Emerging Opportunities of III-Nitride Nanostructures: From Deep UV Nano-optoelectronics to High Efficiency Artificial Photosynthesis

To date, the global investment in III-nitride semiconductors has surpassed any other compound semiconductors and is only next to silicon, which has been fueled largely by GaN-based blue light emitting diodes (LEDs), lasers and power transistors. However, the developments of aluminum and indium-rich nitride semiconductors for applications in the deep UV and near-infrared spectral range have been extremely limited. Many of their fundamental characteristics, including surface charge properties and p-type conduction, have remained elusive. In this context, we have developed a wide range of III-nitride nanoscale heterostructures, including nanowires and quantum dots on low cost, large area Si substrate by plasma-assisted molecular beam epitaxy. We show that such defect-free III-nitride nanostructures exhibit superior quality and can enable a broad range of device applications that were not previously possible. We have discovered that surface Fermi-level pinning, commonly thought as a fundamental property of nitrides, can be precisely tuned by varying the growth conditions and dopant incorporation. We have further achieved, for the first time, p-type conduction in InN and AlN nanowire structures. Our lab has also demonstrated ultralow threshold electrically injected AlGaN nanowire lasers on Si that can operate in the entire UVA-II band (~ 315-340 nm), the shortest wavelengths ever reported for any electrically injected semiconductor lasers.

Another emerging application of III-nitride nanostructures is photosynthesis, i.e. the conversion of sunlight, water and carbon dioxide into useful fuels, which is one of nature’s most fundamental processes. We have developed multi-band InGaN/GaN nanowire heterostructures that can lead to stable hydrogen production from pure water splitting under ultraviolet, blue and green light irradiation (up to ~ 560 nm), the longest wavelength ever reported. We have further identified the definite role of surface band bending on the efficiency and stability of overall water splitting. By tuning the surface Fermi-level, the quantum efficiency of overall water splitting can be enhanced by nearly two orders of magnitude. A detailed study of the photocatalytic activities of InGaN nanowire arrays and their direct correlation with the growth/nucleation process will be presented. The tremendous impact of such unprecedentedly high efficiency photocatalyst on artificial photosynthesis and on recycling anthropogenic carbon dioxide to renewable fuels will also be discussed.
Zetian Mi is an Associate Professor in the Department of Electrical and Computer Engineering at McGill University. He received the Ph.D. degree in Applied Physics from the University of Michigan, Ann Arbor in 2006. Prof. Mi's teaching and research interests are in the areas of solar fuels, III-nitride semiconductors, nanomaterials, and nanophotonics. He has published 7 book chapters, more than 110 journal papers, and ~ 250 conference papers / presentations (including ~ 100 invited talks). He has received many awards, including the Hydro-Québec Nano-Engineering Scholar Award in 2009, the William Dawson Scholar Award in 2011, and the Christophe Pierre Award for Research Excellence (Early Career) in 2012 at McGill University. He has also received the Young Investigator Award from the 27th North American Molecular Beam Epitaxy (MBE) Conference in 2010. Prof. Mi served as the Associate Editor of IEEE J. Lightwave Technol. as well as the Chair of many international conferences regularly.
Prof. Mi has created the field of artificial photosynthesis on metal-nitride nanostructures, with the first demonstration of solar-to-hydrogen conversion on metal-nitride nanowire arrays in 2011. Since then, his group has improved the quantum efficiency for solar-to-hydrogen conversion on metal-nitrides by nearly two orders of magnitude. Such a high efficiency artificial photosynthesis makes it possible for the large scale deployment of solar energy. More recently, he has been selected as one of the finalists of the Grand Challenges program from around the world by the Alberta-based Climate Change and Emissions Management Corporation, a $35-million competition to seek innovative uses for carbon and to transform carbon from a liability into an asset.