Surface electronic structure for various surface preparations of Nb-doped SrTiO3 (001)

Abstract

High-resolution angle-resolved photoemission spectroscopy (ARPES) was used to study the surface electronic structure of Nb-doped SrTiO3 (STO) single crystals prepared using a variety of surface preparations. ARPES measurements show that simple degreasing with subsequent anneal in vacuum is not an adequate surface preparation of STO, rather, preparations consisting of etching with buffered HF or HCl, and to a lesser extent, simple water leaching resulted in surfaces with much less disorder. A non-dispersing, mid-gap state was found ∼800 meV above the top of the valence band for samples which underwent etching. This mid-gap state is not present for vacuum-annealed and water-leached samples, as well as for STO thin films grown using molecular beam epitaxy. Theoretical modeling using density functional theory suggests that this mid-gap state is not related to the SrO- and TiO2-terminated surfaces, but rather, is due to a partial hydrogenation of the STO surface that occurs during etching.