DEFECT INDUCED NOVEL ELECTRICAL, MAGNETIC AND OPTICAL PROPERTIES OF TiO2 THIN FILMS GROWN BY PULSED LASER DEPOSITION TARAPADA SARKAR (M.Tech., INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN SCIENCE DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2015 i DECLARATION I hereby declare that thesis is my original work and it has been written by me in it’s entirety I have duly acknowledged all the sources of information which have been used in the thesis This thesis has not also been submitted for any degree in any university previously Tarapada Sarkar 15 August 2015 ii ACKNOWLEDGEMENTS The completion of my dissertation and subsequent Ph.D has been a long journey It was a journey of yrs and months which taught me a lesson that life doesn’t stand still, nor wait until you are finished Many things have happened and changed in the time in which I have been involved with this project Many have questioned whether I would finish my dissertation, and have doubted my commitment to it I, on the other hand, losing confidence so many times that I could not keep count of, jumping here and there, computers crashing, needing to work as much as possible, so many sleepless night and pure frustration had to push on With all this, I knew that I will complete my Ph.D I just had to it in my own time and on my own terms I am grateful to a lot of people who have been instrumental in enabling me to reach my goal It humbles me to acknowledge them If I have to name one man for whom I am writing this thesis and this acknowledgement, he has to be my supervisor, Prof T Venkatesan Venky, as he is called by one and all has been one of the biggest influences in my life I consider myself to be extremely fortunate to have known, worked together with and been supervised by Venky He has encouraged me in all my efforts and endeavours He has managed to keep me motivated in my research Venky has been extremely patient with me Venky has had a tremendous contribution in my development as an individual I also want to take this opportunity to acknowledge my co-supervisor, Prof Ariando Prof Ariando has been extremely supportive and had taken keen interest in my research activities Thanks to Prof J.M.D Coey for the invaluable feedback and inputs in my research work I thank to Dr Sankar Dhar and Dr Arkajit Roy Barman, my mentors, who helped me a lot in my 1st two years of Ph.D They have been of extraordinary help iii in helping me manage my research work and giving direction to it Their critical inputs have definitely helped me in taking my work to the next level I thank Prof A Rusydi for his help in understanding the use of optical and x-ray spectroscopy for looking at defects in TiO2 They have been of tremendous help with experiments as well as theoretical understandings of my subject I also thank Prof H Yang for the many fruitful discussions and the opportunities to work together I want to thank Dr K Gopinadhan Gopi has been the epitome of sincerity whom all graduate students in our lab have tried to idolize Gopi has helped me a lot with transport measurements and helped me understand the intricate physics related I would also want to thank Dr S Saha We have been good friends in the few days that we have known each other He has helped me with Raman measurements and with understanding of the data I thank Dr C.B Tay- Chuan Beng is a very helpful individual and is always ready to help with PL measurements I must thank Dr W Lú- Weiming has helped me a lot with SQUID measurements and also with PLD depositions I have been fortunate enough to have some of the most wonderful, talented and helpful lab-mates I want to thank Mallikarjuna rao Motapothula, Anil Annadi, Liu Zhiqi, Yong Liang Zhao, Teguh Citra Asmara, Zhihua Yong, Amar Srivastava, Siddhartha Ghosh, Naomi Nandakumar, Abhijeet Patra, Soumya Sarkar, Zeng Shengwei, Ma Haijiao, Abhimunya Rana, Brijesh Kumar, Meenakshi Annamalai, Xiao Wang, Michal Dykas, Masoumeh Fazlali Pranjol kumar Gogoi, and Marlini It is difficult to express love and gratitude to family members I would like to thank my family, my parents who raised me with a love of science and supported me in all my pursuits I love you and will forever be indebted to you for giving me life, and unconditional love that will carry me through hard times iv TABLE OF CONTENTS DECLARATION ii ACKNOWLEDGEMENTS iii TABLE OF CONTENTS v ABSTRACT vii LIST OF PUBLICATION ix LIST OF FIGURES xi LIST OF SYMBOLS .xiv Chapter-1 Introduction 1 Crystal Structure of TiO2 1 Physical Properties of TiO2 4 Electronic Band Structure of TiO2 5 Defects and Substituting at Ti site in TiO2 1.5 Oxygen vacancy 1.5 Titanium vacancy 1.5 Substituting for Ti in TiO2 Applications of TiO2 10 1.6 Photoactivity of TiO2 11 Chapter-2 Growth and Characterization Technique 13 Pulsed Laser Deposition Technique 14 2 X-Ray Diffraction (XRD) 16 Rutherford backscattering-Ion Channelling 18 Magnetic Property Measurement System (MPMS) 20 Physical Property Measurement System 23 Raman spectroscopy 25 Scanning Tunneling Microscopy 29 Atomic Force microscopy 31 Chapter-3 Kondo effect and ferromagnetism 33 Brief history of Kondo effect 34 Experimental Methods 38 3 Magnetic and electrical transport of Ta0.06Ti0.94O2 39 v Thermopower and magnetic susceptibility Measurement 53 Conclusion 57 Chapter-4 Evidence formid-gap states and polarons in anatase TaxTi1-xO2 surface 58 Introduction 59 Experimental 60 Results and Discussion 60 4 Conclusion 71 Chapter-5 Electron transport at the TiO2 surfaces of rutile, anatase and strontium titanates: the influence of corrugation 72 A brief history of two dimensional electron gas in oxides 73 Experimental Methods 77 Experimental results 78 Strontium titanate 81 5 Anatase 83 Rutile 88 Discussion 88 Conclusion 93 Chapter-6 Magneli phases 95 Experimental method 96 Results 97 Dicussion 102 Conclusion 103 Chapter-7 Summary and Future work 104 Simultaneous observation of Kondo effect and magnetism 105 7.2 Electron transport at the TiO2 surfaces of rutile, anatase and strontium titanates 106 7.3 Evidences of surface mid-gap states and polarons in anatase TaxTi1-xO2 107 7.4 Future Work 108 vi ABSTRACT For over half a century, rutile TiO2 has been intensively studied by experimentalist and theoretical scientists due to its interesting physical properties, such as high static dielectric constant and large uniaxial refractive index However, over the last few decades, anatase TiO2 has become the heart of research due to its better electrical properties which make it more suitable over rutile phase for solar photovoltaic, photo catalytic, transparent conductor (TCO), dilute magnetic semiconductor applications The objective of this thesis is to investigate the defect induced electrical, optical, magnetic and structural properties of titanium dioxide (TiO2) thin films grown by pulsed laser deposition (PLD) technique Single crystal TiO2 and tantalum (Ta) doped TiO2 (Ti1-xTaxO2) thin films of different thicknesses were grown epitaxially on lattice matched substrates such as LaAlO3 and SrTiO3 X-ray diffraction (XRD) studies showed very high quality anatase TiO2 thin films Rutherford backscattering-Ion Channelling (RBS) spectroscopy was used to study the crystallinity and defect density of all the films deposited RBS-Ion Channelling studies showed that the defects which arise due to strain in the film at the interface reduced with increasing thickness Ultra Violet-Visible (UV-Vis) Spectroscopy was used to investigate the band gap of the films Recently cationic vacancies in metal oxide has been predicted to form magnetic centers1 Ta substitution in TiO2 results in donor electrons which are believed to enhance the formation of compensating cationic defects such as titanium vacancies (VTi) , Ti3+ and suppress the formation of anionic vacancies such as oxygen vacancies (VO) in a crystal It is inferred that Ti vacancy plays an important role in the observation of ferromagnetism and Kondo scattering in anatase Ti0.94Ta0.06O2 thin films with various thicknesses (5-200 nm) grown on SrTiO3(100) substrate We see ferromagnetism and Kondo scattering vii simultaneously in the same system and by studying the distribution of the defects, magnetism and transport in the films as a function of thickness, we are able to reconcile the observation The systematic variation in the density of states (DOS) of anatase TaxTi1-xO2 near Fermi level is investigated using temperature dependent scanning tunnelling spectroscopy A mid-gap narrow band is seen at ~ 0.6 eV below the Fermi level in undoped TiO2 at low temperature Spatial electronic inhomogeneity is seen at higher temperatures which is significantly reduced with Ta substitution A “gap” in the metallic state of Ta substituted TiO2 is seen in similar strongly correlated metals The mid gap energy is found to be a linear function of chemical potential with temperature The measured value of electron/hole effective mass ratio of undoped TiO2 ~ 0.7, exponentially increases with increase in Ta doping concentration We propose that such large enhancement of effective mass in Ta substituted TiO2 as a strong signature of large polarons at the surface of anatase TiO2 The two-dimensional electron gas in SrTiO3 created by an overlayer of amorphous LaAlO3 is compared with those at TiO2-terminated surfaces of rutile and anatase Differences in conductivity are explained in terms of the limiting Ti-O-Ti bond angles (orbital corrugation), band dispersion and polaron formation At 300 K, the sheet conductivity and mobility for anatase exceed those for SrTiO3 or rutile by one or two orders of magnitude, respectively The electrons in rutile become localized below 25K viii LIST OF PUBLICATION 1) Unexpected observation of simultaneous Kondo scattering and ferromagnetism in Ta alloyed anataseTiO2 thin films as a function of thickness Tarapada Sarkar, K Gopinadhan, M Motapothula, S Saha, Z Huang, S Dhar, A Patra, W M Lu, F Telesio, I Pallecchi, Ariando, D Marré, T Venkatesan (Scientific Report ,5,13011 (2015)) 2) Electron transport at the TiO2 surfaces of rutile, anatase and strontium titanates: the influence of orbital corrugation, Tarapada Sarkar, Kalon Gopinadhan, Zhou Jun1, Surajit Saha, J M D Coey, Yuan Ping Feng, A Ariando, T Venkatesan (ACS Nano (under review)) 3) Evidences of surface mid-gap states and polarons in anatase TaxTi1-xO2, (Abhimanyu Rana, Tarapada Sarkar, Surajit Saha, Xu Hai, Kalon Gopinadhan, Amar Srivastava, A Ariando, Loh Kian Ping, T Venkatesan Advanced materials (submitted)) 4) Effect of Oxygen Vacancy on Water Contact Angle in 3D and 4F Element Based Oxides,Tarapada Sarkar, Siddhartha Ghosh, Abhijeet Patra, Meenakshi Annamalai, Saurav Prakash, Mallikarjuna rao Motapothula, T Venkatesan (Under preparation for energy and environmental science) 5) Anisotropic Magneto Resistance and Planar Hall Effect at the LAO/STO Heterointerfaces: Effect of Carrier Confinement on Magnetic 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deposition (PLD) technique Single crystal TiO2 and tantalum (Ta) doped TiO2 (Ti1-xTaxO2) thin films of different thicknesses were grown epitaxially on