STRAIN DETERMINATION IN HETEROEPITAXIAL GAN

Residual strains have been evaluated in a variety of GaN layers grown on sapphire or 6H-SiC from wafer curvature at 293 K, which avoids need ing to know the unstrained lattice parameters or energy gap of GaN in advance. Estimated strains at 1.7 K are correlated with the energy of the A free exciton to determine its strain dependence. We find that st rain-free GaN has an A exciton energy of 3.468+/-0.002 eV at 1.7 K, an d 293 K lattice parameters a = 3.1912 Angstrom and c = 5.1836 Angstrom . These values imply that GaN on SiC is frequently under net biaxial c ompressive stress due to residual lattice mismatch stress, and that se veral hundred mu m thick GaN layers on sapphire and homoepitaxial laye rs grown on bulk platelets grown at high pressure are both under about 1 x 10(-3) in-plane compressive strain. These conclusions conflict wi th most previous assumptions. (C) 1997 American Institute of Physics.