Strain Dependent Electronic Structure and Band Offset Tuning at Heterointerfaces of ASnO3 (A=Ca, Sr, and Ba) and SrTiO3

John D Baniecki,Nagarajan Valanoor,Toshihisa Anazawa,Norifumi Fujimura,Dan Ricinschi,Hiroyuki Aso,Takashi Yamazaki,Yoshihiko Imanaka,Hiroaki Yamada,Quentin Van Overmeere,Yusuke Miyata,Ronald Maran

doi:10.1038/srep41725

Abstract

The valence band (VB) electronic structure and VB alignments at heterointerfaces of strained epitaxial stannate ASnO3 (A=Ca, Sr, and Ba) thin films are characterized using in situ X-ray and ultraviolet photoelectron spectroscopies, with band gaps evaluated using spectroscopic ellipsometry. Scanning transmission electron microscopy with geometric phase analysis is used to resolve strain at atomic resolution. The VB electronic structure is strain state dependent in a manner that correlated with a directional change in Sn-O bond lengths with strain. However, VB offsets are found not to vary significantly with strain, which resulted in ascribing most of the difference in band alignment, due to a change in the band gaps with strain, to the conduction band edge. Our results reveal significant strain tuning of conduction band offsets using epitaxial buffer layers, with strain-induced offset differences as large as 0.6 eV possible for SrSnO3. Such large conduction band offset tunability through elastic strain control may provide a pathway to minimize the loss of charge confinement in 2-dimensional electron gases and enhance the performance of photoelectrochemical stannate-based devices.

Highlights

The alkaline earth stannates ASnO3 (A=Ba, Sr, and Ca) are rapidly emerging as important materials for a range of applications
The BLSO films were largely strain relaxed at thicknesses as thin as ~2 nm, which is consistent with the small critical thickness reported for BLSO grown on STO17
In contrast to the case of the band gap where volumetric strain plays a dominant role[9,24], the valence band electronic structure of CSO and SSO is dependent on directional bonding in a manner that correlated to a difference in the change of bond lengths between Sn and O atoms at apical and basal plane positions in the unit cell which is a consequence of the two dimensional compression or tension

Summary

Introduction

The alkaline earth stannates ASnO3 (A=Ba, Sr, and Ca) are rapidly emerging as important materials for a range of applications. The large mobility of BLSO has been utilized to fabricate all-perovskite field-effect transistors with an epitaxial BLSO channel layer resulting in large on-off ratios[2] In addition to these applications, the alkaline earth stannates have been considered as photocatalysts[3], where the high mobility may provide for more efficient separation of photogenerated electron-hole pairs that is necessary to drive multi-electron chemistry[4]. To harness the predicted large sensitivity of the band gap of stannates on volumetric strain in a beneficial way, the effect of strain on the electronic structure, including the valence and conduction band alignments, needs to be thoroughly understood. We reveal strain dependent electronic structure of epitaxial stannate thin films and conduction band offset tuning at heterointerfaces of ASnO3 (A=Ca, Sr, and Ba) and SrTiO3.

Methods

Results

Conclusion