Characterization and properties of microwave plasma-treated SrTiO3

Nitrogen was successfully incorporated into the surface of a SrTiO3(1 1 1) single crystal by exposing it to a microwave-induced glow discharge plasma of ammonia (NH3). The layers generated by the flux of active species in the plasma varied in composition and structure depending on the treatment time, t. Substitution of oxygen by nitrogen along with the reduction of the crystal surface was observed by X-ray photoelectron spectroscopy. The 0–150 nm thick top layer showed Sr and O deficiencies and was identified as a mixture of nanocrystalline cubic rocksalt-type TiN, perovskite-type SrTiO3−δ–SrTiO3−xNy and amorphous phases. Deeper nitrogen diffusion into the crystal yielded perovskite-type nitrogen-doped strontium titanate. The diffusion of N into the perovskite structure was characterized by the diffusion coefficient D = (2.1 ± 0.4) × 10−15 cm2 s−1. The growth of the N-containing perovskite layers showed the time dependency L   [μm]=6.3t   [min]. Examination of the electronic properties revealed metallic-like electrical conductivity, superconductivity with Tc of up to 5 K, and negative Seebeck coefficients of up to S = −465 μV K−1 at room temperature.