Electrical conductivity of epitaxial SrTiO3 thin films as a function of oxygen partial pressure and temperature

SrTiO3 (100) epitaxial films with thicknesses of 3, 1 mu m, and 250 nm were prepared on MgO (100) substrates by pulsed-laser deposition. The electrical conductivities of the thin films were systematically investigated as a function of temperature and ambient oxygen partial pressure. This was made possible by using a specially designed measurement setup, allowing the reliable determination of resistances of up to 25 G Omega in the temperature range of 600 degrees-1000 degrees C under continuously adjustable oxygen partial pressures ranging from 10(-20) to 1 bar. The capabilities of the measurement setup were tested thoroughly by measuring a SrTiO3 single crystal. The well-known characteristics, e.g., the decline of the conductivity with a slope of -1/4 under reducing conditions and the opposite +1/4 behavior in oxidizing atmospheres, are found in the log(sigma)-log(pO(2)) profiles of the epitaxial films. However, the p-type conductivity decreases, and the n-type conductivity increases with decreasing film thickness. This phenomenon is attributed to the charge carrier redistribution in the surface space charge layers. Owing to the high surface-to-volume ratio, the space charge layers play an important role in thin films.