Solution-Processed P-N Junction Ultraviolet Photodetectors Based on P-NIO and N-ZNO

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Solution-Processed P-N Junction Ultraviolet Photodetectors Based on P-NIO and N-ZNO
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Ryu, Jiho
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Master's ( M.S.)
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University of Florida
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Materials Science and Engineering
Committee Chair:
So, Franky
Committee Members:
Dempere, Luisa A
Jones, Kevin S

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nickeloxidesolution -- photodetector -- solutionprocess -- ultravioletphotodetector
Materials Science and Engineering -- Dissertations, Academic -- UF
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Abstract:
Ultraviolet (UV) photodetectors have a wide range ofapplications in the industrial and military areas, including biological and environmentalresearch, astronomical studies, missile plum detection, flame sensing, opticalcommunication, and so forth. Although several UV-detectors have recently beendeveloped using Schottky junction diodes and pn-junction of wide-gapsemiconductors such as ZnS, GaN, ZnSe, and diamond, these UV photodetectors still have limitations that make them inappropriatefor many applications due to their high cost and low quantum efficiency of less than 40%. In this study, we report on a low cost of solution-processed and high quantum efficiency UV photodetectors fabricated using solutions of p-type NiO(Eg ~3.7eV) and ZnO(Eg~3.3eV). NiO layer as a p-type were deposited on ITO (Indium Tin Oxide) glass by use of a spin-coat method.Then NiO layers were heated to a temperature at 270°C, 350°C , 450°C , 540°C for 40minutes, respectively. ZnO nanoparticle(NP) layers as a n-type were depositedsame as NiO layer and heated to a temperature at 100°C for 10min. An Aluminum as electrode layer of around 80nm was deposited by thermal evaporation onto the NiO / ZnO layer. The device was investigated by current-voltage (I-V) characteristics. The devices exhibit the dark current density are 36 nA/cm^2, 9.2 nA/cm^2, 6.8 nA/cm^2, 0.7 nA/cm^2 under -1V applied bias and quantumefficiency(QE) are 25,291%, 5,743%, 3,796%, 189% at a wavelength of 350nm with detectivity of 2.08 × 10^13, 9.44 × 10^12, 7.24 × 10^12, 1.11 × 10^12 were achieved at 350nm, respectively.
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by Jiho Ryu.
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Thesis (M.S.)--University of Florida, 2012.
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Adviser: So, Franky.
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1 SOLUTION PROCESSED P N JUNCTION ULTRAVIOLET PHOTODETECTORS BASED ON P NIO AND N ZNO By JIHO RYU A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

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2 2012 Jiho Ryu

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3 To my family and my fiance

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4 ACKNOWLEDGMENTS First of all, I would like to appreciate my advisor, Dr. Franky So for having given me an opportunity to work in his organ ic electronic materials and device group. He also has supported me academically and financ ially during full period of my m a s ter s t hesis course s To my committee members, I am thankful to Dr. Kevin Jones and Dr. Luisa Dempere for their interest in my resea rch and for serving on my thesis committee. I am deeply honored to all my colleagues in Dr. So s group, Dr. Do young Kim, Dr. Dong Woo Song, Dr. Won Hoe Koo, Dr. Sai Wing Tsang Chi Hang Cheung, Lei Qi an, Wonhoe Koo, Sai Wing Tsang, Cephas S mall, Michael H artel, Song Chen Jae Woong Lee, Wooram Youn, Chaoyu Xiang, Sujin Baek, Hyeonggeun Yu, Jesse Manders, Fred Steffy, Tzung Han Lai, Chieh Chun Chaing, and Shuyi Li u. I would like to express my special thanks to Do Young Kim and Jae Woong Lee for their feedb ack in research, life, and creativity in scientific research. I also thank my friends at University of Florida: Chang Woo Jee, James Sejong Kim, Yoo Jin Chung, Jihye Kim, Hyuk Soo Han, Ji Eun Chung, Jae Pil Yoo. I cherished the delightful and happy memori es of our journey. We have had a pleasant time during graduate studies. I also want to express my thanks to Korean friends in MSE: Sangjoon Lee, Jaeseok Lee Minki Hong, Jungbae Lee, Sungwook Min, Jinhyung Lee, Seunghwan Yeo, Chinsung Park, and Kwangwon L ee I hope they will do their best and success I sincerely thank my mom and dad, Joong Ja Won and Kyoung Ho Ryu, for their endless love. My parents have supported me physically and spiritually I want to express very special thank s to my fiance Min Seon Kim. She supported me both physically and mentally although she is also a graduate student at University of Florida.

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5 I graduate this school if she was not here. She has brought me the joy of life. I truly appreciate for your love, your support an d cooking. Finally, I should thank and praise to my Lord, Jesus Christ

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF FIGURES ................................ ................................ ................................ .......... 7 LIST OF ABBREVIATIONS ................................ ................................ ............................. 8 ABSTRACT ................................ ................................ ................................ ..................... 9 CHAPTER 1 MOTIVATION ................................ ................................ ................................ ......... 11 2 BACKGROUND ................................ ................................ ................................ ...... 13 2.1 Ultraviolet Radiation ................................ ................................ .......................... 13 2.1.1 Classification ................................ ................................ ........................... 13 2.1.2 Application of UVR ................................ ................................ .................. 14 2.2 UV Photodetectors ................................ ................................ ............................ 15 2.2.1 Semiconductor Photodetectors ................................ ............................... 15 2.2.2 Semiconductors for UV Photodetection ................................ ................... 15 2.2.3 Photodetector Characterization ................................ ............................... 17 3 EXPERIMENTAL DETAILS ................................ ................................ .................... 21 3.1 NiO Solution ................................ ................................ ................................ ...... 21 3.2 ZnO NPs Solution ................................ ................................ ............................. 21 3.3 UV Detectors Fabrication ................................ ................................ .................. 21 4 RESULTS AND DISCUSSIONS ................................ ................................ ............. 24 5 CONCLUSIONS ................................ ................................ ................................ ..... 37 LIST OF REFERENCES ................................ ................................ ............................... 38 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 40

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7 LIST OF FIGURES Figure page 2 1 Photodetection Principle ................................ ................................ ..................... 18 2 2 Schematic structure of different semiconductor photodetectors ......................... 19 2 3 Typical J V curves of photodetectors in dark and under illumination .................. 20 3 1 Schematic diagrams of UV photodetectors fabrication ................................ ....... 23 4 1 Schematic diagram showing energy level of the device ................................ ..... 28 4 2 Rectifying behavior of I V characteristic ................................ ............................. 29 4 3 I V characteristics of UV photodetector ................................ .............................. 29 4 4 Annealing NiO temperature dependence versus EQE and detectivity ................ 30 4 5 Spectral de tectivity of the UV detector under 1V applied bias ........................... 30 4 6 Life time with EQE and detectivity ................................ ................................ ...... 31 4 7 Transient response of the U V detector with UV illumination on/off ..................... 31 4 8 Temperature dependence with rise and fall time ................................ ................ 32 4 9 XPS Ni2p spectra of each dif ferent NiO films ................................ ..................... 32 4 10 XPS O1s spectra of each different NiO films ................................ ...................... 33 4 11 Picture of different NiO annealing film on ITO subs trate ................................ ..... 33 4 12 Proposed band structure for the high gain photodetector ................................ ... 34 4 13 Schematic of UV photodetector on Quartz substrate ................................ .......... 34 4 14 I V characteristics of UV detector in the dark and under illumination at room temperature ................................ ................................ ................................ ........ 35 4 15 Spectral EQE and detec tivity of the UV detector under 1V applied bias ........... 35 4 16 Transmittance of ITO, NiO and ZnO on quartz substrate ................................ ... 36

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8 LIST OF ABBREVIATION S CCD charged coupled de vice DMSO dimethyl sulfoxide EQE external quantum efficiency HOMO highest occupied molecular orbital ITO indium tin oxide LUMO lowest unoccupied molecular orbital MIS metal insulator semiconductor MSM metal semiconductor metal NP nanoparticle PMT s photomul tiplier tubes TMAH tetramethylammonium hydroxide UV ultraviolet WBG wide band gap XPS x ray photoelectron spectroscopy

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9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requir ements for the Degree of Master of Science SOLUTION PROCESSED P N JUNCTION ULTRAVIOLET PHOTODETECTORS BASED ON P NIO AND N ZNO By Jiho Ryu Aug u s t 2012 Chair: Frank y So Major: Materials Science and Engineering Ultraviolet (UV) photodetectors have a wid e range of applications in the industrial and military areas, including biological and environmental research, astronomical studies, missile plum detection, flame sensing, optical communication, and so forth. Although several UV detectors have recently bee n developed using Schottky junction diodes and pn junction of wide gap semiconductors such as ZnS, GaN, ZnSe, and diamond, thes e UV photodetectors still have limitations that make them inappropriate for many applications due to their high cost and low quan tum efficiency of less than 40% In this study we report on a low cost of solution processed and high qu a ntum efficiency UV photodetectors fabricated using solutions of p t ype NiO(Eg ~3.7eV) and ZnO(Eg~3.3eV) NiO layer as a p type were deposited on ITO (Indium Tin Oxide) glass by use of a spin coat method. Then NiO layers were heated to a temperature at 270 350 450 540 for 40 minutes respectively ZnO nanopart icle( NP ) layers as a n type were deposited same as NiO layer and heated to a temperature at 100 for 10min. A n Aluminum as electrode layer of around 80nm was deposited by thermal evaporation

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10 onto the NiO / ZnO layer. This device was investigated by current voltage (I V) characteristics. The devices exhibit the dark current density are 36 n A/cm 2 9.2 n A/cm 2 6.8 n A/cm 2 0.7 n A/cm 2 under 1V applied bias and quantum efficiency (QE) are 25,291 % 5,743%, 3,796%, 189 % at a wavelength of 350nm with detectivity of 2 08 10 13 9.44 10 1 2 7.24 10 1 2 1.11 10 1 2 were achieved at 350nm respectively

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11 CHAPTER 1 MOTIVATION Many of the important applications of ultraviolet (UV) detection are in high temperature flame detection, space research, gas sensing, missile warning systems, air quality monitoring accurate measurement of radiation for the treatme nt of UV exposured skin, etc. [ 1 4 ] The most common detectors recently in use are the photomultipliers tubes silicon photodetectors, bu t they are not blind and demand costly filters to attenuate unwanted and infrared radiation. With the u se of wide band gap semiconductors such as NiO, GaN, diamond and ZnO, the need for these filters would be eliminated. Among them, NiO is a well known transparent p type semiconductor with a band gap of about 3.7eV It has a rock salt cubic struc ture and a weak absorption band due to the d d transition of 3d 8 electron configuration in the visible region [5 7] Also ZnO, an en vironmentally friendly semiconductor with a room t emperature band gap of 3.35 eV, is of great interest for UV detection because of it s various synthetic methods, diverse processing technologies, a nd the capability of operating at high temperatures and in harsh environments. Liu et al. fabricated Schottky UV photodetectors which exhibit fast response speed by growing high quality ZnO epi taxial films on sapphire substrates [8 ] Nevertheless, achieving high crystal quality of ZnO thin films with suitable metal contacts i s sti ll challenging because of the l ack of high quality and low cost substrates for lattice matched growth. Most ZnO thin films deposited by metal organic chemical vapor deposition, pulsed laser deposition, or radio frequency sputtering have a large density of dislocations and grain boundaries. Transport and UV photoconduction in these polycrystalline ZnO films d epends

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12 sensit ively on stoichiom etry, trap densities, and most importantly, gas adsorption and desorption on the crystal surfaces However, s olution processed electronic and optoelectronic devices have some advantages over conventional crystalline semi conductor devices in terms of ease of fabrication, large device area, physical flexibility, and most importantly, low cost. Therefore, i n this paper, we developed the fabrication of pn junction U V photodetectors by s olution processed p type NiO and n typ ZnO NP s on t he gl ass substrates with pre patterned indium tin oxide (ITO), followed by annealing in air and evaporation of Aluminum contacts through a shadow mask

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13 CHAPTER 2 BACKGROUND 2.1 Ultraviolet Radiation In 1801, t he first reveal of ultraviolet (UV) radiation was made, when J W Ritter discovered that certain chemical reactions were catalyzed by exposure to non visible radiation with a shorter wavelength than violet. After that T Young show e d that chemically active radiation track ed the interference laws in 1804 This observation, together with the work of many other researchers, made it possible to establish that visible and UV emissions both were expression s of the same sort of electromagnetic radiation, solely differing in their wavelength. These days it is bro adly established that the UV region occupies the spectral 10 nm [ 9 ]. It is a greatly ionizing radiation, which trigger s many chemical processes. The most important natural UV source is the Sun. R oughly 9% of the energy obtain e d from the Sun at the higher layers of the atmosphere is in the UV r ange [ 10 ], although the stratospheric ozone layer prevents wave lengths shorter than surface. Besides, remaining UV radiation may be attenuated by pollution in the low troposphere 2.1.1 Classification Ultraviolet radiation (UVR) shows the spectral region within a wavelength interv al [ 11 ] of = 380 10 nm ( hv = 3 2 124 eV) and the UV spectrum is traditionally separated into a near region ( = 380 200 nm, hv = 3 2 6.2 eV) and a far region ( = 200 10 nm, hv = 6 2 124 eV). The near region contains as follows: L ong wavelength UVR (UV A) at 380 3 15nm (hv = 3.2 3.9 eV) This is the less energetic range. It stimulates photosynthesis, and is involved in the synthesis of some vitamins and basic biochemical compounds. Overexposure may lead to erythema and premature ageing.

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14 M iddle wavelength UVR (UV B) at 315 280nm (hv = 3.9 4.4 eV) Even though partially absorbed by the ozone layer, this composes 9 % of the total UV radiation B exposure is risky for human beings (causing bur ns, cataracts, erythema, and skin cancer ), harm fully affects harvests and enhances the ageing of organic materials Short wavelength UVR (UV C) at 280 200nm (hv = 4.4 6.2eV) This is the most energetic range, and it is also the more deleterious. However, it is almost completely absorbed by the stratosp heric ozone layer The following d ivision of the spectrum is frequently used in American publications: near UV, 400 300 nm; middle UV, 300 200 nm; far UV, 200 100nm; extreme UV, 100 10 nm [ 12 ]. 2.1.2 Application of UVR Biologically active UVR action is used in medicine ( solar i rradiation of people ) and in agriculture Vitamin synthesizing U VR action is used in biotechnol ogy (synthesis of vitamins D2 and D 3 ). Bactericidal UVR action is used in water, air and food disinfection, especially for long storage and during epidemics, and for disinfection of blood intended for transfusion. The detection of UV radiation is used in a broad range of military and civil applications, such as biological and chemical analysis (pollutants ozone, and most organic compounds pr esent absorption lines in the UV spectral range), flame detection (including fire alarms, missile warning or combustion monitoring), optical communications (particularly inter emitter calibration (instrumentation, UV lithography), and astronomical studies.

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15 2. 2 UV Photodetectors 2. 2 1 S emiconductor Photodetectors I n semiconductor photodetectors photons are absorbed in the semiconductor, creating electron hole pairs. These photogenerated carriers are separated by the electric field, due either to the built in potential or the applied voltage, producing a current proportional to the photon flux. The schematic operation of th is device is depicted in F igure 2 1 There are different types of semiconductor photodetectors [ 1 3 ]: photoconductors, Schottky barrier photodiodes, metal semiconductor metal (MSM) photodiodes, metal insulator semiconductor (MIS) structures, p n and p i n photodiodes, and field effect and bipolar phototransistors. The schematic structure of these devi ces is depicted in F igure 2 2 2. 2 2 Semiconductors for UV Photodetection UV detection has conventionally been achieve d by photomultiplier tubes (PMTs), thermal detectors, narrow bandgap semiconductor photodiodes or charge coupled devices (CCDs). PMTs show low noise and high gain, and can be reasonably visible blind. However, they are fragile and bulky devices, demanding high power supplies. Thermal detectors are normally used for calibration in the UV region Even though useful as radiometric standards, th ese detectors are slow, and their response is wavelength independent. On the other hand, semiconductor photodiodes and CCDs present the benefit s of solid state devices, demanding only moderate bias. Semiconductor photodetectors are small, lightweight, and insensitive to magnetic fields. Their good linearity low cost, and

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16 sensibility, and capability for high speed operation make them a brilliant advance for UV detection. Since their well silicon photodiodes are the mos t common devices for UV photodetection, even though commercial GaAsP and GaAs based photodiodes are also accessible Silicon based UV photodiodes can be divided into two major families: p n junction photodiodes and charge inversion photodiodes [ 14, 15 ]. Ho wever, Si based UV photodiodes exhibit some limitations natural to silicon technology. The main weak point for these narrow bandgap semiconductor detectors is device ageing, since exposure to radiation of much higher energy than the semiconductor bandgap. B esides passivation layers, typically SiO 2 reduce the QE in the deep UV range, and are also degraded by UV illumination. Another limitation of these devices is their sensitivity to low energy radiation, so that filters are demanded to block out visible a nd infrared photons, resulting in a major loss of effective area of the instrument. Finally, for high sensitivity applications, the detector active area must be cooled to reduce the dark current; the cooled detector behaves as a cold trap for contaminants, which leads to a lower detectivity. UV photodetectors based on wide bandgap (WBG) semiconductors (diamond, SiC, nitrides, and some compounds) are itself a significant benefit for photodetectors, since it allows room temperature operation, and provides intrinsic visible blindness [ 15 18 ] These materials are also apparent that the thermal conductivity of WBG ma terials is in general significantly higher than that of silicon, which makes them suitable for high t emperature and high power applications.

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17 In spite of all their promising characteristics there are still some drawbacks in WBG semiconductors. The main dr awback is crystal quality. The lack of high quality substrates for homoepitaxy or other lattice matched substrates leads to material with a high density of dislocations and grain boundaries. These structural defects induce deleterious effects on device per formance, such as an enrichment of visible detection and leakage currents, and the appearance of persistent effects [19, 20 ] 2. 2 3 Photodetector Characterization Photodetectors are photodiode with rectifying current density voltage (J V) characteristics Typical J V characteristics in the dark and under incident illumination are shown in Figur e 2 3 The responsivity is the most important parameters in p hotodetectors and is calculated by the following formula. R = J net photo /P inc = (J photo J dark )/P inc (1 1) where J net photo inc photocurrent, which subtracts the measured dark current density J dark from the measured photo current density J photo of the p hotodetector The quantum efficiency and detectivity can be calculated from the responsivity using the following expression: QE = (hc/q ) R 100% (1 2) D* = J photo / P inc (2qJ dark ) 1/2 (1 3) photon wavele ngth, respectively.

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18 A B Figure 2 1. Photodetection Principle A) Schematic of UV photodetector structure. B) Phtodetection principles

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19 Figure 2 2 Schematic structure of different semiconductor photodetectors

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20 Figure 2 3 Typical J V curves of p hotodetectors in dark and under illumination

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21 CHAPTER 3 EXPERIMENTAL DETAILS 3.1 NiO Solution The Ni O solution was prepared from a precursor, in which nickel acetate tetrahydrate were dissolved in ethanol. Ethanolamine was added to the precursor as a s tabilizer in equal molar concentration to nickel acetate tetrahydrate 3.2 ZnO NPs Solution W e used ZnO N P s (3 5 nm in size), which were synthesized through a sol gel process using precursors of zinc acetate and tetramethylammonium hydroxide (TMAH). F or a typical process, the ZnO N P s were synthesized by slowly dropwise addition of a stoichiometric amount of TMAH dissolved in ethanol (0.5M ) to zinc acetate dehydrate (0.1M ) dissolved in dimethyl sulfoxide (DMSO), followed by stirring for an hour After that add ethyl ac etate and heptanes to obtain ZnO particles use centrifuge the ZnO N P s were dissolved in ethanol and stored under ambient ltered with a lter 3.3 UV Detectors Fabrication All UV photodetectors were fabr icated on pre patterned ITO glass substrates with a sheet resistance of 2 four ITO glass substrates were first cleaned with acetone and isopropanol in an ultrasonic cleaner for 15minutes each. A nd then blown dry with N2 gas, and subseque ntly treated with UV ozone for 15 minutes. For p type layer, a 130nm thick NiO layer was spin coated on the ITO glass substrates an d the film was subsequently anneale d at 270 C 350 C 450 C 540 C for 40min respectively. After shortly, four samples need t o be cool down until 50 C After that, for n type layer, a 70nm thick ZnO layer was spin coated on the four ITO substrates and

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22 the film was subsequently baked at 100 C for 10min. ZnO film also need to be cool down until 50 C To finish this UV detector fab rication, a 80nm Al was thermally evaporated on top of ZnO layer as an electrode. This layer was deposited in a vacuum chamber with a base pressure of 1 10 6 torr. The deposition rate was 2 /s for Al. The active area of the devices used in this study wa s 0.04 cm2. ITO electrode was always ground and the Al electrode was applied by negative bias. In addition, the device is also fabricated on quartz substrate glass. ITO was sputtered on quartz substrate for 15min. T he devices were tested in air without enc apsulation. Figure 3 1 shows Schematic diagrams of UV photodetectors fabrication. The current voltage (I V) characteristics of the devices were measured with a Keithley 4200 semiconductor parameter analyzer. The devices were irradiated with monochromatic light from a Newport monochromator using an Oriel solar simulator as a source. Illumination between 300 and 400 nm were irradiated on the devices with different power intensities (35.6 The illumination intensities at different wavelengths were measured using two calibrated Newport 918D photodiodes. The EL spectra were collected using an Ocean Optics HR4000 high resolution spectrometer. The spectral re sponse was also used to calculate the spectral detectivity of the devices across the UV wavelengths. To study the stability of the devices, the unencapsulated photodetectors were exposed to ambient conditions. The X ray photoelectron spectroscopy ( XPS ) inv estigations were done on a Perkin Elmer 5100 XPS for NiO film analysis.

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23 Figure 3 1. Schematic diagrams of UV photodetectors fabrication )

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24 CHAPTER 4 RESULTS AND DISCUSSIONS As shown at Figure 4 1 the schematic energy band diagram of such a photode tector with pn junction A thin layer of NiO is used for p type layer and ZnO is used for n type layer. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of a NiO were 1.8 eV and 5.5 eV, respectively and ZnO w ere 4.2 eV and 7.6 eV respectively and work functions of an ITO and a Al also were 4.8 eV and 4.3 eV, respectively Figure 4 2 shows that the device demonstrates a clear diode like rectifying behavior at bias voltages from +0.5 to 1.5V. The dark current d ensities in Figure 4 3 ( A) were 36 nA/cm 2 9.2 nA/cm 2 6.8 nA/cm 2 and 0.7 nA/cm 2 at 1V, respectively. Also, photo current densities as Figure 4 3 ( B) were 8.2 uA/cm 2 1.8 uA/cm 2 1.2 uA/cm 2 and 62 nA/cm 2 at 1V, respectively. The net photo generated cur rent density, the difference of photo current densities and dark current densities decrease gradually with increased NiO annealing temperature. That is, the number of charge carriers flowing through the device under the light illumination. The 350 nm incid en t UV light with the intensity 0.115mW/cm 2 was irradiated on the UV photodetector under applied reverse bias. The changes in dark current and photocurrent led to External Quantum Efficiency ( EQE ) and detectivity. The EQE and detectivity of the UV photode tector are shown in Figure 4 4 with annealed NiO at 270 C 350 C 450 C and 540 C respectively and UV photodetector at a reverse bias of 1V The highest EQE and detectivity are obtained at 270 annealed NiO UV photodetector, whereas the lowest EQE and detec tivity are obtained at 540 annealied NiO UV photodetector. The EQE reaches 25291%, 5743%, 3796% and 189% with 270 C 350 C 450 C and 540 C NiO UV photodetector,

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25 respectively. A lso, detecti vit y (Jones) 210 13 9.410 12 7.210 12 and 1.1110 12 were obtained. Figure 4 5 illustrates the room temperature spectral detectivity of UV detector with NiO annealed at 270 C under 1V applied bias. The spectra show the active range between 320 365nm. I t indicates that this UV detector is visible blindness range. It also can be seen that cutoff occurred at 320nm due to absorption either ITO and glass substrate. As shown at Figure 4 6, life time was measured every week for a month. This UV detector is oxy gen based device so the performance of EQE and detectivity for a month is very stable. To measure rise time and fall time, the temporal responses of the four detectors were measured by shutting on/off the light at the wavelength of 350nm under a 1V applie d bias in Figure 4 7. The four detectors exhibited totally different responses. It is well known that photogenerated electrons will circulate through the external circuit many times before recombining with hole if those photogenerated holes were trapped by the surface state at the interface. Consequently, the temporal photocurrent would not drop sharply when the UV light was turned off like Figure 4 7 (A). This point will discuss later. As increasing NiO annealing temperature, rise time and fall time is get ting be fast in Figure 4 8. As can be seen from the above, photo current multiplication occurred at four UV detectors. At this time, we would like to demonstrate that the photocurrent multiplication or gain mechanism to be controllable by annealing temper ature Each NiO different annealing films were measured by X ray photoelectron spectroscopy (XPS). Figure 4 9 illustrates Ni2p 3/2 XPS spectra of each NiO film For Ni2p 3/2 spectra three clearly

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26 separated peaks can be detected, which corres pond to the bindi ng states of peak NiO state peak Ni(OH) 2 state and shake up peak. Red (853.4 eV) green (855.2eV) and blue peak (860eV) indicate Ni O Ni (OH) 2 and shake up peak, respectively. It can be seen clearly, that the portion of Ni2p 3/2 from Ni (OH) 2 is decreasing with the higher NiO annealing temperature. Ni(OH) 2 come from excess of oxygen in surface of NiO film. As can be seen from Figure 4 10, O1s XPS spectra of each NiO film also show two peaks such as NiO (528.9eV) and Ni(OH) 2 This Figure also show s that Ni(OH ) 2 peak is decreasing with the increasing NiO annealing temperature. It is a well known fact that NiO that is closely stoichiometric appears green like nickel oxide solution whereas material that has an excess of oxygen tends more toward a black appearanc e [21] To illustrate in a quantitative way the nature of this color change on NiO films picture that were shown i n figure 4 11, 270 C NiO film is the darkest film and 540 C film is the lightest film. Non stoichiometric NiO (540 C ) is the darkest due to the presence of a lot of Ni 3+ ions resulting from the appearance of nickel vacancies and/or interstitial oxygen in NiO films and have intrinsically p type conduction [22, 23 ] Nam et al also observed more defective nickel oxide materials containing more Ni 3+ ion states [24]. Moreover, Lee at al proposed that nonstoichiom etric nickel oxide of Ni 3+ ions (directly related to hole concentration) increases with increasing excess of oxygen, which corresponds to the quantity of Ni 2+ ions decreases resulting in a red uced number of reaction sites for OH insertion [25]. Now, we can propose why dark current and photo current decrease with increasing NiO annealing temperature. It has been suggested that the oxidation and reduction reactions of preexisting Ni 3+ ions part icipate in the storage of charge [26]. It

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27 means that Ni 3+ Ions can be act as traps of the UV photodetector. Therefore, there can be a lot of energy level of hole traps near valance band in NiO films. When revere bias is applied to the device, electrons in valance band can be moved to higher energy band which is hole traps by thermal exicitation. Then holes generated by thermal excitation are collected by the applied reverse bias, which correspond to increasing dark current. For the explanation of photocurre nt multiplication, the schematic band diagram as depicted in figure 4 12 illustrate the condition when the UV detector is under reverse bias and under UV illumination The photogenerated holes at ITO / NiO interface are accumulated [27]. In addition, the a ccumulation of photogenerated holes near the interface brings the amount of electrons injected by tunneling barrier under UV illumination from the ITO electrode occurs. As can be seen from Figure 4 13, UV photodetector was fabricated on quarts substrate w ith same procedure to compare between the device with glass substrate and the device with quartz substrate. The spectral EQE and detectivity were shown in Figure 4 14. The response range is similar to the UV photodetector on glass substrate. However, we c an clearly observe that peak of EQE and detectivity within range between 300nm 400nm is located 345nm. The peak of detetectivity is little shifted from the UV detector on glass substrate due to its transmittance. Sharp cut off after 320nm is limited by ITO electrode. Therefore, UV photodetector response can be wider if electrode is replaced with shorter absorption such as Ag nanowire.

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28 Figure 4 1. Schematic diagram showing energy level of the device

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29 Figure 4 2. Rectifying behavior of I V characteris tic A B Figure 4 3. I V chara cteristics of UV photo detector. A) Dark current. B) Photo current under illumination at room temperature

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30 Figure 4 4. Annealing NiO temperature dependence versus EQE and detectivity Figure 4 5. Spectral detectivity of the UV detector under 1V applied bias

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31 A B Figure 4 6. Life time with EQE and detectivity A) EQE B) Detectivity A B C D Figure 4 7. Transient response of the UV detector with UV illuminatio n on/off A) 270 B) 350 C) 450 D) 540

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32 Figure 4 8 T emperature dependence with rise and fall time A B C D Figure 4 9. XPS Ni2p spectra of each different NiO films A) 270 B) 350 C) 450 D) 540

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33 A B C D Figure 4 10. XPS O1s spectra of each different NiO films A) 270 B) 350 C) 450 D) 540 Figure 4 11. Picture of different NiO annealing film on ITO substrate

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34 A B Figure 4 12. Proposed band structure for the high gain photodetector. A ) Dark current mechanism B) Photo current mechanism Fig ure 4 13. Schematic of UV photodetector on Quartz substrate

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35 Figure 4 14. I V characteristics of UV detector in the dark and under il lum ination at room temperature A B Figure 4 15. Spectral EQE and detectivity of the UV detector under 1V applied b ias A) EQE B) Detectivity

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36 Figure 4 16. Transmittance of ITO, NiO and ZnO on quartz substrate

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37 CHAPTER 5 CONCLUSIONS This thesis has investigated the solution processed p n junction ultraviolet photodetectors using p type NiO and n type ZnO Nanoparticle layer The four UV detectors annealed NiO film different temperature exhibits clear rectifying current voltage characteristics under the applied bias from +1V to 2V. Photocurrent multiplication phenomenon at NiO/ITO interface was proposed with X ray phot oelectron analysis. High external quantum efficiency and detectivity were obtained with NiO 270 C annealing temperature of UV detetector.

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38 LIST OF REFERENCES [1] M. Razeghi, A. Rogalski J. Appl. Phys. 1998, 79 7433 [2] H. Fabricius, T. Skettrup, P. Bis gaard, Appl. Opt. 1986, 25 2764 [3] H. Ohta, H. Hosono, Mat er. Today 2004, 7 42 [4] T.H. Moon, M.C. Jeong, W. Lee, J.M. Myoung, Appl. Surf. Sci. 2005, 240 280 [5] R. J. Powell W. E. S picer, Phys. Rev. 1970, B2, 2182 [6] G. A. Sawatzky J. W. Al len, Phys. Rev. Lett. 1984, 53 2339 [7] A. Fujimori F. Minami, Phys. Rev. 1984, B30, 957 [ 8 ] S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, H. Shen, J. Cryst. Growth 2001,110, 113. [9] J. Kolnik, I. H. Oguzman, K. F. Brennan, R. Wang, P. P. Ruden J. App l. Phys. 1995, 78 1033 [10] K. L. Coulso n Solar and Terrestrial Radiation (New York: Academic) 1975. [11 ] Russian Standard Photometry. Terms and Definition GOST 26148 84, modification N 1992. [ 12 ] L. R. Kolle r Ultraviolet Radiation (New York: Wiley) 1 965. [13] P. Bhattacharya Semiconductor Optoelectronic Devices (Englewood Cliffs, NJ: Prentice Hall) 1994. [14 ] Y. A. Goldb erg Semicond. Sci. Technol. 1999, 14 R41 [15 ] M. Razeghi A. Rogalski J. Appl. P hys. 1996, 79 7433 [16 ] J. H. Edgar Propertie s of G roup III Nitrides (London: INSPEC) 1994. [17 ] M. N. Yoder IEEE Trans. 1996, 43 1633 [18 ] S. J. Pearton, J. C. Zol per, R. J. Shul F. Ren J. Appl. P hys. 1999, 86 1 [19 ] R. J. McIn tyre, J. Appl. Phys. 1961, 32, 983 [20 ] A. G. Chynoweth and G. L. Pearson, J. Appl. Phys. 1956, 29, 1103 [21] R. Newman, and R. M. Chrenko, Phys. Rev. 1959, 1507 1513 [22 ] E. Antolini, J. Mater. Sci. 1992, 27 3335

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39 [23 ] H. Sato, T. Minami, S. Takata, T. Yamada, Thin Solid Films 1993, 236 27 [24] K. Nam, W. Yoon, K Ki m, Electrochim. Acta 2002, 47, 3201 [25] S. Lee, C. Tracy, J. Roland, Electroch im Let 2004, 7 10 [26] V. Srinivasan J. Weidner J. Electroch i m. Soc 1997, 144, 8. [27] K. Nakayama, M. Hiramoto, M. Yokoyama J. Appl. Phys 2000, 87, 7.

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40 BIOGRAPHICAL SKETCH Ryu, Jiho was born in 1983, in Inje, Republic of Korea. He was curious about how the images of people were projected on the screen when he watched television as a answer to his question, his curiosity and interest continued to grow from this point. By the time he was in high school the cellular phone was invented, he wondered how the liquid crystal display could fit into such a small machine and move around. To answ er this question, he went to Korea Polytechnic University and majored in advanced materials engineering (2001 2008). He concentrated in the field of display, which has decided his career path. After graduation with B.S. degree, he joined the Department o f Materials Science and Engineering at University of Florida, and completed his M.S. thesis under the advisory of Dr. Franky So in Aug ust 2012. He will start to work for LG Display as a researcher on September in 2012.