Technical: This project aims for greater fundamental understanding and improved quantum efficiency of solid state light emitters in the green-to-amber spectral region (~540 nm to ~610 nm wavelength). The approach involves investigating the material system, AlGaNP grown on GaP, with outcomes anticipated of better material properties and simpler processing technologies for solid state lighting. The approach consists of three parts: investigation of fundamental materials science issues involved in the growth, processing, and optimization of amber emitting GaNP/GaP LEDs, comparison with AlInGaP/GaAs LEDs, exploring AlGaNP/GaP for green emission, and fabricating green emitting AlGaNP/AlGaP LEDs and comparing with InGaN/GaN LEDs. Relevant physical properties, such as effective mass, band alignment, and optical properties, will be systematically investigated. It is well known that the quantum efficiency of InGaN LEDs decreases as the wavelength increases from blue to green because material quality degrades as the In concentration increases. The quantum efficiency of AlInGaP LEDs decreases as the wavelength decreases from red to amber because the band alignment approaches a type II, staircase lineup, resulting in reduced luminescence intensity. With theoretical guidance, the material system, AlGaNP-on-GaP, was chosen for LEDs emitting in this "color gap" region because of its potential advantages: 1) maximum light extraction because of an all-transparent layer structure, except the active region, on a transparent GaP substrate; 2) low threading dislocation density expected because of a lattice-matched, or pseudomorphic, structure on a GaP substrate, in contrast to lattice-mismatched InGaN/GaN on sapphire or SiC; 3) a substrate (GaP) with higher thermal conductivity than sapphire and GaAs (1.1 vs. 0.35 and 0.55 W cm-1 K-1, respectively); 4) better high-temperature characteristics because of stronger carrier confinement from larger conduction-band and valence-band offset, which are 3 times those of AlInGaP/AlInP; 5) higher yield and lower cost because a smaller size is needed for the same power (chips can be made smaller and the yield per wafer increased, reducing cost per chip); 6) simpler, cost-effective one-step epitaxy, instead of substrate removal and wafer bonding needed for AlInGaP LEDs, or heteroepitaxial growth of InGaN/GaN on sapphire or SiC.
The project addresses basic research issues in a topical area of materials science having high technological relevance. The research will contribute basic materials science knowledge at a fundamental level to new understanding and capabilities in electronic devices. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. The research activities will promote interdisciplinary training of graduate students in growth and characterization of novel compound semiconductors, LED device design, fabrication and testing, and provide training in preparation of written and oral presentations. Undergraduate students will be recruited from sources such as the McNair Program, which serves low-income students and/or underrepresented minorities. Undergraduate students will have the opportunity to work on advanced characterizations, such as atomic force microscopy, and participate in undergraduate research conferences at UCSD. Additionally, the PI will participate in the COSMOS (California State Summer School for Mathematics and Science) residential summer program, where a team of high school students will participate in an LED project using commercial LEDs and those fabricated in this program for solid-state lighting experiments. .
