SrTiO sub(3) (001) surface and strained thin films: Atomic simulations using a tight-binding variable-charge model

The tight-binding variable-charge model SMTB-Q was used to study the properties of the (001) surfaces and ultra-thin films of the SrTiCh perovskite. First, the bulk properties of SrTiO sub(3) were successfully reproduced from a set of parameters independently determined for bulk SrO and TiO sub(2). The formation energies, atomic relaxations and charge transfer for SrO- and TiO sub(2)-terminated SrTiO sub(3) (001) surfaces were then derived from 0 to 1200 K. The TiO sub(2)-terminated surface is more stable than the SrO-terminated surface by about 0.15 j m super(-2) under 500 K. This difference decreases by a factor 3 at higher temperature. At 0 K, the surface energies (E sub(TiO2) = 1.10 j m super(-2) and E sub(SrO) = 1.20 j m super(-2)) are in accordance with the mean value yielded by ab initio calculations. A strong Sr displacement towards the SrO-terminated surface (0.33 A) was observed in agreement with both experimental data and DFT calculations. In contrast, the other atomic relaxations included the Ti displacement towards the TiO sub(2)-terminated surface (0.13 A), are in good agreement with ab initio results but strongly differ from experimental data ( approximately 0.00 A). The displacements of surface oxygen planes being small, one observes a splitting of the SrO and TiO sub(2) surface planes by 0.30 and 0.13 A respectively. Moreover, the distance between the Sr and Ti planes is reduced by 19%. The charge transfers at the TiO sub(2)-terminated surface are comparable to those at the pure TiO sub(2) surfaces (-- 0.18 and +0.14, for titanium and oxygen atoms respectively) leading to the increase of the Ti-O bond covalency near the surface. At the SrO-terminated surface, we found negligible charge transfers as at the pure SrO(100) surface. Moreover, we studied the effect of a 1.66% compressive strain (corresponding to the STO/Si(001) lattice mismatch) on the relaxation of thin films from 2 to 40 nm at 273 K. The atomic surface relaxations are not significantly modified apart from the Sr-Ti (resp. Ti-Sr) distances which relaxed by 25%. The ratio between out-of-plane and in-plane lattice parameters is in good agreement with the elasticity theory for a thickness up to 5 nm. Beyond 20 nm thick the film is almost fully relaxed.