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 monotonic 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.