The influence of interdiffusion on strain energy in the GaN-sapphire system

The impact of interdiffusion on strain energy in the GaN-sapphire system was studied. Gallium nitride epitaxial layers were grown on (0001) Al2O3 by low-pressure MOCVD at 850 °C, V/III ratio = 3600, P= 100 Torr using TEGa and NH3 sources in a N2 carrier. The ~0.3 μm thick films were then annealed at growth temperature in N2 for a period of 30 to 120 min. The Al and Ga diffusion coefficients at 850 °C were estimated as DAl = 3.98 × 10 –17 cm/s and DGa = 4.81 × 10 –17 cm/s from SIMS depth profile data. A model was developed to predict the strain energy and describe its relaxation behavior. The calculations assume a gradual transition of lattice parameter near the interface rather than an abrupt transition. The effect of interdiffusion on the strain energy and predicted critical thickness were clearly demonstrated. The estimated strain energy of the first layer was only 176 J/mole, compared to 30 kJ/mole assuming an abrupt interface. For an anneal time of 120 min at 850 °C the model predicts that dislocations are formed when the strain energy reaches 10 kJ/mole after the 4 layer is added, compared to their formation after the 1 layer is formed when an abrupt interface is assumed.