Investigation of nonpolar (1 1 (2)over-bar 0) a-plane ZnO films grown under various Zn/O ratios by plasma-assisted molecular beam epitaxy

Abstract

Structural and optical properties of nonpolar a-plane ZnO films grown with different II/VI ratios on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy were investigated. Even by increasing the II/VI ratio across the stoichiometric flux condition a consistent surface morphology of striated stripes along the ZnO < 0 0 0 1> direction without any pit formation was observed, which is contrary to polar c-plane ZnO films. Root mean square surface roughness, full width at half maximum values of X-ray rocking curves, defect densities, and photoluminescence were changed with the II/VI ratio. The sample grown with stoichiometric flux condition showed the lowest value of rms roughness, the smallest threading dislocation and stacking fault densities of similar to 4.7 x 10(8) cm(-2) and similar to 9.5 x 10(4) cm(-1), respectively, and the highest intensity of D X peak. These results imply that the stoichiometric flux growth condition is suitable to obtain superior structural and optical properties compared to other flux conditions. (C) 2010 Elsevier B.V. All rights reserved.