Ultrasonic characterisation of gallium nitride

Abstract

The authors have calculated the six second and ten third order elastic constants of the material GaN having hexagonal (wourtzite) crystal structure in the temperature range 100–700 K. The temperature variation of the ultrasonic velocities is evaluated along different angles with z axis (unique axis) of the crystal using the second order elastic constants. The ultrasonic velocity decreases with the temperature along particular orientation with the unique axis. Temperature variation of the thermal relaxation time and Debye average velocities is also calculated along the same orientation. The elastic constants are used for the determination of the ultrasonic attenuation in GaN along the unique axis of the crystal in the range 100–700 K. The attenuation is mainly due to the elastic constant and thermal energy density up to 400 K while above 400 K it is due to thermal relaxation time. The temperature variation of the thermoelastic loss along z axis of the crystal is mainly dominated by the thermal conductivity in the range 300–700 K. A brittle to ductile transition occurs at high temperatures. The attenuation peak is obtained at 400 K.