Pseudo-bulk SAW transducers fabricated in GaN epitaxial layers grown on sapphire substrate

Abstract

Gallium nitride is a material commonly applied to the fabrication of various semiconductor devices, naming High Electron Mobility Transistors (HEMT) and Light Emitting Diodes (LED). Its unique feature is that it also possesses good piezoelectric properties. This enables to produce devices that integrate both active transistor operation and signal processing and filtration in one monolithic chip. This aim requires a comprehensive analysis of the acoustic wave propagation in the gan/sapphire system. Particularly attractive in the construction of modern acoustic instruments, due to higher propagation speeds, is the utilisation of pseudo-bulk mods. This paper presents the results of research on Surface Acoustic Wave (SAW) transducers using pseudo-bulk waves propagating in thin (2-6 μm) layers of GaN on sapphire. The layers were made by the MOVPE process using a low-temperature buffer layer. Transducers with a two-finger pitch of 9 and 18 μm with 48 and 24 periods respectively were made using electron lithography. Low finger resistivity was achieved by using Ru/Au/Ti/Au metal alloy. The results of microwave measurements identifying particular pseudo-bulk mods and their dispersion characteristics are presented. Two pseudo-bulk mods were observed, the first one with a propagation velocity range of 8500-7800 ms−1 and the second one of 7200-7300 ms−1. Additionally, pseudo-bulk modes were also compared with classical Rayleigh modes generated in the same transducers at lower frequencies. The results are important from the point of view of possible monolithic integration of the tested transducers with High Electron Mobility Transistors.