White light sources using III-V nitride light-emitting diodes

In this dissertation, white light illumination sources utilizing light-emitting diodes (LEDs) based on III-V nitride materials are discussed. First, we propose and demonstrate a novel type polarization-enhanced ohmic contact to III-V nitride materials. Second, we discuss the white light generation by the mixing of two complementary wavelengths. Last, a comprehensive analysis of color rendering capabilities and luminous efficacies to tricolor LED-based daylight illumination sources is performed.; To overcome the difficulties fabricating high quality ohmic contacts to III-V nitride-based electronics and optoelectronics, the strong internal polarization field is employed for the first time. An appropriate design of the capping layer increases the tunneling probability of carriers between the metal and the semiconductor. A low resistance ohmic contact is thus formed. Polarization-enhanced ohmic contacts to p-type AlGaN/GaN superlattice structures, n-type AlGaN/GaN heterostructures, and p-type InGaN/GaN heterostructures are demonstrated.; Next, a monolithic LED structure with two active regions emitting two different wavelengths is designed and fabricated. Photoluminescence and electroluminescence measurements clearly exhibit the emission of two distinct peaks of this LED. Carrier transport mechanisms of this structure are analyzed in detail. White light sources based on the emission of two complementary wavelengths have the highest luminous efficacies. The luminous efficacy of a gaussian di-chromatic white-light source is calculated assuming a line broadening ranging from 2 kT to 10 kT. Luminous efficacies ranging from 380 to 440 1m/W are obtained for broadened dichromatic sources.; Finally, the performance characteristics of white light sources based on a tri-LED approach are analyzed in detail. Figures of merit such as the luminous efficacy, color temperature, and color rendering capabilities are provided for a wide range of primary emission wavelengths. This analysis reveals that both excellent color rendering abilities and very high luminous efficacies can be achieved by this tri-color LED approach. A luminous efficacy of radiation of 320 lm/W with a color rendering index of 85 are obtained for peak emission wavelengths of 455, 525, and 605 nm with an 8 kT linewidth. The high color rendering capabilities and luminous efficiencies of 64 lm/W for 20 power-efficient devices indicate the high potential of this approach.