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 (112¯0) ZnO films are epitaxially grown on R  -plane (11¯02) 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(112¯0)//Al2O3(11¯20), ZnO[1¯100]//Al2O3[112¯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 [0001] direction. Full width at half maximums of X-ray rocking curves for the on-axis (112¯0) with φ  =0° and 90°, and the off-axis (101¯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 [0001] 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.