Structural, optical, and surface acoustic wave properties of epitaxial ZnO films grown on (011̄2) sapphire by metalorganic chemical vapor deposition

High-quality ZnO films are receiving increased interest for use in low-loss high-frequency surface acoustic wave (SAW) devices, acousto-optic and optical modulators, as buffer layers for III-nitride growth, and as the active material in ultraviolet solid state lasers. In this work, high quality epitaxial ZnO films were grown on $R$ -plane sapphire substrates by metalorganic chemical vapor deposition. The structural, piezoelectric, and optical properties of the ZnO films on $R$ sapphire have been investigated. The epitaxial relationship between ZnO and $\mathrm{R-}{\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ was found to be ${\mathrm{(112̄0) ZnO\parallel (011̄2) Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}},$ and ${\mathrm{[0001] ZnO\parallel [01̄11] Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}.$ The interface between as-grown ZnO and $R$ sapphire was atomically sharp and semicoherent, as evaluated by transmission electron microscopy. On annealing the films at temperatures above $\text{850\hspace{0.05em}°}\mathrm{C},$ a solid state reaction occurred between ZnO and ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}},$ resulting in the formation of ${\mathrm{ZnAl}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ (spinel) at the interface. A 15–20 nm spinel layer formed when the ZnO film was annealed at $\text{850\hspace{0.05em}°}\mathrm{C}$ for 30 min, whereas a 150 nm layer formed when the film was annealed at $\text{1000\hspace{0.05em}°}\mathrm{C}$ for 150 min. To prevent this reaction from occurring, the maximum process temperature should be below $\text{750\hspace{0.05em}°}\mathrm{C}.$ The surface acoustic wave properties of the piezoelectric ZnO were evaluated by fabricating SAW devices on ${\mathrm{(112̄0) ZnO/(011̄2) Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}.$ An effective electromechanical coupling coefficient, $k_{\mathrm{eff}}^2,$ of 6% was achieved for a 1.5 μm thick ZnO film, which is close to the value for bulk single-crystal ZnO. The photoluminescence spectra were obtained both at room temperature and at 11 K. The full width at half maximum of the 3.363 eV band edge emission photoluminescence peak measured at 11 K was 6 meV, which is close to that for single-crystal ZnO. We also evaluated the anisotropic absorption characteristics of the $\text{(112̄0)}$ ZnO film, which can be used for a high contrast ultraviolet light modulator.