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 ¯ 0 ) ZnO films are epitaxially grown on R-plane ( 1 1 ¯ 0 2 ) Al 2O 3 substrates by plasma-assisted molecular-beam epitaxy and structural and optical properties are investigated. Epitaxial relationships between ZnO films and Al 2O 3 substrates are determined to be ZnO ( 1 1 2 ¯ 0 ) / / Al 2 O 3 ( 1 1 ¯ 2 0 ) , ZnO [ 1 ¯ 1 0 0 ] / / Al 2 O 3 [ 1 1 2 ¯ 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 ¯ 0 ) with φ=0° and 90°, and the off-axis ( 1 0 1 ¯ 1 ) reflections are 0.41°, 0.36°, and 0.39°, respectively, for the 300 nm-thick ZnO film. Near-band edge emissions are dominant in low-temperature (12 K) 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.