SBIR-STTR Award

III-V nitride-based, high brightness, UV and visible light emitting diodes (LEDs) are of a great interest for general illumination, but the low light output efficiencies of current high brightness LEDs are still inadequate. A key material issue preventing the achievement of higher light output efficiency in LEDs is the poor crystalline quality of the nitride epitaxial layers that result from lattice-mismatched substrates. This project will develop a novel, physical vapor transport (PVT) technique to grow large diameter, high quality AlN bulk single crystals, which can be used as substrates for the growth of high quality nitride LED epitaxial layers. In Phase I, the PVT technique for AlN single crystal growth will be studied extensively in order to understand the effect of the crucible/insulation set-up, used for the PVT growth, on the quality of AlN crystal boules. In particular, protective TaC coatings on graphite crucibles will be fabricated and tested, a PVT AlN crystal growth process will be developed, defects and impurities in the AlN crystals will be characterized, and AlN crystal boules will be fabricated into wafers.

Commercial Applications and Other Benefits as described by the awardee:
Significant improvements in light output efficiences during the past decade have made GaN-based LEDs the most promising contender for future general illumination application. High brightness LEDs hold great promise for future general illumination applications because of their tremendous energy saving potential, long life time, compactness, and potential cost savings

Nitride-based, high-brightness, ultraviolet, visible, and white light emitting diodes are candidate devices for replacing incandescent light bulbs and fluorescent light fixtures in general illumination applications, due to their tremendous energy saving potential, long lifetime, and high efficiency. However, the poor crystalline quality of the nitride epilayers, resulting from severely lattice-mismatched and crystal-structure-differed substrates, prevents higher-light-output efficiency from being achieved. Native aluminum-nitride single crystal has been shown to have the same crystal structure and a close lattice match to group III-nitride epilayers, and it can be grown in bulk single crystal form. Therefore, this project will develop aluminum nitride substrates suitable for the growth of high quality nitride epitaxial layers, leading to the fabrication of high brightness light emitting diodes for general lighting applications. In Phase I, aluminum nitride crystal boules larger than 20mm in diameter were demonstrated by using a novel, sublimation, physical-vapor-transport growth technique, and aluminum nitride crystal wafers of about 20mm in diameter were fabricated. Phase II will continue to develop and improve the sublimation physical-vapor-transport technique so that single crystal aluminum nitride boules of 2-inch in diameter can be produced. Aluminum nitride wafers and epi-ready aluminum nitride substrates will be fabricated from the aluminum nitride crystal boules. To demonstrate the suitability of the aluminum nitride substrates for high quality nitride epitaxy, III-nitride epi-layers will be grown on aluminum nitride substrates, using a metal-organic chemical vapor deposition technique.

Commercial Applications and Other Benefits as described by the awardee:
The aluminum nitride substrates should be suitable for volume production of high brightness light emitting diodes, with high-light-output efficacies adequate for general illumination applications. In addition, the high-quality aluminum-nitride substrates should find use in the fabrication of other types of nitride-based devices, such as blue laser diodes for optical recording, high frequency devices for telecommunications, and UV detectors for analytical applications (e.g., for the detection of chemical and biological agents for homeland security)