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We report the roles of island coalescence rate and strain relaxation in the development of anisotropic in-plane strains, striation feature, and subsequent degree of polarization in NH3-flow-rate-dependent m-plane GaN. In the high-NH3-flow-rate samples, the results of cathodoluminescence, polarized Raman and in situ optical reflectance measurements reveal that a slower coalescence and unrelieved lattice misfit strain lead to larger anisotropic in-plane strains, striated surface and luminescence patterns, and a lower density of basal-plane stacking fault (BSF) and prismatic stacking fault (PSF). In contrast, a lower NH3 flow rate leads to more rapid island coalescence and fully relaxed lattice misfit strain such that relaxed in-plane strains, a reduced striation surface, and a higher density of BSF and PSF are observed. It is suggested that the anisotropic in-plane strains, striation feature, and BSF and PSF density are consequences of how rapidly coalescence occurs and the degree of relaxation of lattice misfit strain. In addition, the simulation results of the k · p perturbation approach confirm a larger anisotropic strain results in a smaller degree of polarization. The research results provide important information for optimized growth of nonpolar III-nitrides.

This paper reports the effect of controlled growth dynamics, as monitored by in situ optical reflectance, on the microstructure of nonpolar a-plane GaN films grown on r-plane sapphire. The mosaic microstructure of a-plane GaN and its anisotropy are evaluated by X-ray rocking curve (XRC) measurements. By inserting a pronounced islanding stage followed by an enhanced lateral growth, pit-free a-plane GaN has been achieved showing an XRC linewidth of ~0.18 and ~0.3° for on- and off-axes planes, respectively, with only minor anisotropy. The density of basal-plane stacking faults is reduced by ~70% as determined by a modified Williamson–Hall X-ray analysis.