NSF Award
1800139
Nontechnical description: The project explores the electronic and chemical properties of atomic-scale imperfections in the semiconductors zinc oxide, gallium oxide, indium gallium oxide, and scandium nitride with a specific focus on controlling such defects for higher power, speed, and light output of advanced electronic devices. Such defects may degrade semiconductor properties by trapping charge carriers to reduce the current and charge speed. These imperfections depend sensitively on the specific techniques used for growing and/or subsequent thermal, chemical, and plasma treatments. The research, which combines experimental studies with transport modeling, is aimed at measuring the optical properties of lattice structural and chemical imperfections of these relatively unexplored semiconductors, using growth variations, plasma, hydrogen annealing, and irradiation treatments to identify their physical nature and electrical measurements in magnetic fields to identify their ability to donate or accept electrons. The project ultimate goal is to understand the nature of these defects and eventually to eliminate them. The ability to remove these structural and/or chemical defects impacts a range of technologies. Zinc oxide is a prime candidate to replace today's high-cost materials in solar cells, digital displays, and light emitting diodes. Gallium oxide's ability to handle very high voltages can improve power switches for telecommunications and power transmission. Indium gallium oxide can provide higher speed displays and high-resolution TVs. Scandium nitride can lower resistance and power consumption of metallic contacts to semiconductors used in cellphones. The activities provide collaborative research opportunities for a graduate student, several university undergraduates, and high school students from an all-girl's high school.<br/><br/>Technical description: The research focuses on fundamental studies of native point defects in the semiconductors ZnO, Ga2O3, InGaZnO, and ScN, which have emerged as critical materials for advanced high power and optoelectronic display applications. ZnO, doped with Ga or Al, is the prime candidate to replace expensive indium tin oxide in solar cells, displays, light emitting diodes, and touchscreens. Ga2O3 is the dominant new material for power switches because of its record high breakdown voltage. InGaZnO is the dominant amorphous oxide replacing amorphous-Si transistors in displays and high-resolution TVs (e.g., Sharp). ScN can improve ohmic contacts in GaN-based devices and serve as a buffer layer for GaN-on-Si technology. All four can be highly doped with impurity donors, yet all four are impacted by deep level defects that compensate free carriers and introduce scattering that reduces carrier mobility. The nature of native point defects in Ga2O3, InGaZnO, and ScN as well as ZnO is almost completely unexplored, yet these defects can have a major impact on carrier density, mobility, and interface transport. The research team will measure the spatial distribution and physical nature of specific defects using 3-dimensional nanoscale optical spectroscopies coupled with donor /acceptor densities and dielectric properties by temperature-dependent Hall effect and reflectance/transmission measurements, respectively, in order to identify and quantify defect densities on a near-nm scale and understand how to control them through new growth and processing techniques. The goals of this work are to understand the primary compensating defects in these compounds that limit degenerate doping and produce lower mobilities, combining near-surface remote plasma, implantation and thermal processing with optical and surface science techniques to identify these native point defects, correlate them with donor/acceptor densities, and chemically control them. The project also aims to explore the impact of these defects on barriers and transport at Schottky barriers and heterojunctions involving these semiconductors. The overall goal of the project is to control these defects and their impact on carrier densities and junction transport by selected growth and processing techniques that improve conductivity and interface properties.<br/><br/>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.
