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Title: Defect mediated novel structural, optical, electrical and magnetic properties in Ti1-xTaxO2 thin films
Keywords: Defect Mediated Magnetism, Pulsed Laser Deposition, Thin Films, TiO2
Issue Date: 9-Dec-2011
Citation: ARKAJIT ROY BARMAN (2011-12-09). Defect mediated novel structural, optical, electrical and magnetic properties in Ti1-xTaxO2 thin films. ScholarBank@NUS Repository.
Abstract: The main objective of this thesis is to explore the defect mediated structural, optical, electrical and magnetic properties of titanium oxide (TiO2) based alloy thin films grown by pulsed laser deposition (PLD). Such properties can be harnessed for suitable applications in the field of optoelectronics and spintronics as transparent conducting oxides (TCO) and defect induced diluted magnetic semiconductors (DMS) respectively. Single crystal thin films of pure TiO2 and tantalum (Ta) incorporated TiO2 (Ti1-xTaxO2) were grown epitaxially on lattice matched substrates such as LaAlO3 and SrTiO3. By varying the deposition temperatures and the oxygen partial pressures in the PLD process, both anatase and rutile polymorphs of TiO2 were grown. By investigating the growth dependence of the different phases of TiO2 on the deposition parameters, an elaborate phase diagram was developed. Rutherford backscattering-Ion Channeling (RBS) spectroscopy was used to study the crystal structure of all the films deposited. RBS-Ion Channeling studies showed that the crystallinity of the thin films improved with increasing deposition temperature and increasing oxygen partial pressure. Films with higher Ta incorporation also showed higher crystallinity. X-Ray Diffraction studies showed a lattice expansion in TiO2 with Ta incorporation in the out-of-plane direction. This was further supported by the Raman spectroscopy data which showed the softening of the out-of-plane vibrational modes and the hardening of the in-plane vibrational mode. Ultra Violet-Visible (UV-Vis) Spectroscopy was done on both anatase and rutile samples to study the effect of Ta incorporation in TiO2. The band gap of both anatase and rutile samples showed a blue shift with increasing Ta concentration. Using electrical transport data, it was argued that the band structure of TiO2 undergoes a drastic change with Ta incorporation resulting in the formation of a new alloy system. High energy optical reflectivity measurements were done to directly detect the huge spectral weight shift in the spectra for pure TiO2 and Ti1-xTaxO2. This further confirmed the role of Ta in varying the band structure of TiO2. Ta incorporation in TiO2 is believed to enhance the formation of cationic vacancies such as titanium vacancies (VTi) and suppress the formation of anionic vacancies such as oxygen vacancies (VO) in a crystal. The cationic defects in a crystal lattice have been predicted to form magnetic centers. Such magnetic centers scatters electrons resulting in an up-turn of the resistivity curve as function of temperature. This phenomenon, known as Kondo effect has been found in Ti1-xTaxO2 thin films. Thorough and systematic Hall measurements have also been done on Ti1-xTaxO2 thin films to study the variation of carrier density and electron mobility with deposition conditions. Defect mediated magnetism is an alternate and a more trustworthy route to DMS oxides. Ti1-xTaxO2 thin films with cationic defects and enough free carriers showed ferromagnetism (FM) at room temperature (RT). Extensive impurity analysis has been done by RBS, Proton Induced X-Ray Emission Spectroscopy (PIXES), X-Ray Absorption Spectroscopy (XAS) and Secondary Ion Mass Spectrometry (SIMS) to rule out the presence of any magnetic impurities in the thin films. Soft X-Ray Magnetic Circular Dichroism (XMCD) and Optical Magnetic Circular Dichroism (OMCD) measurements were done to confirm the origin of the magnetism to be predominantly cationic defects. Detailed Superconducting Quantum Interference Device (SQUID) Magnetometry measurements were done to study the variation of magnetism with deposition conditions. Based on these measurements, a plausible model is devised for the defect mediated magnetism in Ti1-xTaxO2 thin films.
Appears in Collections:Ph.D Theses (Open)

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