Effect of Si doping on the dislocation structure of GaN grown on the A-face of sapphire

Transmission electron microscopy, x-ray diffraction, low-temperature photoluminescence, and Raman spectroscopy were applied to study stress relaxation and the dislocation structure in a Si-doped GaN layer in comparison with an undoped layer grown under the same conditions by metalorganic vapor phase epitaxy on (11.0) Al2O3. Doping of the GaN by Si to a concentration of 3×1018 cm−3 was found to improve the layer quality. It decreases dislocation density from 5×109 (undoped layer) to 7×108 cm−2 and changes the dislocation arrangement toward a more random distribution. Both samples were shown to be under biaxial compressive stress which was slightly higher in the undoped layer. The stress results in a blue shift of the emission energy and E2 phonon peaks in the photoluminescence and Raman spectra. Thermal stress was partly relaxed by bending of threading dislocations into the basal plane. This leads to the formation of a three-dimensional dislocation network and a strain gradient along the c axis of the layer.