Abstract
A quantitative transmission electron microscopy (TEM) investigation of the relaxation of GaN grown on Al2O3(0001) has been carried out. Terminating {112̄0}-substrate fringes observed at the interface of the highly mismatched system have been characterized and the efficiency of the relaxation was measured. Wurtzite type GaN was grown by plasma induced molecular beam epitaxy (MBE) on the (0001) basal plane of Al2O3. The in-plane orientation between GaN and substrate reveals a high lattice misfit of f=-13.9% and therefore the critical thickness of dislocation formation is reached when the first monolayer of GaN is grown. An expected interfacial relaxation process is characterized by the results of high resolution transmission electron microscopy (HRTEM) which reveals misfit dislocations confined in the GaN/Al2O3 interface region. The quantitative evaluation of the HRTEM images on the one hand and Moiré pattern on the other hand shows the effectiveness of the observed relaxation process: here a degree of relaxation δ=(-11.8±0.9)% and a residual strain of εr=(-2.1±0.9)% was measured, and it seems that only εr causes a dislocation density of estimately 1010 cm-2 in the GaN epilayer.
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