Surface acoustic waves in ZnO/AlxGa1-xN/C-Al2O3 structures

Piezoelectric AlN and ternary Al_xGa_1-xN thin films deposited on (0001) C-plane sapphire substrates are attractive for low-loss and high-frequency surface acoustic wave (SAW) devices. However, growth of epitaxial quality AlN films is difficult due to strong parasitic gas phase reactions between precursors. On the other hand, ZnO is a well-known piezoelectric material with high electromechanical coupling coefficients. Furthermore, high quality epitaxial ZnO films can be grown at temperatures hundreds of degrees lower than AlN. By depositing piezoelectric ZnO and AlN multilayer structures on C-plane sapphire substrates, large coupling coefficient and high SAW velocity can be obtained. In this work, ZnO/Al_xGa_1-xN multilayer structures were epitaxially grown on C-plane sapphire substrates by metal organic chemical vapour deposition (MOCVD). SAW devices were fabricated and tested, which exhibited good agreement between theoretical and experimental results. The SAW velocity and coupling coefficients of the ZnO/Al_xGa_1-xN(0≤x≤1)C-Al₂O₃ multilayer systems are analyzed as a function of the Al mole percentage, x in Al_xGa_1-xN and of the ZnO (h₁) to Al_xGa_1-xN (h₂ thickness ratio. It has been found that the hf region where the coupling coefficient is close to K²_max broadens with increasing Al content, while k²_max decreases slightly. When the thickness ratio h₁/h₂ is 0.5, a wide hf region where coupling is close to k²_max is obtained.