  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,
oxygen 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 oxygen 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 structure 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 oxygen vacancy
concentrations, respectively, indicating a decoupling between the two phase
transitions. In particular, oxygen 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.
