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 4.0×10¹⁷ up to 1.6×10¹⁹ cm^-3. As the Si-doping concentration increases, a monotonie decrease of the E₂ 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 ×10¹⁹ cm^-3. The linear coefficient of shift in Raman frequency (ω) induced by the in-plane biaxial compressive stress (σ_∥) is estimated to be Δω/Δσ_∥=7.7 cm^-1/GPa. We suggest that Si doping increases density of misfit dislocation, judging from linewidth of x-ray rocking curve.