Thermal stability of GaN investigated by Raman scattering

We have investigated the thermal stability of GaN using Raman scattering. Noninvasive optical monitoring of structural damage to GaN by high-temperature anneals in nitrogen ambient has been demonstrated. Characteristic features in the Raman spectrum identify three thermal stability regimes. Thermal damage between 900 and $\text{1000\hspace{0.17em}°}\mathrm{C}$ results in the appearance of a broad Raman peak between the $E_2$ and $A_1(\mathrm{LO})$ phonon. For anneals at temperatures higher than $\text{1000\hspace{0.17em}°}\mathrm{C}$ emerging macroscopic disorder gives rise to distinct Raman modes at 630, 656, and $\text{770\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}.$ Below $\text{900\hspace{0.17em}°}\mathrm{C}$ no thermal damage has been observed. The evolution of the Raman spectrum of GaN with increasing annealing temperature is discussed in terms of disorder-induced Raman scattering. We find clear indications for a reaction at the GaN/sapphire interface for anneals higher than $\text{1000\hspace{0.17em}°}\mathrm{C}.$