The morphology and optical properties of Cr-doped ZnO films grown using the magnetron co-sputtering method

Undoped ZnO and Zn0.9Cr0.1O films were prepared on Al2O3 (0 0 0 1) substrates using the magnetron co-sputtering technique. X-ray diffraction scans show that all films exhibit nearly single-phase wurtzite structure with c-axis orientation. Both chromium doping and growth ambient have a significant impact on the lattice constants and nucleation processes in ZnO film. A large quantity of subgrains (10 nm in size) has been observed on Zn0.9Cr0.1O film grown under Ar + O2, while irregular plateau-like grains 40-50 nm in size were observed on Zn0.9Cr0.1O film grown under Ar + N2. The ultraviolet-visible transmittance and optical bandgap of all films were also examined. The photoluminescence spectra of all films exhibit a broad emission located around 400 nm, which is composed of one weak ultraviolet luminescence and another rather intense near-violet one, as determined by Gaussian peak fitting. The near-violet emission centered on 400 nm might originate from the electron transition between the band tail state levels of surface defects and/or lattice imperfection in the ZnO film.