Piezoelectric properties of III-V nitrides

Abstract

The main objective of this work is to measure the piezoelectric coefficients of III-V nitride thin films, namely aluminum nitride (AIN) and gallium nitride (GaN), by an interferometric technique. The nitride films were grown on Si substrates by Molecular Beam Epitaxy (MBE) and were characterized by Rutherford Backscattering Spectrometry (RBS) and X-ray Diffraction (XRD). Through these measurements, the nitride films were found to consist of mainly a hexagonal wurtzite structure. In order to grow epitaxial gallium nitride film on silicon, an aluminum nitride buffer layer is often needed, hence, some of the samples we measured are GaN/AIN/Si composite films. When a voltage is applied to a multilayer thin film, the voltage drop across each layer is determined by the ratio of the resisitivity and permittivity of the individual layer. A method to estimate the voltage drop across each layer as a function of frequency is described and used to evaluate the voltage drop across each piezoelectric layer. A single beam Mach-Zehnder type heterodyne interferometer was used to measure the displacements induced in the piezoelectric AIN and GaN samples. Using the voltage drop estimated above, the d33 and d31 coefficients of AIN were found to be (5.10 +- 0.1) pmV-1 and (2.55 +- 0.1) pmV-1 , respectively and the d33 and d31 coefficients of GaN were found to be (2.60 +- 0.1) pmV-1 and (1.30 +-0.1) pmV-1, respectively. By collating data in the literature, together with the measured d33 and d31, computer programs were compiled to calculate the velocity curves, power flow angles and electromechanical coupling coefficients in AIN, GaN, GaAs and ZnO. These results will be used for our future work in designing surface acoustic wave (SAW) devices based on these materials.