Properties of zinc oxide thin films grown on R-plane alumina by metal organic chemical vapor deposition.

摘要

High-quality ZnO films are desired for applications such as buffer layers for III-nitride growth, low-loss high-frequency surface acoustic wave devices, and UV lasers. In this project, ZnO thin films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures between 350 to 500°C. The MOCVD reactor was redesigned by introducing separate injectors for DiEthyl Zinc and O₂, which reduced the gas-phase reactions between the reactants. The orientation relationship between the ZnO films and the R-plane sapphire substrates was determined to be (11 20) ZnO//(0112) Al₂O₃, and [0001] ZnO//[0111] Al₂O₃. The films had a smooth surface morphology without showing grain boundaries. The ZnO/Al₂O₃ interface was investigated by high resolution transmission electron microscopy and it was found to be atomically sharp and semicoherent. The strain due to the 18.3% lattice mismatch along the [1 100] direction of ZnO was relieved by a regular array of misfit dislocations. On average, the misfit dislocations had a separation of 1.4 nm, which corresponds to five (1100) ZnO planes.; The surface acoustic wave (SAW) properties of the piezoelectric ZnO films were studied by fabricating SAW testing devices with the propagation direction parallel to the c-axis of ZnO. A maximum effective electromechanical coupling coefficient $k2eff$ of 6% was obtained for a 10 μm wavelength device fabricated on a 1.5 μm thick ZnO film, which is close to the $k2eff$ value for bulk single crystal ZnO, indicating the high quality of the film. The achieved results are promising for the fabrication of low-loss surface acoustic wave devices operating in the giga hertz (GHz) frequency range. The low temperature (11 K) photoluminescence spectra consisted of a sharp excitonic peak at 3.363 eV with a full width at half maximum of about 6 meV, which is comparable to the values for bulk ZnO. The anisotropic absorption characteristics for light polarized parallel to and perpendicular to the c-axis of ZnO were also evaluated.; In order to investigate the thermal stability of the as-grown ZnO, films, annealing in an O₂ + N₂ atmosphere was conducted. While the crystal quality of the films improved on high temperature annealing, ZnO reacted with Al₂O₃ forming a spinel (ZnAl₂O ₄) layer at the interface. The thickness of the spinel layer formed was 15 nm and 35 nm on annealing the films for 30 min at 850°C and 1000°C respectively. Since this layer is confined to the interface between sapphire and ZnO, it is not expected to affect the use of ZnO as a buffer layer for the deposition of III-Nitrides. The kinetics of the solid state reaction between ZnO and Al₂O₃ was also investigated and the structures of the ZnO/ZnAl₂O₄ and ZnAl₂O₄/Al ₂O₃ interfaces were evaluated. It was found that the reaction was interface-controlled. It was determined that the ZnO/Al₂O₃ interface is stable if annealing is carried out below 750°C.