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.