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 850C, V/III ratio = 3600, P = 100Torr using TEGa and NH3 sources in a N2 carrier. The approximately 0.3µm thick films were then annealed at growth temperature in N2 for a period of 0.5 to 2h. The Al and Ga diffusion coefficients at 850C were estimated to be DAl = 3.98 x 10-17 and DGa = 4.81 x 10-17cm2/s from secondary ion mass spectrometry 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 176J/mole, compared to 30kJ/mol assuming an abrupt interface. For an anneal time of 2h at 850C, the model predicts that dislocations were formed when the strain energy reaches 10kJ/mol after the 4th layer was added, compared to their formation after the 1st layer was formed when an abrupt interface was assumed.

The Influence of Interdiffusion on Strain Energy in the GaN-Sapphire System. S.W.Kang, H.J.Park, T.Kim, T.Dann, O.Kryliouk, T.Anderson: Physica Status Solidi C, 2005, 2[7], 2420-3