The development of high brightness blue LEDs and blue laser diodes required many breakthroughs of GaN growth, p-type conductivity control, InGaN growth and device structures using InGaN/GaN double heterostructures. I will discuss the history and background story of the key scientific issues solved in order to realize high efficiency solid state lighting. The fundamental discovery of high quality p-type doping by removing hydrogen passivation, and the role of the InGaN layer in achieving high brightness blue LEDs and Laser Diodes will be described. I will also discuss the latest results of InGaN-based emitting devices at the University of California at Santa Barbara.
There is an intrinsic problem of the LEDs that cannot be easily overcome. When we increase the current densities so high, a reduction in efficiency with increasing the current density is observed. This phenomena, referred to as efficiency droop, forces LED manufactures to operate LEDs at lower current densities (and hence reduced light output) than would be possible to prevent excess heating of the device. An alternative method to produce white light is by using a blue laser, as opposed to an LED, in combination with a phosphor. Above the lasing threshold, the carrier density is clamped at threshold, fixing its density. Increases in carrier density beyond the threshold density immediately contribute to stimulated emission, or lasing. Thus, the carrier density is maintained at the lower, threshold density, prohibiting it from reaching densities where the Auger recombination process becomes the dominant recombination process. Auger recombination, with the resulting efficiency droop, does not appreciably occur in blue laser diodes.
Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 201.
Refreshments in the lobby of Varian Physics at 4:15 pm.
Autumn 2015/2016, Committee: A, Linde (Chair), S. Chu, P. Hayden, M. Schnitzer, L. Senatore