III‐nitride quantum dots for ultra‐efficient solid‐state lighting

III‐nitride light‐emitting diodes (LEDs) and laser diodes (LDs) are ultimately limited in performance due to parasitic Auger recombination. For LEDs, the consequences are poor efficiencies at high current densities; for LDs, the consequences are high thresholds and limited efficiencies. Here, we present arguments for III‐nitride quantum dots (QDs) as active regions for both LEDs and LDs, to circumvent Auger recombination and achieve efficiencies at higher current densities that are not possible with quantum wells. QD‐based LDs achieve gain and thresholds at lower carrier densities before Auger recombination becomes appreciable. QD‐based LEDs achieve higher efficiencies at higher currents because of higher spontaneous emission rates and reduced Auger recombination. The technical challenge is to control the size distribution and volume of the QDs to realize these benefits. If constructed properly, III‐nitride light‐emitting devices with QD active regions have the potential to outperform quantum well light‐emitting devices, and enable an era of ultra‐efficient solid‐state lighting.

     

Auger carrier leakage in III‐nitride quantum‐well light emitting diodes

Auger induced leakage is shown to be a contributing factor for the internal quantum efficiency (IQE) droop in III‐nitride quantum‐well light emitting diodes (LEDs). The mechanism is based on leakage current from carrier spill‐out of the well originating from energy transfer during Auger recombination. Adding this leakage reduces the Auger coefficient by 50% when compared to a standard Auger model with cubic density dependence. As reference, experimental data of a green quantum‐well LED are taken. Direct leakage due to non‐ideal carrier capture and re‐emission out of the well affects the IQE at current densities much larger than the maximum IQE point. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impact of surface recombination on efficiency of III‐nitride light‐emitting diodes

The paper considers surface recombination at the free active region surface as the mechanism of carrier losses which has not yet been discussed with regard to III‐nitride LEDs despite of its evident importance for AlGaInP‐based light emitters. Using advanced thin‐film and triangular volumetric chip designs reported in literature as prototypes, we have demonstrated by simulation a noticeable impact of surface recombination on the wall‐plug efficiency of InGaN‐based LEDs. Various types of LEDs whose efficiency may be especially affected by surface recombination are discussed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

A facile method for flexible GaN‐based light‐emitting diodes

Flexible GaN‐based light‐emitting diodes (LEDs) on polyethylene terephthalate (PET) substrates are demonstrated. The process uses commercial LEDs on patterned sapphire substrates, laser lift‐off (LLO), wet etching for additional surface roughening, and mounting of the freestanding LED on a PET substrate. Electrical and optical properties from the free‐standing LLO‐LEDs mounted on the flexible PET substrates were characterized. The process is scalable to large wafer diameters. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Research challenges to ultra‐efficient inorganic solid‐state lighting

Solid‐state lighting is a rapidly evolving, emerging technology whose efficiency of conversion of electricity to visible white light is likely to approach 50% within the next several years. This efficiency is significantly higher than that of traditional lighting technologies, giving solid‐state lighting the potential to enable significant reduction in the rate of world energy consumption. Further, there is no fundamental physical reason why efficiencies well beyond 50% could not be achieved, which could enable even more significant reduction in world energy usage. In this article, we discuss in some detail: (a) the several approaches to inorganic solid‐state lighting that could conceivably achieve “ultra‐high,” 70% or greater, efficiency, and (b) the significant research questions and challenges that would need to be addressed if one or more of these approaches were to be realized.