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Growth and Ferromagnetic Semiconducting Properties of Titanium Dioxide Thin Films: An Oxide-Diluted Magnetic Semiconduct...

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Title: Growth and Ferromagnetic Semiconducting Properties of Titanium Dioxide Thin Films: An Oxide-Diluted Magnetic Semiconductor (O-DMS) for Spintronics
Physical Description: Mixed Material
Copyright Date: 2008

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Source Institution: University of Florida
Holding Location: University of Florida
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System ID: UFE0004240:00001

Permanent Link: http://ufdc.ufl.edu/UFE0004240/00001

Material Information

Title: Growth and Ferromagnetic Semiconducting Properties of Titanium Dioxide Thin Films: An Oxide-Diluted Magnetic Semiconductor (O-DMS) for Spintronics
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0004240:00001


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GROWTH AND FERROMAGNETIC SEMI CONDUCTING PROPERTIES OF TITANIUM DIOXIDE THIN FILM S: AN OXIDE-DILUTED MAGNETIC SEMICONDUCTOR (O-DMS) FOR SPINTRONICS By BYOUNG-SEONG JEONG 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 Byoung-Seong Jeong

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I dedicate this dissertation to my wife, Youngse, and to my family.

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ACKNOWLEDGMENTS I would especially like to express my sincerest gratitude to the chair of my supervisory committee, Professor David P. Norton, for his generous support when I needed it most, for the critical reading of this dissertation, and for kindly allowing me to participate as a member of his group. I would also like to thank the members of my supervisory committee, Professor Cammy Abernathy, Stephen Pearton, Rajiv Singh, and Andrew Rinzler, for having given me indispensable guidance and support. My wife Youngse has been a source of so many things to me in the process of graduate school. Motivation, encouragement, strength, and love have all been given in excess. Her patience and support have helped ease the writing process. There are many people to thank in the Dr. Norton group: Youngwoo, Seh-Jin, Jennifer, Hyung-Jin, Yongwook, V.J., Mat., Jean-Marie, Semmant, and Yuan-jie. They helped me a lot in experimental advice and discussion. Whenever I needed their support, they were pleased to help me. Especially, I would like to thank to Dr. Youngwoo Heo. He helped me a lot in several characterizations. Of course no acknowledgement would be complete without thanking my family. I gratefully acknowledge my mother, and my father, and my older brother who supported me all through graduate school. My father and mother endowed me and my brother and sister with their intellectual sensitivities. There are not enough words in the world to thank them. My only older brother, Byoung-Jo Jeong, supported me in finance and in iv

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spirit throughout graduate school. I truly thank him. I also would like to thank my father-in-law and mother-in-law. v

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TABLE OF CONTENTS Page ACKNOWLEDGMENTS .................................................................................................iv LIST OF TABLES ...........................................................................................................viii LIST OF FIGURES ...........................................................................................................ix ABSTRACT .....................................................................................................................xiii CHAPTER 1 INTRODUCTION........................................................................................................1 1.1 Transition Metal Oxide, TiO 2 .................................................................................1 1.2 Three Different Polymorphs of TiO 2 ......................................................................4 1.3 Transition Metal (TM)-Doping Effect....................................................................8 1.3.1 Cobalt (Co).................................................................................................10 1.3.2 Chromium (Cr)...........................................................................................12 1.3.3 Other Impurities..........................................................................................14 2 PREVIOUS WORKS IN UNDOPED AND TRANSITION METAL DOPED ANATASE..................................................................................................................17 2.1 Epitaxial Growth of Anatase TiO 2 ........................................................................17 2.2 Transport Properties of Oxygen-Deficient TiO 2 Thin Films................................20 2.3 Optical Properties of Anatase and Rutile..............................................................24 2.4 Transition Metal-Doped Anatase and Related Compounds.................................26 3 EXPERIMENTAL APPROACH AND TECHNIQUES............................................32 3.1 The Growth of Epitaxially Stabilized Anatase.....................................................32 3.2 Transport Measurement Techniques.....................................................................33 3.2.1 Hall Effect Measurement............................................................................33 3.2.1.1 van der Pauw Method.......................................................................33 3.2.1.2 Definitions........................................................................................33 3.2.2 Magnetoresistivity Measurement...............................................................36 3.3 Transition Metal Doping Techniques Using Co-sputtering..................................38 3.4 Characterization....................................................................................................40 3.4.1 Profilometry................................................................................................40 3.4.2 X-ray Diffraction (XRD)............................................................................40 vi

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3.4.3 Atomic Force Microscopy (AFM)..............................................................40 3.4.4 Scanning Electron Microscopy (SEM).......................................................40 3.4.5 X-ray Photoelectron Spectroscopy (XPS)..................................................41 3.4.6 Auger Electron Microscopy (AES)............................................................41 3.4.7 Superconducting Quantum Interference Device (SQUID).........................42 3.4.8 High-Resolution Transmission Electron Microscopy (HRTEM)...............42 4 EPITAXIAL STABILIZATION OF SINGLE CRYSTAL ANATASE....................43 4.1 The Use of Oxygen...............................................................................................43 4.1.1 Introduction................................................................................................43 4.1.2 Experimental Procedures............................................................................44 4.1.3 Results and Discussion...............................................................................44 4.2 The Effect of Water Vapor as an Oxidant............................................................51 4.2.1 Introduction................................................................................................51 4.2.2 Experimental Procedures............................................................................52 4.2.3 Results and Discussion...............................................................................52 4.3 Phase Stability of Rutile TiO 2 on Si(100).............................................................60 4.3.1 Introduction................................................................................................61 4.3.2 Experimental Procedures............................................................................62 4.3.3 Results and Discussion...............................................................................63 4.4 Transport Properties of Oxygen-Deficient Anatase.............................................67 4.4.1 Introduction................................................................................................67 4.4.2 Experimental Procedures............................................................................68 4.4.3 Results and Discussion...............................................................................68 5 EPITAXIALLY GROWN ANATASE Co x Ti 1-x O 2 ....................................................74 5.1 Ferromagnetic Semiconducting Properties of Co-doped Anatase Films..............74 5.1.1 Introduction................................................................................................74 5.1.2 Experimental Procedures............................................................................75 5.1.3 Results and Discussion...............................................................................76 6 HALL EFFECT AND MAGNETORESISTANCE...................................................96 6.1 Hall Effect and Magnetoresistance of Undoped and Co-doped Anatase.............96 6.1.1 Introduction................................................................................................96 6.1.2 Experimental Procedures............................................................................97 6.1.3 Results and Discussion...............................................................................97 7 SUMMARY..............................................................................................................106 LIST OF REFERENCES.................................................................................................109 BIOGRAPHICAL SKETCH...........................................................................................114 vii

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LIST OF TABLES Table page 1-1. Quantum number characteristics of TiO 2 .....................................................................2 1-2. Crystal structure of three different polymorphs of TiO 2 ..............................................6 3-1. Units of Measurement................................................................................................34 4-1. Deposition conditions and semiconducting properties of TiO 2 films deposited using water vapor...............................................................................................................71 5-1. Hall effect measurement for Co x Ti 1-x O 2 thin film(x=0.07, 0.02, 0)...........................78 viii

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LIST OF FIGURES Figure page 1-1. Relative energies of the electrons for the various shells and subshells........................2 1-2. Qualitative electron energy-level diagrams for transition metal oxides.......................4 1-3. Crystal diagrams for the two most stable polymorphs of TiO 2 ....................................4 1-4. Unit cell of polymorphs of TiO 2 ...................................................................................5 1-5. Calculated band structures............................................................................................7 1-6. Calculated total and partial DOSs of rutile and anatase..............................................7 1-7. Solubility limits of different transition metal (Tm) ions in anatase and rutile thin films............................................................................................................................9 1-8. Phase relations and stabilities of compounds in the system CoO-TiO 2 .....................10 1-9. Electrical resistivity (curves 1, 2) and response time (curve 3) as a function of dopant concentration at 1250K............................................................................................13 1-10. Conductivity as a function of reciprocal temperature for Nb-doped rutile..............15 2-1. Anatase plan views.....................................................................................................19 2-2. Typical XRD pattern of TiO 2 thin film deposited on LaAlO 3 (001) single substrate.19 2-3. Temperature dependence of the resistivity and Hall coefficient of an anatase single crystal.......................................................................................................................21 2-4. Carrier concentration versus temperature for an anatase crystal and static susceptibility s of anatase and rutile sample...........................................................23 2-5. Room temperature optical absorption and photoconductivity spectra of anatase and rutile films................................................................................................................25 2-6. Photoluminescence spectrum of an anatase film at 4K..............................................26 2-7. Spin Field Effect Transistor (SFET)...........................................................................28 ix

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2-8. Computed values of the Curie temperature Tc for various p-type semiconductors...29 2-9. An M-H curve of an x=0.07 for the Ti 1-x Co x O 2 thin film on SrTiO 3 .........................30 3-1. The geometry in which the Hall effect is most easily measured................................35 3-2. The free path of a charged particle.............................................................................37 3-3. Co-sputtering system..................................................................................................39 4-1. X-ray diffraction patterns for TiO 2 thin films deposited at an Ar pressure of 15mTorr and a oxygen partial pressure of 10 -3 Torr................................................................45 4-2. X-ray diffraction scans for epitaxial anatase films grown in various oxygen pressures at 700C....................................................................................................................46 4-3. X-ray diffraction -2 scans of anatase film grown at 700C in P(O 2 ) = 10 -3 Torr...47 4-4. X-ray diffraction rocking curve through the (004) and -scan through the (204) peak peaks.........................................................................................................................48 4-5. Atomic force microscopy image of anatase film........................................................50 4-6. AES analyis for TiO 2 thin film grown at 700C, PO 2 =15mTorr for 2h.....................50 4-7. Depth profile analysis for TiO 2 thin film grown at 700C, PO 2 =15mTorr for 2h......51 4-8. Deposition rate of TiO 2 films on LaAlO 3 as a function of substrate temperature with H 2 O...........................................................................................................................53 4-9. X-ray diffraction patterns for TiO 2 films deposited at an Ar pressure of 15mTorr and a water vapor partial pressure of 10 -3 Torr on LaAlO 3 ............................................54 4-10. X-ray diffraction patterns for TiO 2 films deposited at an Ar pressure of 15mTorr and a water vapor partial pressure of 10 -3 Torr on Si...............................................54 4-11. X-ray diffraction data for TiO 2 on LaAlO 3 using H 2 O.............................................55 4-12. The scan through the anatase {204} peaks..........................................................56 4-13. The X-ray diffraction patterns for films deposited with 15mTorr Ar (no hydrogen) as a function of water vapor pressure.......................................................................57 4-14. X-ray diffraction patterns for TiO 2 films deposited in the presence of both H 2 O and H 2 ..............................................................................................................................58 4-15. AFM images for TiO 2 film surfaces.........................................................................59 4-16. RMS data for TiO 2 films deposited on LaAlO 3 at various substrate temperatures..60 x

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4-17. Deposition rate of TiO 2 films on Si(100) at different temperature...........................63 4-18. X-ray diffraction patterns and the relative intensity of rutile and anatase for TiO 2 films on LaAlO 3 .......................................................................................................64 4-19. X-ray diffraction patterns and the relative intensity of rutile and anatase for TiO 2 films on LaAlO 3 .......................................................................................................65 4-20. The X-ray diffraction patterns for films deposited with 15mTorr Ar (no hydrogen) as a function of water vapor pressure.......................................................................66 4-21. The X-ray diffraction patterns for TiO 2 films as a function of oxygen partial pressure.....................................................................................................................66 4-22. Hall coefficient and magnetoresistance results for the TiO 2 on Si(100)..................67 4-23. Phase map showing crystalline phases and conductivity behavior as a function of deposition conditions................................................................................................69 4-24. X-ray diffraction patterns for Ti-O film deposited at an Ar pressure of 15mTorr and an oxygen partial pressure of 10 -4 Torr on LaAlO 3 ...................................................70 4-25. Hall data for TiO 2 films grown at 700C in oxygen.................................................70 4-26. X-ray diffraction data for TiO 2 on LaAlO 3 using H 2 O.............................................71 4-27. Atomic force microscope scan of anatase film grown in water vapor at different temperatures.............................................................................................................72 5-1.The X-ray diffraction pattern for Co x Ti 1-x O 2 on LaAlO 3 (001)(x=0.07, 0.02).............77 5-2. Rocking curve and phi-scan for Co 0.07 Ti 0.93 O 2 thin films on LaAlO 3 (001)...............78 5-3. An M-H curve for Co x Ti 1-x O 2 (x=0.07, 0.02) thin films on LaAlO 3 (001) ................79 5-4. XPS spectrum of Co 0.07 Ti 0.93 O 2 on LaAlO 3 (001)......................................................81 5-5. Backscattered image and AES survey for the surface of Co 0.07 Ti 0.93 O 2 on LaAlO 3 (001).............................................................................................................82 5-6. Backscattered image of Co 0.02 Ti 0.98 O 2 on LaAlO 3 (001).............................................83 5-7. BSE image and EDS mapping of Co 0.07 Ti 0.93 O 2 .........................................................84 5-8. BSE image and line scan of Co 0.07 Ti 0.93 O 2 .................................................................85 5-9. A cross-sectional HRTEM image and an enlarged image taken from a Co 0.07 Ti 0.93 O 2 thin film....................................................................................................................88 xi

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5-10. A cross-sectional HRTEM image of a Co 0.07 Ti 0.93 O 2 thin films and a magnified image for surface segregration of a Co 0.07 Ti 0.93 O 2 thin films...................................89 5-11. Selected-area diffraction patterns(SADPs) taken from a Co 0.07 Ti 0.93 O 2 thin films..90 5-12. Selected-area diffraction patterns(SADPs) taken from a Co 0.07 Ti 0.93 O 2 thin films..91 5-13. A lattice image taken from Co 0.07 Ti 0.93 O 2 thin films away from segregated area....92 5-14. HRTEM image taken from secondary phase particle and EDS data for Co 0.07 Ti 0.93 O 2 film....................................................................................................93 5-15. EDS data for Co 0.07 Ti 0.93 O 2 film...............................................................................94 5-16. EDS mapping taken from Co 0.07 Ti 0.93 O 2 anatase thin films.....................................94 5-17. EDS data taken from the cross-sectional area of Co 0.07 Ti 0.93 O 2 anatase..................95 6-1. Schematic view from top of sample showing placement of voltage and current leads. .....................................................................................................97 6-2. The Hall resistivity result for undoped TiO 2 thin films..............................................98 6-3. The Hall resistivity result for Co 0.02 Ti 0.98 O 2thin films............................................99 6-4. The Hall resistivity and anomalous Hall effect........................................................100 6-5. The Hall coefficient and carrier density of Co 0.02 Ti 0.98 O 2......................................100 6-6. Magnetoresistance data for undoped and Co 0.02 Ti 0.98 O 2obtained at 200K............101 6-7. The temperature dependence of xx for the Co 0.02 Ti 0.98 O 2.....................................101 6-8. The magnetic field dependence of Hall resistivity xy (H) for the Co 0.10 Ti 0.90 O 2...102 6-9. Normalized magnetoresistance ( xx (H)/ xx (0)) in zero magnetic field for the Co 0.10 Ti 0.90 O 2films...............................................................................................103 6-10. The temperature dependence of xx (0T) and xy (1T) for the Co 0.10 Ti 0.90 O 2thin films........................................................................................................................104 6-11. An M-H curve for Co 0.10 Ti 0.90 O 2thin films..........................................................104 xii

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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 GROWTH AND FERROMAGNETIC SEMICONDUCTING PROPERTIES OF TITANIUM DIOXIDE THIN FILMS: AN OXIDE-DILUTED MAGNETIC SEMICONDUCTOR (O-DMS) FOR SPINTRONICS By Byoung-Seong Jeong May 2004 Chair: David P. Norton Major Department: Materials Science and Engineering Single-phase (001) anatase thin films have been realized via epitaxial stabilization on (001) LaAlO 3 substrates using reactive sputter deposition. Phase-pure anatase can be achieved using either water vapor or oxygen as the oxidizing species, although crystallinity is slightly degraded for films grown with water vapor. The use of hydrogen during growth to manipulate Ti valence appears possible, although controlling phase formation remains challenging. The RF reactive sputtering method is used to prepare titanium dioxide thin films on Si(100) p-type at temperature ranging from 550 to 750C. For the temperature range examined, only the rutile (200) phase was observed in the range of temperature between 600 and 650C with Ar gas. The intensity of rutile (200) phase was much less with P(O 2 ) than that with P(H 2 O) at a total pressure of 15 mTorr with Ar gas. Hall coefficient and xiii

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magnetoresistance results for the TiO 2 on Si(100) with 10 -3 Torr of a water vapor at 300K shows typical n-type semiconductor behavior at 300K. Epitaxial Co x Ti 1-x O 2 anatase thin films were grown on (001)LaAlO 3 by a reactive RF magnetron co-sputter deposition with water vapor serving as the oxidant. The use of water as the oxygen source proves useful in growing oxygen-deficient, semiconducting Co x Ti 1-x O 2 by reactive sputter deposition, with undoped and Co-doped TiO 2 thin films showing n-type semiconductor behavior, and carrier concentrations of 10 17 to 10 18 cm -3 Magnetization measurements of Co x Ti 1-x O 2 (x=0.07) thin films reveal ferromagnetic behavior in the M-H loop at room temperature with a saturation magnetization on the order of 0.6 Bohr magnetons/Co. X-ray photoemission spectrometry indicates that the Co cations are in the Co 2+ valence state. However, chemical analysis of surface structure indicates that the cobalt segregates into a Co-enriched particles on the surface of films. From SADPs, we confirm that the nanoclusters observed on the surface of the Co 0.07 Ti 0.93 O 2 are Co-enriched anatase. From the resistivity measurement as a function of magnetic field, the ordinary Hall effect is dominant in the undoped and Co 0.02 Ti 0.98 O 2thin films. For each film, the magnetoresistance was positive and increase monotonically with increasing magnetic field. The anomalous Hall effect contribution is observed for the Co 0.10 Ti 0.90 O 2films grown under lower water vapor pressure due to more oxygen deficiency. xiv

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CHAPTER 1 INTRODUCTION In recent years, significant interest has emerged in the synthesis of TiO 2 thin films. Wide band gap semiconducting oxides, such as TiO 2 are useful for a number of applications, including transparent conducting oxides, chemical sensors, and optical components. This chapter will present a general introduction to TiO 2 especially emphasizing the importance of these materials in the field of correlated electron systems. An overview of the transition metal oxide, TiO 2 and three different polymorphs of TiO 2 and electron transport properties of oxygen-deficient anatase is given in Section 1.1 and 1.2, respectively. Transition metal(TM) doping studies will be discussed in Section 1.3. In Chapter 2, a literature review of experimental results for epitaxial growth of anatase, oxygen-deficient anatase, and transition metal-doped anatase and related compounds will be discussed. In Chapter 3, a review of the experimental approaches and techniques and characterization in these studies will be discussed. A presentation of results (such as epitaxial stabilization of single crystal anatase, transport properties of oxygen-deficient anatase, transition metal-doped anatase thin film, and Hall effect and magnetoresistance) is given in Chapter 4, 5, 6, and Chapter 7. 1.1 Transition Metal Oxide, TiO 2 The elements in Groups IIIB through IIB are termed the transition metals, which have partially filled d electron states. Figure 1-1 shows a complete energy-level diagram for the various shells and subshells using a wave-mechanical model. The smaller the principal quantum number, the lower the energy level. 1

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2 f d p d s p s p s s 1 2 3 4 Energy Principal quantum number, n Figure 1-1. Relative energies of the electrons for the various shells and subshells However, there is overlap in the energy of a 3d state in the M shell with an adjacent 4s state in the N shell as shown in Figure 1-1 and Table 1-1. Therefore, the energy of a 3d state is greater than that of a 4s state. As shown in Table 1-1, electrons fill up 4s state first and proceed to the 3d state. Therefore, the element Ti has an electron configuration of 3d 2 and 4s 2 Oxygen is seen in Table 1-1 to have four 2p-electrons in its outermost shell. Two more electrons will bring O 2into the closed-shell configuration. Four electrons are Table 1-1. Quantum number characteristics of TiO 2 Z Element K L M N 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 8 O 2 2 4 22 Ti 2 2 6 2 6 2 2 needed to accomplish the same for two oxygen ions, such as in TiO 2 These four electrons are provided by the titanium(from its 3dand 4s-shells). Thus, in the case of

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3 TiO 2 all involved elements are in the noble gas configuration. Since ionic bonds are involved, any attempted removal of electrons would require a considerable amount of thermal energy. Therefore, TiO 2 is an insulator having a wide band gap. TiO, however, is not an insulator. Since only two titanium valence electrons are needed to fill the 2p-shell of one oxygen ion, two more titanium electrons are free to serve as conduction electrons. Thus, TiO has metallic properties [1]. One of the transition metal oxides, TiO 2 is also termed a d 0 insulators [2]. The d 0 insulators are stoichiometric oxides with a d 0 electron configuration. They are good insulators with quite large band gaps (e.g. 3eV in TiO 2 ), and they show other properties expected of insulators: they show optical transparency at photon energies less than the band gap, and are diamagnetic with paired electrons. Their properties can be understood according to the qualitative energy-level diagram of Figure 1-2. The band gap is between a filled band of bonding orbitals, with a predominantly oxygen 2p atomic character, and an empty metal d band of antibonding orbitals. Many d 0 insulators are susceptible to loss of oxygen, which gives rise to semiconducting properties. Semiconducting properties are, in principle, a feature of all the insulating oxides, but in most cases the band gaps are large enough that there is some difficulty in observing intrinsic semiconduction behavior. In practical terms, semiconduction is characteristic of defective or non-stoichiometric oxides. These include reduced d 0 compounds such as TiO 2-x At a qualitative level, Figure 1-2 (c) shows how an electron introduced by chemical reduction can exist in a defect level with an energy slightly below the empty conduction band, indicating an n-type semiconductor.

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4 Metal d conduction band Energy Band gap Oxygen 2p valence band (a) (b) (c) (d) Figure 1-2. Qualitative electron energy-level diagrams for transition metal oxides. (a) The bands of a d 0 compound, with a gap between the oxygen 2p valence band and the empty metal d conduction band. (b) Localized d levels appropriate to a transition metal impurity. (c) Donor level associated with semiconduction in a non-stoichiometric oxide. (d) Partially filled conduction band of a metallic oxide. 1.2 Three Different Polymorphs of TiO 2 There are three different polymorphs of TiO 2 : rutile, anatase, and brookite. The crystal diagrams and unit cell of two most stable polymorphs of TiO 2 anatase and rutile phases, are shown in Figure 1-3 [3] and 1-4. Figure 1-3. Crystal diagrams for the two most stable polymorphs of TiO 2 (a) Anatase. (b) Rutile. The lattice constants, the Ti-O bond length, and the O-Ti-O bond angles for the three phases are summarized in Table 1-2. Rutile and anatase phases are both tetragonal, containing six and 12 atoms per unit cell, respectively. In both structures, each Ti +4 cation

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5 is coordinated to six O 2anions; and each O 2anion is coordinated to three Ti +4 cations. In each case, the TiO 6 octahedron is slightly distorted, with two Ti-O bonds slightly greater than the other four, and with some of the O-Ti-O bond angles deviating from 90. The distortion is greater in the anatase than in the rutile phase. Figure 1-4. Unit cell of polymorphs of TiO 2 (a) Anatase. (b) Rutile. Anatase and rutile phases have the same symmetry(tetragonal 4/m 2/m 2/m) despite having different structures. In rutile phase, the structure is based on octahedrons of titanium oxide that share two edges of the octahedron with other octahedrons and form chains. It is the chains themselves that are arranged into a four-fold symmetry. In anatase phase, the octahedrons share four edges hence the four-fold axis. Rutile is thermodynamically stable phase at high temperatures, and is the most widely studied. In polycrystalline films, rutile is the more common phase observed. Anatase is a metastable polymorph that can be realized in polycrystalline films deposited at low temperatures. Despite being metastable, single crystals of anatase have been realized via chemical transport reaction processes. Undoped anatase is an anisotropic, tetragonal insulator (a=3.78 c=9.52 ) with a bandgap of 3.2 eV. The static dielectric constant of anatase is reported as 31 [4-5]. Limited work has been done on brookite, which is orthorhombic (b) (a) O O Ti Ti [001] [010] [TiO 6 ] 8 a[100] [100]

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6 with 2/m 2/m 2/m symmetry. This is probably because of the difficulty in the calculation due to the larger number of atoms in the unit cells of these two phases. Table 1-2. Crystal structure of three different polymorphs of TiO 2 Rutile Anatase Brookite Crystal structure Tetragonal Tetragonal Orthorhombic Lattice constants() a = 4.59 a = 3.78 a = 9.18 c = 2.96 c = 9.52 b = 5.45 c = 5.15 Space group P4 2 /mnm I4 1 /amd Pbca Molecule/cell 2 4 8 Volume/molecule( 3 ) 31.22 34.06 32.17 Density(g/cm 3 ) 4.13 3.79 3.99 Ti-O bond length() 1.95(4) 1.94(4) 1.87 ~ 2.04 1.98(2) 1.97(2) O-Ti-O bond angle 81.2 77.7 77.0 ~ 105 90.0 92.6 The calculated band structures and the density of states (DOS) of the TiO 2 perfect crystals are shown in Figure 1-5 and 1-6 [5-6]. For rutile, the lowest-band gap is located at a point with value of 1.78eV. This band gap energy is much smaller than that experimentally observed(3.0eV) due to the well-known shortcoming of the local density approximation (LDA) which generally underestimates the experimental band gap for insulators and semiconductors. For rutile, as shown in Figure 1-6(a), the upper valence band is composed of O 2p orbitals and has a width of 6.22. The lowest conduction band consists of two sets of Ti 3d bands and has a width of 5.9eV. These two sets of bands have their atomic origin from the hybridized states of t 2g and e g Double-peak structure between the two major features in the valence band has its origin in the separation between the nonbonding and bonding O 2p states. A similar two-peak feature is also found in the CB due to crystal-field splitting of the Ti 3d band states. There is also a substantial degree of hybridization between O 2p and Ti 3d in both the CB and VB regions, indicating

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7 strong interactions between Ti and O atoms in rutile. It means that excitation across the band gap involves both O 2p and Ti 3d states. Figure 1-5. Calculated band structures. (a) Rutile. (b) Anatase. Figure 1-6. Calculated total and partial DOSs of rutile and anatase: (a) Total. (b) O2s. (c) O2p. (d) Ti3d.

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8 In contrast, the calculated minimal band gap for anatase is 2.04eV showing a direct-gap insulator. The reported experimental gap value of 3.2eV for anatase is 0.2eV larger than that of rutile. The DOS of anatase is compared in Figure 6(b) with that of rutile. The only difference appears to be that the double-peak feature is less distinct. 1.3 Transition Metal(TM)-Doping Effect TiO 2 thin films have been widely studied for many applications, such as photocatalytic properties of water and environmental pollutants and fabrication of solar cells. Transparent dielectric TiO 2 thin films with high reflection indices have excellent antireflective properties for thermal protection coating and selectively reflecting layers for building glass. Titanium dioxide is also one candidate for thin-film capacitors because of its high dielectric constant. In undoped materials, a wide band gap transition metal oxide, TiO 2 shows the nature of oxygen-deficient when it is nonstoichiometric. 2x-22O2x TiO TiO (1-1) According to the literature studies, the following reaction may be proposed : (1-2) ikkiTi0Ti O Ti 2O2 where k = 3, 4, x = 2[ ]Tiki (1-3) ijej00V O where j=1,2 ; x = [ ]Vj0 O 0 = CSP + TiO 2-x + 22Ox (1-4) Where CSP = crystallographic shear planes.

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9 To improve the sunlight efficiency in photodecomposition of water and to extend the light absorption spectrum into the visible region in solar cells, it is necessary to modify the properties of TiO 2 thin-film doping with other impurities. Numerous efforts have been focused on the surface structure, electronic and chemisorption properties of pure TiO 2 showing that surface area, band gap, porosity and surface defects strongly affect the photocatalytic activity. It has been reported that the addition of a small amount of WO 3 or MoO 3 powder to TiO 2 samples considerably improved the photocatalytic activity of TiO 2 Adding small quantities of Nb 2 O 5 to TiO 2 significantly increased photocatalytic activity. These studies indicate that photocatalytic activity may be enhanced by adding impurity atoms. Based on the XRD measurement, the solubility limits of different transition metal ions in anatase and rutile TiO 2 thin films are compared in Figure 1-7 [7]. Figure 1-7. Solubility limits of different transition metal (Tm) ions in anatase and rutile thin films

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10 It is noteworthy that the solubility limits of the dopants vary significantly depending on the 3d ion species and on the involved crystal structure of TiO 2 (anatase or rutile). For example, V and Fe dopants, which are well known to form bulk solid solution alloys in high concentrations in the rutile phase, are not so soluble in the anatase phase. However, Co is much more soluble in anatase phase than in rutile; Ni is not soluble in either phase. Because of the longer non-equilibrium state in thin film process, the tendency of solubility limits is different from that of bulk, and it is possible to make highly doped films that are not obtainable under equilibrium conditions. 1.3.1 Cobalt(Co) Figure 1-8 shows the phase relations and stabilities of compounds in the system CoO-TiO 2 Figure 1-8. Phase relations and stabilities of compounds in the system CoO-TiO 2 Phase relations in the system CoO-TiO 2 (Figure 1-8) [8] in the temperature range 1130 to 1569C was determined by the quenching technique. Three crystalline phases(Co 2 TiO 4

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11 CoTiO 3 and CoTi 2 O 5 ) are stable in equilibrium. The phase CoTi 2 O 5 is unstable relative to the phase assemblage CoTiO 3 + TiO 2 below 1140C. In the thin film growth of Co-doped TiO 2 phase, we can not exclude the formation of CoTi 2 O 5 known to be unstable below 1140C in the above bulk Co-Ti-O phase diagram, because of the quasi-equilibrium nature of sputtering. Recently, ferromagnetism was observed at room temperature in Co-doped semiconducting anatase thin films [9]. To achieve ferromagnetic properties, as in other DMS materials, the Co ions must be magnetically aligned by some means. To determine the mechanism of magnetic coupling, we must know the formal charge of the Co ions and the majority carrier type in the material. Chamber et al. [3, 10] reported that the oxidation state of Co in the lattice is +2. They also revealed that the local structural environment for Co in anatase is very similar to that of Co in CoTiO 3 (highly distorted octahedral coordination with oxygen ligands). In the structural similarities between the cation site in anatase and the Co site in CoTiO 3 this result corroborates the XRD-based conclusion that Co substitutes for Ti in the anatase lattice. The presence of substitutional Co at cation sites in the TiO 2 lattice requires an equal number of oxygen vacancies to maintain overall electrical neutrality. This fact can be understood by recognizing that substituting Co +2 for Ti +4 is equivalent to reducing the formal charge of a cation from +4 to +2. Doing so requires the removal of one O -2 for each Co to achieve charge balance. The resulting oxygen vacancies are expected to be electrically neutral. Chamber et al. also reported that nonmagnetic Co x Ti 1-x O 2 films are found to be insulating. Pure, bulk TiO 2 is n-type by virtue of charged oxygen vacancies, which create shallow electron donor levels in the band gap. Therefore, Co impurities in the lattice are

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12 locally compensated by electrically inactive oxygen vacancies. Additional oxygen vacancies not associated with Co sites, if present, are charged and produce the n-type behavior. These experiments suggest that Co impurities are magnetically coupled by electrons in the conduction band, resulting in ferromagnetism via an electron-mediated exchange interaction. 1.3.2 Chromium(Cr) Several studies on Cr-doped TiO 2 thin films have focused on metal oxide gas sensors. Electrical conduction has been studied in pure rutile and in doped samples. The resistivity for these materials changes upon exposure to oxygen gas. Chromium has been used to improve the sensitivity of these semiconductor gas sensors. Doping with trivalent chromium has been chosen because such a cation does not introduce crystallographic shear planes within the range of 5 mol. % Cr 2 O 3 yielding a solid solution. The cation Cr 3+ having an ionic radius of 0.63, similar to Ti 4+ (0.68), may be substituted for titanium. The trivalent Cr acts as an acceptor type impurity which can be expressed as shown in Equation *00Ti323OV2CrOCr (1-5) where V 0 represents oxygen vacancies and Cr Ti is Cr substitution in Ti sites. Doping with suitable cations was proven to have the shortest route by altering electronic and catalytic properties for gas interaction at the interface, improving response time. It has been reported that the concentration of oxygen vacancies increases in the TiO 2 lattice by addition of these trivalent dopants. Bernasik et al. [11] measured the electrical resistivity of the films by a DC four-point method for the undoped TiO 2 Crand Nb-doped TiO 2 thin films. Even though the measurement temperature was considered high(1250K), they showed electrical

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13 conductivity results as a function of dopants concentration. As the concentration of Nb ions increases, the resistivity of the films decreases. However, Cr increases the resistivity up to 0.4 at% and then decreases at concentration above 0.8at%, indicating nto p-type transition. They assumed that Cr ions act as an acceptor in the TiO 2 thin film, forming substitutional solid solution. Some interesting results were shown at curve 3 on Figure 1-9, which is the time response determined at 95% of the new equilibrium resistivity value at 1250K. Figure 1-9. Electrical resistivity (curves 1, 2) and response time (curve 3) as a function of dopant concentration at 1250K Response time is faster for undoped and Cr-doped samples than for Nb-doped samples. This difference in re-equilibration kinetics was thought to be a result of the different motion of the predominant lattice defect : fast diffusivity of oxygen vacancies in the case of undoped and Cr-doped TiO 2 and low mobility of cation vacancies in Nb-doped samples. Zakrzewska et al. [12] showed that no secondary phases resulted from doping up to 4 at. % Cr and 6 at.% Nb. This is due to formation of a substitutional solid solution. Yongxiang Li et al. [13] reported on the XPS scan spectra of Cr-doped TiO 2 thin film.

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14 The Ti 2p and Cr 2p spectra indicated the chemical states of Ti and Cr to be Ti 4+ and Cr 3+ respectively. 1.3.3 Other Impurities Most previous studies about impurities doped into titanium dioxide were limited to rutile structure. The rutile structure of TiO 2 is an anisotropic insulator with a band gap of 3eV. It is a tetragonal crystal with a dielectric constant of 170 in the c-direction and 86 in the a-direction at room temperature. Even though a pure rutile structure is an insulator with resistivity less than 10 13 -cm 3 it is easily doped with many different impurities up to 10 19 /cm 3 of carrier concentration. Some of them show n-type semiconducting behavior. So far it has not been possible to produce p-type semiconductor material at room temperature. The electron mobility at room temperature is in the 1 cm 2 /Vs range. Many cation impurities(Nb, Ta, V, W, Cr, Mn, Fe, and Mo) occupy substitutional sites under oxidizing conditions. Iron and aluminum, which has smaller ion diameters, may diffuse as interstitials under reducing conditions. Aluminum and other divalent and trivalent impurities act as acceptors, with electron-trapping levels below the middle of gap. Li and H, however, occupy only the interstitial, acting as donor impurities. Mo is known to exist in 3+, 4+, 5+, and 6+ states. Nb is also known have 4+ and 5+ states. These multiple charge states are observed because the large dielectric constant of TiO 2 reduces the energy difference between charge states of typical impurities relative to that of the free ion, so that more than one such state appears in the gap of a given impurity. A main problem to be solved in many electrical applications of rutile is impurities and lattice defects which tend to migrate in the presence of electric field, resulting in unstable

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15 electronic properties. Hydrogen is on example. Therefore, to obtain reproducible electronic measurements on TiO 2 such mobile species must be eliminated. Deford et al. [14] suggested that Ta and Nb, which are substituted to the Ti site as donor impurities with very shallow trapping levels, were known to diffuse slowly at 1300C. One can assume that little motion of these dopants occurs, in the presence of an electric field at room temperature. Figure 1-10. Conductivity as a function of reciprocal temperature for Nb-doped rutile, oriented so that electric field and current are in the a direction. The solid line is a linear fit to the data. Figure 1-10 shows that in the temperature range from 12 to 28K, the conductivity is accurately given by kTE0lnln (1-5) where 0 is independent of temperature. Moreover, the high Fermi level by doping with Ta or Nb dramatically reduces the equilibrium concentration of donor lattice defects and interstitial H donors, entirely eliminating almost all mobile defects and impurities. Hence, Ta or Nb-doped material is considered the obvious choice for manipulating electron transport properties. Deford et al. [14] also reported that these materials may be

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16 reversibly doped with H without introducing measurable concentrations of lattice defects. As expected, H doping introduces unstable electronic properties, but in Nbor Ta-doped material, the change was relatively small, which was useful information.

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CHAPTER 2 PREVIOUS WORKS IN UNDOPED AND TRANSITION METAL DOPED ANATASE This chapter provides a literature review of the epitaxial growth, and transport properties of oxygen-deficient, and transition metal doped anatase titanium dioxide and related compounds, and their potential use for spintronics. 2.1 Epitaxial Growth of Anatase TiO 2 There is significant interest in the synthesis of phase-pure anatase thin films, both for applications and fundamental studies. In catalysis, photocatalysis, and dye-sensitized solar cells, anatase has proven advantageous over the rutile phase [15-17]. Electronic properties, surface structure, and morphology are key elements that must be controlled in these applications. Numerous efforts have focused on the formation of anatase films that are phase pure and highly crystalline. In general, epitaxial stabilization offers a means by which anatase thin films can be obtained on lattice-matched substrates, often for processing conditions where the phase is thermodynamically unstable in bulk. Understanding how growth conditions impact phase development, film morphology, and crystallinity is important in exploring these applications. For many applications, even though rutile phase represents the stable phase at high temperatures and is the easiest to realize as phase-pure crystals or thin films, anatase phase displays interesting properties and performance. For applications dependent on the semiconducting properties of anatase, carrier transport will depend critically on crystallinity. Grain boundaries and/or secondary phase boundaries are unacceptable for optimizing mobility and conductivity. As a low-temperature polymorph, anatase thin 17

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18 films are typically realized via deposition at low temperature where crystallinity is not optimal. In most cases, polycrystalline TiO 2 films possess either a mixture of rutile and anatase, or solely rutile [18]. For fundamental studies of material properties, the synthesis of single crystal, phase-pure anatase is highly desirable. Recent efforts have shown that phase-pure anatase thin films can be realized via epitaxial stabilization on single crystal substrates [19]. There are some reports on epitaxial anatase thin films deposited on SrTiO 3 (001) substrate by metalorganic chemical vapor deposition, or molecular-beam epitaxy(MBE) [20-21]. The considerable lattice mismatch between SrTiO 3 and anatase (lattice mismatch(f) : -3.1%), however, makes it difficult to obtain high-quality epitaxial films. Furthermore, optical properties of these anatase thin films were difficult to evaluate because of the very close band gap energies of 3.2 eV for both anatase and SrTiO 3 Therefore, another susbstrate to be considered is LaAlO 3 (001). Although the crystal structure of LaAlO 3 is rhombohedral, it can be regarded as a perovskite having displaced O ions. In terms of a pseudocubic cell, the unit cell parameter is 0.379 nm, which is close to the a-axis length of anatase (0.3748 nm). Chamber et al. showed schematic plan views of LaAlO 3 (001) and TiO 2 (001) anatase (Figure 2-1). Anatase displays a cubic surface mesh along the (001) orientation. The perovskites SrTiO 3 and LaAlO 3 are a=0.390 and 0.379 nm respectively. Therefore, the lattice mismatch [(a anatase -a LaAlO3 )/a LaAlO3 ] is -0.26%. Murakami et al. [22] reported the crystallinity and optical properties of anatase thin films fabricated on LaAlO 3 (001) substrates by using a laser MBE. The XRD spectrum of the TiO 2 film on LaAlO 3 (001) substrate in Figure 2-2 shows (004) peaks of anatase without any impurity phase.

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19 Figure 2-1. Anatase plan views. (a) LaAlO 3 (001). (b) TiO 2 (001). The full width at half maximum of the (004) peak rocking curve is 0.11, while that of anatase film on SrTiO 3 (001) substrates under the same condition is larger than 0.6. Consequently, the lattice mismatch affects the crystal quality of the film. Figure 2-2. Typical XRD pattern of TiO 2 thin film deposited on LaAlO 3 (001) single substrate. Rocking curve at anatase (004) is 0.11. A key experiment used LaAlO 3 (001) substrate, which has a wider energy gap and smaller lattice mismatch(f:-0.2%) with anatase than SrTiO 3 an atomically flat substrate surface, and optimization of the laser MBE conditions with in situ monitoring of the

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20 growth process by reflection high energy electron diffraction (RHEED). Chen et al. prepared epitaxial TiO 2 films by ultra high vacuum metalorgainc chemical vapor deposition growth and in situ analyses of epitaxial titanium dioxide thin films prepared by the thermal decomposition of titanium(IV) isoropoxide, Ti(O-i-Pr) 4 on various single crystal oxide substrates. They used SrTiO 3 (001) and Al 2 O 3 (0001) as substrates. As it is known, the lattice mismatch between TiO 2 and SrTiO 3 (f:-3.1%) is much higher than that between TiO 2 and LaAlO 3 A full width half maximum(FWHM) value from a rocking curve scan of (004) anatase peak was 1.47. This value is high. Even though there were no peaks contributable to the rutile structure, the FWHM value of high crystalline epitaxial films should be much lower than this value. The mechanism that caused the preferential growth of the anatase-type TiO 2 over the rutile-type TiO 2 has not yet been clarified. Recent work on epitaxially-stabilized anatase has focused on pulsed-laser deposition or molecular beam epitaxy. While yielding promising results, neither technique is attractive for application. For this reason, the synthesis of high-quality anatase using deposition techniques that are amenable to large-scale synthesis is highly desirable. 2.2 Transport Properties of Oxygen-Deficient TiO 2 Thin Films The electronic transport properties of transition metal oxides is a topic of long-standing interest. However, the well-known difficulty of preparing large single crystals of transition metal oxides has frequently precluded definitive determination of their electronic transport properties. Among the three crystalline forms of TiO 2 the rutile phase is the most widely investigated; mainly because of the ease in synthesizing large single crystals of this phase. In contrast to the extensive studies on the rutile phase, very

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21 little is known about the other two less stable phases. The electronic properties of anatase are often assumed to be similar to those of rutile. As with rutile, anatase can be made an n-type semiconductor with a carrier density, n, on the order of 10 19 /cm 3 via cation substitution or by Ti interstitials. Hall mobility has been measured as high as 20cm 2 /Vs with the effective mass of anatase, m =m e These properties for anatase yield attractive material performance in photocatalysis and dye-sensitized photovoltaics. For applications dependent on the semiconducting properties of anatase, carrier transport will depend critically on crystallinity. (a) (b) Figure 2-3. Temperature dependence of the resistivity and Hall coefficient of an anatase single crystal. (a) Temperature vs resistivity for as-grown anatase single crystal. (b) Temperature vs Hall coefficient. The carrier density calculated from the Hall coefficient is plotted vs reciprocal temperature in the inset. The carrier density is thermally activated in the whole temperature range with an activation temperature of 50K. The recent interest in anatase was motivated by its key role in the injection process in a photochemical solar cell with a high conversion efficiency. The recent success in growing large single crystals of anatase allowed the study of the bulk electronic properties of this lower temperature phase of TiO 2

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22 L. Forro et al. [23] reported on the dc resistivity, thermoelectric power, and Hall mobility measured on large and high purity crystals from 10 to 300K. These measurements were taken by four-probe dc resistivity measurements with contacts produced by ultrasonic indium soldering. As shown in Figure 2-3(a), the two temperature-dependent regimes of anatases resistivity are similar to those of a conventional semiconductor. At low temperatures, below 50-70K, the resistivity falls with increasing temperature. However, at higher temperatures, the resistivity rises with increasing temperature. At higher temperatures (above 50-70K), the carrier concentration saturates (the exhaustion regime), the fall of the mobility of conducting electrons with increasing temperature provides the dominant temperature dependence of the resistivity. The Hall coefficient was found to be negative, indicating n-type semiconductor, and decrease with increasing temperature as shown in Figure 2-3(b). The density of n-type conducting carriers rises with increasing temperature to a maximum value of 10 18 cm -3 The activation energy for carrier generation is 4.210 -3 eV. Thus, the donors appear to be very shallow. They revealed a very shallow donor level and a high n-type mobility in anatase crystals. The n-type doping of anatase is due to oxygen off-stoichiometry. They showed the room temperature Hall mobility of the electrons is high in comparison with other transition metal oxides. Both static and magnetic susceptibility measurements of anatase single crystals was reported by O. Chauvet et al. [24] as shown Figure 2-4. The static susceptibility reveals the electrons excited into the conduction band. They showed a very shallow donor level located 67K below the conduction band with a donor concentration 10 17 cm -3 However,

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23 the precise nature of the donor center was unknown. The static susceptibility was dominated by Van Vleck paramagnetism above 100K. H. Tang et al. [5] have studied photoluminescence in the same anatase crystals. Band gap exicitation of pure and Al-doped TiO 2 anatase crystals results in visible broad band luminescence which is interpreted as the emission from the self-trapped excitons localized on TiO 6 octahedra. With increasing temperature up to room temperature, the photoluminescence spectra of the undoped crystal shift to higher energies, while the spectra of the crystal doped with Al shift to lower energies. The influence of the Al dopant centers was understood as an extrinsic self-trapping effect. The luminescence in TiO 2 anatase was found to be essentially different from that in TiO 2 rutile because of significant structural differences. Luminescence due to the recombination of self-trapped excitons is observed in anatase crystals, but not in rutile crystals. Optical investigations have shown that the band gap is slighltly wider in anatase than in rutile. (b) (a) Figure 2-4. Carrier concentration versus temperature for an anatase crystal (a) and static susceptibility s of anatase and rutile sample (b): (a) The solid line is the fit with equation (1) of the text. M. Radecka et al. [25] reported annealing effect for a variety of optical properties for nonstoichiometric TiO 2-x thin films where x varied from 0.08 to -0.2 grown by

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24 plasma-emission-controlled DC magnetron and conventional RF reactive sputtering. All as-sputtered films were amorphous independently of x except annealed sample at 1173K, which is rutile structure. They found the electrical conductivity the refractive index n, and the absorption coefficient were correlated with the deviation from stoichiometry x. So far, several experimental results have showed that a very shallow donor level and a high n-type mobility exist in anatase crystals. The n-type doping of anatase is due to oxygen off-stoichiometry. These properties have been obtained from different processes such as molecular beam epitaxy or metal-organic chemical vapor deposition. With sputtering system, its expected to be easier to control semiconductor anatase film properties with the use of water vapor and hydrogen in depositing the thin-film materials. This is useful in manipulating the valence of cations in the depositing films. The presence of hydrogen during deposition can result in oxygen deficiency and semiconducting behavior. The sputtering method is amenable to large-scale synthesis. 2.3 Optical Properties of Anatase and Rutile The optical absorption near the absorption edge was derived from the transmittance(T) and reflectivity(R) measured on the anatase and rutile film, using the following equation, ]1[])1[(222ddeReRT The absorption coefficient above the threshold of fundamental absorption follows the (E-E g ) 2 energy dependence characteristic of indirect allowed transitions, as shown by the 1/2 versus photon energy(E) plot in Figure 10(a). The extrapolated optical absorption gaps of anatase and rutile films are found to be 3.2 and 3.0eV, respectively, at room temperature.

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25 Figure 2-5 shows the photoconductivity spectra of anatase and rutile films. The photoconductivity threshold energy of the anatase film is higher than that of the rutile film. The threshold energies approximately match with the optical band gap energies. The energy band structure of rutile has been extensively studied. TiO 2 rutile has a direct forbidden gap (3.0eV), which is almost degenerate with an indirect allowed transition (3.05eV). Due to the weak strength of the direct forbidden transition, the indirect allowed transition dominates in the optical absorption just above the absorption edge. (b) (a) Figure 2-5. Room temperature optical absorption (a) and photoconductivity spectra of anatase and rutile films (b) However, there are few experimental results on the band structure of anatase. The fundamental absorption edge of anatase crystals was determined at 3.2eV at room temperature and at 3.3eV at 4K. Figure 2-6 shows the photoluminescence spectrum of an undoped anatase film. The luminescence is a visible emission (green-yellow) with a broad spectral width. The spectrum resembles the luminescence spectrum of anatase single crystals which has been attributed to the radiative recombination of self-trapped excitons(STE).

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26 Figure 2-6. Photoluminescence spectrum of an anatase film at 4K (b) The STE is supposed to be localized on a TiO 6 octahedron which is the unit structure of anatase TiO 2 Therefore the origin of the luminescence is unlikely to be related to oxygen vacancies or impurities. As in anatase crystals, the visible emission in the film is from self-trapped excitons. The self-trapping of excitons causes loss of exciton energy through lattice relaxation. As a result, the luminescence band maximum (at 2.4eV) shifts far below the energy gap (at 3.3eV). The emission band undergoes a Stokes shift of 0.9eV. The Stokes shift is determined as the difference between the emission peak energy and the optical absorption edge. Such Stokes-shifted broad emission bands can be explained as resulting from the fundamental Ti 4+ to O 2charge transfer transition localized on the TiO 6 octahedron. In rutile thin films as well as in crystals, such visible emission of self-trapped excitons is not observed. Instead, a sharp free exciton emission characteristic of the dipole-forbidden direct transition (at 3.0eV) is observed in high quality rutile crystals at very low temperature. 2.4 Transition Metal-Doped Anatase and Related Compounds As device shrinkage in size continues to achieve higher speeds, alternative concepts are being investigated that would reduce device size and power consumption. One is the

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27 spin field effect transistor (SFET) which is operated based on the spin of electrons, holes, nuclei to gain functionality in electronic devices [26-27]. A spin FET has several advantages over a conventional FET. Flipping an electrons spin takes much less energy and can be done much faster than pushing an electron out of the channel [28]. This new innovative device can not be realized without the ability to generate, preserve, and transmit the long-lived a well-defined spin state in semiconductor. To combine spin and charge, or use spin to achieve new functionality, its highly desirable to be able to inject spins into semiconductor heterostructures electrically. Figure 2-7. shows a proposed design of a SFET( spin field-effect transistor). SFET has a source and a drain, separated by a narrow semiconducting channel, the same as in a conventional FET. In the spin FET, both the source and the drain are ferromagnetic. The source sends spin-polarized electrons into the channel, and this spin current flows easily if it reaches the drain unaltered (top). A voltage applied to the gate electrode produces an electric field in the channel, which causes the spins of fast-moving electrons to rotate(bottom). The drain impedes the spin current according to how far the spins have been rotated. Spintronic devices rely on differences in the transport of spin up and spin down electrons. In a ferromagnet, such as iron or cobalt, the spins of certain electrons on neighboring atoms tend to line up. In a strongly magnetized piece of iron, this alignment extends throughout much of the metal. When a current passes through the ferromagnet, electrons of one spin direction tend to be obstructed. The result is a spin-polarized current in which all the electron spins point in the other direction.

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28 Figure 2-7. Spin Field Effect Transistor (SFET) Fiederling et al. [29] and Jonker et al. [30] developed diluted magnetic semiconductor(DMS), Mn-doped ZnSe as a spin injector for a GaAs/AlGaAs quantum well. A strong electron spin population imbalance in the MnZnSe conduction band, induced by an external magnetic field at 4.2K, leads to circularly polarized light emission from the GaAs quantum well when recombination with holes from the p-AlGaAs layer occurs in the well. Spin injection efficiencies, namely, light polarization of at least 50%, have been achieved. However, the polarization effects quickly disappeared above 4.2K because the Mn-doped ZnSe is a paramagnet and the magnetization depends on temperature very strongly. It is, therefore, necessary to develop spin injectors that can function at and above room temperature for spintronics technology to become a reality. It is attractive to use ferromagnetic metals (FM) as spin injectors. Their major advantage is a Curie temperature which is much higher than those of most DMSs. However, spin injection efficiencies for FMs are noticeably lower than for DMSs. This poor efficiency causes a decrease in conductivity in passing from the FMs into a semiconductor so that the large discrepancy in spin polarization in the FMs would be realized. This results in an extremely low(<1%) spin injection efficiencies [31-33]. However, it has been reported

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29 that a tunnel junction between a FMs and a semiconductor may increase the spin injection efficiency by decoupling metal and semiconductor states at the interface, removing conductivity mismatch [34]. A spin injection efficiency of ~30% has been measured for a spin-LED structure(GaAs/AlGaAs) with epitaxial Fe, rather than MnZnSe, as the spin injector. Even though it was not as high as that measured for a DMS injector, an injection efficiency of 30% is an important result. The goals of spin injector materials research are room temperature operation and high injection efficiencies. The most promising materials are DMS materials with a Curie temperature above 300K that are crystallographically well-matched to important nonmagnetic semiconductors such as Si and III-Vs. Dielt et al. [35] reported that hole-mediated exchange interaction could induce ferromagnetic behavior. Figure 2-8. shows the computed values of the Curie temperature Tc for various p-type semiconductors. 0100200300400 0100200300400 AlPAlAsGaNGaPGaAsGaSbInPInAsZnOZnSeZnTeSiSiGe Curie Temperature(K) Figure 2-8. Computed values of the Curie temperature Tc for various p-type semiconductors containing 5% of Mn and 3.5 20 holes per cm 3 Two p-type semiconductors, GaN and ZnO, were predicted to be ferromagnetic at temperature above 300K. Based on this prediction, several experimental results have

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30 been reported. It has been shown that Co x Zn 1-x O films grown on sapphire(1120) by PLD are ferromagnetic with Curie temperature of up to 280K [36]. Recently, Matsumoto et al. [9] have reported that Co doped TiO 2 (Co x Ti 1-x O 2 ) in the anatase form, grown by PLD on SrTiO 3 (001) and LaAlO 3 (001), is ferromagnetic above 400K. The magnetization was found to increase with increasing Co concentration( Co doping level(x) :0.01~0.08). Figure 2-9 shows the M-H curve measured for a film with x=0.07 film on SrTiO 3 taken at room temperature. Figure 2-9. An M-H curve of an x=0.07 for the Ti 1-x Co x O 2 thin film on SrTiO 3 taken at room temperature. Magnetic field was applied parallel to the film surface. Hysteresis is observed, indicating that the Co-doped anatase film is ferromagnetic even at room temperature. Even though the Curie temperature was found to be at least 400K, the magnetic moment was low (~0.25 B per Co ion at saturation). The remanence at zero external magnetic field was also low(~3%). Chambers et al. [10] have grown thin films of Co x Ti 1-x O 2 on SrTiO 3 (001) and LaAlO 3 (001) by oxygen plasma assisted molecular beam epitaxy(OPA-MBE). They found ferromagnetism and report evidence that the magnetic Co impurities substitute for

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31 Ti ions and reside on the cation sites in anatase. They showed that magnetic behavior coincides with the free electrons produced by oxygen vacancies resulting from the growth conditions. Co substitution for Ti in the anatase lattice results in oxygen vacancies to maintain charge neutrality. The magnetic moment per Co ion measured from SQUID was ~1.25 B per Co ion, 5 times larger than that measured for PLD-grown Co x Ti 1-x O 2 The Curie temperature was above 500K. Additionally, the remanence is ~30%. The magnetization saturates at a much lower field value, indicating that domain-pinning defect shown by the Matsumoto group are probably not present in the films grown by MBE. Such a high value would enable the use of this materials as an ambient temperature spin injector. However, the magnetic properties of Co-doped anatase film are still controversial. In particular, the relationship between Co clusters (segregation) and ferromagnetism can not be ruled out. In recent years, several reports have focused on pulsed laser deposition and molecular beam epitaxy. Even though these efforts have yielded promising results, PLD and MBE are not attractive for applications. For this reason, the synthesis of high-quality anatase using a deposition technique that is amenable to large-scale synthesis is highly desirable. In order to understand the origin of ferromagnetism and the magnetic behavior depending on growth parameters, the observation of microstructure is very critical.

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CHAPTER 3 EXPERIMENTAL APPROACH AND TECHNIQUES This chapter discusses the experimental approach and techniques for the growth of epitaxially stabilized anatase, transport measurements, and transition metal doping techniques using a co-sputtering method. 3.1 The Growth of Epitaxially Stabilized Anatase The film growth experiments were performed in a reactive RF magnetron sputter deposition system equipped with a load-lock for substrate exchange. A quartz lamp heater provided for substrate heating up to 750C. Two-inch diameter Ti sputtering targets were used. The target-to-substrate distance was approximately 15 cm. The base pressure of the deposition system was on the order of 5x10 -8 Torr. For epitaxially stabilized anatase film growth, (001) LaAlO 3 was chosen as the substrate as it provides a lattice mismatch on the order of 0.2%. For comparison, Si substrates were also used so as to produce polycrystalline films under the same conditions. The substrates were cleaned in trichloroethylene, acetone, and ethanol prior to mounting on the sample platen with Ag paint. The oxygen was provided through a mass flow control valve. A water source was created by freezing and evacuating a water-filled stainless steel cylinder that was attached to the deposition chamber via a leak valve. Upon returning to room temperature, the vapor pressure of water is sufficient to provide an H 2 O source. The use of water vapor and hydrogen in depositing oxide materials is useful in manipulating the valence of cations in the depositing film. The presence of hydrogen during deposition can result in oxygen deficiency and semiconducting behavior. 32

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33 3.2 Transport Measurement Techniques 3.2.1 Hall Effect Measurement 3.2.1.1 van der Pauw Method This test method requires a test specimen that is homogeneous in thickness, but of arbitrary shape. The contacts must be sufficiently small and located at the periphery of the specimen. The measurement is most easily interpreted for an isotropic semiconductor whose conduction is dominated by a single type of carrier [37]. 3.2.1.2 Definitions Hall coefficient: the ratio of the Hall electric field (due to the Hall voltage) to the product of the current density and the magnetic flux density Hall mobility: the ratio of the magnitude of the Hall coefficient to the resistivity ; its readily interpreted only in a system with carriers of one charge type. Resistivity: the ratio of the potential gradient parallel to the current in the material to the current density. For the purpose of this method, the resistivity shall always be determined for the case of zero magnetic flux. Units: in these test methods SI units are not always used. For these test methods, its convenient to measure length in centimeters and to measure magnetic flux density in gauss. This choice of units requires that magnetic flux density be expressed in Vscm -2 where : 1Vscm -2 =10 8 gauss (3-1) The units employed and the factors relating them are summarized in Table 3-1 [38]. The motion of free electrons in metals and semiconductors can be influenced significantly by external magnetic fields. Their effect is most rigorously treated by introducing magnetic forces into the Boltzmann transport equation via the laws of classical electromagnetism. The most important of these is the Hall effect, which is most easily studied in the geometry shown in Figure 3-1.

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34 Table 3-1. Units of Measurement Quantity Symbol SI unit Factor Units of Measurement Resistivity m 10 2 cm Charge carrier concentration n, p m -3 10 -6 cm -3 Charge e, q C 1 C Drift mobility, Hall mobility H m 2 V -1 s -1 10 4 cm 2 V -1 s -1 Hall coefficient R H m 3 C -1 10 6 cm 3 C -1 Electric field E Vm -1 10 -2 Vcm-1 Magnetic flux density B T 10 4 Gauss Current density J Am -2 10 -4 Acm -2 Length L, t, w, d m 10 2 cm a, b, c Potential difference V V 1 V In this case, a steady current I x flows along the longer axis of a sample in the form of a thin rectangular plate, and a constant external magnetic field B z is applied normal to the surface of the plate as shown. The drift motion of the electrons in the x-direction then give rise to a transverse magnetic force in the vertical y-direction which deflects them downward and causes them to accumulate at the lower edge of the sample, leaving a deficiency in electron concentration at the top edge. An electric field is generated by this charge distribution, which increases until it exactly conteracts the original magnetic force and restores a state in which charges and currents again move horizontally. F = qE + q(vB) (3-2) The y-component of this total force is in the final state, zero, which allows us to write F y =qE y -qv x B z =0, or E y =v x B z (3-3) However, the current density is given by Jx=nqv x which permites this to be stated as zxzxyBRJnqBJ E (3-4)

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35 Figure 3-1. The geometry in which the Hall effect is most easily measured, showing the drift directions of electrons and holes, the directions of electric and magnetic forces acting on them. where nqR1 (3-5) The field E y is referred to as the Hall field ; its seen to be proportional to the product of the current density and the magnetic induction. The coefficient of proportionality R is called the Hall Coefficient. The Hall field can be measured by measuring the Hall voltage V H across the sample as shown in the diagram. This potential difference is related to the Hall field by V H = E y w (3-6) B(+) + J L W (p-type) +(n-type) +(p-type) (n-type) E H (n-type) E H (p-type) z y x

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36 where w is the sample width. Clearly the Hall coefficient R can be determined from measured values of V H J x and B z The Hall effect is useful because it allows us to measure the density of the free charges in the sample. Moreover, combined with measurements of the electrical conductivity, it permits one to determine both the density of the free charges in the sample and their mobility. Its important in this regard to note that conductivity measurements alone give only the product of n and but provide no information about the value of either quantity separately. However, since the conductivity e can be written e = nq (3-7) the mobility is obtained as R e = (3-8) For electrons the charge is q=-e. For holes the charge is q=e. The Hall coefficient is thus negative in sign for an ideal free electron gas. 3.2.2 Magnetoresistivity Measurement In the Hall effect measurement, one can also usually detect a small decrease in the resistance of the sample when the magnetic field is applied. This effect, in which the change in resistance is found to be proportional to the square of the magnetic field, is called magnetoresistance. Its observed because the magnetic forces that act on the charges cause them to follow circular paths rather than straight lines in the x-direction. Such a path, of arc length equal to the mean free path has a projection on the x-axis smaller than by the amount shown in Figure 3-2 [39]. Consider a charged particle whose path in the absence of a magnetic field is a straight line of length in the x-direction, as represented above as OA.

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37 Figure 3-2. The free path of a charged particle shown as OB in the presence of a magnetic field normal to the page, and as OA in the absence of such a field When a magnetic field B normal to the page is applied, the path becomes a circular arc of radius r and length shown as OB. The projection of this path OB on the x-axis is now the effective mean free path for current flowing along the sample, and it is less by an amount than the free path in the absence of the field. The linear velocity of the particle is unaltered by the field, since the magnetic force is normal to both v and B, as shown by F=qE + q(vB), and therefore has no component in the direction of motion. The angular velocity w is obtained by equating the magnetic force qvB, which is in the radial direction, the centripetal force mv 2 /r. Then, since v=rw, we have mrw 2 =q(rw)B or w=qB/m (3-9) This quantity is called the cyclotron frequency. The change in resistivity is clearly proportional to the amount by which the effective free path is shortened, so we may now write = r rsin B eff. = r A O = r

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38 sin1sinrrr (3-10) where 0 is the zero-field Resistivity. 3.3 Transition Metal Doping Techniques Using Co-sputtering The film-growth methods were carried out using a reactive RF magnetron sputter deposition system equipped with a load-lock for substrate exchange because this system has potential for technological application. Figure 3-3 shows a schematic diagram of the co-sputtering system using two tilting guns. Co x Ti 1-x O 2 anatase films were epitaxially grown by reactive RF magnetron co-sputter deposition with cation sputtering targets of Ti (99.995%) and Co (99.95%). The film-growth system was equipped with multiple 2" sputtering sources and a load-lock for substrate exchange. The base pressure of the deposition system was on the order of 5x10 -8 Torr. For epitaxially stabilized anatase TiO 2 film growth, (001) LaAlO 3 was chosen as the substrate as it provides an excellent in-plane lattice match (a = 3.788 ) with c-axis oriented anatase. The lattice mismatch for anatase on LaAlO 3 (001) is 0.2%. The substrates were cleaned in trichloroethylene, acetone, and methanol prior to loading on the sample holder. Argon was provided as the sputter gas. Oxygen gas, water vapor and water vapor plus H 2 gas will be used as oxidizing species. The oxygen is provided through a mass flow control valve. A water source is created by freezing and evacuating a water-filled stainless steel cylinder that is attached to the deposition chamber via a leak valve. Upon returning to room temperature, the vapor pressure of water is sufficient to provide an H 2 O source. The use of water vapor and hydrogen in depositing oxide materials is useful in manipulating the valence of cations in the depositing films.

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39 Substrate Heater Ti target Co targetRF 13.56 MHz Vacuum Chamber PlasmaReactive RF Magnetron Sputtering Substrate Heater Ti target Co targetRF 13.56 MHz Vacuum Chamber PlasmaReactive RF Magnetron Sputtering Substrate Heater Ti target Co targetRF 13.56 MHz Vacuum Chamber PlasmaReactive RF Magnetron Sputtering Figure 3-3. Co-sputtering system The presence of hydrogen during deposition can result in oxygen deficiency and semiconducting behavior. A water source was created by freezing and evacuating a water-filled stainless cylinder that was attached to the deposition chamber via a leak valve. The total pressure during growth was fixed at 15 mTorr, whereas the water vapor pressure was varied from 10 -4 and 10 -2 Torr. A water vapor pressure of P(H 2 O)=10 -3 Torr was found to be optimal in realizing oxygen deficiency and semiconductor transport behavior. A substrate temperature during the deposition of 650C resulted in the growth of highly crystalline epitaxial Co-doped TiO 2 thin films in the anatase phase. The growth rate was on the order of 2 nm/min. For electronic applications, the roughness will significantly limit the utility of films grown using H 2 O. For catalysis, the enhanced surface area realized with rough films could prove advantages. Its expected for oxygen gas to give smooth surface of films. The

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40 epitaxial stabilization of anatase on LaAlO 3 could be achieved over a large range of growth conditions. In general, epitaxial anatase films were realized for growth temperatures between 500 and 700C and oxygen partial pressure 10 -4 P(O 2 ) 10 -2 Torr. 3.4 Characterization 3.4.1 Profilometry In order to measure thickness of sputtered thin films, a portion of the substrate was masked with silver paint. After depositing the films, the silver paint was removed. An Alphastep profilometer was used to measure the thickness of deposited thin films. 3.4.2 X-ray Diffraction (XRD) X-ray Diffraction was used to identify the specimens crystallinity and orientation of crystallites. A Philips APD 3720 powder diffractometer using Cu K was used to survey a 2 range of 20 to 80. The measurement of full width at half maximum(FWHM) was performed using an Philips Xpert X-ray diffractometer. 3.4.3 Atomic Force Microscopy (AFM) In order to observe the surface roughness and topography of deposited thin films, Atomic Force Microscopy(AFM) micrographs were taken with a Digital Intruments, Inc. Nanoscope III and Dimension 3100. The surface morphology was taken in the tapping mode with Si 3 N 4 tips. Typical data taken from AFM height images include root mean square(RMS) roughness and grain size. 3.4.4 Scanning Electron Microscopy (SEM) In order to investigate the surface morphology and defects as well as segregation of doping materials, scanning electron microscopy was employed using a Jeol JSM-6335F and Jeol JSM-6400F.

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41 3.4.5 X-ray Photoelectron Spectroscopy (XPS) XPS is a broadly applicable general surface analysis technique because of its surface sensitivity, combined with quantitative and chemical state analysis capabilities. It can detect all elements except hydrogen and helium with a sensitivity variation across the periodic table of only about 30. A photon of sufficiently short wavelength, i.e., high energy, can ionize an atom, producing an ejected free electron. The kinetic energy KE of the electron (the photoelectron) depends on the energy of the photon hv expressed by the Einstein photoelectric law: KE = hv-BE (3-11) where BE is the binding energy of the particular electron to the atom concerned. All of photoelectron spectroscopy is based on the above equation. Since hv is known, a measurement of KE determines BE. By experimentally determining a BE, one is approximately determining an value, which is specific to the atom concerned, thereby identifying that atom [40]. 3.4.6 Auger Electron Microscopy (AES) Auger electron spectroscopy(AES) is a technique used to identify the elemental composition, and in many cases, the chemical bonding of the atoms in the surface region of solid samples. It can be combined with ion-beam sputtering to remove materials from the surface and to continue to monitor the composition and chemistry of the remaining surface as this surface moves into the sample. It uses an electron beam as a probe of the sample surface and its output is the energy distribution of the secondary electrons released by the probe beam from the sample, although only the Auger electron component of the secondaries is used in the analysis.

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42 The basic Auger process involves the production of an atomic inner shell vacancy, usually by electron bombardment, and the decay of the atom from this excited state by an electronic rearrangement and emission of an energetic electron rather than by emission of electromagnetic radiation. The complete description of the number of Auger electrons that are detected in the energy distribution of electrons coming from a surface under bombardment by a primary electron beam contains many factors. They can be separated into contributions from four basic processes, the creation of innershell vacancies in atoms of the sample, the emission of electrons as a result of Auger processes resulting from theses inner shell vacancies, the transport of those electrons out of the sample, and the detection and measurement of the energy distribution of the electrons coming from the sample. In fact, Auger electrons are generated in transition back to the ground state of atoms with inner shell vacancies, no matter what process produced the inner shell vacancy. Therefore, it is possible to conduct accurately, nondestructively quantitative analysis of major(host), minor(doping materials). So its feasible to measure how much Co exactly doped and the ratio of Co and Ti. 3.4.7 Superconducting Quantum Interference Device (SQUID) The magnetization properties of the films with increasing Co was measured using SQUID magnetometry. This characterization is an important tool in probing the relationship between ferromagnetism and Co doping in the TiO 2 films. 3.4.8 High-Resolution Transmission Electron Microscopy (HRTEM) A Jeol 2010F HRTEM was employed to investigate the microstructure of the films.

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CHAPTER 4 EPITAXIAL STABILIZATION OF SINGLE CRYSTAL ANATASE This chapter discuss on the epitaxial stabilization of anatase on (001) LaAlO 3 using reactive sputter deposition. Single crystal-like TiO 2 films possessing the anatase crystal structure have been realized on (001) LaAlO 3 using reactive sputter deposition. 4.1 The Use of Oxygen 4.1.1 Introduction The anatase polymorph could be epitaxially stabilized over a wide range of deposition temperatures and oxygen pressures. Both in-plane and out-of-plane X-ray diffraction measurements reflect a high degree of crystallinity for the deposited films. Thickness oscillations in the X-ray diffraction intensity and atomic force microscopy measurements indicate that the films are remarkably smooth, with surface morphology that is limited by that of the substrate. This result illustrates the effectiveness for epitaxy in stabilizing metastable phases, in particular for anatase, through matching of lattice spacing and nucleation chemistry. Film properties are examined as a function of deposition conditions. X-ray diffraction results indicate that the TiO 2 films are essentially single crystal-like anatase based on the in-plane and out-of-plane rocking curves. Film morphology measurements indicate that the films are extremely smooth as well. As reactive sputter deposition is applicable to large-area deposition and high throughput, these results will have significant impact with regards to potential electronic applications of anatase thin-film materials. 43

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44 4.1.2 Experimental Procedures The film-growth experiments were carried out using a reactive RF magnetron sputter deposition system equipped with a load-lock for substrate exchange. A quartz lamp heater provided substrate heating up to 750C. Two-inch diameter Ti disks were used as the sputtering targets. The target to substrate distance was approximately 12 cm. The base pressure of the deposition system was on the order of 5x10 -8 Torr. Single crystal (001) LaAlO 3 was chosen as the substrate material as it provides an excellent in-plane lattice match (a = 3.788 ) with c-axis oriented anatase. The (001) LaAlO 3 substrates were cleaned in trichloroethylene, acetone, and ethanol prior to mounting on the sample platen with Ag paint. Oxygen was used as the oxidizing species for the experiments reported in this paper. A typical deposition time was 2 h. Sputter deposition was performed at an Ar pressure of 15 mTorr and an RF power of 250 W. 4.1.3 Results and Discussion The epitaxial stabilization of anatase on LaAlO 3 could be achieved over a large range of growth conditions. In general, epitaxial anatase films were realized for growth temperatures between 500-700C and oxygen partial pressure 10 -4 P(O 2 ) 10 -2 Torr. X-ray diffraction patterns are shown in Figure 4-1 for TiO 2 thin films deposited at an Ar pressure of 15mTorr and a oxygen partial pressure of 10 -3 Torr on LaAlO 3 (001) from 550C to 700C. As shown in the Figure, there exists only anatase (004) at various temperatures on LaAlO 3 At the lower growth temperatures, there is a distinguishable shift to longer lattice spacing for the anatase, indicating a defect structure. The -peak of LaAlO 3 (001) was observed at 43 due to absence of hybrid mirror.

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45 303540455055 600C550CLaAlO3(001)650C700C LaAlO3(002)*A(004)Intensity(arb.unit) Figure 4-1. X-ray diffraction patterns for TiO 2 thin films deposited at an Ar pressure of 15mTorr and a oxygen partial pressure of 10 -3 Torr on LaAlO 3 (001) substrate. The highest intensity of anatase(004) peak was obtained from X-ray data taken at 650C. However, the crystal quality of thin films as determined via CuK 1 x-ray diffraction was excellent at 700C. Figure 4-2 shows x-ray diffraction data taken along the surface normal for films deposited in various oxygen partial pressures at 700C. Epitaxial anatase is observed over the entire range of pressures, with evidence for a small amount of secondary rutile phase for the films grown at P(O 2 ) 10 -4 Torr. Conductivity could be controlled via oxygen partial pressure during film growth. The film deposited at P(O 2 ) 10 -4 Torr exhibited a slight coloration indicating mobile carriers due to oxygen deficiency. Conductivity measurement confirms that these films are semiconducting.

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46 Figure 4-2. X-ray diffraction scans for epitaxial anatase films grown in various oxygen pressures at 700C. Locations of diffraction peaks for anatase (A) and rutile (R) phases are indicated. 102103104105106intensity (arb)P(O2) = 10-4 Torr 102103202530354045intensity (arb)2(deg)P(O2) = 10-2 Torr 102104intensity (arb)P(O2) = 10-3 Torr LaAlO3 (001)A (004)R (200)R (111) a) b) c) Anatase films with excellent crystal quality as determined via CuK 1 x-ray diffraction were obtained at 650-700C with P(O 2 ) = 10 -3 Torr. X-ray diffraction -2 scans for a film deposited at P(O 2 )=10 -3 Torr are shown in Figure 4-3.

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47 1021031041051062030405060intensity (arb)2(deg)LaAlO3(002)LaAlO3(001)TiO2(004)(a)* 100101102103104105353637383940intensity (arb)2(deg)anatase (004)thickness oscillations d = 0.2523 nm(b) Figure 4-3. X-ray diffraction -2 scans of anatase film grown at 700C in P(O 2 ) = 10 -3 Torr. (a) Low-resolution. (b) High-resolution (Ge111 analyzer crystal) scans. The plot shows no evidence for rutile as a secondary phase. X-ray intensity Laue oscillations observed in the high-resolution scan shown in Figure 4-3(b) through the anatase (004) peak are due to the finite number of TiO 2 (001) planes in the thin film. These coherency oscillations are indicative of a film thickness that is extremely uniform and a surface that is extremely flat. From the spacing of the oscillation peaks, the film thickness was calculated to be 705 which is in agreement with surface profilometer measurements. The low-angle shoulder near the LaAlO 3 may indicate some interfacial reaction between the substrate and film and can also be influenced by film-substrate

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48 interference effects. The origin of the impurity peak at d = 2.523 is also unknown, but may be strained rutile(101). Note that the intensity scale is logarithmic, indicating that only a small percentage of this impurity phase is present. The rocking curve through the 0400080001200018.618.81919.219.4intensity(arb)(deg) = 0.09o(a) 0 1005 1041 1051.5 105-180-9009018 0 intensity (arb)(deg)-scan through TiO2 (204)(b) Figure 4-4. X-ray diffraction. (a) Rocking curve through the (004). (b) -scan through the (204) peak peaks. TiO 2 (004) peak is shown in Figure 4-4(a). The full-width-half-maximum (FWHM) is only 0.09, reflecting the high degree of crystallinity for the film. The in-plane crystallinity of the film was measured via a -scan through the TiO 2 (204) peak as shown in Figure 4-4(b). The four-fold symmetry of the diffraction peaks indicates a cube-on-cube in-plane alignment of the (001) anatase film

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49 with the (001) LaAlO 3 substrate. The in-plane FWHM =0.10, again reflecting a high degree of crystallinity in the film. The single crystal-like nature of the film is remarkable considering that anatase is a metastable phase. From the high-resolution x-ray diffraction data, the lattice parameters for the tetragonal film are a=b=3.790 and c=9.495 This indicates that the film is slightly compressed in-plane and expanded along the c-axis relative to bulk anatase lattice constants. In-plane compressive strain is consistent with the films in-plane spacing being clamped to that of the substrate lattice spacing during cooling. Note that the films deposited at 700C and P(O 2 )=10 -3 Torr are insulating. For potential electronic and optical applications, surface morphology is extremely important as it determines optical scattering loss and heterojunction properties. Atomic force microscopy (AFM) was used to measure the film roughness. Figure 4-5 shows an AFM image for the film grown at P(O 2 ) = 10 -3 Torr and T = 700C. The RMS roughness of the film is only 2.1 nm, which is on the order of that observed for the LaAlO 3 substrate. This result is consistent with the x-ray intensity oscillations shown in Figure 4-3, indicating that the film surface is extremely flat. In order to measure surface composition of TiO 2 thin films grown at 700C, PO 2 =15mTorr for 2h, AES(Auger electron spectroscopy) analysis was taken. As shown in Figure 4-6, this thin films shows complete TiO 2 stoichiometric. This results match well with depth profile data taken from same sample. Figure 4-7 shows the results of the depth profile. As depth of thin film increased, the oxygen and titanium peak maintained consistent intensity, indicating stoichiometric TiO 2 composition through the entire thin film.

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50 Figure 4-5. Atomic force microscopy image of anatase film. RMS roughness is 2.1 nm. 200nm Kinetic Energy (eV)dN(E) 20243466689912113513581581 C1O1Ti1 Figure 4-6. AES analyis for TiO 2 thin film grown at 700C, PO 2 =15mTorr for 2h

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51 Depth (angstroms) 0080160240320400480 C1O1Ti1C1O1Ti1C1O1Ti1 Figure 4-7. Depth profile analysis for TiO 2 thin film grown at 700C, PO 2 =15mTorr for 2h 4.2 The Effect of Water Vapor as an Oxidant 4.2.1 Introduction The growth of TiO 2 films in the anatase crystal structure was investigated using reactive sputter deposition with H 2 O serving as the oxidizing species. With water vapor, the formation of phase-pure anatase TiO 2 thin films via epitaxial stabilization on (001) LaAlO 3 was achieved, although crystallinity was slightly inferior to that obtained when O 2 was employed. Films grown using water vapor exhibited a rougher surface morphology indicating a difference in growth mechanisms. At low H 2 O pressure, the formation of a Ti n O 2n-1 Magnli phase was observed. When hydrogen was employed during growth, mixed phase films of rutile and anatase resulted. The development of crystallinity and phase as a function of deposition temperature and oxidant pressure are discussed. This chapter discuss on the synthesis and properties of epitaxial anatase, focusing on the use of water vapor as the oxidizing species during reactive sputter deposition. The

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52 introduction of hydrogen-bearing species into the growth process produces the potential to modify the Ti valence state, thus introducing charge carriers. Here, the chapter addresses the phase development, crystallinity, and surface morphology of anatase as a function of oxidizing species, oxidation conditions, and temperature using reactive sputter deposition. 4.2.2 Experimental Procedures A water source was created by freezing and evacuating a water-filled stainless steel cylinder that was attached to the deposition chamber via a leak valve. Upon returning to room temperature, the vapor pressure of water is sufficient to provide an H 2 O source. As mentioned before, the use of water vapor and hydrogen in depositing oxide materials is useful in manipulating the valence of cations in the depositing film. The presence of hydrogen during deposition can result in oxygen deficiency and semiconducting behavior. 4.2.3 Results and Discussion For films deposited using water vapor, anatase was obtained with comparable deposition rate, but slightly inferior crystallinity, as compared to that achieved with O 2 The epitaxial relationship between the film and substrate, as determined by four-circle x-ray diffraction, was (001)[110] LaAlO3 //(001)[110] anatase as has been reported elsewhere. The deposition rate for TiO 2 showed a slight dependence on substrate temperature as is shown in Figure 4-7. The rates are for an applied RF power of 250 W, P(H 2 O) = 10 -3 Torr, and a total pressure of 15 mTorr. The background gas was either argon or a 4% H 2 /96% Ar mixture. The deposition rate did not change significantly when oxygen was used in place of water vapor. The increase in deposition rate with temperature reflects the limiting surface reaction of Ti with the oxidizing species to yield TiO 2 since the

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53 sticking coefficient of Ti should be near unity. For most of the films discussed below, the deposition time was 2 h, yielding a film thickness on the order of 350 nm. 0102030405060500550600650700750800 d epos it i on ra t e ( angs t rom / m i n ) temperature (C)Ar/H2Ar Figure 4-8. Deposition rate of TiO 2 films on LaAlO 3 as a function of substrate temperature with H 2 O serving as the oxidizing species The effectiveness of epitaxy in stabilizing the anatase crystal structure can be seen in Figure 4-9 and 4-10, where the x-ray diffraction patterns for TiO 2 films deposited at various temperatures on either LaAlO 3 or Si are shown. For these films, the total pressure during deposition was 15 mTorr, with 10 -3 Torr of H 2 O vapor pressure. For films deposited at a substrate temperature ranging from 500-750C, the dominant TiO 2 phase observed on (001) LaAlO 3 was (001)-oriented anatase. Figure 4-11(a) shows the X-ray diffraction intensity as a function of deposition temperature for the anatase (004) peak. The films deposited at the higher temperatures possess a c-axis lattice parameter

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54 that is essentially the same as bulk crystals. However, at the lower growth temperatures, there is a distinguishable shift to longer lattice spacing for both the anatase and rutile 10110310510710910113436384042intensity (arb)2(deg)450 C500 C600 C650 C700 C750 C550 CA(004)R(111)R(200) Figure 4-9. X-ray diffraction patterns for TiO 2 films deposited at an Ar pressure of 15mTorr and a water vapor partial pressure of 10 -3 Torr on LaAlO 3 10110310510710920253035404550intensity (arb)2(deg)450 C500 C600 C650 C700 C750 C550 C A (004)R(200)R(110) Figure 4-10. X-ray diffraction patterns for TiO 2 films deposited at an Ar pressure of 15mTorr and a water vapor partial pressure of 10 -3 Torr on Si

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55 components, indicating a defect structure. Plotted in Figure 4-11(b) are the relative ratios of the (004) anatase peak to the (200) and (111) rutile peaks. Anatase is clearly the dominant TiO 2 phase with the strongest relative (004) anatase peak at a growth temperature of 750C. 0 1004 1038 1031.2 104400500600700800A(004) Peak Intensity (cps)Temperature (oC)(a) 0.00.20.40.60.81.01.2400500600700800 A(004)/A(004) + R(200) A(004)/A(004)+R(111)A(004) Peak Intensity (cps)Temperature (oC)(b) Figure 4-11. X-ray diffraction data for TiO 2 on LaAlO 3 using H 2 O. (a) The anatase (004) peak intensity. (b) The relative intensity of anatase peaks to the rutile peaks. In contrast, the diffraction data for polycrystalline TiO 2 films deposited on silicon substrates are predominantly rutile, particularly at the elevated temperatures, with only a minority anatase component present. The exception to this are the films deposited at T

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56 500C, where the crystallinity is poor and the anatase structure appears more prominent. Clearly, the epitaxial nucleation of TiO 2 on a lattice-matched substrate provides a robust processing window for realizing anatase phase selection and crystallinity. Epitaxy of the films with respect to the substrate was examined using four-circle X-ray diffraction. A low resolution -2 scan along the TiO 2 (001) shows that the predominant phase is (001) anatase, although some minor peaks attributable to rutile are evident. From a high-resolution -2 scan through the anatase (004), the lattice parameters for the anatase film deposited at 700C and 10 -3 Torr of H 2 O are determined to be a=b=3.78 c=9.522 which is essentially that reported for bulk material. A -scan rocking curve through the (004) yields a =0.58 full-width-half-maximum. Figure 4-12 shows the -scan through the anatase {204} peaks showing that the film is in-plane aligned. 103104-180-120-60060120180Intensity (arb)(deg)= 0.6o scan through the TiO2 (204) Figure 4-12. The scan through the anatase {204} peaks, showing in-plane alignment for the films grown using H 2 O The FWHM for the in-plane peaks is 0.6. The in-plane and out-of-plane mosaic widths for anatase grown using water vapor are somewhat broader than that observed for films grown using oxygen as the oxidant. Anatase films deposited using oxygen display

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57 out-of-plane and in-plane mosaic spreads on the order of 0.1. This slight degradation in crystallinity does not significantly impact the stabilization of the anatase crystal structure. In addition to investigating phase dependence on temperature, the effect of water vapor partial pressure on crystallinity was also explored. Figure 4-13 shows the X-ray diffraction patterns for films deposited on (001) LaAlO 3 at 700C with 15 mTorr Ar at a water vapor pressure ranging from 10 -4 to 10 -2 Torr. 10210410610820304050607080intensity (arb)2(deg)P(H2O) = 10-2 TorrP(H2O) = 10-3 TorrP(H2O) = 10-4 Torr*****A(004)R1R2 LAOLAOLAO Figure 4-13. The X-ray diffraction patterns for films deposited with 15mTorr Ar (no hydrogen) as a function of water vapor pressure. The anatase (A) and rutile peaks are indicated, with R1= rutile (200) and R2 = rutile (111). The asterisk indicate the Magnli phase peaks. The films deposited at the highest water vapor pressure considered (P(H 2 O) = 10 -2 Torr) were phase-pure anatase with little or no evidence of rutile peaks. At a water vapor pressure of P(H 2 O) = 10 -4 Torr, the phase assemblage changes significantly. Additional peaks in the x-ray diffraction pattern reflects the presence of crystalline Magnli phases of the type Ti n O 2n-1 [41-42]. Given the density of diffraction peaks observed for these triclinic phases, it is not possible to unequivocally identify which phase is present. The

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58 most likely candidates are Ti 4 O 7 and Ti 6 O 11 Note that the Magnli phase(s) displays some degree of preferred texture. Similar film-growth experiments were performed with sputter deposition occurring in the presence of H 2 O and hydrogen. For these experiments, the Ar sputter gas was replaced with a 4%H 2 /96%Ar mixture, yielding a H 2 pressure of 6x10 -4 Torr. The motivation for investigating the effect of hydrogen and H 2 O on growth and properties is the possibility of increasing the carrier density in TiO 2 films via oxygen vacancies. In fact, films deposited with H 2 O and hydrogen did show an apparent increase in oxygen vacancies, based on dark coloration of the films and increased conductivity. Unfortunately, the introduction of hydrogen also led to mixed-phase films. Figure 4-14 shows the X-ray diffraction patterns for TiO 2 films deposited in the presence of both H 2 O and H 2 1011021031041051061071083436384042intensity (arb)2(deg)600 C650 C700 C750 C550 CA(004)R(111)R(200) Figure 4-14. X-ray diffraction patterns for TiO 2 films deposited in the presence of both H 2 O and H 2

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59 A significant fraction of the rutile phase is observed in the films. The maximum anatase phase density is realized at a deposition temperature of approximately 650C. In addition to crystallinity, we also examined the TiO 2 film morphology for films deposited using H 2 O with and without H 2 and compared these to films deposited with oxygen. Atomic force microscopy images of films deposited in O 2 H 2 O H 2 and H 2 O are shown in Figure 4-15, and clearly indicate a significant change in nucleation and growth when H 2 O is used. (a)(b)(c) 5 m 400 nm 0 nm400 nm 0 nm400 nm 0 nm 5 m 5 m Figure 4-15. AFM images for TiO 2 film surfaces with (a) O 2 (b) H 2 O, and (c) H 2 O-H 2 ambients

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60 For the films deposited in O 2 films are extremely smooth, with a RMS roughness of 2.1 nm. With H 2 O and H 2 O-H 2 ambients, the RMS roughness was 18.5 nm and 30 nm, respectively. For electronic applications, the roughness will significantly limit the utility of films grown using H 2 O. For catalysis, the enhanced surface area realized with rough films could prove advantageous. The RMS roughness also increased with increasing deposition temperatures. Figure 4-16 shows the RMS roughness, measured with AFM, as a function of deposition temperature for films grown by H 2 O and H 2 O with hydrogen. 051015202530500550600650700750800 H2/Ar ArRMS Roughness (nm)Temperature (oC) Figure 4-16. RMS data for TiO 2 films deposited on LaAlO 3 at various substrate temperatures. For these experiments, total pressure was 15 mTorr, RF power was 250 W, and water vapor pressure was 10 -3 Torr. 4.3 Phase Stability of Rutile TiO 2 on Si(100) The effects of substrate temperature on the characteristic of TiO 2 films grown on the Si(100)p-type by RF magnetron sputtering were investigated using a mixture of H 2 /Ar gases and Ar gas with water vapor. It was found that the crystalline structures strongly depend on the substrate temperature and on the sputtering gas. The intensity of rutile (200) phase was much higher with Ar/H 2 (4%) mixed gas than that with Ar gas at same

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61 water vapor pressure, 10 -3 torr. The intensity of rutile (200) phase was much less with PO 2 than that with PH 2 O at a total pressure of 15mTorr with Ar gas 4.3.1 Introduction One factor limiting progress in integrated circuit technology has been the inability to reduce the size of the capacitive elements. For this purpose, high dielectric constant thin-film materials are required. Ferroelectrics, such as barium titanate, have dielectric constants on the order of 10 3 However, production of these complex multiple-cation oxides and their subsequent instability during use limits their practical value. Attention has been turned to less complex materials. For example, the rutile phase of titanium dioxide(TiO 2 ), a nonferroelectric, has a dielectric constant on the order of 10 2 [43]. Its electrical properties make TiO 2 an excellent candidate for applications in semiconductor devices [44]. Rutile is not only valued because of its higher dielectric constant, but also for its high index of refraction and chemical stability [45-46]. In thin-film form, it is often used as a component in multiplayer optical coatings. In bulk form, titanium dioxide(TiO 2 ) is known to exist in three crystalline structures : two tetragonal phases, anatase and rutile; and an orthorhombic phase, brookite. Brookite, however, was not observed in any of this research, and so it will not be considered in this paper. In thin-film form, an amorphous phase of titanium dioxide is also stabilized. With respect to the tetragonal crystalline phases, rutile is the high-temperature, stable phase, and anatase is a low-temperature polymorph. A transformation from anatase to rutile can be thermally induced at temperatures above 800C. Rutile, the technologically preferred form, has a higher index of refraction and higher dielectric constant than anatase and is desired for these properties.

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62 Previous work reporting highly crystalline anatase films via reactive sputtering focused on the phase development of TiO 2 particularly on anatase structure with LaAlO 3 (001) substrate when oxygen or H 2 O are used as the oxidan.These studies showed that epitaxial stabilization of anatase was exploited on the LaAlO 3 substrate because the lattice mismatch between anatase and LaAlO 3 was only 0.26%. In this study, I briefly report the phase stability of rutile TiO 2 via reactive sputter deposition with various growth conditions, such as, growth temperature, water vapor and oxygen gas as an oxidant, Ar or Ar/H 2 mixture as a sputtering gas. 4.3.2 Experimental Procedures The TiO 2 films were deposited on Si (100) p-type substrates by RF magnetron sputtering at 13.56MHz. A metal Ti (99.98 % purity), a diameter of 5cm, was used as a target. The target to substrate distance and the base pressure were 12cm, less than 510 -7 Torr, respectively. A mixture of Ar/H 2 (4%) and Ar (5N) was used as a sputtering gas. The sputtering power was 250W that corresponds to a power density of 10Wcm -2 After pre-sputtering the target for 5min with a closed shutter, the TiO 2 films were deposited at temperature between 550C and 750C for 2h. Si(100) wafers were prepared by solvent cleaning using sequential rinses of acetone, methanol in an ultrasonic bath for 5min., respectively, and then etched in a 5% HF solution for 30sec. The substrates were cleaned with D.I.water and dried in a stream of dry nitrogen gas prior to being mounted on a heater block, centrally located above the target in on-axis geometry. Oxygen was provided through a mass flow control valve. A water source was created by freezing and evacuating a water-filled stainless steel cylinder that was attached to the deposition

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63 chamber via a leak valve. Upon returning to room temperature, the vapor pressure of water is sufficiently to provide an H 2 O source. The deposition rates of films were measured by stylus profilometer. The structure of the films was examined by using X-ray diffraction with Cu K radiation in standard X-ray diffractometer. 4.3.3 Results and Discussion Figure 4-17 shows the deposition rates of TiO 2 films as a function of deposition temperature in two different sputtering gases. As the temperature is increased, the deposition rate is faster. The deposition rate was much higher in P(H 2 /Ar) than that in P(Ar). 5506006507007501015202530 Deposition Rate(/min.)Deposition Temperature(C) PAr PH2/Ar Figure 4-17. Deposition rate of TiO 2 films on Si(100) at different temperature. Applied RF power and water vapor were constant at 250W and 10 -3 Torr, respectively. Deposition time was 2h. Total pressure was 15mTorr. Figure 4-18 shows the dependence of XRD patterns of deposited films on the substrate temperature with Ar gas. As shown in Figure 4-18(a) and Figure 4-19(b), as temperature is increased, the intensity of rutile (200) peak is increased. Note also that only rutile(200) peak appears between 600C and 650C. A mixed phase, however,

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64 appears below 550C and above 700C. With Ar sputtering gas, the optimum condition for rutile phase was 600C and 650C. Figure 4-19 shows the X-ray diffraction patterns for TiO 2 films on Si(100) prepared at a total pressure of 15mTorr with H 2 /Ar mixed gases. There are only rutile(200) phase in the all range of temperature except for 700C. The anatase(004) peak that appears at 700C is weak. Below 650C, the intensity of rutile(200) phase with Ar Figure 4-18. X-ray diffraction patterns (a) and the relative intensity of rutile and anatase (b) for TiO 2 films on LaAlO 3 at an Ar pressure of 15mTorr and a water vapor partial pressure of 10 -3 Torr. Applied RF power and water vapor were constant at 250W and 10 -3 Torr, respectively. Deposition time was 2h. gas is similar to that with H 2 /Ar mixed gas. Above 700C, the intensity of rutile phase with Ar gas is less than that with H 2 /Ar. It seems that hydrogen gas contributes to the reduction of the intensity of rutile phase. Figure 4-20 shows X-ray diffraction data for films grown at a total pressure of 15mTorr with Ar gas. Applied RF power was constant at 250W. Deposition temperature and time was 700C, 2h, respectively. At 10 -3 torr of water vapor pressure, the intensity of the rutile(200) peak was the highest. At a water vapor pressure of P(H 2 O)=10 -4 Torr, the

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65 phase group changes significantly due to the presence of crystalline Magnli phases of the type Ti n O 2n-1 which is not possible to easily identify which phase is exist. Figure 4-21 shows X-ray diffraction patterns for TiO 2 films grown at a total pressure of 15mTorr as a function of oxygen partial pressure. All of the peaks are less than those with water vapor. It is apparent that hydrogen helps in the deposition of TiO 2 films during deposition. Figure 4-19. X-ray diffraction patterns (a) and the relative intensity of rutile and anatase (b) for TiO 2 films on LaAlO 3 grown in the presence of both H 2 O and H 2 Applied rf power and water vapor were constant at 250W and 10 -3 Torr, respectively. Deposition time was 2h.

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66 20253035404550556065707580 A(004)R(200)PH2O=10-3torrPH2O=10-2torrSiPH2O=10-4torr Intensity(arb. unit) Figure 4-20. The X-ray diffraction patterns for films deposited with 15mTorr Ar (no hydrogen) as a function of water vapor pressure. Applied RF power was constant at 250W. Deposition temperature and time was 700C, 2h. 20253035404550556065707580 R(200)A(004)PO2=10-3torrPO2=10-4torrPO2=10-2torrSiIntensity(arb.unit) Figure 4-21. The X-ray diffraction patterns for TiO 2 films as a function of oxygen partial pressure. Applied RF power was at 250W. Deposition time and temperature were 2h, 700C.

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67 -1.0-0.50.00.51.0-0.3-0.2-0.10.00.10.20.3 (a) RH=-25 (cm3/C) at 300K N-type Rxy,odd(-cm)(x10-2)Magnetic Field(0H/T) 0123456702468 (b) 300K Magnetoresistance((H,T)-(0,T)/(0,T))x100%MagnetoresistanceMagnetic Field(0H/T) Figure 4-22. Hall coefficient (a) and magnetoresistance results (b) for the TiO 2 on Si(100) with 10 -3 Torr of a water vapor at 300K Figure 4-22 shows the Hall coefficient(a) and magnetoresistance results for the TiO 2 on Si(100) with 10 -3 Torr water vapor at 300K. The magnetic field dependence of R xy, odd shows typical n-type semiconductor behavior at 300K. Magnetoresistance is 8% at a magnetic field of 6 Tesla. 4.4 Transport Properties of Oxygen-Deficient Anatase 4.4.1 Introduction Defect doping of the anatase polymorph that is epitaxially stabilized on (001)LaAlO 3 was explored using either oxygen or water vapor as the oxidizing species. For films grown in oxygen, a transition from insulating to metallic conductivity of the films is observed as the O 2 pressure is reduced. X-ray diffraction measurements show the presence of the Ti n O 2n-1 phase when the oxygen pressure is reduced sufficiently to induce conductive behavior. Hall measurements indicate that these materials are p-type. In contrast, the use of water vapor as the oxidizing species enabled the formation of n-type semiconducting TiO 2 with carrier density on the order of 10 18 cm -3 and mobility of 10-15 cm 2 /V s.

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68 A key issue in exploiting TiO 2 for spintronic applications is to understand the film growth conditions that yield mobile carriers in the TiO 2 matrix. The focus of this section is on the synthesis and properties of semiconducting TiO 2 In particular, we have investigated the transport properties of sputter-deposited TiO 2 films using either O 2 or H 2 O a the oxidizing species. Here, we address the issues of phase development, crystallinity, conductivity, and surface morphology of TiO 2 as a function of oxidizing species, oxidation conditions, and temperature using reactive sputter deposition. 4.4.2 Experimental Procedures Previous work reporting highly crystalline anatase films via reactive sputter deposition focused on the phase development of TiO 2 when oxygen or H 2 O are used as the oxidant [47-48]. The introduction of the hydrogen-bearing species into the growth process provides the potential of modifying the Ti valence state in the TiO 2 matrix. 4.4.3 Results and Discussion For the experiments focusing on the growth of TiO 2 using oxygen as the oxidizing species, films were deposited at substrate temperatures ranging from 400 to 700C in an oxygen ambient of 10 -2 to 10 -4 Torr. The total pressure was 15 mTorr for most experiments. Over this range of conditions, the formation of semiconducting TiO 2 was not observed. Instead, a transition from insulating TiO 2 to metallic films, with Ti n O 2n-1 as a secondary phase was observed as the oxygen partial pressure was reduced. Figure 4-23 shows a compilation of sputtering conditions in which the transport properties are characterized as a function of O 2 and Ar pressure.

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69 1.0x10-22.0x10-23.0x10-210-410-310-2 PO2(Torr) Total Pressure(Torr) Anatase(insulating) Anatase & TinOm(metallic) TinOm(metallic) Figure 4-23. Phase map showing crystalline phases and conductivity behavior as a function of deposition conditions The films shown were grown at 700C. The most significant parameter in determining transport properties is oxygen pressure as this determines the formation of defects in the TiO 2 matrix that leads to conducting behavior. For growth at P(O 2 ) 7.5 -4 Torr, the films are insulating and transparent. For P(O 2 )7.5 -4 Torr, the films become conductive and black. Both the X-ray diffraction data and Hall measurements indicate that the observed conductivity is not due to defect-doped anatase or rutile, but due to a metallic Ti n O 2n-1 impurity phase. Ti n O 2n-1 phases, such as Ti 4 O 7 are metallic at room temperature. Figure 4-24 shows the X-ray diffraction pattern for a Ti-O film grown at 700 C, P(O 2 )=10 -4 Torr, PAr=15 mTorr. The diffraction data shows both TiO 2 anatase and Ti n O 2n-1 peaks. In addition, the Hall measurements indicate that these films are metallic and p-type. The Hall measurements yield a carrier density on the order of 10 23 cm -3 a resistivity of 7 -5 -cm and a mobility of 0.36 cm 2 /Vs. Figure 4-25 shows the results from Hall measurements

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70 for the films grown at a P(O 2 ) 10 -3 Torr. In all cases, the Hall voltage sign is positive, indicating p-type conductivity. 3035404550556065 LaAlO3(002)TinOmTinOmAnatase(004)Intensity(arb.)2(deg.) Figure 4-24. X-ray diffraction patterns for Ti-O film deposited at an Ar pressure of 15mTorr and an oxygen partial pressure of 10 -4 Torr on LaAlO 3 at 700C, showing the presence of both anatase and Ti n O 2n-1 10-410-31020102110221x1023 Carrier Density MobilityOxygen Pressure(Torr)Carrier Density(cm-3)0.1110100 Mobility(cm2/V-s) Figure 4-25. Hall data for TiO 2 films grown at 700C in oxygen

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71 For films grown at P(O 2 ) 10 -3 Torr, the films are insulating. This metal-insulator transition seen in the transport with varying oxygen during growth corresponds to the appearance of Ti n O 2n-1 as a second phase in the TiO 2 matrix. We have also examined the use of H 2 O as the oxidizing species in synthesizing TiO 2 thin films. Previous work showed that phase-pure anatase films could be realized using water vapor as the oxidant. In contrast to the case of O 2 processing conditions are identified in which n-type semiconducting films can be realized. For films grown at 600-650C in10 -3 Torr H 2 O, the films are transparent, n-type, and exhibit no Ti n O 2n-1 peaks in the X-ray diffraction patterns. Table 4-1 lists the specific properties of films grown under these conditions. Table 4-1. Deposition conditions and semiconducting properties of TiO 2 films deposited using water vapor Growth Temperature(C) PH 2 O(Torr) Carrier Type n(cm -3 ) (cm) Hall (cm 2 /Vs) 650C 10 -3 n 210 18 0.29 13 600C 10 -3 n 2.510 18 3 0.7 Figure 4-26 shows the X-ray diffraction pattern for a TiO 2 film grown at 650 C in 10 -3 Torr H 2 O. Figure 4-26. X-ray diffraction data for TiO 2 on LaAlO 3 using H 2 O

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72 Figure 4-27. Atomic force microscope scan of anatase film grown in water vapor at different temperatures Note that the strongest peaks can be assigned to anatase with a much smaller peak corresponding to rutile. Deposition at 10 -4 Torr H 2 O yielded Ti n O 2n-1 peaks and metallic behavior. In addition to transport and structural properties, film morphology for TiO 2 grown using H 2 O was examined using atomic force microscopy (AFM). Figure 4-27 shows the AFM images of TiO 2 films grown in P(H 2 O)=10 -3 Torr at various substrate temperatures. Two items should be noted. First, the grain size increases significantly as the growth temperature is increased from 550 to 750C.The average grain size for films grown at 550C is on the order of 80 nm. At 750C, the grain size is on the

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73 order of 250 nm. With this increase in grain size, an increase in RMS roughness is also observed, increasing from 3.1 nm at 550 C to 9.3 nm at 750C.

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CHAPTER 5 EPITAXIALLY GROWN ANATASE CO X TI 1-X O 2 5.1 Ferromagnetic Semiconducting Properties of Co-doped Anatase Films 5.1.1 Introduction In recent years, there has emerged considerable interest in the development of electronics based on the spin of electrons. One approach to realizing novel spin-based electronic devices involves the exploitation of spin-polarized electron distributions in dilute magnetic semiconductor (DMS) materials. In most semiconductors doped with transition metals, the magnetic behavior, if any, is observed at temperatures well below room temperature [49-52]. Among the conventional semiconductors, (Ga, Mn)As exhibits the highest Curie temperature (Tc), on the order of 110K [53-54]. In recent years, it has been reported that cobalt-doped semiconducting anatase (TiO 2 ) is ferromagnetic at room temperature. This may provide for the development of spin-based DMS electronics that operate at room temperature if the ferromagnetic spin moments are present in the free electron distribution. Ferromagnetic thin films of Co x Ti 1-x O 2 epitaxially stabilized in the anatase structure, have previously been realized on SrTiO 3 (001) and LaAlO 3 (001) by pulsed laser deposition(PLD) and oxygen plasma assisted molecular-beam epitaxy(OPA-MBE). Spectroscopic studies on the MBE-grown TiO 2 thin films suggests that the cobalt exists in the Co 2+ oxidation which is consistent with ferromagnetism originating from Co substitution on the Ti site. However, other studies of Co-doped TiO 2 films deposited by pulsed laser deposition suggest that the formation of Co nanoclusters may be responsible 74

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75 for the ferromagnetic properties [55]. Thus, it remains an open question as to the origin of ferromagnetism in these materials, as well as the probable role that the specific processing technique plays in yielding these results. In this chapter, the growth and properties of Co-doped TiO 2 (Co x Ti 1-x O 2 ) epitaxial thin films by reactive co-sputter deposition is discussed. The use of water vapor(H 2 O) as the oxidant facilitates the formation of carriers via the creation of oxygen vacancies. We have previously reported on the transport properties of undoped semiconducting in transparent anatase TiO 2 thin films epitaxially grown by reactive sputtering deposition employing water vapor [56]. The primary focus of the present study is to investigate the magnetic properties of sputter-deposited Co x Ti 1-x O 2 with particular attention given to delineating the origin of ferromagnetism relative to Co substitution or metal precipitate formation. 5.1.2 Experimental Procedures Co x Ti 1-x O 2 anatase films were epitaxially grown by a reactive RF magnetron co-sputter deposition with cation sputtering targets of Ti (99.995%) and Co(99.95%). The total pressure during growth was fixed at 15mTorr, whereas the water vapor pressure was varied from 10 -4 and 10 -2 Torr. A water vapor pressure of P(H 2 O)=10 -3 Torr was found to be optimal in realizing oxygen deficiency and semiconductor transport behavior. A substrate temperature during the deposition of 650C resulted in the growth of highly crystalline epitaxial Co-doped TiO 2 thin films in the anatase phase. The growth rate was on the order of 2nm/min. The crystalline structure of the Co-doped TiO 2 films was characterized by X-ray diffraction (XRD) with a Cu Ka radiation source. Quantitative analysis of chemical composition was performed by electron probe microanalysis (EPMA). In order to extract

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76 information regarding chemical states of the element, X-ray photoelectron spectroscopy (XPS) was used with Al-K radiation (h=1486.6eV). The surface morphology, back-scattered images and chemical mapping were performed by field emission scanning electron microscopy (FESEM). Hall effect measurements were performed to measure transport properties of oxygen-deficient Co-doped TiO 2 anatase films. Room temperature magnetization was measured by a Quantum Design superconducting quantum interference device (SQUID) magnetometer for films with different Co content (x=0.07, 0.02). 5.1.3 Results and Discussion Co-doped epitaxial anatase films realized for a growth temperature of 650C and water vapor pressure at 10 -3 Torr exhibited crystalline quality similar to that seen in previous work on undoped films. Figure 5-1 shows the -2 X-ray diffraction data taken along the surface normal for films deposited under these conditions with 2 at% and 7 at% Co doping levels. The films are near single phase epitaxial anatase with a small amount of secondary rutile phase seen in the data. From in-plane and out-of-plane X-ray diffraction measurement, the anatase lattice parameters were a=3.790 and c=9.495. As shown in Figure 5-1, there was no evidence for metallic Co or cobalt oxides phases seen in the diffraction data. This does not preclude the presence of the secondary phases since a small volume fraction of a randomly-oriented impurity phase would be difficult to detect. However, metallic Co has been observed in X-ray diffraction scans of Co-doped ZnO. There is little difference observed for the X-ray diffraction patterns for these 2 at%

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77 and 7 at% doped films. Figure 5-2 shows the rocking curve and -scan for Co 0.07 Ti 0.93 O 2 thin films. FWHM was 0.24 from the rocking curve and 0.3 from the -scan. 3040506070102104106 A(004)LaAlO3(002)R(200)x=0.02x=0.07Intensity(arb.unit)deg. Figure 5-1.The X-ray diffraction pattern for Co x Ti 1-x O 2 on LaAlO 3 (001) (x=0.07, 0.02) This value is a little higher than that for undoped anatase (FWHM=0.09). Table 5-1 shows the results from room temperature Hall effect measurements for the Co x Ti 1-x O 2 (x=0.07, 0.02, 0) thin films. The current used for the measurement was 100A. Both doped and pure anatase TiO 2 thin films show n-type semiconductor behavior. The carrier concentration was typically in the range 10 17 -10 18 cm -3 which is lower than that shown for MBE-grown films. The resistivity of the 7 at% Co-doped thin film is slightly higher than that for doped or undoped anatase films. The carrier mobility clearly decreases as the cobalt concentration is increased. However, we note that the carrier concentration in these materials is not strongly dependent of Co concentration. The magnetization properties of the Co-doped films are shown via the M-H plots shown in Figure 5-3. For both the 2 at%

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78 and 7 at% Co-doped TiO 2 sample, hysteresis loops are observed at room temperature for film grown at 650C with water vapor as a oxidant. 17.518.018.519.019.520.020.5040080012001600 (a) FWHM=0.24oIntensity(arb)(deg.) 050100150200250300350050100150 (b)FWHM=0.3o Intensity(arb)(Phi.) Figure 5-2. Rocking curve and phi-scan for Co 0.07 Ti 0.93 O 2 thin films on LaAlO 3 (001) Table 5-1. Hall effect measurement for Co x Ti 1-x O 2 thin film(x=0.07, 0.02, 0). Co 0.07 Ti 0.93 O 2 Co 0.02 Ti 0.98 O 2 TiO 2 (undoped) Resistivity(-cm) 7.18 0.186 0.283 Hall Coefficient(cm 3 /C) -8.39 -4.18 -8.85 Hall Mobility(cm 2 /Vs) 1 22 31 Carrier Concentration(cm -3) 8.0910 17 1.4810 18 7.4610 17 Type N-type N-type N-type Both the total magnetization and calculated magnetization per Co cation are plotted. Clearly, the 7 at% Co-doped TiO 2 film exhibits stronger ferromagnetic behavior relative to the 2 at% doped sample. These results are consistent with both the MBE and PLD results reported earlier [9-10]. Magnetic moment was relatively low (0.25B per Co ion) at saturation. S. A. Chamber et al. [3, 10] have grown thin films of Co x Ti 1-x O 2 on SrTiO 3 and LaAlO 3 by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). They showed

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79 -1000-50005001000-8.0x10-5-4.0x10-50.04.0x10-58.0x10-5 (a) 300K Magnetization(emu)H(Gauss) x=0.07 x=0.02-1000-50005001000-0.6-0.30.00.30.6 (b) 300K M(Bohr magneton/Co atom)H(Gauss) x=0.07 x=0.02 Figure 5-3. An M-H curve for Co x Ti 1-x O 2 (x=0.07, 0.02) thin films on LaAlO 3 (001) taken at room temperature. Magnetic field was applied parallel to the film surface. (a) Magnetization(emu) vs. Magnetic field(H, gauss), (b) Magnetization(Bohr magneton/Co atom) vs. Magnetic field(H, gauss) that ferromagnetic properties were originated from Co substitution on the Ti site. The spontaneous magnetic moment per Co atom derived from the saturated magnetization

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80 was 0.6 B.This value is 2 times higher than that reported by Y. Matsumoto et al. [9] They showed that the Curie temperature is at least 400K, while the The elemental composition for the Co-doped TiO 2 thin films was determined by XPS. An XPS spectrum for the Co 0.07 Ti 0.93 O 2 film is shown in Figure 5-4. The following peaks are observed in Figure 5-4(a) : Ti 2p1 at 464.24eV and Ti 2p3 at 458.49eV, Co 2p1 at 796.21eV and Co 2p3 at 780.12eV, O 1s at 529.9eV and a carbon peak (C 1s at 285. 16eV). The shift in all peaks caused by charging effects has been corrected using the standard peak, O 1s at 529.9eV from TiO 2 as a reference as seen in Figure 5-4(d). The oxidation and spin state of the Ti atoms shown in Figure 5-4(b) match well with standard Ti 2p1 and Ti 2p3 peaks, indicating that these two peaks correspond to Ti +4 from TiO 2 thin films. The line separation between Ti 2p1/2 and Ti 2p3/2 was 5.75eV, which is consistent with 5.7eV as the standard binding energy. For the Co-related peaks shown in Figure 5-4(c), the oxidation and spin state of the Co atoms can be inferred from the shape of the Co 2p lines. The satellite peak structure that is observed on the high binding energy side of the principal 2p1/2 and 2p3/2 lines is indicative of high spin Co 2+ [57]. The shake-up peak is a more easily identified characteristic of the chemical state of the Co than either the absolute binding energy or the line separation between Co 2p 1/2 and Co 2p 3/2 The results from the XPS analysis taken from the as-grown surface is consistent with the Co existing in the +2 formal oxidation state in these sputter-deposited Co x Ti 1-x O 2 anatase thin films. In order to further investigate the properties of these materials, the surface topography and chemistry was determined using field-emission SEM with energy dispersive spectrometry.

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81 5355305254.0x1038.0x1031.2x1041.6x104 (d)O1s Counts(Arb.)Binding Energy(eV)8108007907804.0x1044.4x1044.8x104 (c)Shake-up peaks Co2p3/2Co2p1/2 Counts(Arb.)Binding Energy(eV)4704654604553.0x1036.0x1039.0x103 (b)Ti2p1/2Ti2p3/2 Counts(Arb.)Binding Energy(eV)8006004002000.02.0x1044.0x1046.0x104 (a)C1sTi2pO1sCo2p Counts(Arb.)Binding Energy(eV) Figure 5-4. XPS spectrum of Co 0.07 Ti 0.93 O 2 on LaAlO 3 (001): (a) General spectrum, (b) Ti 2p band, (c) Co 2p band, (d) O 1s band Figure 5-5 shows the backscattered electron image obtained by the FESEM and the result of auger electron spectroscopy survey for the surface of 7 at% Co-doped TiO 2 anatase thin films grown on (001)LaAlO 3 The sample surface is decorated with apparent segregated nanoclusters. As shown in Figure 5-5, however, sputter deposited undoped TiO 2 and Co 0.02 Ti 0.98 O 2 films do not show these nanoclusters from the surface, indicating that nanoclusters formation directly correlates with Co concentration. In order to further investigate these segregated particles, selected area Auger electron spectroscopy (AES) was also performed. Results from AES, also given in Figure 5-5, indicate that the Co dopant is principally located in the segregated areas, with significantly less Co located in the film

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82 region between the nanoclusters. However, from the Co 0.0 2 Ti 0.98 O 2 there is no evidence of the surface segregation as shown in Figure 5-6. 200400600800100012001400 (b) Co0.07Ti0.93O2 on LaAlO3(001)Co1O1Ti2Ti1C1 dN(E)Kinetic Energy(eV)200400600800100012001400 O1Co1Co2Co3Ti2Ti1C1 Co0.07Ti0.93O2 on LaAlO3(001)(c) dN(E)Kinetic Energy(eV) Figure 5-5. Backscattered image and AES survey for the surface of Co 0.07 Ti 0.93 O 2 on LaAlO 3 (001): (a) Backscattered image. (b) (c) AES survey.

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83 Figure 5-6. Backscattered image of Co 0.02 Ti 0.98 O 2 on LaAlO 3 (001) 1m In order to further elucidate the composition of these segregated particles, we employed spatial chemical mapping using EDS in the FESEM for the Co-doped TiO 2 film (x=0.07). The results are shown in Figure 5-7. First note that the Ti and oxygen peaks were uniformly distributed across the film surface. If these segregated clusters were Co metal, we would expect a drop in Ti signal intensity in the segregated areas relative to the remainder of the films. This is not observed. However, when the Co signal is mapped, the nanoclusters clearly contain higher Co content than the TiO 2 film area between the particles. These results indicate that the segregated particles contain Co, Ti, and O elements. Figure 5-8 shows the intensity of each element peak plotted as a line scan across the Co-doped TiO 2 film surface (x=0.07). Clearly, the Co intensity is significantly higher in the segregated particles region. Note also that the oxygen peak intensity is somewhat higher in the segregation areas as well.

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84 Co map BS image Ti map Figure 5-7. BSE image and EDS mapping of Co 0.07 Ti 0.93 O 2 : (a) BSE image. (b) Co mapping. (c) Ti mapping. (d) O mapping. O map

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85 Co K ( b ) ( a ) O K Ti K ( d ) ( c ) Figure 5-8. BSE image and line scan of Co 0.07 Ti 0.93 O 2 : (a) BSE image. (b) Line scan for Co. (c) Line scan for Ti. (d) Line scan for O. In order to further investigate these segregated nanoclusters on the surface, high resolution transmission electron microscopy (HRTEM) was taken as shown in Figure 5-9 and Figure 5-10. Several segregated nanoclusters were observed in the cross-sectional image shown in Figure 5-9(a). Figure 5-10(b) shows an enlarged HRTEM image of segregated particles. Note that the segregated particles do not nucleate at the substrate surface, but rather within the continuous anatase film surface after films have started to grow. S. A. Chamber et al. [3] reported similar nanoparticles grown on LaAlO 3 (001) by PLD. From their diffraction patterns, it was realized that the nanoparticles were crystalline rutile, whereas the continuous film was anatase. S. R. Shinde et al. [58] also reported clusters formed across the entire surface of their thin

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86 films. However, those small particles were observed only within the thin films and at the substrate surface, showing a Co signal with no oxygen or titanium peaks. In order to characterize the segregated nanoclusters, we show a HRTEM bright-field image and selected-area diffraction patterns(SADPs) in Figure 5-11(a) and 5-11(b). From SADPs taken from region 1(LaAlO 3 (001) substrate), region 2((Co, Ti)O 2 thin film, region 3(interface between LaAlO 3 (001) substrate and (Co, Ti)O 2 thin films), we confirm the epitaxial relationship with (001)LaAlO 3 // (002)anatase and [001]LaAlO 3 //[001]anatase. These results match a high-resolution lattice image for the same film in Figure 5-13. Figure 5-12 shows SADPs taken from region 2 and 4 in Figure 5-10(b) for the Co 0.07 Ti 0.93 O 2 thin films. Area 2 and 4 correspond with (Co, Ti)O 2 region(no segregation) and segregated region, respectively. Both SADPs are completely matched together. As mentioned previously, EDS mapping, line scan and AES results show higher Co intensity on these particles than that of the rest of the films. Therefore, from these results and SADPs data, we confirm that the segregated particles observed on the surface of Co 0.07 Ti 0.93 O 2 thin films are Co-enriched anatase. Similar results have been reported by S. A. Chamber et al. [59] that under certain conditions, highly co-enriched nanoclusters were observed on the surface of the films which are grown by PLD. The inhomogeneous distribution of the Co ions should be carefully checked for films with a metastable phase, as in our Co doped TiO 2 anatase films. Note that the most stable phase is the rutile of the three different polymorphs of TiO 2 As known so far, although the anatase phase is difficult to make in bulk, it is metastable enough to be grown in thin film form on lattice matching substrates. Because the anatase is not

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87 thermodynamically stable, defects can be easily formed with a slight change in the film growth conditions. In addition, the Co ions are known to diffuse easily at a relatively low temperature, such as 400C. Therefore, the possibility of Co or Co oxide clusters also should be considered carefully in the case of oxide films grown at higher temperature. However, from our SADPs results, metallic Co or Co oxide clusters was not observed from the surface of films. Note that, from the thermodynamic point of view, cobalt oxide is less stable than TiO 2 at a low PH 2 O. The oxygen vacancies in the anatase TiO 2 thin films grown at low PH 2 O may help the diffusion of the Co ions, resulting in the formation of the nanoclusters. This result matches EDS results taken in the cross-section area in Figure 5-14 and Figure 5-15. If one assumes that the shape of nanoclusters is a cylindrical, the volume of one segregated particle is estimated as 9.510 7 nm 3 Approximately, there are 210 16 precipitates for the area of the film (0.2510 14 nm 2 ). Therefore, the total volume of the nanoclusters is 2.410 14 nm 3 The total volume of the sample is 5.7510 15 nm 3 The remainder of the film(not including nanoclusters) is 5.510 15 nm 3 Therefore, the total amount of Co from segregated particles per volume is 25at.% Co /(2.410 14 nm 3 ) = 1.0410 -13 at.%Co/nm 3 The total amount of Co from the remainder of the film per volume is 7at.% Co/(5.5110 15 nm 3 ) = 1.2710 -15 at.%Co/nm 3 Figure 5-14 shows a HRTEM image taken from a segregated particle, along with EDS results. EDS data shown in Figure 5-14(b) acquired from each cluster confirms a large concentration of Co in these clusters. These segregated particles show a stronger Co signal than the area of TiO 2 film between the particles. In addition, as shown in Figure

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88 Figure 5-9. A cross-sectional HRTEM image (a) and an enlarged image (b) taken from a Co 0.07 Ti 0.93 O 2 thin film

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89 Figure 5-10. A cross-sectional HRTEM image of a Co 0.07 Ti 0.93 O 2 thin films (a) and a magnified image for surface segregration of a Co 0.07 Ti 0.93 O 2 thin films (b): Region 1 (LaAlO 3 (001) substrate), region 2 ((Co, Ti)O 2 thin film, region 3 (interface between LaAlO 3 (001) substrate and (Co, Ti)O 2 thin films), region 4 (segregated particles on the surface of films) (a) LaAlO 3 (001) Co:TiO 2 (b) 1 3 2 4

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90 (a) (b) A(202) A(200) Figure 5-11. Selected-area diffraction patterns(SADPs) taken from a Co 0.07 Ti 0.93 O 2 thin films in Figure 5-10 (b). (a) Region 1. (b) Region 2. (c) Region 3. A(002) A(200) A(202) [010] L(100) L(001) (c) 3 1 [010] L(001) L(100) A(002) [010] 2

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91 Figure 5-12. Selected-area diffraction patterns(SADPs) taken from a Co 0.07 Ti 0.93 O 2 thin films in Figure 5-10(b). (a) Region 2. (b) Region 4. (a) 2 (b) 4

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92 Figure 5-13. A lattice image taken from Co 0.07 Ti 0.93 O 2 thin films away from segregated area (200) (101) (002) Co:TiO 2 5-14 and Figure 5-15, the intensity of the Co and Ti peaks decrease at same position, indicating that these segregated nanoclusters contains Ti elements. We also measured EDS on several different areas between the segregated particles. There were weak Co signals away from the clusters, indicating the presence of some Co inside the TiO 2 matrix in the form of substituted Co on the Ti sites. Figure 5-15 also show EDS data taken from the HRTEM image for Co 0.07 Ti 0.93 O 2 film shown in Figure 5-14(a). The intensity of Ti and Co peaks are high on the segregated particles relative to that in the rest of the anatase film.

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93 Figure 5-14. HRTEM image taken from secondary phase particle and EDS data for Co0.07Ti0.93O2 film La L1Ti K1Co K1O K1Al K1Distance(nm)Intensity(arb) La L1Ti K1Co K1O K1Al K1Distance(nm)Intensity(arb)(b) (a)

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94 Figure 5-15. EDS data for Co0.07Ti0.93O2 film Figure 5-16. EDS mapping taken from Co0.07Ti0.93O2 anatase thin films

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95 Figure 5-17. EDS data taken from the cross-sectional area of Co0.07Ti0.93O2 anatase Figure 5-16 shows EDS mapping data taken from Co0.07Ti0.93O2 anatase thin films. Ti was uniformly distributed through entire cross-section area. The Co peak showed higher intensity from the surface segregation of the films. The oxygen peak was observed over the entire film area. Figure 5-17 shows EDS data taken from the cross-sectional area of Co0.07Ti0.93O2 anatase. Co peaks were observed from the uniformly distributed area of anatase film and the segregated phase on the surface of the films, indicating the presence of some Co ion inside the TiO2 matrix. This suggests the possibility of a small quantity of Co occupying on Ti substitutional sites as a form of solid solution in anatase TiO2 thin film.

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CHAPTER 6 HALL EFFECT AND MAGNETORESISTANCE 6.1 Hall Effect and Magnetoresistance of Undoped and Co-doped Anatase 6.1.1 Introduction Anomalous Hall Effect(AHE) [60], observed in ferromagnetic semiconductors, have attracted considerable attention due to the interest in spin dependent transport phenomena caused by charge carriers. Critical information about the physical mechanism responsible for ferromagnetism can be obtained by measurement of AHE. The Hall resistivity of magnetic materials can be expressed as a sum of two terms-the ordinary part and anomalous one: xy = R0B + Rs0M (6-1) where R0 and Rs are the ordinary and anomalous Hall coefficients, respectively, B is the magnetic field, 0 is the magnetic permeability and M is the magnetization. While the ordinary Hall effect results from the Lorenz force, the AHE is related with the asymmetric scattering of the charge carriers where the spin-orbit interaction plays the important role [61]. It is known that two mechanisms of scattering are responsible for the anomalous Hall effect: the skew-scattering (anisotropic amplitude of scattered wave packet in the presence of spin-orbit coupling) and the side-jump(the changes in paths of charge carrier due to a lateral displacement) [62]. Recently, Jungwirth et al. [63] reported a new approach to the theory of AHE in semimagnetic semiconductors. This theory relates the AHE to Berry phase acquired by a quasiparticle wave function upon traversing closed paths on the spin-split Fermi surface. 96

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97 In this study, Hall effect and magnetoresistance measurement for undoped and (Co, Ti)O2 films has been performed at PPMS(Physical Properties Measurement System). 6.1.2 Experimental Procedures Both undoped geometry shown in Fig and 2 at.% Co-doped TiO 2 samples were cut in the ure 6-1. Figure 6-1. Schematic view from top of sample showing placement of volta ge and current leads. The applied magnetic field comes out of page. (a) Hall effect the Physical Property Measurement System (PPM inch dn 0C under water vapor of 10-3Torr. measurement. (b) Magnetoresistance measurement. The measurements were taken in S)manufactured by Quantum Design Inc. The sample pucks were loaded into a 1iameter cylinder kept at 7.6 Torr pressure of helium exchange gas. This cylinder is enclosed by a liquid helium Dewar which itself is enclosed by another Dewar filled with liquid nitrogen. The sample can be cooled down to 1.9K. The measurements were performed automatically by computer-controlled. Two types of sequences were used to analyze the samples. The first type of sequence was a magnetic field sweep from -6T to +6T while the temperature was kept constant. The following temperatures were chosefor magnetic field sweeps : 100, 200, and 300K. The second was a temperature sweep between 10K and 300K holding the magnetic field constant at a value of 0T and 1T. 6.1.3 Results and Discussion Figure 6-2 shows magnetic field dependence of xy at 300K for undoped TiO 2 thin films grown on LaAlO 3 (001) substrates at 65

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98 -2-1012-800-600-400-200 0 200 400 600 RH=-2.7cm3/C 300Kxy()Magnetic Field(0H/T) TiO2 on LaAlO3(001) -cm Figure 6-2. The Hall resistivity result for undoped TiO2 thin films The negatiall coefficient, which is independent on the magnetic field, was -2.7cm3/C at 300K. Figure 6-3 shows the Hall resistivity results for CoTiOthin films grown at 650C under water vapor of 10-3Torr. cannot be clearly seen because ordinary part of s l d term ect, where the latter term dominates over the former term in typ ve linear slope indicates n-type majority carrier in TiO 2 matrix. The H 0.020.982AH xy ( OH ) is dominant. However, the contribution of AH can be seen by subtracting OH It is known that anomalous part of xy ( AH ), that is proportional to magnetization, idominant for lower magnetic field whereas ordinary part of xy ( OH ), that is proportionato inverse of n overcomes ( AH ) for higher magnetic field [64]. In order to further investigate magnetic field dependence of xy the Hall effect measurement was performein lower scale of magnetic field. From equation (6-1), the first term denotes ordinary Hall effect and the second denotes the anomalous Hall eff ical ferromagnetic materials. If the charge carrier is ferromagnetically spin polarized, the Hall resistivity xy vs. B (B~ 0 H) curve should behave like the

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99 -2-1012-800-600-400-2000200400600800 RH(cm3/C)-3.87(100K)-2.58(200K) -2.21(300K) xy(-cm)Magnetic Field(0H/T) 300K 200K 100Kn thougn Figure 6-3. The Hall resistivity result for Co 0.02 Ti 0.98 O 2thin films magnetization hysteresis curve. As shown in Figure 6-4, xy shows a linear curve against magnetic field. Eve h the ordinary Hall effect is dominant, nonlinear anomalous Hall effect terms can be seen by extracting ordinary part 0 ( H) from xy xy 0 as shown in Figure 6-4(b), is anomalous Hall effect for Co 0.02 Ti 0.98 O 2. However, obvious anomalous Hall effect contribution was not observed, suggesting the spin-orbital interaction between electrocarriers and Co ions is weak. xy also increase with decreasing temperature. This phenomenon could be interpreted as an appearance of skew scattering at low temperature, which is anisotropic scattering of charge carrier by impurities, such as Co ions inside TiO 2 matrix. Figure 6-5 shows the Hall coefficient and carrier density of Co 0.02 Ti 0.98 O 2. The Hall coefficient increases as temperature decreases. However, the carrier density decreases as temperature decreases.

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100 Figure 6-6 shows normalized magnetoresistance for undoped and Co0.02Ti0.98O2at 200K. For each film, the magnetoresistance is positive and increases monotonically with increasing magnetic field. The Co0.02Ti0.98O2show stronger magnetoresistance than that of undoped anatase. This may originate from the skew scattering due to Co ions at low temperature. -40-1001020304050 -2000-1000010002000 -50 -30-20 (a) Co0.02Ti0.98O2-cm) -600-400-2000200400600-1.0-0.50.00.51.0 (b) Co0.02Ti0.98O2 xy( (xy) -(0)(-cm) 200K 100K 100K Magnetic Field( 0 H/Oe) Magnetic Field( 0 H/Oe) Figure 6-4. The Hall resistivity in low scale of magnetic field (a) and anomalous Hall effect (b) 100150200250300 2 4 Co0.02Ti0.98O2 Hall CoefficientHl(32.02.8 Temperature(K)1.6 lRcm/C)2.4 Carrier Densitycmarrier density of Co0.02Ti0.98O2Carrier Density(x1018-3) Figure 6-5. The Hall coefficient and c

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101 1.00151.00201.0025 -6-4-202461.00001.00051.0010 200Kxx(HMagnetic Field(T) )/xx(0) CoTiO 0.020.982 Undoped TiO2 Figure 6-6. Magnetoresistance data for undoped and Co0.02Ti0.98O2obtained at 200K 1.0 0.8 0501001502002503000.00.20.40.6 xx(-cm)Temperature(K) Resistivity Figure 6-7. The temperature dependence of xx for the Co0.02Ti0.98O2-Figure 6-7 shows temperature dependence of xx for the Co0.02Ti0.98O2-. The change in slope with temperature indicates the decrease of carrier concentration with decreasing temperature. Figure 6-8 shows magnetic field dependence of Hall resistivity xy(H) at 100K,

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102 200K, and 300K for the Co0.10Ti0.90O2grown on LaAlO3(001) substrates at 650C under PH2O=410-4Torr. The segregated particles from the surface of films were not observed in FESEM. This sample contains more n-type majority carrier than that of sample grown under PH2O=10-3Torr because of the low resistivity due to more oxygen deficiency. -2-1012-0.9-0.6-0.30.00.30.60.9 Co0.10Ti0.90O2-xy( -cm)Magnetic Field(H/Telsa) 300K 200K 100K Figure 6-8. The magnetic field dependence of Hall resistivity xy(H) at 100K, 200K, and 300K for the Co0.10Ti0.90O2grown under PH2O=410-4Torr AH is dominant for this 10 at.% Co-doped TiO2 films, while OH is dominant for Co0.02Ti0.98O2-. From the equation (6-1), the latter term dominates over the former term in Co0.10Ti0.90O2thin films. Therefore, the Hall resistivity xy(H) vs. B (~0H) curve ecreasing tempeg B behave like the magnetization hysteresis curve. xy also increase with d rature. This phenomenon can be explained by an appearance of skew scatterin(anisotropic scattering of charge carrier by impurities, such as Co ions) at low temperature.

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103 -6-4-202460.99920.99961.0000 Co0.10Ti0.90O2-xx(H)/ xx() 0Magnetic Field(0H/T) 300K 200K 100K Figure 6-9. Normalized magnetoresistance (xx(H)/xx(0)) in zero magnetic field at various temperature for the Co0.10Ti0.90O2films Figure 6-9 shows normalized magnetoresistance (xx(H)/xx(0)) in zero magnetic field at various temperature for the Co0.10Ti0.90O2films. For each film, the magnetoresistance is negative and decreases monotonically with decreasing temperature and increas attering re. t 300K. rly shows strong ferromagnetic hysteresis behavior at room temperature. The saturated magnetization (Ms) was 20emu/cm3. ing magnetic field. Figure 6-10 shows the temperature dependence of xx (0T) and xy (1T) for the Co 0.10 Ti 0.90 O 2thin films. xy (1T) corresponds to the anomalous part of Hall effect. Both xx and xy increase with decreasing temperature. The increase of xy with decreasing temperature can be interpreted as an appearance of skew scattering (anisotropic scof charge carrier by impurities in the presence of spin-orbit coupling) at low temperatuFigure 6-11 shows an M-H curve for Co 0.10 Ti 0.90 O 2thin films obtained aThis film clea

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104 0501001502002503001.0x10-35.0x10-31.0x10-2 xx xyTemperature(K)xx(-cm)1.0x10-52.0x10-44.0x10-4 xy(-cm) Figure 6-10. The temperature dependence of xx(0T) and xy(1T) for the Co0.10Ti0.90O220 thin films -2000-1000010002000-20 -10 0 10 Magnetization (emu/cm 3) 300KApplied Field (Oe) Figure 6-11. An M-H curve for Co0.10Ti0.90O2thin films. Magnetic field was applied oresistance was positive and increase parallel to the film surface. In conclusion, the ordinary Hall effect is dominant over the undoped and Co 0.02 Ti 0.98 O 2thin films. For each film, the magnet

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105 monotonically with increasing magnetic field. The anomalous Hall effect contribution is dominant for the Co0.10Ti0.90O2grown under lower water vapor pressure due to more oxygen deficiency. An M-H curve show strong ferromagnetic behavior at room temperature.

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CHAPTER 7 SUMMARY In many diluted magnetic semiconductor oxide systems(O-DMS), the room temperature ferromagnetic materials has opened up a way for spintronics devices. The TiO2 compounds has been extensively studies as a host matrix for being doped essentially with Co. Most of the films obtained exhibit ferromagnetism (FM). However, the mechanism of these ferromagnetic properties is still in controversy. Some authors claimed that it arises from Co clusters. Other groups suggested that it originate from the uniform distribution of Co which can be substituted with Ti cations in the anatase films. Several dopants, such as Co, Cr, Fe, have been utilized to realize ferromagnetism above the room temperature. Theory predicted FM for these doped materials. However, only Co doped anatase films have been reported the Curie temperature above the room temperature. So far, a clear mechanism of the origin of magnetism in O-DMS has not been obtained. It seems that not only the growth condition, including the pressure of oxidants during the growth, such as water vapor and oxygen gases, but also the growth technique (MBE, PLD, co-sputtering) and intrinsic effect(substitutional dopants with matrix cations) have a strong effect on the properties of these films. Therefore, more study for these materials is needed to explain the origin of the ferromagnetism above the room temperature. This chapter briefly summaries the major results of this work. Single-phase (001) anatase thin films have been realized via epitaxial stabilization on (001) LaAlO3 106

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107 substrates using reactive sputter deposition. Phase-pure anatase can be achieved using either water vapor or oxygen as the oxidizing species, although crystallinity was slightly degraded for films grown with water vapor. The llization of the films is excellent, with rocking curves as narrow as 0.09ess was essentially that of the substrnd ains 2e of temperature except for 700C. The intensity of rutile (200) phase was much less with PO2 than that with PH2O at a total pressur of 15mTorr with Ar gas. Hall coefficient and magnetoresistance results for the TiO2 on Si(100) with 10 Torr of a water vapor at 300K shows typical n-type semiconductor behavior at 300K. Epitaxial CoxTi1-xO2 anatase thin films were grown on (001)LaAlO3 by a reactive RF magnetron co-sputter deposition with water vapor serving as the oxidant. The use of water as the oxygen source proves useful in growing oxygen-deficient, semiconducting CoxTi1-xO2 by reactive sputter deposition, with undoped and Co-doped TiO2 thin films showing n-type semiconductor behavior, with carrier concentrations of 10-10cm. crysta Film roughn ate. This result should prove enabling for future activities in understanding aexploiting the electronic and optical properties of anatase without the interfering effectsof grain boundaries or rutile secondary phase. The use of hydrogen during growth to manipulate Ti valence appears possible, although controlling phase formation remchallenging. The RF reactive sputtering method is used to prepare titanium dioxide thin films onSi(100) p-type at temperature ranging from 550C to 750C. Substrate temperature has a significant effects on the rutile and anatase phase of TiO films. For the temperature range examined, the only rutile (200) phase was observed in the range of temperature between 600C and 650C with Ar gas. The only rutile(200) phase, however, was observed in the all rang -3 1718-3

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108 Magnetization measurements of Co x Ti 1-x O 2 (x=0.07) thin films reveal ferromagnetic behavior in M-H loop at room temperature with a saturation magnetization on the order of 0.6 Bohr magnetons/Co. X-ray photoemission spectrometry indicates that the Co cations are in the Co 2+ valence state. However, chemical analysis of surface structurindicates that the cobalt segregates into a cobalt-enriched particles. From SADPs, we confirm that the nanoclusters observed on the surface of the Co e ll 0.02Ti0.98O2thin films. For each film, the magn 0.07 Ti 0.93 O 2 are Co-enriched anatase. From the resistivity measurement as a function of magnetic field, the ordinary Haeffect is dominant over the undoped and Co etoresistance was positive and increase monotonically with increasing magnetic field. The anomalous Hall effect contribution is dominant for the Co 0.10 Ti 0.90 O 2grown under lower water vapor pressure due to more oxygen deficiency.

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BIOGRAPHICAL SKETCH ngest son of Jum-Soo Jeong and Sook-Ja Kim. He graduated from Myung-Shin High School in 1992.tion with aical Engineering in February of 1996. His education continued Engineering. He also attended Korean Institute of Science and Technology (KIST), one Metalt receivresear Byoung-Seong Jeong was born in Hadong, South Korea August 15, 1973, you He attended The SungKyunKwan University from March of 1992 to his gradua BS in Metallurg with graduate studies at the Yonsei University in the Department of Metallurgical of the most advanced national laboratories in Korea, as a research assistant. He performed several national projects successfully at KIST. He received his MS in lurgical Engineering in 1998. He continued his research at KIST until he wen abroad to study in the United States in August of 2000. He has participated in research concerning electronic oxide materials in the Department of Materials Science and Engineering at the University of Florida. He ed his MS in 2003. He hopes to obtain future employment either in industrial ch or at a national laboratory. 114