Extensional piezoelectric coefficients of gallium nitride and aluminum nitride

Measurements of piezoelectric coefficients d(33) and d(31) in wurtzite GaN and AlN using an interferometric technique are presented. We report on the clamped values, d(33)(c) of these coefficients found in GaN and AlN thin films, and we derive the respective bulk values, d(33)(b). The clamped value of d(33)(c) in GaN single crystal films is 2.8 +/- 0.1 pm V-1 which is 30% higher than in polycrystalline films grown by laser assisted chemical vapor deposition. The value of d(33)(b) in bulk single crystal GaN is found to be 3.7 +/- 0.1 pm V-1. The value of d(33)(c) in plasma assisted and laser assisted chemical vapor deposited AlN films was 3.2 +/- 0.3 and 4.0 +/- 0.1 pm V-1, respectively. The bulk value estimate of d(33)(b) in AlN of 5.6 +/- 0.2 pm V-1 was deduced. The values of d(31), both clamped and bulk, were calculated for wurtzite GaN and AlN. We have also calculated the values of d(14) in cubic phase film and bulk GaN and AlN. Interferometric measurements of the inverse piezoelectric effect provide a simple method of identifying the positive direction of the c axis, which was found to be pointing away from the substrate for all films. (C) 1999 American Institute of Physics. [S0003-6951(99)01552-1].