Critical Assessment 23: Gallium nitride-based visible light-emitting diodes

Solid-state lighting based on light-emitting diodes (LEDs) is a technology with the potential to drastically reduce energy usage, made possible by the development of gallium nitride and its alloys. However, the nitride materials family exhibits high defect densities and, in the equilibrium wurtzite crystal phase, large piezo-electric and polarisation fields arising at polar interfaces. These unusual physical properties, coupled with a high degree of carrier localisation in devices emitting visible light, result in ongoing challenges in device development, such as efficiency 'droop' (the reduction in efficiency of nitride LEDs with increasing drive current density), the 'green gap' (the relatively low efficiency of green emitters in comparison to blue) and the challenge of driving down the cost of LED epitaxy.