Structural and optical properties of non-polar A-plane ZnO films grown on R-plane sapphire substrates by plasma-assisted molecular-beam epitaxy

Abstract

Non-polar, A-plane (1 1 (2) over bar 0) ZnO films are epitaxially grown on R-plane ( 1 (1) over bar 0 2) Al(2)O(3) substrates by plasma-assisted molecular-beam epitaxy and structural and optical properties are investigated. Epitaxial relationships between ZnO films and Al(2)O(3) substrates are determined to be ZnO(1 1 (2) over bar 0)//Al(2)O(3)) (1 (1) over bar 2 0), ZnO[(1) over bar 1 0 0]//Al(2)O(3)[1 1 (1) over bar 0]. The misfit is relaxed by the regularly spaced misfit dislocations. Structural properties are anisotropic and surfaces of films show stripes running along the ZnO [0 0 0 1] direction. Full width at half maximums of X-ray rocking curves for the on-axis (1 1 (2) over bar 0) with phi = 0 degrees and 90 degrees, and the off-axis (1 0 (1) over bar 1) reflections are 0.41 degrees, 0.36 degrees, and 0.39 degrees, respectively, for the 300nm-thick ZnO film. Near-band edge emissions are dominant in low-temperature (12K) photoluminescence while emissions from deep levels are negligible indicating a high optical quality of the layers. The near-band edge emissions are strongly polarized perpendicular to the [0 0 0 1] axis. Transitions of neutral-donor bound excitons (D(0)X) and allowed free excitons are observed in photoluminescence and photoreflectance spectra, which are significantly blue-shifted compared to the transitions in C-plane ZnO. (C) 2007 Elsevier B.V. All rights reserved.