A chemical imaging and Nano-ARPES study of well-ordered thermally reduced SrTiO3(100)

TL;DR

This study uses advanced synchrotron-based microscopy and ARPES to analyze the chemical and electronic properties of thermally reduced SrTiO3(100), revealing localized Ca segregation and its limited penetration depth, challenging previous assumptions about surface reconstructions.

Contribution

It provides high-resolution imaging and spectroscopic evidence clarifying the nature and localization of Ca segregation in SrTiO3(100), and its impact on electronic structure.

Findings

Ca segregation is highly localized with minimal penetration depth.

Long-range surface reconstructions are unlikely caused by Ca.

Ca interacts with intragap states related to oxygen vacancies.

Abstract

The structural and electronic properties of thermally reduced SrTiO3(100) single crystals have been investigated using a probe with real- and reciprocal-space sensitivity: a synchrotron radiation microsopic setup which offers the possibility of Scanning Photoemission Microscopy and Angle Resolved Photoelectron Spectroscopy (ARPES) down to the nanometric scale. We have spectroscopically imaged the chemical composition of samples which present reproducible and suitable low-energy electron diffraction patterns after following well-established thermal reduction protocols. At the micrometric scale, Ca-rich areas have been directly imaged using high-energy resolution core level photoemission. Moreover, we have monitored the effect of Ca segregation on different features of the SrTiO3(100) electronic band structure, measuring ARPES inside, outside and at the interface of surface inhomogeneities with the identified Ca-rich areas. In particular, the interaction of Ca with the well-known intragap localized state, previously attributed to oxygen vacancies, has been investigated. Moreover, the combination of direct imaging and spectroscopic techniques with high spatial resolution has clarified the long-standing dilemma related to the bulk or surface character of Ca segregation in SrTiO3. Our results present solid evidence that the penetration depth of Ca segregation is very small. In contrast to what has been previously proposed, the origin of long-range surface reconstructions can unlikely be associated to Ca due to strong local variations of its surface concentration.