Effect of metallisation on surface acoustic wave velocity in GaN-on-sapphire structures

Surface acoustic wave (SAW) velocities have been measured on the free and metallised surfaces of GaN layers grown by metal organic chemical vapour deposition on (0001) sapphire substrates. The measurements were performed using the acousto-optic diffraction technique in the frequency range 100 – 400 MHz for SAW propagation direction along the [112¯0] sapphire axis. The deposition of thin aluminium films on the GaN surface reduced the SAW velocity up to 1% as compared to that on the free surface of the GaN-on-sapphire structure. The level of reduction was found to increase linearly with the acoustic frequency and GaN layer thickness.