Development of high efficiency light emitting diodes for solid state lighting

The solid-state lighting revolution has begun. Traditional lighting sources are destined to be replaced by semiconductor light emitting diodes (LEDs) because their potential efficiency and longevity far exceeds that of existing lighting technologies. In theory, LEDs can convert electrical energy into optical energy with no loss. However, current state-of-the-art LED performance is far below this limit, and reaching it requires that several major technical obstacles be overcome. This work describes research into improving the efficiency of LEDs through advances in materials technology, device design and processing, and emission optimization.; With respect to materials technology, the problem of poor p-type conductivity in nitride semiconductors has been addressed. Superlattice doping has been employed to demonstrate drastic improvement in the conductivity of p-type GaN and AlGaN materials. Superlattice materials have been incorporated into homojunction diodes which show reduced forward voltage. Studies on these superlattice materials have also lead to the observation of a two-dimensional hole gas. Finally, a new ohmic contact is proposed and demonstrated based on polarization induced electric fields inherent to nitride semiconductors. These improvements are expected to improve injection efficiency of existing LEDs, and enable the next generation of UV emitting devices.; Device design and processing improvements have been realized in the phosphide materials system. A new type of AlGaInP LED is demonstrated that incorporates an omnidirectional reflector to improve extraction efficiency. Metal-based mirrors are shown to exhibit improved performance over dielectric reflectors employed in existing technology. Experimental work has included development of a metal-metal wafer bonding and substrate removal processes. AlGaInP LEDs incorporating omnidirectional reflectors show performance greater than traditional absorbing substrate LEDs as measured by output power and high current density operation.; Finally, the suitability of monolithic multi-wavelength LEDs for use in white light sources has been has been investigated. Structures consist of a blue GaInN primary active region that pumps a secondary set of green GaInN quantum wells. Photoluminescence studies reveal a primary to secondary emission ratio of near one to one, which is required for use in white light sources. The potential for such monolithic integration of a semiconductor based white-light is discussed.