Small Polaron Formation on the Nb-doped SrTiO$_\textbf{3}$(001) Surface

TL;DR

This study investigates how small polarons form on Nb-doped SrTiO3 surfaces using density functional theory, revealing their stability depends on doping levels and impurity configurations, which impacts material properties.

Contribution

The paper provides the first detailed computational analysis of small polaron formation on Nb-doped SrTiO3 surfaces, considering various doping levels and surface configurations.

Findings

Small polarons are less stable at low Nb-doping concentrations.

Polaron stability varies with impurity types and configurations.

Surface polarons influence the electronic and spectroscopic properties of SrTiO3.

Abstract

The cubic perovsike strontium titanate SrTiO$_3$ (STO) is one of the most studied, polarizable transition metal oxides. When excess charge is introduced to this material, e.g., through doping or atomic defects, STO tends to host polarons: Quasi-particles formed by excess charge carriers coupling with the crystal phonon field. Their presence alters the materials properties, and is a key for many applications. Considering that polarons form preferentially on or near surfaces, we study small polaron formation at the TiO$_2$ termination of the STO(001) surface via density functional theory calculations. We model several supercell slabs of Nb-doped and undoped STO(001) surfaces with increasing size, also considering the recently observed as-cleaved TiO$_2$ terminated surface hosting Sr-adatoms. Our findings suggest that small polarons become less stable at low concentrations of Nb-doping, in analogy with polarons localized in the bulk. Further, we inspect the stability of different polaron configurations with respect to Nb- and Sr-impurities, and discuss their spectroscopic properties.