown on YSZ substrate at different deposition
temperatures: (a) 750 C, (b) 700 C, (c)650 C, (d)600 C.

3.3.2 Cathodoluminescent Properties

With respect to luminescent properties, the characteristic PL and EL emission

wavelength of Zn2GeO4: Mn is a broad peak with a maximum at 540 nm [22,35,37] as

discussed in Chapter 2. The CL emission spectra for Zn2GeO4: Mn on MgO deposited at

various temperatures are shown in Figure 3.8. The CL emission peak intensity was

observed to be at 540 nm, as well. The highest 540 nm CL intensity came from films that







46


were grown at 650 and 700 C. For 600 C sample, no CL emission at 540 nm is noted.

Rather, the emission peak has shifted to 650 nm.


200 300 400 500 600 700 800
Wavelength (nm)


Figure 3-9. CL emission spectrum of Zn2GeO4: Mn on MgO substrate at various
deposition temperatures.

The CL emission spectra from film deposited onto Si substrates are shown in

Figure 3.9. CL emission at 540 nm resulted only from the film grown at 750 and 700 C.

Films grown at temperatures of 600 and 650 C did not show any detectable CL

emission at 540 nm, but did show emission at 650 nm.








47



1600


1400
700 C

1200


1000


800



600grown at 750 C. There was no CL response at 540 nm from the m grown at 600 C,

400

750 C




0
200 300 400 500 600 700 800 900
Wavelength (nm)




Figure 3-10. CL emission spectrum of Zn2GeO4: Mn on a Si substrate for various
deposition temperatures.

The CL emission spectra from film deposited onto YSZ are presented in Figure


3.10. The highest CL intensity, in this case, results from the film grown at 750 'C. In


contrast to the results for films grown on MgO and Si, the intensities for CL emission


from the films deposited at 700 and 650 'C on YSZ are less than that from the film


grown at 750 'C. There was no CL response at 540 nm from the film grown at 600 'C,


but again the emission peak was shifted and broadened to 650 nm.











3500

750C
3000 750



2500



2000



1500 -


700
1000

650 C
500 600 C


0 ,
200 300 400 500 600 700 800 900
Wavelength (nm)




Figure 3-11. CL emission spectrum of Zn2GeO4: Mn on YSZ substrate at various
deposition temperatures.

3.3.3 Zinc to Germanium Ratio in the Films

The Zn to Ge atomic percent ratio was measured using energy dispersive X- ray


fluorescence (EDX). These ratios, given in Table 3.2, were measured for films deposited


at T > 650 C. All of the deposited films exhibited a low Zn/Ge ratio, ranging from 0.31


to 0.89, suggesting a Zn deficiency. As discussed earlier, the absolute values of these


ratios are subject to error. Nonetheless, the relative changes in the ratios correlate well


with the changes in crystalline quality and CL intensities, suggesting that the ratios


present valid trends. The fact that the XRD patterns indicate that Zn2GeO4 with varying


degrees of crystallinities was present is consistent with these ratios having large absolute


errors, but accurate trends. As shown in Table 3.2, films deposited on MgO and Si









substrates at 650 and 700 C have a higher Zn/Ge ratio than films deposited at 750 C.

Because of the low Zn/Ge ratio for films deposited at 750 C on MgO or Si substrates,

the crystalline quality decreases. The Zn/Ge ratio is highest (0.89) for the (110) textured

film deposited at 750 C on YSZ, and this film has the best diffraction pattern, indicating

the highest crystalline quality.

Table 3-2 Zn/Ge Atomic Ratio in Deposited Zn2GeO4: Mn Film vs. Deposition
Temperature on Various Substrates
750 oC 700 oC 650 C
MgO 0.31 0.60 0.36
Si 0.42 0.47 0.57
YSZ 0.89 0.65 0.44


3.3.4 Photoluminescence Excitation and Emission

The photoluminescence (PL) emission and the excitation (PLE) spectra for a

Zn2GeO4 sample that emits green light are shown in Figure 3.11. The emission excited

with 325 nm radiation showed a peak at 540 nm. The excitation spectrum, monitored at

540 nm, exhibited an excitation peak at 310 nm with a small shoulder at 265 nm. The

results are similar to what has been noted for green emission from Zn2GeO4: Mn [35].

The PL emission and excitation spectra for Zn2GeO4 samples that have longer

wavelength emission are shown in Figure 3.12. The PL emission for these samples,

when excited with 325 nm radiation, results in a broad peak at 625 nm with a smaller

peak showing at 535 nm. The excitation spectrum, monitored at 625 nm, exhibited a

peak at 270 nm with a small shoulder at 330 nm.























265 nm







10000 200 00 30000 40000 50000 60000 70000 80000
Wavelength (nm)




Figure 3-12. Green emission spectrum of Zn2GeO4: Mn excited with 325 nm radiation at
540 nm and excitation spectrum monitored at 540 nm.


3.4 Discussion

In summary, Zn2GeO4: Mn was successfully pulsed laser deposited onto MgO, Si,

and YSZ substrates. The X- ray diffraction data show improved crystallinity in films

deposited at T > 650 C. For the films deposited at 600 C, on all substrates, the structure

of films was mixed amorphous and polycrystalline. The quality of the diffraction peaks

was good for films deposited at 650 and 700 C on MgO and Si substrates. The best

diffraction pattern was for the textured film deposited at 750 C on YSZ substrate.

A relationship between the CL emission maximum intensity and the film crystal

quality and stoichiometry is obvious. The film with the best CL emission intensity at 540









nm was the (110) textured film grown on the YSZ substrate at 750 C. The Zn/Ge ratio

is highest (0.89) for this film and this film has the best diffraction pattern, indicating the

highest crystalline quality.


270 nm



Excitation
Emission






Snm640 nm





10000 200 00 30000 40000 50000 60000 70000 80000
Wavelength (nm)

Figure 3-13. Red emission spectrum of Zn2GeO4: Mn excited with 325 nm radiation at
625 nm and excitation spectrum monitored at 625 nm.

The films deposited at 700 and 650 C on MgO, which were determined to have

good crystal quality and Zn/Ge ratios -0.5 also exhibited the highest CL emission

intensities at 540 nm for films deposited on this substrate. For films deposited on a Si

substrate, the CL emission was also highest for the films with the best crystal quality,

which again correlated with higher Zn/Ge ratios. The fact that higher Zn/Ge ratios lead

to brighter CL emission is consistent with observation of better crystallinity for the

Zn2GeO4 crystal structure.









The highly textured film grown on the YSZ substrate at 750 C had the highest

crystallinity, highest Zn/Ge ratio, and the best CL properties. It has been suggested that

grain boundaries may limit the luminescent performance of a phosphor [63], which is

consistent with emission from more randomly oriented polycrystalline film being lower

when compared to the textured film on YSZ.

A red shift in wavelength of emission from green to red has also been noted in Mn-

doped ZnGa204 phosphors [67]. It was reported that Mn4+ ions in the octahedral sites led

to red emission, while Mn2+ ions that occupied the tetrahedral sites in the spinel structure

resulted in green emission [67]. At 600 C, where the Zn2GeO4: Mn films exhibit mixed

short and long range order, the dominant CL emission was at 650 nm. It is postulated

that the Mn4+ ions are responsible for red emission at 650 nm. The rhombohedral crystal

structure of Zn2GeO4 has only tetrahedral sites [68]. Therefore, a change in substitution

site must occur to accommodate the change in valency of Mn in Zn2GeO4: Mn. The

Mn4+ ions may substitute for Ge4+ at the lower temperature. This substitution is possible

since the ionic radii of both ions are 0.39 A [69]. At higher deposition temperatures (650,

700, & 750 C), the films develop a polycrystalline structure and the Mn2+ ion gains

enough energy to move into the expected tetrahedral site and substitute for Zn2+. This is

again possible since the ionic radii of these atoms are close, 0.60 A versus 0.66 A for

Zn2+ and Mn2+, respectively [69]. The Mn2+ activator ions resulted in emission at 540

nm, as expected.

The difference in photoluminescence excitation spectra for green (540 nm) and red

(650 nm) luminescence indicates that excitation of different states are responsible for

each type of emission. This correlation between the excitation of different ionic states






53


and the PL excitation spectra has also been observed in for theZnGa204 host [67]. The

excitation spectra are consistent with the Mn ion exhibiting different valence states and

substituting for either Zn or Ge, depending upon the temperature during deposition.














CHAPTER 4
DEVELOPMENT OF ZINC SILICATE DOPED WITH MANGANESE THIN FILM
PHOSPHORS

4.1 Introduction

This chapter describes the development of zinc silicate doped with manganese

(Zn2SiO4: Mn) thin film phosphors. The overall effects of the processing conditions on

the resulting films will be evaluated. In the first section, the experimental procedures that

were used to grow the films will be described. The films were produced by sputter

deposition, pulsed laser deposition, or combustion chemical vapor deposition. The

luminescent and structural properties of these films have been evaluated and are

discussed in the results section. The resulting composition of the films will be discussed

as it relates to the film growth method and post- processing conditions. These growth

methods are excellent choices for growing thin film phosphors, however it was found that

the zinc loss must be controlled to obtain the best performance from the phosphor film.

The thin film phosphors were also studied for cathodoluminescence degradation, as will

be discussed in Chapter 5.

4.2 Experimental Procedures

4.2.1 Substrates and their Preparation

Clear fused quartz and (100) Si were chosen as the substrates for film growth.

These substrates were chosen since they could withstand the high annealing temperatures

required to crystallize zinc silicate. The substrates were cleaned by a solvent wash in

trichloroethylene, acetone, and then in methanol. The substrates were placed in a beaker









with each solvent for fifteen minutes inside a sonicator and then removed. The substrates

were then placed in the next solvent, and the wash was repeated. The substrates were

then blown dry by compressed air.

4.2.2 Phosphor Processing

4.2.2.1 Sputter deposition

The Zn2SiO4: Mn sputter targets were made from raw material produced at

Shanghai Yuelong New Materials Co. Ltd. in Shanghai, China, and sold primarily for

plasma display panel applications. The 2" sputter target was made by Plasmaterials, Inc.

by hot pressing and sintering. The target was also bonded to a 0.125" thick Cu backing

plate using metallic bonding by Plasmaterials, Inc. The density of the resulting target

was estimated to be -90% of theoretical.

The sputter deposition was completed with two different targets, however the

starting material was the same in both cases. The initial target was changed after cracks

propagated through the target, and films deposited from it had an uncharacteristic brown

tint, which was determined by EDX analysis to be carbon contamination. The source of

the carbon contamination was the bonding material for adhering the target to the copper

backing plate. The films that were carbon contaminated are not included in this study. A

second target was used in a sputter gun that did not require a copper backing plate. This

adjustment eliminated the bonding material as a source of carbon contamination.

The deposited thin films were treated to a post- deposition thermal anneal. The

rapid thermal anneal (RTA) was completed in an AG Associates Heatpulse Model 4100

furnace with a high- purity nitrogen atmosphere. The anneal recipe used for the heat

treatments is given below in Table 4-1. The annealing temperature for the samples was

chosen as 1100 C, since it was high enough to cause crystallization of Zn2SiO4









[26,27,29]. The duration of the anneal was limited to five minutes due to the operating

capabilities of the furnace. Upon completion of the anneal, the heat source was turned

off and an increase in N2 flow helped to cool the samples before removal from the

furnace.

Table 4-1 Rapid thermal anneal recipe for heat treatment of thin film samples.

Segment Type Time Temp. (C) N2 Flow (slpm)

1 Steady 10 sec 0 10

2 Ramp 125 C/sec 1100 2.5

3 Steady 300 sec 1100 2.5

4 Steady 60 sec 0 10


4.2.2.2 Pulsed laser deposition

Thin film samples were also made by pulsed laser deposition (PLD). The PLD

target was made from the commercial Zn2SiO4: Mn phosphor powder obtained from

Shanghai Yuelong New Materials Co. Ltd. This target was pressed using a 1" stainless

steel die. The thin film processing was completed at North Carolina A & T State

University.

The pulsed laser deposited phosphor films were all grown at the same processing

conditions on (100) Si substrates. The oxygen pressure during growth was at 300 mTorr

and the substrates were heated to and maintained at 700 C during the deposition cycles.

After deposition, the thin film PLD samples were heat treated with a rapid thermal anneal

in N2 atmosphere for five minutes at 1100 C. The same apparatus and program

described in section 4.2.2.1 was used for the annealing of these samples.









4.2.2.3 Combustion chemical vapor deposition

Zn2SiO4: Mn thin film phosphors were deposited onto quartz substrates by

combustion chemical vapor deposition (CCVD) at Georgia Institute of Technology.

Combustion chemical vapor deposition is a novel growth method that produces

crystallized films at high temperatures, eliminating the need for a post- deposition anneal

[70]. In CCVD, the precursors are sprayed near or in a flame that causes the precursors

to chemically react leading to the vapor deposition of a film onto a substrate [71]. The

substrates are heated during the deposition through exposure to the open flame and the

temperature is controlled by the substrate distance from the flame end. The films are

grown at atmospheric pressure using ambient air for the reaction, making this method

suitable for oxide materials [70].

The precursor solution consisted of zinc nitrate, tetraethyl orthosilicate (TEOS),

and manganese nitrate [70].

4.2.3 Thin Film Phosphor Characterization

4.2.3.1 X- ray diffraction

The crystalline quality of the films was investigated using a Philips APD 3720 X-

ray diffractometer. X- ray diffraction (see section 3.2.3.1) was completed with Cu KUc

radiation (0.15406 nm wavelength) generated by a 40 keV and 20 mA electron beam.

The 20 scan range for the samples was from 10 to 80 degrees, while the scan rate was

0.080 0/ sec in a continuous scan mode. The resulting X- ray diffraction patterns were

indexed using a collection of patterns from the Joint Committee on Powder Diffraction

Standards (JCPDS) catalog, where the rhombohedral crystal structure was found to

correlate with the Zn2SiO4 developed in this study.









A high resolution X- ray diffractometer was also used to measure the crystal

structure and quality of the thinner films (less than 1000 A). This method was useful for

the thin films that were too thin for the powder diffraction method. The 20 scans were

taken from 20 to 40 degrees at a glancing angle of 0.7 A Philips MRD X' Pert X-ray

diffraction system was used for the measurements.

4.2.3.2 Scanning electron microscopy

Scanning electron microscopy (SEM) is useful for characterizing the sample

morphology. A scanning electron microscope provides a magnified image of the surface.

The magnification for this type of microscopy may go as high as 300,000 [65].

A JEOL scanning electron microscope model 6400 was used for obtaining the

micrographs. The SEM was operated at 15 keV. Because of the high accelerating

voltage and the low conductivity of the samples, the samples were coated with carbon to

reduce the effects of charging. The carbon layer was deposited onto the samples by

electron beam evaporation.

4.2.3.3 Wavelength dispersive X- ray spectrometry

Wavelength dispersive X- ray (WDX) spectroscopy is an elemental analysis

technique that is used to provide information about the composition of a sample. It can

be used to detect elements from beryllium to the actinides. This method relies on the

generation of characteristic X- rays by a focused electron beam, as in energy dispersive

spectroscopy (EDX). However, WDX (versus EDX) relies for energy analysis by the

constructive diffraction of X- rays by a crystal [65] for elemental analysis. Several

crystals with various d- spacings are used to incorporate a broad energy range.

Correction factors for X- ray intensity are usually applied in the quantitative analysis.









The ZAF [65] correction takes into accounts the effects related to the atomic number of

an element (Z), X- ray absorption (A), and secondary fluorescence (F).

4.2.3.4 X- ray photoelectron spectroscopy

X- ray photoelectron spectroscopy (XPS) is a very beneficial analytical method for

examining the surface chemistry of a sample [65,72]. Energetic X-rays, greater than

1000 eV [72], bombard a sample causing characteristic photoelectrons to be ejected.

Chemical identification is made by analyzing the kinetic energies of the ejected

photoelectrons. Electron binding energies are sensitive to the chemical state of the atom.

With sensitivity to small changes in kinetic energy, information about the chemical state

of the identified element can be determined. This method is good for detecting most

elements, except for hydrogen and helium.

The kinetic energy of the photoelectron is dependent upon the impinging X- ray

photon as dictated by Einstein photoelectric law:

KE = h v BE (4-1)

where KE is the kinetic energy, h vis the energy of the impinging photon, and BE is the

binding energy of the electron in the atom. Since the energy of the photon is known and

the kinetic energy is measured, the binding energy can be determined. An XPS spectra is

usually given as the binding energy versus peak intensity. The peak intensities in an XPS

plot are dependent upon the photoionization cross- section. The photoionization cross -

section is dependent upon the probability for photoejection, which is different for each

quantum orbital [65]. The probability depends upon the orbital of ejection for each

element and on the energy of the X-ray. A quantitative analysis can be made if the

photoionization cross- section is known, along with other experimental parameters.









4.2.3.5 Cathodoluminescence characterization

The cathodoluminescence was measured in the ultra- high vacuum chamber

described in Chapter 3. A Kimball Physics model EFG- 7 electron gun was used as the

energy source to stimulate cathodoluminescence. The accelerating voltage was varied for

these experiments from 500- 5000 eV. The current level also varied from 2.5 to 8 ptA

typically in an area of 0.0314 cm2, corresponding to current densities of 80 to 255 |jA/

cm2. A fiber optic inputted Ocean Optics S2000 spectrometer was used to detect the

visible light emission from the samples.

4.3 Results from Pulsed Laser Deposited Phosphors

The thickness of the films was measured by secondary ion mass spectrometry

(SIMS) profiling followed by profilometer measurements of the sputter crater and depth.

The films were grown to two thicknesses: 0.85 and 1.4 |tm. The increased thickness was

a result of a longer deposition time. The deposition rate for both growth conditions was

about 500 A/ min. The samples were characterized for structure and morphology,

composition, and cathodoluminescence emission spectrum and peak intensity.

4.3.1 Structural Characterization

The as- deposited samples and annealed samples were examined by X- ray

diffraction. The XRD diffraction patterns for the thin film samples were compared with

the Zn2SiO4: Mn powder that was used to make the ablation target. This pattern for the

powder has been indexed with its corresponding planes, and is shown in Figure 4.1. The

powder pattern matches closely to the JCPDS file for rhombohedral zinc silicate. All of

the peaks from the powder sample can be attributed to Zn2SiO4. Figure 4.2 compares the

diffraction pattern for the 0.85 jtm thick sample for as- deposited at 700 C and annealed







61


at 1100 C for 5 minutes. The as- deposited film had no discernable diffraction peaks

from Zn2SiO4. However, diffractions peaks for the (100) Si substrate are observed along

with a peak at 44 20 degrees. This peak has been identified as an impurity SiO2 phase

and is denoted by '*' in Figure 4.2. The XRD pattern for the annealed film indicates that

Zn2SiO4 is present and has a polycrystalline structure. Figure 4.3 shows the improvement

in structural quality for the thicker film, 1.4 |tm, after the anneal and the corresponding

zinc silicate diffraction peaks are denoted. The pattern for the as- deposited at 700 C

film only shows diffraction peaks resulting from the Si substrate. Impurity SiO2 phases

are also present in this film, as indicated by the diffraction peaks at 22, 27, and 44

degrees and is denoted in the diffraction patterns by an asterisk '*'




100 "



80










0
20-
CM






0 C



10 20 30 40 50 60 70
2 theta (degrees)




Figure 4-1. Powder XRD pattern for Zn2SiO4: Mn. This is consistent with the
rhombohedral crystal structure reported in JCPDS 37- 1485.





































2 theta (degrees)



Figure 4-2 XRD pattern of Zn2SiO4: Mn thin film phosphor (0.85 |tm thick) before and
after annealing. designates peaks from SiO2.





























10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2 theta (degrees)


Figure 4-3. XRD pattern of Zn2SiO4: Mn thin film phosphor (1.40 |tm thick) before and
after annealing. designates peaks from SiO2.

4.3.2 Morphological Characterization

All films were specular reflecting before the anneal at 1100 C. After the anneal in

N2, the thinner sample retained its reflectivity, while the thicker sample had a dull

appearance. The morphologies were investigated by scanning electron microscopy

(SEM) to better understand the differences in the samples, and the micrographs are

shown in Figure 4.4. The micrographs indicate that different surface morphologies exist

for the two different conditions. Figure 4.4 (a) shows that the thinner Zn2SiO4: Mn film

was produced with particulates on the surface. It is









A)





















B)






















Figure 4-4. SEM pictures of the surface of the phosphor films after annealing at 1100 C
for the A) 0.85 atm sample and for B) 1.4 atm thick sample.









common for particulates to form on the surface during pulsed laser deposition [45]. The

thicker film was investigated further to determine what the clusters on the surface were.

It was found to compose of a silicon- oxygen rich matrix, much like the remaining

surface. A small amount of zinc (1- 3 atomic%) was also present.

4.3.3 Cathodoluminescence Results

The CL emission spectra for annealed Zn2SiO4: Mn for each thickness are shown in

Figure 4.5. The primary beam voltage and current for these spectra was 5 keV and 8.0

JA. The spectra have the characteristic Gaussian profile. The maximum peak intensity

was observed at a wavelength of 530 nm. The thinner sample is significantly brighter

than the thicker sample. The cathodoluminescent brightness is 9 times greater for the

0.85 tm film, compared to the 1.4 |tm sample.

The maximum peak intensity was also measured for each film from 500- 5000eV

as is shown in Figure 4.6. The trend indicates that the thinner film has a higher CL

intensity than the thick film at all voltage levels. Previous researchers have observed the

opposite trend [3,34], where the CL intensity is higher as the thickness of the films

increase. CL intensity increases with an increasing accelerating voltage for the thinner

sample. However, the slope of change in CL intensity for the thicker sample with beam

voltage is very small.




















1500



1000



500




400 500 600 700
Wavelength (nm)



Figure 4-5. Cathodoluminescent emission spectra from Zn2SiO4: Mn at V= 5keV, i= 8.0
[LA, with inset of CL spectrum for 1.4 |tm thick sample.






3000

0.85 urn PLD (530 nm) U 1.4 urn PLD (530 nm)

2500



2000



". 1500



1000



500





0 1000 2000 3000 4000 5000 6000
Accelerating Voltage (eV)


Figure 4-6 CL brightness versus primary beam voltage for PLD Zn2SiO4: Mn thin films.









4.3.4 Composition of Films

The composition of the films was measured by electron probe microanalysis

(EPMA). The atomic % of each element for the two samples is shown in Figure 4.7. It

can clearly be seen that the thin sample has a higher zinc content (12 at %) than the

thicker sample (3 at %). The Zn/Si ratios were 0.58 and 0.11 for the 0.85 and 1.40 jtm

thick films, respectively. Both samples exhibit a lower than expected Zn/Si ratio,

suggesting a Zn deficiency. The XRD diffraction patterns indicate that Zn2SiO4 is

present, but to varying degrees. The 1.40 jtm thick film had a diffraction pattern that

included several peaks resulting from SiO2 and Zn2SiO4. This correlates to the trend of

the composition data suggesting that less Zn is present in this sample versus the 0.85 jtm

thick film. These data suggest that the trends from the EPMA data are correct although

the absolute values may not be accurate.















U


Atomic %










Zn Si 0




Figure 4-7 Comparison of composition in the PLD thin film samples.

4.4 Results from Sputter Deposited Phosphors

4.4.1 Target 1

Two samples were grown with Target under different sputter power (80 versus

100W) while all other variables remained the same. The films were deposited onto

quartz substrates. The growth parameters are defined in Table 4.2. A rapid thermal

anneal at 1100 C was necessary for detectable cathodoluminescence.









Table 4-2 Processing conditions for sputter deposition of Zn2SiO4: Mn thin films using
Target 1.


Processing Parameter Selected Conditions

Sputter power 80- 100 W

System pressure 18 mTorr

02 partial pressure 3 mTorr

Substrate temperature room temperature- no heating

RTA temperature 1100 C



The thickness of a step in the thin films was measured by a profilometer. The

sample that was sputtered at 80W had a deposition rate of 16 A/ min, resulting in a

thickness of 1925 A. The sample sputtered at 100 W had a measured thickness of 2262

A, corresponding to a deposition rate of 18.8 A/ min.

4.4.1.1 Structural characterization

X- ray diffraction data from the films was collected before and after the 1100 C

anneal for 5 minutes. Both as- deposited films were amorphous with no discernable

diffraction peaks. The X- ray diffraction patterns for two samples that were annealed at

1100 oC are shown in Figure 4.8. All of the diffraction peaks were indexed to zinc

silicate, indicating that a polycrystalline solid exists for both growth conditions after the

post- deposition anneal.


























20 25 30 35 40
2 theta (degrees)



Figure 4-8. XRD pattern of 1100 C annealed sputtered films at two different sputter
conditions.

4.4.1.2 Cathodoluminescence results

The cathodoluminescent (CL) spectra are compared in Figure 4.9 for the two

different growth conditions. The emission spectra resulted from an electron beam

excitation source with an energy of 5000 eV and 7.0 [tA of beam current. There were no

observable differences in the two spectra. The maximum emission peak for these

samples was at 526 nm consistent with visible green emission. The film deposited at the

higher power (100 W) was brighter than the film deposited at 80 W. In the case of

sputter deposited films, the thicker film resulted in brighter luminescence. As expected,

the CL brightness increased with accelerating voltage (Figure 4.10). The CL emission

was evaluated for accelerating voltages from 1000- 5000 eV at a beam current of 2.5 |tA.

However, at the low voltages there is little difference in the maximum emission intensity

for the two samples. In contrast, at 5000 eV the difference in the maximum intensity is

1.3 times as shown in Figure 4.11.







71


The cathodoluminescence from 500- 5000 eV and a beam current of 8.0 [tA from


films grown by sputter (0.1925 [tm) versus pulsed laser deposition (0.85 [tm) is compared


in Figure 4.12. The thinner pulsed laser deposited sample was chosen since it had the

best brightness for that growth method. It is compared with the less bright sample

created by sputter deposition. The sputter sample is still more than 30% brighter than the

thin film phosphor sample made by pulsed laser deposition, even though it is a factor of

four times thinner.








1 -



0.8 -


-100W -80W
0.6-


E
o 0.4



0.2



0
400 450 500 550 600 650 700
wavelength (nm)


Figure 4.9. Cathodoluminescent spectra from sputtered deposited thin film samples.








72






1400

| 100W H 80W
1200


1000


8000


S600


400


200


0
0 1 2 3 4 5 6
Voltage (keV)





Figure 4-10. Maximum CL brightness for sputter deposited and annealed Zn2SiO4: Mn
thin films from 1000 to 5000 eV.














2500



2000


o100 W E8OW




i


U
5


Current (uA)


Figure 4-11. Maximum CL intensity at V= 5keV for i2


2.5 to 7 A.
















0.8-





0 0.6-

Z


0.2
z




*0.85 urn PLD (530 nm) 0O.1925 umSpD (525 nm)

0
0 1000 2000 3000 4000 5000 6000
Beam voltage (eV)





Figure 4-12. Maximum CL brightness for PLD vs. sputter deposited Zn2SiO4: Mn thin
films from 500 to 5000 eV.

4.4.2 Target 2

As discussed earlier, a second target was used after it was noticed that the first

target had become a source of carbon contamination due to the cracking and exposure of

the carbon based bonding material. A different sputter gun was used with target 2, which

eliminated the need for bonding the target to a backing plate with a carbon based

material. A different deposition chamber was also used for these film growth cycles.

The growth parameters are outlined in Table 4.2. The films were deposited onto

quartz substrates. Each deposition cycle lasted 60 minutes. A rapid thermal anneal at

1100 C for 5 minutes was necessary to detect cathodoluminescence.









Table 4-3 Processing conditions for sputter deposition ofZn2SiO4: Mn thin films using
Target 2.

Processing Parameter Selected Conditions

Sputter power 40- 70W

Sputter pressure 13 mTorr

02 partial pressure 2.5 mTorr

Substrate temperature room temperature- no heating

RTA temperature 1100 C


4.4.2.1 Structural characterization

The as- deposited samples were all amorphous as indicated by their respective

diffraction patterns. A representative X- ray diffraction pattern for the annealed at 1100

oC phosphors is shown in Figure 4.13. All of the diffraction peaks are indexed to zinc

silicate, indicating that a polycrystalline solid exists after the anneal.

4.4.2.2 Cathodoluminescence results

The cathodoluminescence spectra from the annealed sputter deposited films are shown in

Figure 4.14. The maximum emission peak for these samples was at 526 nm consistent

with visible green emission. The two brightest phosphors were from sputter conditions at

the higher sputter powers, 70 and 60 W. As the sputter power decreased, a decrease in

the cathodoluminescence is also observed. The composition was measured by X- ray

photoelectron spectroscopy (XPS) to further investigate the drop in luminescence at the

lower sputter power. However, the results did not accurately correlate to the trends for

cathodoluminescence.




































25 30 35 40
2 theta (degrees)


Figure 4-13 X- ray diffraction pattern from Zn2SiO4: Mn thin film sputter deposited with
target 2.


S800

600

400

200


500 600
Wavelength (nm)


Figure 4-14 Cathodoluminescent emission from sputter deposited Zn2SiO4: Mn at
electron beam V= 5keV, i= 15[tA for sputter powers of 40- 70W.









The composition was measured by XPS which has a detection depth of about 3 nm,

while the CL data result from the electron beam voltage of 5 keV penetrating more than

100 nm into the film. The XPS results indicate that a zinc deficiency exists at the surface

for all films, but may not be representative of the bulk composition.

4.5 Results from Combustion Chemical Vapor Deposition Phosphors

The phosphor films grown on quartz substrates by combustion chemical vapor

deposition (CCVD) contain Mn at two concentration levels, 2 and 4 mol %. The as-

deposited films were investigated with X- ray diffraction, and their structure was

determined to be polycrystalline Zn2SiO4 [70]. Therefore, no post deposition heat

treatment was completed for these samples.

4.5.1 Cathodoluminescence Results

The cathodoluminescence spectra for the CCVD phosphor films are shown in

Figure 4.15, and their peak maximum at 525 nm is consistent with the spectra from PLD

and sputter deposited Zn2SiO4: Mn. The emission from the 4 mol% Mn concentration

phosphor is 5% brighter than the emission from the 2 mol% Mn phosphor. Additionally,

the CL intensity was compared with the brightest sputter deposited film in Figure 4.13.

The films produced by CCVD were at least 40% brighter.













4500
-CCVD 4% Mn
4000 -
4000- -Sputter Dep from commercial phosphor
3500 CCVD 2% Mn

3000

J 2500 -

| 2000 -

1500 -

1000

500

0-
400 500 600 700
Wavelength (nm)





Figure 4-15 CL spectra from CCVD Zn2SiO4: Mn phosphor films at beam V= 5keV, i=
7.0 [tA.

4.6 Discussion

Zn2SiO4: Mn thin film phosphors were successfully made by pulsed laser

deposition, sputter deposition, and combustion chemical vapor deposition. The X- ray

diffraction data indicated that the as- deposited films for PLD and sputter deposition were

amorphous, even though the PLD films were deposited onto substrates at 700 C. The

thin films were polycrystalline after a rapid thermal anneal at 1100 C in N2 atmosphere.

The CL performance of the PLD films was compared for two thicknesses. The

thinner film was actually 94% brighter than the thicker film. This indicates that the

brightness of the films does not depend solely on the thickness, since thicker films are

expected to result in brighter luminescence. The low CL performance of the thick film









was correlated to a low Zn to Si ratio, compared to the brighter film which had a higher

Zn to Si ratio. Additionally, the impurity SiO2 phase is strongly represented in the

diffraction pattern for the thicker film indicating that the film is a mixture of non-

luminescent SiO2 and luminescent Zn2SiO4. The Mn activator is expected to substitute

for the Zn atoms in the lattice structure. However, there is less Zn present in this film.

The reduction in cathodoluminescence is attributed to a reduction in the luminescent

center density. This is a reasonable assumption since the CL intensity changes only

slightly as the excitation volume increases.

The cathodoluminescence was also characterized for films that were sputtered

deposited. The results of the sputter deposited films were divided according to the

sputter target and deposition chamber used for film growth. The films that used target 1

were sputter deposited at two different powers and were compared based upon growth

condition and film thickness. The 20% higher sputter power resulted in a film that was

15% thicker. Green cathodoluminescence was observed from the samples after the

anneal was completed, with the thicker sample being 25% brighter than the thin sample.

The sputter deposited films from target 2 were grown at a slightly lower pressure and a

lower sputter power range, 40- 70W. The cathodoluminescence was brightest for films

deposited at 60- 70W and dimmest for those films deposited at 40- 50W sputter power.

The increase brightness is due to increased thickness of the phosphor films as the sputter

power is increased.

The films created by sputter and pulsed laser deposition were also compared. The

cathodoluminescence was at least 30% brighter for the films made by sputter deposition









versus pulsed laser deposition. The pulsed laser deposited samples has an SiO2 impurity

phase, which limits the activator density.

The films grown by combustion chemical vapor deposition developed a

polycrystalline structure during growth at 1200 C at atmospheric conditions. Therefore,

no post- growth heat treatment was completed for these films. The films were made at

two dopant levels where the 4 mol% Mn had a brighter cathodoluminescent emission

than the film doped with 2 mol% Mn. The CCVD films were brighter than the films

made by sputter deposition. The brightest films from this study were those made by

CCVD and were chosen to study the degradation ofZn2SiO4: Mn thin films, as discussed

in the following chapter.














CHAPTER 5
DEGRADATION OF ZINC SILICATE DOPED WITH MANGANESE POWDER AND
THIN FILM PHOSPHORS

5.1 Introduction

The degradation behavior of thin film and powder Zn2SiO4: Mn phosphors is

evaluated and discussed in this chapter. The degradation was observed under low (95

ptA/ cm2) and high (460 ptA/ cm2) current excitation densities for a period up to 24 hours.

Continuous exposure to the electron source over the 24-hour period has been shown to

affect the brightness of sulfide phosphors [19,73].

The first section in this chapter discusses the experimental procedures used to

monitor the degradation of the phosphors. The changes in the surface chemistry of the

phosphors have been observed by Auger electron spectroscopy. The decrease in

cathodoluminescence observed during electron beam exposure will also be discussed.

The differences between the degradation behaviors of the thin film phosphors versus the

powder phosphors are discussed. It was found that the excitation density also affects the

rate of degradation for both types of phosphors.

5.2 Experimental Procedures

The thin film phosphors were grown by a novel combustion chemical vapor

deposition (CCVD) method [70]. The Zn2SiO4: Mn phosphor was introduced and

evaluated for its cathodoluminescent emission in Chapter 4. It was determined to be the

brightest thin film phosphor of all phosphors developed in Chapter 4, and therefore

deserves further evaluation. The thin film phosphor on a quartz substrate was mounted









onto a stainless steel holder. The powder phosphor is a commercial plasma display panel

(PDP) phosphor obtained from Shanghai Yuelong New Materials Co., Ltd. The powder

samples were cold pressed into 4 mm deep, 6 mm diameter holes in a stainless steel

holder.

The degradation experiments took place in an ultra- high vacuum stainless steel

chamber with a base pressure of 3- 5 X10-9 Torr. The phosphors were exposed

continuously to an electron beam for 24 hours. The electron source was from a coaxial

electron gun in a PHI model 545 scanning Auger electron spectrometer. During electron

beam exposure, the cathodoluminescence was measured every minute to monitor changes

in luminescence leading to degradation. An Oriel model 77400 multi- spectrometer was

used to detect the emission spectra and peak intensities. Additionally, the surface

chemistry was simultaneously observed through Auger electron spectroscopy (AES). A

cylindrical mirror analyzer (CMA) model 15- 110 was used as the Auger electron

detector. The spectra were taken every 5 minutes during the continuous beam exposure

for 24 hours. The program created for AES analysis scanned the kinetic energy range of

Zn, Si, 0, and C for the range as specified in Table 5-1. Consecutive scans were taken

for each of the species during 5- minute intervals of data collection.



Table 5-1 Species monitored during AES analysis.
SPECIES ENERGY RANGE SCANNED (eV)

C 200-400

0 400-600

Si 70-100









Table 5-1. Continued
SPECIES ENERGY RANGE SCANNED (eV)

Znl 900-1050

Zn2 40-80

Survey of All Energies 30- 1100


5.3 Results

5.3.1 Thin Film Zn2SiO4: Mn Phosphor

5.3.1.1 24 hour CL degradation

Thin film Zn2SiO4: Mn was exposed to a 2000 eV, 3.0 ptA electron beam for 24

hours. Over this time period, the thin film phosphor decreased by 14% from its original

cathodoluminescent intensity at the low current density of 95.5 [A/ cm2 as shown in

Figure 5.1. The spikes in CL intensity are due to system fluctuations, which have been

correlated to instabilities with the Peltier cooler system that helps to reduce electrical

noise. Figure 5.2 shows the CL intensity during the first 20 minutes of degradation. The

phosphor actually decreased to its final intensity in less than ten minutes. The

cathodoluminescent spectra before and after degradation were compared in Figure 5.3 to

determine any difference in the spectral distribution. No shifts in the spectral distribution

or changes in peak shape were noted. Changes in the surface chemistry were also

observed during the degradation through monitoring of the Auger electron spectra. The

before and after spectra are shown in Figure 5.4, indicating composition changes of the

phosphor that evolved during beam exposure. A strong emission from carbon was noted

initially, but decreased significantly by the end of the experiment. The carbon Auger

electron signal results from adventitious surface contamination. It has been shown that










electron beam bombardment in oxidizing gases may remove this carbon during the initial

beam exposure [18,19]. The zinc and oxygen signals are stronger at the end of

degradation. This increase is due to the removal of the surface layer of carbon, where

now the signal comes exclusively from Zn2SiO4. There is no evidence of a chemical

reaction or change within the phosphor. Charging of the phosphor during electron beam

exposure is evident from the before and after AES spectra as illustrated by the charging

feature at the beginning of both spectra and the shift in the individual Auger signals.


cl degradation on thin film(i= 3.0- 3.5 uA)






S08

system fluctuations
06

z04


02

0
0 120 240 360 480 600 720 840 960 1080 1200 1320 1440 1560 1680
time (min.)


Figure 5-1. Cathodoluminescent degradation of thin film phosphor at V= 2 keV, i= 3.0
[tA (95.49 pA/ cm2).

A second CL degradation experiment was completed on the thin film phosphor at

the same beam voltage (2000 eV), but with a higher current density of 461 + 48 pA/ cm2

(electron beam current of 14.5 + 1.5 tA, spot size of 2 mm). The phosphor decreased to

26% of its original cathodoluminescent intensity at the higher current density as indicated

in Figure 5.5. In this case, it took a longer period of time (- 14 hours) for the phosphor











brightness to decrease to its final intensity level. The before and after

cathodoluminescent spectra are shown in Figure 5.6, and no changes in the spectral

distribution or peak shape is observed. The composition of the phosphor was measured

by AES during exposure to the electron beam. The electron beam assisted in the removal

of carbon from the surface, as indicated by the decrease in the carbon signal in the AES

after spectra shown in Figure 5.7. There is no Auger signal from carbon at the end of the

degradation experiment. There is a slight increase in the Auger signals from oxygen and

zinc. Again, this increase is expected based on the nearly complete removal of the

adventitious carbon from the surface which will reduce Auger electron scattering from

Zn2SiO4 by C. The charging features at the low energies and the shift in the Auger

signals indicate that the surface is charging during electron beam exposure.


cl degradation on thin film (i= 3.0- 3.5 uA)

1.2


1



0 0.0
0.8






0





0 2 4 6 8 10 12 14 16 18 20
time (min.)


Figure 5-2. Degradation within the first 20 minutes for the thin film phosphor at i= 3.0
[tA.
'a 0 .6 ------------------------------------






0 2 ----------------------------------










|tA.