Dependence of the stress-temperature coefficient on dislocation density in epitaxial GaN grown on α-Al2O3 and 6H-SiC substrates

We report measurements of stress in GaN epitaxial layers grown on 6H-SiC and alpha-Al2O3 substrates. Biaxial stresses span +1.0 GPa (tensile) to -1.2 GPa (compressive). Stress determined from curvature measurements, obtained using phase-shift interferometry (PSI) microscopy, compare well with measurements using accepted techniques of x-ray diffraction (XRD) and Raman spectroscopy. Correlation between XRD and Raman measurements of the E-2(2) phonon gives a Raman-stress factor of -3.4+/-0.3 cm(-1)/GPa. We apply PSI microscopy for temperature dependent stress measurements of the GaN films. Variations found in the stress-temperature coefficient correlate well with threading dislocation densities. We develop a phenomenological model which describes the thermal stress of the epitaxial GaN as a superposition of that for ideal GaN and the free volume existing in the layers due to the threading dislocations. The model describes well the observed dependence. (C) 2004 American Institute of Physics.