Single crystal strontium titanate surface and bulk modifications due to vacuum annealing

Vacuum annealing is a widely used method to increase the electric conductivity of SrTiO3 single crystals. The induced oxygen vacancies act as intrinsic donors and lead to n-type conductivity. Apart from the changed electronic structure, however, also crystal structure modifications arise from this treatment. Hence, electronic properties are determined by the interplay between point defects and line defects. The present paper provides a survey of the real structure of commercially available SrTiO3 single crystals and the changes induced by reducing vacuum heat-treatment. Therefore, all investigations were performed ex situ, i.e., after the annealing process. Used characterization methods include atomic force microscopy, transmission electron microscopy, spectroscopic ellipsometry, infrared spectroscopy, and photoluminescence spectroscopy. Besides the expected variation of bulk properties, especially surface modifications have been detected. The intrinsic number of near-surface dislocations in the samples was reduced by vacuum annealing. X-ray photoelectron spectroscopy proves the existence of a layer of adsorbed molecules, which influences the SrTiO3 work function. Also, the interaction between adsorbates and surface point defects as well as laser annealing due to local oxygen absorption are discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3638692]