Electrical conductivity of sputtered films of strontium titanate

Abstract

Among the most critical parameters in the fabrication of rf‐sputtered SrTiO₃ films is the argon/oxygen ratio in the sputter gas and the substrate temperature, both during sputtering and during postannealing. After sputtering at 500 °C, SrTiO₃ layers on Al₂O₃ substrates are partially crystalline. In this case the SrTiO₃ lattice is enlarged compared to the bulk material. With increasing temperatures of postannealing, the crystallites grow and the lattice shrinks. In particular, films that are sputtered with a sputter gas containing oxygen show an additional phase of TiO₂ when they are annealed at temperatures of more than 1000 °C. This paper shows for the first time results of the investigation of sputtered SrTiO₃ films on Al₂ O₃ substrates as a function of oxygen partial pressure P(O₂ ). The measurements performed on the sputtered films indicate semiconducting properties in agreement with the bulk material: At temperatures between 700 and 1100 °C the electrical conductivity of sputtered layers changes from p‐type to n‐type as the oxygen partial pressure decreases from 1 to 10⁻¹⁵ bar. The enthalpy of oxidation in the SrTiO₃ films is 1.24 eV, and the energy gap is 3.30 eV. These values are in agreement with those reported for bulk materials.