White light emitting diodes (LEDs) based on III-nitride $\mathrm{InGaN}/\mathrm{GaN}$ quantum wells currently offer the highest overall efficiency for solid state lighting applications. Although current phosphor-converted white LEDs have high electricity-to-light conversion efficiencies, it has been recently pointed out that the full potential of solid state lighting could be exploited only by color mixing approaches without employing phosphor-based wavelength conversion. Such an approach requires direct emitting LEDs of different colors, including, in particular, the green-yellow range of the visible spectrum. This range, however, suffers from a systematic drop in efficiency, known as the “green gap,” whose physical origin has not been understood completely so far. In this work, we show by atomistic simulations that a consistent part of the green gap in $c$-plane $\mathrm{InGaN}/\mathrm{GaN}$-based light emitting diodes may be attributed to a decrease in the radiative recombination coefficient with increasing indium content due to random fluctuations of the indium concentration naturally present in any InGaN alloy.

We have investigated pattern formation of thin PMMA films during both hot embossing and demoulding of micro- and nanostructures. During filling of the stamp cavities, compressive and capillary effects were observed, and under certain conditions periodic patterns with characteristic length scales formed. In unstructured stamp regions the form of these patterns is affected by local differences in the pressure. For structured stamps, the self-assembly is strongly influenced by the size and the shape of the stamp cavities. Rapid expansion of trapped air resulted in viscous fingering patterns and a dewetting behaviour of the polymer could be observed. (Some figures in this article are in colour only in the electronic version; see www.iop.org)

Based on numerical simulation and comparison with measured current characteristics, we show that the current in InGaN/GaN single-quantum-well light-emitting diodes at low forward bias can be accurately described by a standard trap-assisted tunneling model. The qualitative and quantitative differences in the current characteristics of devices with different emission wavelengths are demonstrated to be correlated in a physically consistent way with the tunneling model parameters.

In this paper, we present a framework for the simulation of electronic devices based on a multiscale and multiphysics approach. A formal description is provided that includes both multiscale and multiphysics problems and which can be linked to already established multiscale methods. We present a set of simulations of an AlGaN/GaN nanocolumn based on a multiscale coupling between atomistic descriptions and continuous media models, illustrating the application of such a multiscale approach to electronic device simulation.

Top-down fabricated GaN nanowires, 250 nm in diameter and with various heights, have been used to experimentally determine the evolution of strain along the vertical direction of 1-dimensional objects. X-ray diffraction and photoluminescence techniques have been used to obtain the strain profile inside the nanowires from their base to their top facet for both initial compressive and tensile strains. The relaxation behaviors derived from optical and structural characterizations perfectly match the numerical results of calculations based on a continuous media approach. By monitoring the elastic relaxation enabled by the lateral free-surfaces, the height from which the nanowires can be considered strain-free has been estimated. Based on this result, NWs sufficiently high to be strain-free have been coalesced to form a continuous GaN layer. X-ray diffraction, photoluminescence, and cathodoluminescence clearly show that despite the initial strain-free nanowires template, the final GaN layer is strained.