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Materials Science at Oregon State University

Defects, Built-In Potential, and Functionality of Complex Oxides

Date: Thursday, Nov 1st
Presenter: Dr. Peter Sushko, Physical Sciences Division, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA


Interface-defined materials exhibit novel phenomena and tunable functions which can be enabled or suppressed by defects and deviations from ideality that emerge during their synthesis. To take full advantage of this class of materials, it is essential to characterize individual defects and extended defect structures, understand their origin and the nature of interaction with internal interfaces, and to develop protocols for their control. Epitaxial thin films and heterostructures of complex oxides are convenient materials systems to introduce and study such defect structures.

In the first part of the talk I will examine how intentional and unintentional defects in thin films of complex oxides affect the properties of these materials. I will focus on chromium ferrite spinel Fe3–xCrxO4 (FCO) synthesized using molecular beam epitaxy. Replacing Fe atoms with Cr converts FCO from a metal (x=0) to a semiconductor (x=1) to an insulator (x=2), which creates a possibility to manipulate the electron distribution in order to engineer materials with low-energy optical absorption, promote electron-hole separation, and control electron transport channels.

In the second part of the talk I will discuss how potential gradients in oxide/semiconductor heterojunctions can be reconstructed from hard x-ray photoelectron spectroscopy (HAXPES) spectra. Functionality of such structures, e.g., their ability to separate photo-induced electrons and holes, is determined by the off-plane profile of the electrostatic potential. However, quantitative characterization of these potential profiles remains challenging. Our method is based on fitting the HAXPES spectra for relevant core levels while ensuring continuity of the potential function and its smoothness without assuming band-bending direction or an analytical form of the potential profile. The physical insights generated through this analysis are discussed for prototype systems SrTiO3/Ge(001) and SrTiO3/Si(001).

Peter Sushko received his BSc and MSc degrees in Physics from St. Petersburg State University, Russia, and his PhD in Physics from the University College London (UCL), United Kingdom. After completing his PhD in 2000, he held several research positions at the Department of Physics and Astronomy and, from 2006, at the London Centre for Nanotechnology, UCL. In 2008, he was awarded the Royal Society University Research Fellowship and joined UCL faculty; he was promoted to a Reader (Associate Professor) in 2012. In 2014, he moved to the Pacific Northwest National Laboratory (PNNL), as an Associate Division Director with responsibilities for materials sciences at PNNL’s Physical Sciences Division. His research is focused on computational modeling of atomic-scale mechanisms of processes that underpin synthesis, functionality, and evolution of materials systems, and predicting the effects of defects and disorder on materials properties and functions.