Enhanced SAW characteristics of a-plane AlN epitaxial films using ZnO buffer layer

Abstract

High-quality a-plane AlN epitaxial films were successfully deposited on r-plane sapphire using a thin ZnO buffer layer for surface acoustic wave (SAW) applications. Structural properties of AlN epitaxial films on different thicknesses of ZnO buffer and the related SAW characteristics were investigated systematically. The results demonstrate that AlN films were epitaxially grown on ZnO buffered-substrate with orientation relationship of $(11\overline {2} 0){\text{ }}[10\overline {1} 0]$ AlN//$(11\overline {2} 0){\text{ }}[10\overline {1} 0]$ ZnO//$(01\overline {1} 2){\text{ }}[\overline {2} 110]$ Al₂O₃. A thin ZnO buffer layer with moderate thickness significantly facilitated the crystal quality of a-plane AlN films, and in turn enhanced the SAW characteristics. High frequency SAW devices with a center frequency of ~ 1500 MHz corresponding to a high phase velocity of 6000 m/s were achieved on the obtained AlN films. The as-grown a-plane AlN epitaxial films with 40 nm optimized ZnO buffer exhibit high crystal quality with a FWHM value of the rocking curve of 0.76° and good performance of SAW devices with low insertion loss and large out-of-band rejection. This work offers an effective approach to achieve high-quality a-plane AlN epitaxial films on sapphire substrates for the AlN-based SAW devices applications.