Abstract
The electrical properties of SrTiO3(100) single crystals were investigated in-situ at different stages of thermal reduction by means of a 4-tip STM. Using the tips of the STM as electrical probes, distance-dependent four-point measurements were performed at the surface of the crystal at room temperature after reduction by thermal treatment. For annealing temperatures T ≤ 700 °C, charge transport is confined to a surface region <3 μm below the surface. For reduction at T ≥ 900 °C a transition from a conducting 2D sheet with insulating bulk to a system with dominant 3D bulk conductivity is found. At an intermediate reduction temperature of T = 800 °C, a regime with mixed 2D/3D contributions is observed in the distance-dependent resistance measurements. Describing the depth-dependent conductivity with an analytical N-layer model, this regime of mixed 2D/3D conductivity is evaluated quantitatively under the assumption of an exponentially decaying conductivity profile, correlated with the previously observed depth-dependent dislocation density in the sample. A non-monotonous temperature dependence of the 3D conductivity in the respective conducting layer is found and possible underlying mechanisms are discussed, particularly with regard to non-intrinsic material properties depending on details of the sample preparation.
Highlights
Strontium titanate (SrTiO3) has become a prototype ternary transition metal oxide due to its unique electronic properties
SrTiO3 has been used recently to generate systems of confined metallicity such as LaAlO3/SrTiO3 interfaces[6] showing the properties of a 2D electron gas, or high-temperature superconductors, such as FeSe grown on SrTiO3 7
The macroscopic electronic transport properties of SrTiO3 can be described by the point defect chemistry model. This model takes into account the formation of oxygen vacancies and strontium vacancies as a function of the oxygen activity and temperature in thermodynamic equilibrium[2]. While this model is well-established and finds agreement with experimental investigations performed under defined oxygen activity by employing gas mixtures[19], in this paper we focus on the initial non-equilibrium thermal reduction under ultra high vacuum conditions
Summary
Strontium titanate (SrTiO3) has become a prototype ternary transition metal oxide due to its unique electronic properties. We use in-situ distance-dependent four-point measurements in Valdes configuration to investigate the electronic transport properties on the microscale[20], we demonstrate that the initial thermal reduction leads to the formation of a 2D conducting sheet in the surface region.
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