Abstract
The conductive behavior of the perovskite SrTiO 3 is strongly influenced by the presence of oxygen vacancies in this material, therefore the identification of such defects with spectroscopic methods is of high importance. We use density functional theory to characterize the defect-induced states in SrTiO 3 and Sr 2 TiO 4 . Their signatures at the surface, the visibility for scanning tunneling spectroscopy and locally conductive atomic force microscopy, and the core-level shifts observed on Ti atoms in the vicinity of the defect are studied. In particular, we find that the exact location of the defect state (e.g., in SrO or TiO 2 planes relative to the surface) are decisive for their visibility for scanning-probe methods. Moreover, the usual distinction between Ti 3 + and Ti 2 + species, which can occur near defects or their aggregates, cannot be directly translated in characteristic shifts of the core levels. The width of the defect-induced in-gap states is found to depend critically on the arrangement of the defects. This also has consequences for the spectroscopic signatures observed in so-called resistive switching phenomena.
Highlights
The electronic properties of many insulating oxides, such as SrTiO3, are strongly determined by defects, in particular oxygen vacancies
E.g. in LaAlO3 /SrTiO3 heterostructures, where the presence of Ti3+ is expected at the interfaces, binding energy shifts of about 1.5 eV were found in X-ray photoelectron spectroscopy [30]
It is believed that oxygen vacancies in insulating perovskites such as SrTiO3 are decisive for the conductive properties observed in these materials, their characterization is far from trivial
Summary
The electronic properties of many insulating oxides, such as SrTiO3 , are strongly determined by defects, in particular (but not exclusively) oxygen vacancies. Several studies based on density functional theory (DFT) have investigated this problem and, rather counter-intuitively, under certain conditions tendencies for vacancy clustering were found [8,9]. This behavior is not unknown in other oxides, e.g., TiO2 , where it is well accepted that extended defects act as precursor phases for the formation of Magnèli phases such as Ti5 O9 or Ti4 O7 [10,11]. DFT calculations find that such tendencies towards linear ordering of defects exist and relate that to observed resistive switching phenomena [12]
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