Acoustic-Wave Velocities and Refractive Indices in an m-Plane GaN Single-Crystal Plate and c-Axis-Oriented ScAlN Films Measured by Brillouin Scattering Techniques

We have experimentally investigated wave velocities and refractive indices in bulk and film samples [a GaN single-crystal plate and c-axis-oriented Sc_xAl_(1-x)N (x= 0.00-0.63) films] by Brillouin scattering (BRS). All of the piezoelectrically unstiffened elastic constants and the ordinary refractive index of the GaN single-crystal plate were determined from the reflection-induced θA(RIθA) scattering geometry and the combination of 90R and 180° scattering geometries. The uncertainties of the measured wave velocities were approximately 0.17% (RIθA) and 2.5% (combination technique). In addition, the longitudinal-wave velocities of Sc_xAl_(1-x)N films propagating in the normal direction were obtained by the combination technique. The maximum uncertainty was approximately 3.3%. The shear-wave velocities and refractive indices of Sc_xAl_(1-x)N films were also investigated by the 90R scattering geometry using velocities measured by high-overtone bulk acoustic resonators. The softening trends of the elasticity were obtained from the measured longitudinal- and shear-wave velocities, although there were large uncertainties in the Brillouin measurement system owing to thermal instability.