Highly conductive and optically transparent GZO films grown under metal-rich conditions by plasma assisted MBE

We demonstrate a critical effect of a metal-to-oxygen ratio on the electrical, optical, and structural properties of ZnO films heavily doped with Ga (carrier concentration in the range of 10-10 cm) grown by plasma-assisted molecular beam epitaxy. The as-grown layers prepared under the metal-rich conditions exhibited resistivities below 3 × 10 Ω cm and an optical transparency exceeding 90% in the visible spectral range as well as a large blue shift of the transmission/ absorption edge attributed to the Burstein-Moss shift of the Fermi level deep into the conduction band, indicating high donor concentration. In contrast, the films grown under the oxygen-rich conditions required thermal activation and showed inferior properties. Furthermore, electrical measure- ments point to the nonuniform depth distribution of free car-riers. An oxygen-pressure-dependent surface disordering is suggested to be responsible for the drastic effect of the metal- to-oxygen ratio on the film properties. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.