Stress relaxation in Si-doped GaN studied by Raman spectroscopy

We report the Si-doping-induced relaxation of residual stress in GaN epitaxial layers grown on (0001) sapphire substrate by the metalorganic vapor phase epitaxy technique. Micro-Raman spectroscopy is used to assess stress situation in the films with systematically modulated doping concentration from 4.0 x 10(17) up to 1.6 x 10(19) cm(-3). As the Si-doping concentration increases, a monotonic decrease of the E-2 phonon frequency is observed, which signifies gradual relaxation of the stress in the film. The layers are fully relaxed when electron concentration exceeds 1.6 x 10(19) cm(-3). The linear coefficient of shift in Raman frequency (omega) induced by the in-plane biaxial compressive stress (sigma(parallel to)) is estimated to be a Delta omega/Delta sigma(parallel to) = 7.7 cm(-1)/GPa. We suggest that Si doping increases density of misfit dislocation, judging from linewidth of x-ray rocking curve. (C) 1998 American Institute of Physics. [S0021-8979(98)06411-1].