Engineering Band Gap Energy Through Structural Motifs in Nitride Semiconductors

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Nontechnical SummarySemiconductor materials form the basis for practically all electronics, from solar cells and solid state lighting, to complex integrated circuits such as microprocessors. Nevertheless, despite a century of investigation, researchers are still making fundamental discoveries in this field – discoveries which lead to improved device performance, reduced manufacturing cost, or streamlined integration with other technologies. The project activity is based on the unexpected discovery by the research team that the fundamental properties of semiconductor materials stem directly from atomic-scale structural motifs, and not deviations from periodicity as is generally accepted. By shifting that viewpoint, it is possible to achieve a much larger range of parameter values than previously realized. The result is a completely new approach to selecting materials for specific applications. The project also incorporates development of outreach activities for both middle school children and teachers, involving the community directly in the project by investigating how artificial intelligence/machine learning techniques are applied to image recognition and data analysis, thereby helping to ensure a diverse and motivated pool of young students for careers in science/technology/engineering/math (STEM) fields.Technical SummaryAs part of a previous study on how cation disorder changes the band gap energy of the earth abundant element, sustainably-sourced ternary heterovalent semiconductor ZnSnN2, the research team has discovered the unexpected role played by structural motifs in determining this most fundamental of all semiconductor parameters. Carefully designed experiments indicate that it is not only possible to close the band gap of this material through systematic variation of the type and concentration of motifs that make up the lattice, but it is also possible to access "negative" band gap energies, corresponding to inverted bands. Further, preliminary evidence indicates that the same effect is possible in other compound semiconductors, including the commercially important InGaN family of materials, again through the action of structural motifs. The project investigates the full range of achievable band gap energies of both of these nitrogen-based material systems using plasma-assisted molecular beam epitaxy based crystal growth in conjunction with a complementary suite of optical absorption, Hall effect and electron and x-ray diffraction techniques, with a view towards understanding the ramifications for charge carrier transport, optical properties, and ultimately material selection choices for devices.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.