Deposition and characterization of ZnO thin films on GaAs and Pt/GaAs substrates

This work reports the deposition and characterization of piezoelectric ZnO thin films on semi-insulating GaAs substrates for the fabrication of bulk acoustic waves sensors. ZnO films are deposited at 350 degrees C and low deposition rate using reactive radio frequency magnetron sputtering. The use of a Pt bottom electrode, between ZnO and GaAs, with and without Ti buffer layer, as well as the effect of the substrate crystallographic orientation are investigated. The characterization of the deposited films is performed to determine the optimal parameters for obtaining high-quality films and ZnO residual conductivity. ZnO films are textured along the c-axis for all GaAs cuts. The highest structural quality is obtained on (100) GaAs substrates. Moreover, the presence of the Ti buffer layer improves the texture quality, surface roughness and residual stresses. The lowest residual conductivity is determined for the ZnO/Pt/Ti/GaAs structure. However, a slight diffusion of Ga and Ti into Pt is observed, which may be of concern if Pt is used as a bottom electrode. Therefore, for thickness excitation functioning, further optimization of the Pt/GaAs interface has to be considered.