High Power High Efficiency Semipolar InGaN Light Emitting Devices for Solid State Lighting
The impact of solid state lighting on the world has been remarkable. The improvement in efficiency and lifetime, the reduced environmental impact and the new design space of solid state lighting when compared to traditional incandescent or compact fluorescent lighting is dramatic. Still, improvements can be made to improve and accelerate the adoption of this technology.
Light Emitting Diodes (LEDs), the basis of solid state lighting, are impacted by the problem of efficiency droop, whereby the efficiency of LEDs peaks at low current density and declines dramatically with increasing current. This limits the power out per chip and requires many chips to be used to create an incandescent equivalent replacement fixture. In this dissertation, several ways of addressing this problem and other efficiency limits of solid state lighting will be addressed.
Utilizing novel properties of alternative planes of the gallium nitride (GaN) crystal, LEDs can be fabricated with reduced droop. These semipolar planes differ from the c-plane (the plane all commercial devices are grown on) in that they have reduced polarization-related effects, which increases the quantum efficiency. Semipolar LEDs with wider active regions to reduce carrier density and limit Auger recombination are demonstrated.
Laser Diodes (LDs) can also be used to improve the efficiency of solid state lighting. LDs have higher brightness than LEDs and are not impacted by efficiency droop above threshold, so it is possible to run LD chips at high currents and obtain orders of magnitude higher power per chip, with the potential to reduce size and thus cost of solid state lighting fixtures. Semipolar planes can again be used to overcome efficiency limits of c-plane LDs as well.
The design of LEDs and LDs are presented in this dissertation. Beginning with metalorganic chemical vapor deposition (MOCVD) growth, continuing through nanofabrication, packaging, and testing, all facets of semipolar nitride devices will be discussed. Simulations are used throughout to gain additional insight into experimental results and predict future behavior.
Semipolar LEDs with low droop and power of 1 W from a small 0.1 mm2 chip and continuous wave (CW) semipolar LDs with 11 % wall plug efficiency are demonstrated. The internal loss and injection efficiency of semipolar LDs is reported for the first time. An accurate absorption model is used to predict gain and transparency current density. Reduced threshold current and improved differential efficiency LDs with fewer quantum wells in the active region are demonstrated. Many considerations to improve the wall-plug efficiency of LDs are discussed, and a path to world record wall plug efficiency is described at the conclusion.