Residual strain dependence of optical characteristics in GaN layers grown on (0001) sapphire substrates

GaN layers were grown on (0001) sapphire substrates by metalorganic chemical vapor deposition with various growth conditions. Some samples were grown with pressures higher than atmospheric. The residual strain in the epitaxial layer was estimated by measuring the lattice constants using x-ray diffraction. The optical quality was evaluated in terms of the threshold power density of stimulated emission. The residual strain perpendicular to C face, εzz, ranged from 0.058% to 0.125%. The strain ratio under biaxial stress (Δc/c)/(Δa/a), is estimated to be −0.46 from the relation between the lattice constants a and c. From the photoluminescence spectra with weak excitation, δEA/δεzz for undoped GaN is estimated to be 16.4 eV. The threshold power density decreased from 2.77 to 0.59 MW/cm2 as the strain increased, suggesting that the strain relaxation process is accompanied by a generation of defects which act as nonradiative recombination centers.