Structural, optical, and surface acoustic wave properties of epitaxial ZnOfilms grown on (01(1)over-bar2) 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 opti cal modulators, as buffer layers for III-nitride growth, and as the active material in ultraviolet solid state lasers. In this work, high quality epit axial ZnO films were grown on R-plane sapphire substrates by metalorganic c hemical vapor deposition. The structural, piezoelectric, and optical proper ties of the ZnO films on R sapphire have been investigated. The epitaxial r elationship between ZnO and R-Al2O3 was found to be (11 (2) over bar 0) ZnO parallel to(01 (1) over bar 2) Al2O3, and [0001] ZnO parallel to[0 (1) ove r bar 11] Al2O3. The interface between as-grown ZnO and R sapphire was atom ically sharp and semicoherent, as evaluated by transmission electron micros copy. On annealing the films at temperatures above 850 degrees C, a solid s tate reaction occurred between ZnO and Al2O3, resulting in the formation of ZnAl2O4 (spinel) at the interface. A 15-20 nm spinel layer formed when the ZnO film was annealed at 850 degrees C for 30 min, whereas a 150 nm layer formed when the film was annealed at 1000 degrees C for 150 min. To prevent this reaction from occurring, the maximum process temperature should be be low 750 degrees C. The surface acoustic wave properties of the piezoelectri c ZnO were evaluated by fabricating SAW devices on (11 (2) over bar 0) ZnO parallel to(01 (1) over bar 2) Al2O3, An effective electromechanical coupli ng coefficient, k(eff)(2),of 6% was achieved for a 1.5 mu m thick ZnO film, which is close to the value for bulk single-crystal ZnO. The photoluminesc ence spectra were obtained both at room temperature and at 11 K. The full w idth at half maximum of the 3.363 eV band edge emission photoluminescence p eak measured at 11 K was 6 meV, which is close to that for single-crystal Z nO. We also evaluated the anisotropic absorption characteristics of the (11 (2) over bar 0) ZnO film, which can be used for a high contrast ultraviole t light modulator. (C) 1999 American Institute of Physics. [S0021-8979(99)0 5304-9].