Instability and Surface Potential Modulation of Self-Patterned (001)SrTiO₃ Surfaces

Abstract

ABSTRACT:The (001)SrTiO 3 crystal surface can be engineered to display a self-organized pattern of wellseparated and nearly pure single-terminated SrO and TiO 2 regions by high temperature annealing in oxidizing atmosphere. By using surface sensitive techniques we have obtained evidence of such surface chemical self-structuration in as-prepared crystals and unambiguously identified the local composition. The contact surface potential at regions initially consisting of majority single terminations (SrO and TiO 2 ) is determined to be Φ(SrO) <Φ(TiO 2 ), in agreement with theoretical predictions, although the measured difference ΔΦ ≤ 100 meV is definitely smaller than theoretical predictions for ideally pure single-terminated SrO and TiO 2 surfaces. These relative values are maintained if samples are annealed in UHV up to 200 °C. Annealing in UHV at higher temperature (400 °C) preserves the surface morphology of self-assembled TiO 2 and SrO rich regions, although a non-negligible chemical intermixing is observed. The most dramatic consequence is that the surface potential contrast is reversed. It thus follows that electronic and chemical properties of (001)SrTiO 3 surfaces, widely used in oxide thin film growth, can largely vary before growth starts in a manner strongly dependent on temperature and pressure conditions. INTRODUCTIONElectronic redistributions at solid interfaces occur to reduce the chemical potential gradients, and the energy cost to bring electrons in/out across these interfaces is strongly influenced by the intrinsic properties of materials involved, namely, the work function (Φ) in metals and the electron affinity (χ) and the ionization potential (IP) in intrinsic semiconductors and insulators.The work function of metals and degenerated semiconductors depends on the electron density and surface dipoles formed at the corresponding free surface. Similarly, the IP of insulators and intrinsic semiconductors depends on the crystallographic plane considered. Modulation of the substrate work function, as required in most common planar electronic devices or active surfaces, can be achieved by changing the carrier concentration by chemical or electrostatic doping, or by chemically engineering the surface. A beautiful example is the LaAlO 3 −SrTiO 3 interface, where a high mobility two-dimensional electron gas (2DEG) can be formed only if the STO surface is TiO 2 terminated [1]. Although, following the pioneering work by Kawasaki et al. [2], TiO 2 singleterminated (001)SrTiO 3 (STO) single-crystalline surfaces can be prepared, as-received crystals display

Quantifying work function differences using low-energy electron microscopy: The case of mixed-terminated strontium titanate

“…Here the canonical method finds an apparent WF difference of ∼5 eV, while our refined method yields only 0.5 eV. This value is close to other experimental findings [3,4] but much lower than the theoretically predicted values of up to ∼3 eV [19,20]. Such high values that were calculated for unreconstructed surfaces are energetically unfavorable and might be one of the driving forces behind the formation of the 2×2 and the √ 13× √ 13R33.7°r econstructions of the SrO and TiO 2 -terminated surfaces, respectively [18,27,28].…”

Lubrication Mechanism of Phosphonium Phosphate Ionic Liquid in Nanoscale Single‐Asperity Sliding Contacts

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