MOCVD growth and SAW properties of epitaxial ZnO thin films

ZnO thin films deposited on sapphire substrates are attractive for low-loss, high frequency devices. Single crystal quality of the piezoelectric ZnO film and an abrupt interface between the film and the substrate are desired for improving device performance. MOCVD has many advantages over other deposition techniques. We have grown epitaxial ZnO films on c- and r-sapphire substrates using MOCVD. The epitaxial relationships between the ZnO films and Al₂O₃ substrates were established by a combination of Transmission Electron Microscopy and X-Ray Diffraction techniques. The ZnO-sapphire interface was atomically sharp. The strain due to lattice mismatch is accommodated by the presence of misfit dislocations at the interface region within 6 nm. SAW test devices with various film thickness to wavelength ratios (h/λ) were fabricated. The acoustic velocity, coupling coefficient (k²) and temperature coefficient of frequency (TCF) were determined as functions of h/λ. The coupling coefficient obtained for a 1.5 μm thick ZnO thin film was about 6% at 10 μm wavelength, comparable to bulk single crystal ZnO. The results are promising for low-loss, high frequency filters