Epitaxial films, heterostructures and composites of a, b and m phases of VO2 2

EPITAXIAL FILMS, HETEROSTRUCTURES AND COMPOSITES OF A, B AND M PHASES OF VO2 AMAR SRIVASTAVA (M. TECH, INDIAN INSTITUTE OF TECHNOLOGY KANPUR, INDIA M.Sc, INDIAN INSTITUTE OF TECHNOLOGY DELHI, INDIA) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN SCIENCE DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2014 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for a degree in any university previously. Amar Srivastava 20 August 2014 i ACKNOWLEDGEMENTS This thesis, a truly life-changing experience for me, is not only the end of my journey in obtaining my Ph.D., but also has opened up the doors of new opportunities for me. It is a milestone of nearly years of my research work at NUS and specifically within the NanoCore Laboratory. My experience at NanoCore has been nothing short of amazing. I have been blessed with ample of opportunities, and have taken advantage of them. This thesis is also the result of many experiences I have encountered at NUS from dozens of remarkable individuals who I also want to acknowledge. First and foremost I wish to thank my advisor, Professor T. Venkatesan, director of NUSNNI-NanoCore at NUS, who has encouraged and influenced me in all my efforts and endeavors. I consider myself extremely fortunate to have worked together with and been supervised by Venky. His personality and gesture are contagious and has influenced in developing my personality as an individual. His knowledge and experience that he imparted onto me in research and career will forever support me in pursuing my goals. I also want to take this opportunity to acknowledge my co-supervisor, Prof. Jun Ding. Prof. Jun Ding has been extremely encouraging and had taken keen interest in my research activities. He has always helped me out with his invaluable inputs about my work. I thank Prof. D.D. Sharma, Prof. Daniel Khomskii, Prof Michael Coey and Prof. A. Rusydi for their invaluable support. There is no doubt whatsoever, that my work would not have been possible without them. They have been of tremendous help with experiments as well as theoretical understandings of my subject. ii I would like to thank Dr. Surajit Saha, my good friend and colleague. Dr. Saha is a focused individual with very sharp instincts of a researcher. Whenever I had felt totally lost with my research, I had blindly turned to Dr. Saha for help. His critical inputs have definitely helped me in taking my work to the next level. I feel happy to thank him for all his help. I thank Dr. C.B. Tay and Dr Herng Tun Seng. Both of them are very helpful individuals and have helped me with PL and with understanding the data. I have been fortunate enough to have some of the most wonderful, talented and helpful lab-mates. I want to thank Banabir Pal, Kalon Gopinadhan, Sinu Mathew, Xiao Wang , Mallikarjunarao Motapothula, Lv Weiming, Huang Zhen, Anil Annadi, Zeng Shengwei, Liu Zhiqi, Michal Dykas, Yong Liang Zhao, Tarapada Sarkar, Naomi Nandakumar, Masoumeh Fazlali and last but not the least Abhimanyu Singh Rana. Over the years we have been more of good friends and less of colleagues. I guess we will always remember the night outs in the lab. I also warmly remember all the Summer Internship students who have worked with me during my stay at NUSNNI-NanoCore. It has been an honor to know and work with you all. I definitely want to thank all the Lab officers and lab staff who have supported in running the lab smoothly throughout the period of my research. I want to thank all the other staffs at the NUSNNI NanoCore office specially Syed Nizar, Teo Ngee Hong and Marlini Binte Hassim. I would like to take this opportunity to mention my friends in Singapore. Most importantly, Prashant, Ajeesh, Rajesh, Dolly, Orhan, Ekta, Mrinal, and Olga, I thank you all from the bottom of my heart for the much necessary distractions. It has been a pleasure knowing all of you. iii I particularly want to thank Dr. Helene Rotella who joined NanoCore when I was in my last year of Ph.D. Her experimental expertise and analytical skills have improved my understanding on my research work. I cannot be thankful enough to her for encouraging me and giving me moral support during the most difficult times while writing this thesis. Finally and most importantly, I want to express my love and gratitude to family members. My parents, brother Gaurav, sisters Garima and Pooja – you are the source of my sustenance. I could not have asked for anything more from you. It is all because of you. Thank you for being so patient and supportive especially during the time of my Ph.D. iv TABLE OF CONTENTS DECLARATION . i ACKNOWLEDGEMENTS . ii ABSTRACT . viii LIST OF PUBLICATIONS . x LIST OF TABLES . xii LIST OF FIGURES xiii LIST OF SYMBOLS xix Chapter Introduction 1. Crystal structure of VO2(M1) and VO2(R) 1. Transition Mechanism: Peierls vs Mott-Hubbard? . 1. Development in the understanding of VO2 field in chronological order . 1. VO2 polymorphism and phase transition . 11 1.4. VO2(M2) monoclinic phase 13 1.4. VO2(A) Tetragonal Phase . 15 1.4. VO2(B) Monoclinic Phase 16 1. Substrate and buffer layer materials for film growth 18 1.5. Aluminum Oxide (Al2O3) . 18 1.5. Zinc Oxide (ZnO) . 19 1.5. Perovskite LaAlO3, SrTiO3, LSAT, LSAO substrates 20 Chapter Sample Preparation and Various Characterization Technique 22 2. Sample preparation technique: Pulsed Laser Deposition 23 2. Different growth modes and surface kinetics for thin film . 24 2. Thin Film Epitaxy . 26 2. Structure characterization techniques 28 2.4. X-ray diffraction . 28 2.4. Rutherford Backscattering Spectrometry (RBS) and Ion Channeling 31 2.4. Transmission Electron Microscopy (TEM) 34 2. Optical band gap- Ultraviolet-visible Spectroscopy 37 2. Transport properties study technique: Physical Property Measurement System 38 2. Raman Spectroscopy . 40 Chapter A, B and M Single Phase VO2 Films by Tuning Vanadium Arrival Rate and Oxygen Pressure . 44 v 3. Pulse Laser Deposition of VO2 Polymorphs . 45 3. Structural Characterization of different Polymorphs of VO2 46 3.2 X–Ray Characterization . 46 3. Phase Diagram for the different phases of VO2 48 3. Microscopic Studies 50 3.4. Cross Sectional TEM of VO2(A) film 51 3.4. Cross Sectional TEM of VO2(B) film . 56 3.4. High resolution X-ray diffraction analysis of VO2(A) and VO2(B) thin film 60 3. Raman spectroscopy studies 63 3.5. Raman spectroscopic analysis of VO2(M) Phase . 64 3.5. Raman spectroscopic analysis of VO2(A) and VO2(B) films . 65 3. Transport Properties 66 3.6. Temperature dependent Resistivity measurement 66 3.6. Hall measurement for the carrier density and mobility 70 3. X-ray photoelectron Spectroscopy analysis . 73 3. Conclusion . 74 Chapter Effect of Modified Orbital Occupancy on the Electrical Behavior of VO2 Polymorphs on SrTiO3-Si Substrate 76 4. Characterization of VO2 polymorphs deposited on SrTiO3 (28nm)-Si substrate 77 4.1. X-Ray characterization 78 4.1. Oxygen resonance Rutherford backscattering spectra 81 4.1. Mid and Far infrared spectroscopy . 82 4.1. Comparison of Raman and Infrared Spectra of films deposited on STO and STO-Si substrate . 84 4. Temperature dependent Raman of VO2(A) . 86 4. Temperature dependent Raman of VO2(B) . 88 4. Conclusion . 97 Chapter Vertical Nanocomposite Heterostructure Thin Films of VO2(A) and VO2(B) . 98 5. Hetrostructures of VO2(A), VO2(B) 99 5. Deposition of vertical nanocomposite heterostructure thin films 99 5. Electrical transport nanocomposite heterostructure thin films . 100 vi 5. Structural characterization of vertical nanocomposite heterostructure thin films 101 5.4. X-Ray measurement . 101 5.4. TEM analysis of nanocomposite heterostructure thin films . 104 5. HAXPES analysis of nanocomposite heterostructure thin films . 106 5. Conclusion . 108 Chapter Coherently Coupled ZnO and VO2 Interface Studied by Photoluminescence and Electrical Transport across a Phase Transition 110 6. Introduction . 111 6. Pulse Laser Deposition of VO2 . 112 6. Growth of ZnO and VO2 . 112 6. Structural Characterization 114 6.4. X–Ray Diffraction Studies . 114 6. Electrical Characterization 114 6. Photoluminescence 115 6.6. PL of VO2 and ZnO/VO2 coherently coupled interface . 115 6. Conclusion . 120 Chapter Rectifying Behavior of VO2(A), VO2(B) on Nb-STO Substrate 121 7. Introduction . 122 7. Deposition of VO2 polymorphs on Nb-SrTiO3 122 7. Transport Measurement of VO2(B) films of different thickness . 123 7. Rectifying behavior of VO2(B)/ Nb-SrTiO3 124 7. I-V and C-V measurement for VO2(B)/ Nb-STO film . 126 7. Rectifying behavior of VO2(A)/ Nb-SrTiO3 129 7. Conclusion . 130 Chapter Summary and Future Work . 132 8. Summary . 132 8. Future Work 133 BIBLIOGRAPHY 135 vii ABSTRACT Transition metal oxides exhibit various polymorphic structures, among which many are neither stable in ambient conditions nor can be easily synthesized. Integration of these metastable phases on Si substrates promises novel device functionalities. Prime among them is metal insulator transition based functionality using transition metal oxides such as VO2(M). VO2 exhibits two other layered polymorphs which are promising materials to study strong electronic correlations resulting from structure [VO2(A)] or their use as electrode materials for batteries [VO2(B)]. However, growing single crystal thin films of these novel metastable phases have remained a challenge. I demonstrate for the first time that high quality single phase films of VO2(A, B, and M) can be grown on Si substrate by controlling the vanadium arrival rate (laser frequency) and oxidation of the V atoms. Single phase monoclinic VO2(M), tetragonal VO2(A) and monoclinic VO2(B) thin films were grown on (100) SrTiO3 (STO) and (100) STO (28 nm) buffered Si substrates using PLD. A phase diagram has been developed (oxygen pressure versus laser frequency) for various phases of VO2. A detailed structural analysis, coupling X-ray diffraction and transmission electron microscopy, revealed a [011]VO2(M)||[100]STO, [110]VO2(A)||[100]STO, [001]VO2(B)||[100]STO epitaxial relationship and the presence of 90° oriented domains for VO2(A) and VO2(B) thin films respectively. The transport measurement showed that B is semi-metallic, A is insulating while M is semiconducting which was corroborated by the HAXPES measurements. Furthermore, the presence of the V-V dimers (present in all phases with varying amounts) probed by Raman and infrared spectroscopic measurements in the three polymorphs underscores the importance of dimerization that strongly influences the electronic properties of VO2. Considering the R/M system, orbital band diagram and relative position of different bands for the VO2(A) and VO2(B) with respect to viii VO2(M) are proposed. In order to corroborate our model a deep study on the behavior of these two polymorphs grown on STO and STO-Si substrate, in term of structural behavior as well as electronic transport behavior is performed. I present a detailed study on composite films of VO2(A) and VO2(B) phases and show that these composite films exhibits a metal insulator transition similar to the VO2(M/R) phase transition. However, extensive TEM and temperature dependent XRD studies reveal that the film is mainly comprised of VO2(A) and VO2(B) phases and very little of M phase. The A phase is under compressive stress while the B phase is under tensile stress and we believe this stress leads to the dimer induced metal insulator transition in this system presumably triggered by the small amount of M phase present. This raises the question “Is a structural phase transition necessary for the metal to insulator transition (MIT) in VO2(M)?” I report the study on a coherently coupled interfaces of ZnO/VO2(M) in a heterostructure form to study the effect of strain exerted due to the structural phase transition of VO2(M) on the over-layer. This strain induced defects in the over layer (ZnO) was monitored by measuring the photoluminescence from ZnO which exhibited a temperature dependent hysteresis similar to the hysteresis in transport exhibited by the VO2 layer below. Considering the strong potential application in devices of the two polymorphs VO2(A and B), I report on the electronic properties of the junctions formed in VO2(A)/ NbSrTiO3 and VO2(B)/ Nb-SrTiO3. Both the junctions showed rectifying behavior while temperature dependent I-V and 1/C2-V behaviors confirmed that for VO2(B)/ NbSrTiO3 rectified junction, the surface electronic structure of VO2(B) is distinct from that of the interface of the film to substrate and does not undergo the transition seen in bulk. ix Chapter Rectifying Behavior of VO2(A), VO2(B) temperature. Large values of n can be attributed to the effects of the bias voltage drop across the interfacial insulator layer, the particular distribution of interface states at the insulator/ semiconductor interface, and the special barrier inhomogeneities at the metal/semiconductor interface [155, 157-159]. For 0.5 wt % Nb:SrTiO3 with a correspondingly thinner depletion layer, this effect is even more pronounced, suppressing the relative temperature dependence of I-V. The reverse breakdown voltage is estimated to be -19 V and -16 V for VO2(B) / 0.01 wt % and 0.5 wt % Nb:SrTiO3 Shottky junctions respectively at room temperature as shown in the inset of Figure 7.3 (a) & (b). This could be attributed to the sensitivity of the avalanche breakdown effect to Nb concentration [160]. Figure 7. Temperature dependence of the built-in potential Vbi of the VO2(B) /Nb: SrTiO3 junctions, as derived from C-V measurements as in Fig. 3(c) and 3(d) for cooling (circle) and heating (square) cycle. Depletion width at zero bias depends on the doping concentration as in Eq. 𝑉 𝑊𝑑𝑒𝑝 = �2𝜀𝑠 (𝑞𝑁𝑏𝑖 ) 𝐷 128 (3) Chapter Rectifying Behavior of VO2(A), VO2(B) Where εs, ND, Vbi are semiconductor permittivity, carrier concentration, and built in potential, respectively. Using ND, εs from references [161-163] and Vbi from Figure 7.4, we find depletion width to be 79 nm for VO2(B)/ 0.01 % Nb-SrTiO3 and nm for VO2(B)/ 0.5 % Nb-SrTiO3 schottky junction. Similar results were also observed for Au/Nb:SrTiO3 junctions [164]. Aside from the difference in Vbi arising from the difference in work function between Au and VO2(B), our results match previous studies of metal Schottky junctions formed with Nb:SrTiO3. These results indicate that VO2(B) behaves as a Schottky metal at all temperatures despite exhibiting the first-order MI transition shown in Figure 7.2. Temperature dependence of barrier height Vbi has been extracted by fitting Eq. to the capacitance data in Figure 7.3 (c) & (d) in the voltage range of V-0.5 V and has been shown in Figure 7.4 for VO2(B)/[0.01, 0.05, 0.5%] Nb:SrTiO3. In Figure 7.4 we neglect the nonlinear permittivity of SrTiO3, leading to the unphysical drop in Vbi below 150 K for the junction. Barrier height is less sensitive to the temperature for VO2(B)/ 0.5 wt % Nb:SrTiO3 junction because in the VO2(B)/ 0.5 wt % Nb:SrTiO3 junction, the increase in Nb doping by a factor of 50 decreases the relative contribution of the depletion region in SrTiO3 to the junction capacitance hence high internal electric field in the junction drives the permittivity to the high field limit, which is relatively temperature independent hence the Vbi is less temperature sensitive [162, 164]. However, the smooth variation of junction capacitance and Vbi through the metal-to-insulator transition in Figure 7.4 with no hysteresis indicate no change in electronic structure of VO2(B) at the interface because of the clamping effect from the substrate. 7. Rectifying behavior of VO2(A)/ Nb-SrTiO3 The junction behavior is also tested for VO2(A) by directly depositing the VO2(A) films on Nb-STO substrate. Figure 7.5 (a) and (b) shows temperature dependent I-V 129 Chapter Rectifying Behavior of VO2(A), VO2(B) measurements for VO2(B)/ 0.01 wt % and 0.01 wt % Nb:SrTiO3 junctions. Considering electron affinity (χ) of Nb-SrTiO3 to be 4.0 eV [156], the estimated work function of VO2(A) film is ~4.3 eV and the schottky barrier height ~0.3 eV while the work function of VO2(B) film is ~4.5 eV and the schottky barrier height ~0.5 eV. Figure 7.5 I-V characteristics of (a) VO2(A)/ 0.01 wt% Nb: SrTiO3 and (b) VO2(B)/ 0.01 wt% Nb: SrTiO3. 7. Conclusion VO2(A) and VO2(B) are two interesting metastable polymorphs in family of VO2 system. The transport measurement suggest VO2(A) to be insulator while VO2(B) exhibits a strong first-order transition from a high temperature metal to a low temperature insulator. In this work we investigated the electronic properties of the junctions formed in VO2(A)/ Nb-SrTiO3 and VO2(B)/ Nb-SrTiO3. Although the junctions are rectifying at room temperature for both the junction, for VO2(B) the current-voltage and capacitance characteristics in the temperature range from 300 K150 K show no indication of the metal-insulator transition clearly observed in the transport measurement. In conclusion this study suggests that in VO2(B) the MI transition is strongly suppressed at the interface. This is also coherent with the systematic evolution of TMI with film thickness. The STO substrate clamps the VO2(B) 130 Chapter Rectifying Behavior of VO2(A), VO2(B) to a higher symmetry phase at the interface and prevents the distortions which are characteristics of MI transition in VO2(B). Contrary to heterostructures which consist of non-conducting surface of dead layer, these results exhibit a persistent metallic surface with an unusual characteristic at the interface incorporating strongly correlated electron systems. 131 Chapter Summary and Future Work Chapter Summary and Future Work 8. Summary We have investigated the different VO2 phases- M (R), A and B which are characterized by different V-V bond distances. This work presents a truly original idea for the growth of VO2(M), (A) & (B) phases in thin film form. The main idea, which is very unique and novel, is that by changing the vanadium arrival rate (in other words laser frequency of the PLD process) and the oxygen background pressure one can grow the various important phases of VO2 selectively. During the film formation process if the V-V interaction is strong (with less interference from oxygen), this leads to selective formation of phases containing shorter bonds. We also explored the possible suitable substrates for the growth of these phases. In the process, we have discovered that both the phases can be grown only on the SrTiO3 (100). Taking into account the possible technological importance of these materials, it was necessary to investigate the growth of these phases on Si substrate. Hence, SrTiO3 (100) (28 nm) buffer-layered Si substrates have been used to grow these polymorphs to make these phases suitable for Si based device technological applications. A molecular orbital picture is used to demonstrate the effect of modified orbital occupancy which is a consequence of different V-V dimer distances and apical V-O distances, on the electronic transport properties of VO2 polymorphs. We also investigated, whether in VO2(M) structural phase transition is a necessary condition to have metal insulator transition or not. For that we created an artificial VO2(M) like MIT using vertical nanocomposite heterostructures of VO2(A) and VO2(B) 132 Chapter Summary and Future Work for the first time. We believe that the lateral strain induced by column of VO2(A) on VO2(B) and vice- versa could be responsible of inducing a metal insulator transition very similar to VO2(M). The negligible amount of VO2(M) (< 1%) in these composite and no structural phase transition in the nanocomposite suggest structural phase transition is not necessary driving MIT in VO2(M). We demonstrated a way to use the phase transition of VO2 to exert a strain on an overlayered ZnO film across the MIT that induced defects in the over layer which was monitored by measuring PL. If the (VO2) phase transition can induce a strain on the over layer and change its optical or magnetic property then the phase transition can be monitored by other than transport measurement which will give an extra degree of freedom in device design. On the other hand such strain can also induce defects in the over layer which could also be monitored by measuring the over layer property leading to a better understanding of the behavior of materials under stress. We also showed the electronic properties of the Schottky junctions formed in VO2(A)/Nb-SrTiO3 and VO2(B)/Nb-SrTiO3 and showed that both the junctions show rectifying behavior. The temperature dependent 1/C2-V behaviors for VO2(B)/NbSrTiO3 showed a smooth variation of junction capacitance and Vbi through the metal-toinsulator transition with no hysteresis indicating no change in electronic structure of VO2(B). Hence we have confirmed that the surface electronic structure of VO2(B) is distinct at the interface of the film and does not undergo the bulk transition. 8. Future Work VO2 and its polymorphs have proved to be an extremely rich system to work on. Besides extremely interesting physics, they also gives ample technological device 133 Chapter Summary and Future Work application scopes. Here are some of the very exciting opportunity that could be pursued: • Investigate the doping effect in VO2(A) and VO2(B) films. The most probable candidates for doping are W6+, Cr3+, Mo5+, Ti4+ etc.; It will be interesting to investigate if by doping we can induce MIT in VO2(A) or can suppress the MIT in case of VO2(B). • In our study we found that these phases can be grown only on SrTiO3 substrate specially the VO2(A) phase. It would be very exciting to look for other substrates on which these phases can be established. Establishing these phases will allow us to study the effect of strain on the physical properties of these films. One exciting way to study the strain effect could be, as these phases can be grown on STO substrate only, using a buffer layer on top of STO. For example using BaTiO3 buffered STO with different thickness of BTO and then depositing VO2(A) or VO2(B) films. 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(28 nm)/Si substrate From the simulation and fitting we confirmed the composition for the two films VO2( A) and VO2 (B) deposited on two different substrate SrTiO3 and SrTiO3 (28 nm)/Si is VO2 0. 02 82 Figure 4.4 The far infrared transmittance (%) of VO2 (M) , VO2( A), VO2 (B) films deposited on SrTiO3 (28 nm)/Si substrate 83 Figure 4.5 Comparison of Raman spectra of VO2 (M) , VO2( A), VO2 (B) films deposited... 72 Table 4.1 Comparison of the rocking curves and the calculated d spacing’s of M, A and B phase of VO2 deposited on SrTiO3 and SrTiO3 (28 nm)/Si substrate 81 Table 4 .2 Comparisons of Raman and Infrared active modes present in the polymorphs VO2 (M) , VO2( A) and VO2 (B) films deposited on buffered STO-Si substrate 84 Table 4.3 Comparison of the resistivity of M, A and B phase of VO2 deposited... 25 nm and 50 nm thickness) and VO2 (M) (50nm) films The inset shows pattern of (a) VO2 (M) (black), (b) VO2 (B) (blue) thin films on SrTiO3 (100) substrate 123 Figure 7 2 (a) Schematic density of states (above) for Insulating VO2 (B) and the following band diagram (below) of a VO2 (B) / Nb: SrTiO3 junction for T TMI 125 Figure 7 3 Temperature dependent... trigonal bipyramid and distorted octahedron based on which a phase diagram has been generated to understand the growth of different 11 Chapter 1 Introduction vanadium oxide phases [24 ] as shown in Figure 1.4 In the class of vanadium oxide, VO2 exhibits a number of polymorphic forms, such as VO2 (M1 ), VO2 (M2 ), VO2 (M3 ), VO2( R), VO2( A), VO2 (B) and VO2( C) Various preparation techniques have been used and developed... temperature range (400 K- 150 K) 101 xvi Figure 5 2 X-Ray diffraction θ - 2 spectra for (a) VO2( A), VO2 (B) and the different composite of B and A, (b) calculated d spacing (b) grain size for VO2( A) and VO2 (B) for the composites 1 02 Figure 5 3 (a) 2D XRD plot χ vs θ -2 for M, B0 .25 A0.75 and B0 .71A0 .29 thin film Pole figure for (b) M phase film, (c) B0 .25 A0.75 and (d) B0 .71A0 .29 ... stabilize these phases in bulk and thin film forms Figure 1.4 Experimental phase diagram of the VOx system [24 ] Pyrolysis of vanadium precursor [25 ], soft-chemical route [26 ], reduction of V2O5 into VO2 (M) [27 ], transforming the VO2( A) or VO2 (B) powders using heat treatment into VO2 (M) and hydrothermal synthesis technique [28 -31] have been used to study these phases in the bulk and nanostructured forms... tetragonal VO2 5 Figure 1.4 Experimental phase diagram of the VOx system 12 Figure 1.5 Phase diagram of V1−xCrxO2 and M1 , M2 , and M3 indicate the metallic rutile and the three insulating monoclinic phases, respectively 13 Figure 1.6 (a) Monoclinic M1 ~M (b) Monoclinic M2 structure of VO2 14 Figure 1.7 Comparison of lattice parameters of M1 , M2 and R phases 14 Figure 1.8 Bulk crystal... TaxTi1-xO2: role of polarons” (Manuscript in preparation) xi LIST OF TABLES Table 2. 1 List of some materials deposited for the first time by PLD after 1987 and references 24 Table 3.1 Raman and Infrared active modes predicted by group theory for three different polymorphs VO2 (M) , VO2( A), VO2 (B) 64 Table 3 .2 Comparison of hall carrier density and mobility of different polymorphs of VO2. .. B0 .71A0 .29 films 103 Figure 5 4 (a) θ -2 XRD measurement at 45° in φ and 7° in χ for B0 .25 A0.75 and pure VO2 (M) film (b) θ -2 XRD of the B0 .25 A0.75 nanocomposite film at 0° in φ and 0° in χ 104 Figure 5 5 (a) Cross sectional TEM image of the VNH B0 .25 A0.75 (b) Zoomed images of the top left (TL) and top right (TR) recangular area (c) Ball & stick model of crystallographic VO2 (B) /VO2( A)... characteristics of (a) VO2 (B) /0.01 wt% Nb: SrTiO3 and (b) VO2 (B) / 0.5 wt% Nb: SrTiO3 1/C2 characteristics of (c) VO2 (B) /0.01 wt% Nb: SrTiO3 and (d) VO2 (B) /0.5 wt% Nb: SrTiO3 126 Figure 7 4 Temperature dependence of the built-in potential Vbi of the VO2 (B) /Nb: SrTiO3 junctions, as derived from C-V measurements as in Fig 3(c) and 3(d) for cooling (circle) and heating (square) cycle 128 Figure . Introduction 122 7. 2 Deposition of VO 2 polymorphs on Nb-SrTiO 3 122 7. 3 Transport Measurement of VO 2 (B) films of different thickness 123 7. 4 Rectifying behavior of VO 2 (B) / Nb-SrTiO 3 124 7 for VO 2 (B) (10 nm, 25 nm and 50 nm thickness) and VO 2 (M) (50nm) films. The inset shows pattern of (a) VO 2 (M) (black), (b) VO 2 (B) (blue) thin films on SrTiO 3 (100) substrate. 123 Figure. composite of B and A, (b) calculated d spacing (b) grain size for VO 2 (A) and VO 2 (B) for the composites. 1 02 Figure 5. 3 (a) 2D XRD plot χ vs θ -2 for M, B 0 .25 A 0.75 and B 0.71 A 0 .29

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