Point defect distribution in high-mobility conductive SrTiO(3) crystals

Among perovskites, SrTiO_3 (STO) is one of the most widely studied oxides because of its potential in many applications in oxide electronics. Recently it has been shown that irradiated STO surfaces exhibit high-mobility conduction in contrast to the insulating behaviour of stoichiometric STO single crystals. The possibility of modifying the properties of solids just by etching their surfaces opens up new perspectives for engineering of the functional properties of materials. But for that purpose, a deeper knowledge of damage extension and its consequences on the physical properties is highly desired. Bearing this in mind, we have characterized the spatial distribution and the nature of vacancy defects in insulating as-received as well as in ion-irradiated STO substrates exhibiting high-mobility conduction. Because tiny amounts of oxygen vacancies can trigger substantial modifications of the physical properties of STO, positron annihilation spectroscopy techniques appear as an appropriate characterization tool. We show that Ti vacancies are native defects homogeneously distributed in as-received substrates. In contrast, the dominant vacancy defects consist of non-homogeneous distributions of cation-oxygen vacancy complexes in ion-etched substrates. Their spatial extension is tuned over a few microns in ionetched samples. Our results shed light on the transport mechanisms of conductive STO crystals and on strategies for defect-engineered oxide quantum wells, wires and dots.