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

Non-polar, A-plane (1 1 over(2, -) 0) ZnO films are epitaxially grown on R-plane (1 over(1, -) 0 2) Al₂O₃ substrates by plasma-assisted molecular-beam epitaxy and structural and optical properties are investigated. Epitaxial relationships between ZnO films and Al₂O₃ substrates are determined to be ZnO(1 1 over(2, -) 0) / / Al₂ O₃ (1 over(1, -) 2 0), ZnO[over(1, -) 1 0 0] / / Al₂ O₃ [1 1 over(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 over(2, -) 0) with φ=0° and 90°, and the off-axis (1 0 over(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 (D⁰X) and allowed free excitons are observed in photoluminescence and photoreflectance spectra, which are significantly blue-shifted compared to the transitions in C-plane ZnO.