Measurements of piezoelectric coefficients of nitride semiconductor films

Piezoelectric coefficients of the wurtzite phase of the important semiconducting materials GaN and AlN have been measured using optical interferometry. Both the extensional and shear strain coefficients have been evaluated from direct measurements of the relevant displacements of the film surface. The measured value of the thickness coefficient d₃₃, was 2.0 pm V^-1 for polycrystalline GaN and 2.8 pm V^-1 for single crystal material. The measured d₃₃ coefficient for polycrystalline AlN was 4.0 pm V^-1. The measured values of the d₁₅ coefficient were -3.1 pm V^-1 for GaN and -3.6 pm V^-1 for AlN. The electromechanical response is found to include a significant contribution from the nonlinear effect of electrostriction. The measured value of the electrostrictive coefficient M₃₃ in a polycrystalline sample of GaN was 1.2×10^-18 m² V^-2. The piezoelectric measurement also provides a simple method for identifying the positive direction of the c axis, which was found to be pointing away from the substrate for all films studied