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.

Al x Ga 1−x N layers with 0.05⩽x⩽0.25 were studied using spectrally and time resolved cathodoluminescence (CL). Continuous wave spectra were taken at temperatures ranging from 5 to 300 K. The near-band-edge peak emission energy exhibits an s-shaped temperature dependence characteristic of disordered systems. This effect is quantitatively explained within a model of potential fluctuations caused by alloy disorder. An s-shape temperature dependence has been observed in other alloy systems including InGaN, however, no systematic study exists for AlGaN. In this work, the s-shape temperature dependence is systematically analyzed as a function of aluminum content and quantitatively correlated with a model of alloy disorder. The shift in the luminescence peak position with respect to the usual temperature dependence of the band gap has been quantified by −σE2/kBT, where σE is the standard deviation of the potential fluctuations. Its dependence on aluminum concentration, x, was found to systematically increase from 7 meV at x=0.05 to 21 meV at x=0.25, following the theory for alloy disorder. The recombination and relaxation kinetics investigated using time-resolved CL are fully consistent with our potential fluctuation model. At 5 K, when the excitons are strongly localized, the exciton lifetime increases monotonically with aluminum content. At elevated temperatures, when the excitons are delocalized, the decay is significantly faster and preferentially nonradiative, regardless of the aluminum content.

[1] First ever in-situ measurements of black carbon (BC) aerosols in the troposphere (up to 9 km) made over central India and the resulting atmospheric impact as revealed by the environment lapse rate are presented. The altitude distribution of BC showed multiple peaks; two surprisingly large peaks, one at ∼4.5 km, and another above 8 km. Associated with these, rapid decrease in the environmental lapse rate and a sharp increase in the atmosphere stability were observed, probably caused by the atmospheric warming by the BC layers. This important observation calls for extensive high altitude profiling of BC to quantify the resultant warming, increase in stability and consequent increase in BC lifetime. Citation: Babu, S.

We report on the presence of optically active stacking faults on basal and prismatic planes in epitaxially laterally overgrown GaN (ELOG) on {112¯2} facets. The structure of the faults has been analyzed using diffraction contrast electron microscopy. We show that stacking faults on {112¯0} prismatic planes involve a lattice displacement of 12⟨11¯01⟩, parallel to the fault plane. They appear as jogs connecting basal-plane stacking faults, the latter with a lattice displacement of 16⟨202¯3⟩. These faults are observed only in the laterally overgrown regions that grow on {112¯2} planes, which indicates that the stacking fault formation is closely related to the orientation of the growth surface. Possible formation mechanisms of these faults are discussed. Direct correlation between TEM and cathodoluminescence shows that these prismatic-plane and basal-plane stacking faults are optically active with light emission at 3.30 and 3.41eV, respectively.

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