Probing Vacancy Behavior in Complex Oxide Heterostructured Films

Abstract

Oxygen vacancies (Vo•• ) play a critical role in the transport mechanisms within complex oxides, analogous to electrons and holes within semiconductors. Systems including memristors, all-oxide electronics, and electrochemical cells comprise substrate-supported thin films either in metal-insulator-metal or complex oxide heterostructure configurations. As well-studied defect chemistry dictates mixed electronic/ionic functionality, improving oxide-oxide interfaces necessitates a direct, spatial understanding of vacancy distributions that define electrochemically active regions. Here we show that vacancies deplete over large, micron-level distances within single crystalline perovskite Nb-doped SrTiO3 substrate (Nb:SrTiO3) substrates after typical vacuum film deposition and post-annealing processes. We demonstrate the conversion of the surface potential across a four-layer strontium titanate / yttria-stabilized zirconia (STO/YSZ) heterostructured film to spatially defined (< 100 nm) [Vo•• ] profiles within STO through a unique combination of high temperature (500 °C), in situ scanning probes and classic semiconductor energy band diagram model analysis. Further comparison between room temperature and high temperature potential profiles clearly distinguishes between electronic-dominant and activated, ionic-dominant transport characteristics within the oxide layers. Consequently, we determined that oxygen scavenging by deposited films during pulsed laser deposition significantly reduce the Nb:STO, which is then partially reoxidized in the ambient environment during cooling. The results presented herein i) introduce the means to spatially resolve quantitative vacancy distributions across oxide films, and ii) pose the mechanism by which oxide thin film getters both enhance then deplete vacancies within the underlying substrate.