We have studied the microstructure of InGaN layers grown on two different GaN substrates: a standard GaN film on sapphire and an epitaxial lateral overgrown GaN (ELOG) structure. These two materials exhibit two distinct mechanisms of strain relaxation. InGaN epilayers on GaN are typically pseudomorphic and undergo elastic relaxation by the opening of threading dislocations into pyramidal pits. A different behavior occurs in the case of epitaxy on ELOG where, in the absence of threading dislocations, slip occurs with the formation of periodic arrays of misfit dislocations. Potential slip systems responsible for this behavior have been analyzed using the Matthews-Blakeslee model and taking into account the Peierls forces. This letter presents a comprehensive analysis of slip systems in the wurtzite structure and considers the role of threading dislocations in strain relaxation in InGaN alloys.

The In x Ga 1-x N system has electronic band gaps extending from under 1eV to 3.4 eV, and as such they are used as the active layer in commercially available visible-light emitting devices. There are many interesting features that make these nitride semiconductor alloys especially useful for efficient light emitters. It has been conjectured that the combination of piezoelectric fields and local composition inhomogeneities may be responsible for the observed high emission efficiencies, in spite of their characteristic high dislocation densities. But it is very difficult to grow In x Ga 1-x N layers with high indium composition. This paper presents an overview of the properties of In x Ga 1-x N epilayers based on a systematic study of thick layers and of quantum well structures. We find that the microstructure of thick films varies significantly with indium composition. For x < 0.08, the composition is uniform and unperturbed by dislocations. For 0.10 < x < 0.20, secondary phases nucleate at threading dislocations. For x > 0.20, spontaneous phase separation occurs resulting in a polycrystalline, inhomogeneous layer. A correlation between optical properties and microstructure is presented. It is observed that the misfit strain is affected by threading dislocations. Mechanisms of misfit strain relaxation are presented for In x Ga 1-x N layers grown on standard GaN on sapphire and on epitaxial-lateral-overgrowth GaN layers. In addition, we have studied the properties of quantum well structures using several novel techniques. The electrostatic fields across the wells have been profiled using electron holography in the TEM. The effect of well thickness on the strength of the fields is reported. The effects of localization by compositional fluctuations and of internal field screening have been studied using time-resolved cathodoluminescence spectroscopy. In spite of significant progress that has been made in the last ten years, much work remains ahead in order to master the science and technology of these alloys.

A direct correlation between the structural and luminescence properties of thick InxGa1−xN layers has been achieved on a microscopic scale using highly spatially resolved cathodoluminescence. Surface roughening is typically observed in growth by metalorganic vapor phase epitaxy of thick InxGa1−xN layers for x⩾0.1. Although the film remains highly planar, craters and protrusions appear on the surface. These surface defects are associated with redshifted luminescence indicative of indium segregation, and are related to threading dislocations in the films.

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We have observed a systematic nucleation of misfit dislocations at the InGaN/GaN heterointerface. This occurs when InGaN films are grown on an epitaxially laterally overgrown GaN substrate with a reduced dislocation density. The misfit dislocations are aligned along 〈1100〉 directions forming a symmetric hexagonal array. Potential wurtzite slip systems were analysed by extending the Matthews-Blakeslee model to include Peierls forces. Due to an inactive basal plane in the c-growth direction, non-basal slip is necessary for plastic relaxation. The active slip system was identified to be {1122} 〈1123〉. The possibility of activation of other slip systems is also discussed. However, such a periodic nucleation of misfit dislocations has not been observed in InGaN alloys. These materials undergo strain relaxation usually by forming the so-called V-defects, which nucleate from the threading dislocations in the GaN substrate [4]. Misfit dislocations are observed for higher indium compositions ([In] > 1 0%), but these are random and do not provide a systematic strain relief. Lattice strain can severely affect the properties of InGaN [5] and it is important to obtain a clear understanding of strain relaxation mechanisms.In this work, we have observed a systematic strain relief in InGaN by growing the films on GaN substrates with reduced dislocation density. To our knowledge this is the first such observation in these materials. This was achieved by using epitaxially laterally overgrown GaN (ELOG) substrates. The formation of the misfit dislocations for various slip systems is analysed by extending the MatthewsBlakeslee model [1] to include the effect of Peierls forces. The active slip system was identified to be {1122}〈1123〉. We show that in the c-plane heteroepitaxy configuration the basal-plane glide is inactive. Under these conditions, non-basal glide along {1122} slip planes is activated. The possibility of other slip systems being activated is also discussed.