Nanoheteroepitaxy of (In,Ga)N: Toward a Phosphor-Free White LED

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0424161SANDSGoal: The proposed research is directed toward the goal of demonstrating bright red (~650 nm) electroluminescence from an (In,Ga,Al)N light-emitting diode (LED) employing an active region consisting of an array of dislocation-free strained (In,Ga)N nanorod heterostructures. Significance: National and international solid-state lighting initiatives are aimed at replacing fluorescent and incandescent lighting technologies with an LED alternative. A white LED replacement technology with luminous efficiency of 200 lm/W, color temperature comparable to the midday sun, and color rendering equivalent to conventional illumination sources has the potential to reduce overall electric power consumption and the associated power-plant emissions by 10%. Combining discrete red (In,Ga,Al)P LEDs with green and blue (In,Ga,Al)N LEDs may suffice for large outdoor displays, but this approach is too costly for general white-light illumination. Today's most promising approaches utilize down-conversion of blue or UV LED light with phosphors. However, efficiency and lifetime limitations inherent to phosphor-based white LEDs suggest that a phosphor-free alternative will be required. Approach: The phosphors could be eliminated if one materials system could be utilized for emission across the entire visible spectrum. Conventional planar (In,Ga,Al)N LED heterostructures can be designed to emit from the UV into the yellow. However, the large lattice mismatch between InN and GaN (10.8%) prevents the fabrication of bright orange and red LEDs. The proposed research program is based on the concept that lateral elastic strain relief in the nanorod topology can extend the range of lattice-mismatch that can be accommodated coherently in an (In,Ga)N heterostructure, thereby enabling the fabrication of bright (In,Ga,Al)N LEDs across the entire visible spectrum. Self-organized GaN and (In,Ga)N nanorods have been grown by halide vapor phase epitaxy (HVPE) and rf-plasma molecular beam epitaxy without the use of foreign metal catalysts, suggesting that the nanorod approach to a red LED may be feasible. The proposed research will fund the P.I. and two of his graduate students (one fully dedicated to this project, and a second who will split his or her effort with another project that requires the same skill set) to identify the factors that control the nanorod growth regimes in HVPE, metalorganic chemical vapor deposition and reactive pulsed laser deposition (PLD). The theories of nanoheteroepitaxy will be tested, and methods to control the nanorod diameter distribution using porous anodic alumina templates will be explored. The work will culminate in an attempt to demonstrate an (In,Ga)N nanorod-array LED with a peak emission wavelength of ~650 nm (red). The P.I. and his students have unique facilities to pursue this research, including a custom HVPE reactor for (In,Ga)N nanorod synthesis, and a load-locked PLD system with ammonia process gas and group-III metal targets.Broader ImpactsInterdisciplinary Research Environment: The proposed research program will support graduate student research in an interdisciplinary environment. The P.I. has a joint appointment in ECE and MSE at Purdue, and his research group is evenly populated with MSE and ECE graduate and undergraduate students, all of whom are actively involved in the Birck Nanotechnology Center. Outreach: The P.I. proposes to complement the proposed research program with the development of a 30-min hands-on presentation than can be tuned for elementary, secondary and adult layperson audiences. The focus of the presentation will be on LEDs as an emerging nanotechnology success story, taking advantage of the sensory stimulation and curiosity naturally evoked by these bright light sources. The presentation will also bring in elements of other energy conversion technologies that might benefit from nanotechnology, including solid-state cooling, and the potential impacts of these new technologies on society. Senior Design Projects: The P.I. will invoke the scientific and technological contexts of the proposed research program in an annual MSE Senior Project, involving 4-6 undergraduate students each academic year. The students will utilize the laboratory facilities and interact with industrial mentors in the design of novel growth, etching and nanofabrication processes for III-V nitride heterostructures.