It's not easy being green: Strategies for all-nitrides, all-colour solid state lighting

The design strategy presently employed to obtain 'white' light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining 'true' red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1. the green gap in the internal quantum efficiency (IQE) of LEDs; 2. the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wave-function overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim