Abstract
Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, O vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understanding the role of defects in complex oxides have been largely limited to controlling O vacancies. In this study, we report on the selective formation of different types of elemental vacancies and their individual roles in determining the atomic and electronic structures of perovskite SrTiO3 (STO) homoepitaxial thin films fabricated by pulsed laser epitaxy. Structural and electronic transitions have been achieved via selective control of the Sr and O vacancy concentrations, respectively, indicating a decoupling between the two phase transitions. In particular, O vacancies were responsible for metal-insulator transitions, but did not influence the Sr vacancy induced cubic-to-tetragonal structural transition in epitaxial STO thin film. The independent control of multiple phase transitions in complex oxides by exploiting selective vacancy engineering opens up an unprecedented opportunity toward understanding and customizing complex oxide thin films.
Highlights
As with many transition metal oxides, O vacancies have been a center of interest among traditional STO defect studies, since they are thought to affect both the structural and electronic properties of the material[15,16,17]
PLE is a well-established technique for the growth of perovskite oxide thin films and is advantageous since it transfers the target material to the substrate with high crystalline quality
The x-ray diffraction (XRD) results indicate that the tetragonal STO thin films were homoepitaxially grown on cubic STO substrates with the elongated c-axis lattice constant due to the interplay between lattice expansion and epitaxial strain
Summary
As with many transition metal oxides, O vacancies have been a center of interest among traditional STO defect studies, since they are thought to affect both the structural and electronic properties of the material[15,16,17]. A significant amount of Ti vacancies gives rise to Ruddlesden-Popper-type planar faults, with the collapse of the perovskite structure[23,24,25]. While both oxygen and cation vacancies play critical roles in STO, systematic studies with regard to individual vacancy types and their coupling to distinctive physical properties have been largely unexplored in STO. While Sr vacancies are responsible for structural phase transition in epitaxial STO thin films, O vacancies are not mainly related to lattice expansion but induce electronic metal-insulator transition
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