Bridging the Miscibility Gap in InGaN Alloys

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"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)." Technical. This project addresses the epitaxial growth of high crystalline quality InGaN across the complete composition range. The aim is to gain greater understanding of the properties of the miscibility gap and to explore methods that can consistently provide single phase InGaN films. Theoretical studies suggest that phase separation in layers having the interface oriented parallel to the c-axis (such as a- or m-plane InGaN) is dramatically suppressed. Thus, the approach is to develop MOCVD processes for the growth of InGaN epilayers on a-plane through different epilayer templates (AlN, GaN, and InN). Studies to understand fundamental effects of growth orientation and strain on the miscibility gap in InGaN are expected to lead to a new approach for achieving single phase device quality InGaN epilayers. These films will then allow for studies of fundamental optical and transport properties of InGaN in composition ranges which were previously inaccessible. Detailed studies concerning the optical and transport properties of InxGa1-xN in the miscibility gap region (0.45 x 0.75) have not been possible because these InxGa1-xN films generally have been of very low crystalline quality and exhibit weak or negligible photoluminescence. It is anticipated that many of the important physical properties (structural, optical, and electrical) of InxGa1-xN in the miscibility gap region will be characterized through these studies. Non-Technical. The project addresses fundamental research issues in a topical area of electronic/photonic materials science having technological relevance. The realization of device quality InGaN epilayers over a more complete alloy range would yield significant benefits for many III-nitride based optoelectronic devices. The bandgap of InGaN expands from about 0.7 eV to 3.4 eV, which covers the entire solar spectrum. In principle, a multijunction solar cell or photoelectrochemical cell based on multi-layers of InGaN with different In-contents is highly efficient at capturing different wavelengths of the sunlight passing through the cell. InGaN alloys could also be potentially important thermoelectric (TE) materials and may be an attractive alternative to other materials for the development of TE generators that are able to directly convert heat to electricity in new generation automobiles, radioisotope TE generators in spacecraft or cooling modules for enhanced efficiency and lifetime of micro/nano-scale sensor networks. Attainment of high quality InGaN in the previously predicted miscibility gap region would also significantly benefit the development of high efficiency LEDs with wavelengths longer than 550 nm. The successful attainment of highly efficient green/yellow InGaN LEDs would then enable the technology for white LEDs through the R-G-B three color chip integration approach providing highly efficient light sources for general illumination. The bandgap of In-rich InGaN could also be engineered to match the fiber optic communication wavelength around 1.5 um. Postdoctoral and graduate students will be actively involved in a research program that is highly interdisciplinary in nature including state-of-the-art epitaxial research of photonic materials and structures to advanced materials characterization and micro/nano-scale prototype photonic/optoelectronic device research. Undergraduates will also participate in this research using Whitacre Endowment funds. Plans for education and outreach include the development of hands-on activities to be presented to teams of high school teachers and students on 'Nanophotonics' in conjunction with activities conducted by the Center of Engineering Outreach at TTU.