Phase transitions via selective elemental vacancy engineering in complex oxide thin films

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.

Tuning electromagnetic properties of SrRuO3 epitaxial thin films via atomic control of cation vacancies

“…For example, for the growth of STO thin films, low P(O2) growth results in Sr (heavier element) vacancies. 10 In addition, recent studies show that the same trend can be observed for the growth of BaTiO3, CaTiO3, La0.4Ca0.6MnO3, EuAlO3, and LiMn2O4 thin films. [22][23][24] For SRO, Ru is heavier than Sr, so Ru vacancies prevail in the highly energetic plume condition (low P(O2) growth), consistent with other oxide thin film growth.…”

“…The formation of Ru-O vacancy in SRO thin film is quite different from the case of STO thin film, where the cation and oxygen vacancies can be separately controlled. 10 With an increasing Sr/Ru ratio, the unit cell volume also shows a monotonically increasing behavior. Indeed, the unit cell volume can be a measure of the Sr/Ru ratio, regardless of the value being greater or less than one.…”

“…This is again in contrast to the case of STO, where the unit cell volume increases when the Sr/Ti ratio deviates from one. 10,21 The correlation between the cation stoichiometry and unit cell volume in SRO thin film can be understood in terms of subtle internal structural distortion induced by the Ru vacancy site. 18,21 It has been suggested that as Sr atoms substantially relax towards vacant Ru sites, the Ru-O-Ru bond angle flattens, giving a positive contribution to the expansion of the unit cell volume.…”

“…[3][4][5] Oxygen vacancies, which can be easily manipulated during the deposition, are among the most prominent examples that induce changes in electronic and optical properties of transition metal oxide thin films. [6][7][8][9][10] On the other hand, cation vacancies, e.g., A-and/or B-site vacancies in ABO3 perovskites, can also be employed for the control of different physical properties such as lattice structure, ferroelectricity, ferromagnetism, and thermoelectricity. [11][12][13][14] To properly design and take advantage of desirable material characteristics of oxide thin films and heterostructures, it is essential to comprehend the formation and the roles of various elemental defects.…”

Influence of RF power on the opto-electrical and structural properties of gallium-doped zinc oxide thin films

“…It may be explained this deposition power was not sufficient to verifying of the compositional stoichiometry. As known, in case off-stoichiometry GZO films which has Zn vacancies, lattice expansion and increasing of electrical resistivity can be occurred due to increasing Coulomb repulsion between neighboring transition metal ions [37]. However, at this power, Ga incorporation into the layer, i.e.…”