Epitaxy and Device Design for High Efficiency Blue LEDs and Laser Diodes

The (Al,Ga,In)N materials system has impacted energy efficiency on the world-wide scale through its application to blue light-emitting diodes (LEDs), which were invented and developed in the 1990s. Since then, cost reductions and performance improvements have brought GaN-based LEDs into the mainstream, supplanting outdated lighting technology and improving energy efficiency.One of the main challenges that still limits commercial LEDs, however, is “efficiency droop,” which refers to the reduction in efficiency as the input current density (and with it, the carrier density) increases. This phenomenon especially plagues high power LEDs, which operate in the current density range of 100-1000 A/cm2.Few practical options exist to directly eliminate efficiency droop, however we investigated two complementary approaches to circumvent the phenomenon. The first “high power solution” would employ blue laser diodes as the engine of solid state white lighting in lieu of LEDs. When laser diodes reach the threshold current density for stimulated emission, the carrier density in the active region clamps, simultaneously clamping droop. The wall plug efficiency of the laser diodes can then continue to rise as input current density increases until another effect (usually thermal) overrides it. The second “low power solution” maintains the blue LED as the solid state lighting engine, but shifts the operation point to low current density (and low carrier density) where efficiency droop effects are negligible and other thermal and electrical constraints in the device design are alleviated, enabling designs for high wall-plug efficiency. Both approaches to circumventing efficiency droop are likely to find a home in diverse future technologies and applications for lighting and displays.The challenge to produce high performance blue laser diodes was approached from an m-plane epitaxy platform. m-Plane is a non-polar orientation of the wurtzite (Al,Ga,In)N, which is free from deleterious pol