The critical thickness gives the transition point between fully strained and relaxed heteroepitaxial films and determines the onset of defect generation, including misfit dislocations, cracks, and V-pits. An important variable in critical thickness calculations concerning misfit dislocations is the dislocation energy. It consists of two contributions: the elastic energy of the bulk material around a dislocation and the energy of the dislocation core. The latter part is often neglected. Recent atomistic calculations have estimated this quantity together with the radius of dislocation cores in wurtzite III-nitrides. The effect of the dislocation core energy on equilibrium critical thickness values for III-nitrides is investigated theoretically and is shown to be significant. The calculated values of the critical thickness are compared with experimentally determined values of the critical thickness for misfit dislocations in the InGaN/GaN system using transmission electron microscopy and x-ray diffraction techniques. A comparison of the present model with experimental observations, together with a wide range of data from the literature for both AlGaN/GaN and InGaN/GaN systems, shows reasonable agreement. Finally, we speculate on a possible reason for differences between theory and experiment.

A series of GaN films were grown by metalorganic vapor phase epitaxy on nitrided sapphire using an initial annealed low-temperature nucleation layer (LT-NL), without employing any conventional threading dislocation (TD) reduction methods. Film thicknesses ranging from the LT-NL to 500 nm were used. The island network morphology was investigated at each growth stage using atomic force microscopy. Data from cathodoluminescence studies showed initially uniform luminescence, followed by the gradual development of bright (low TD) regions which had lateral sizes different from the island sizes at all times and which continued to increase in size after coalescence. The formation of low-energy arrays of a-type TDs also continued after island coalescence. X-ray diffraction, transmission electron microscopy (TEM) and AFM data indicated that the highest (a+c)-type TD densities were found in the LT-NL, but subsequently decreased due to TD loop formation (promoted by island facets) and reaction to produce a-type TDs. a-type TD densities were also high in the LT-NL but subsequently increased slightly, due to the reaction of (a+c)-type TDs. A very sharp dynamical ‘correlation’ peak was also observed in XRD of the LT-NL, related to TDs with an a-component. Furthermore, defect formation was observed within the LT-NL using high-resolution TEM. These data are consistent with TD formation predominantly in the LT-NL, followed by TD movement at elevated growth temperatures. Initially, coalesced films had a high TD density with a spatially random TD arrangement, but progressively altered into a lower TD density, spatially clustered arrangement during growth. This type of microstructure may mistakenly be interpreted as arising from island coalescence.

A tandem absorber of TiAlN∕TiAlON∕Si3N4 is prepared using a magnetron sputtering process. The graded composition of the individual component layers of the tandem absorber produces a film with a refractive index increasing from the surface to the substrate, which exhibits a high absorptance (0.95) and a low emittance (0.07). The tandem absorber is stable in air up to 600°C for 2h, indicating its importance for high temperature solar selective applications. The thermal stability of the tandem absorber is attributed to high oxidation resistance and microstructural stability of the component materials at higher temperatures.

In this paper we report on the optical properties of a series of GaN/AlGaN multiple quantum well structures grown on a-plane (112¯0) GaN, which had been deposited on r-plane (11¯02) sapphire substrates, compared to a reference GaN template of the same orientation. The low temperature photoluminescence spectrum of the template layer is dominated by two emission bands, which we attribute to recombination involving excitons in the bulk of the layer and electrons and holes trapped at basal-plane stacking faults, designated X1 and X2, respectively. The photoluminescence spectra from the quantum well structures show similar emission bands except that both X1 and X2 shift to higher energy with decreasing quantum well thickness. The shift to higher energy is due to the effects of quantum confinement on carriers trapped at the stacking faults that intersect the quantum wells, as well as those excitons that are localized within the quantum wells. This assignment is based partly on excitation spectroscopy that reveals exciton transitions associated with electrons from the n=1 and n=2 quantum well confined states.

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