2018 AFOSR MURI
Gallium Oxide Materials Science and Engineering GAME
PO: Dr. Ali Sayir, Aerospace Materials for Extreme Environments: extreme.environment@us.af.mil
Lead PI: James Speck (UCSB, Materials)
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β-Ga2O3 is the largest bandgap (~4.5 eV) semiconductor that can be grown directly from the melt in single crystal form. The ease of n-type doping with tetravalent cations, and a wide variety of bulk single crystal and epitaxial film growth techniques have triggered worldwide interest in β-Ga2O3. The predicted breakdown electric field (6-8 MV/cm) is higher than that of GaN or SiC (~3 MV/cm), which when combined with electron mobility (predicted 250-350 cm2/Vs) yields amongst the best figures of merit for power electronic devices. As a result, β-Ga2O3 possesses transformative potential for not only power electron devices but also high frequency devices, and deep UV optoelectronics. Thus, the Gallium Oxide Materials Scienceand Engineering (GAME) MURI objective is to develop the basic materials understanding that will enable these applications.

Early progress in β-Ga2O3 has demonstrated the potential of this system for high voltage and power applications. However, many of the foundational materials issues require attention. The epitaxial growth is in its infancy. Doping studies have only started recently. The nature of point defects and impurities requires a focused effort. The β-gallia structure is unique in crystallography and thus alloying with other sesquioxides such as Al2O3 and In2O3 requires understanding of phase stability and solubility. The experimentally realized electron mobility is less than half of theoretically predicted value – studies are needed to reveal the key carrier scattering mechanisms that limit mobility and whether they are due to intrinsic limitations or purity of the material. The high electric field properties, namely breakdown field and saturation electron velocity needs to be measured and related to material quality.

Our proposing team of experimentalists, theorists, materials scientists and semiconductor researchers from UCSB, OSU, Cornell and Georgia Tech blends a highly collaborative set of leading researchers whose synergies create a dynamic research team such that the whole is far greater than the sum of its parts. The GAME team members have published 45 journal papers on β-Ga2O3 to date. The team integrates established pioneers and leaders in epitaxial growth of β-Ga2O3 and related oxides by MBE (Speck, Rajan) and CVD growth (Zhao), electronic and structural defect characterization (Ringel, Hwang), theory regarding the formation and properties of defects from the electronic, optical and structural perspectives (Van de Walle), which together are essential to understand collective phenomena such as carrier scattering, thermal transport (Graham), and semiconductor transport (Jena).

The GAME MURI program will be comprised of three major themes on Defect Science, Transport and Optical Properties, and Alloys and Interfaces. The program has Cross-Cutting Areas in Growth Science and Theory that are closely interwoven in the three themes. The GAME MURI team is built on the significant connections and overlap between these themes, and deep interaction between the themes and team members is essential for the success of the program.

There is a natural interdependence between all aspects of the research proposed, and we believe that the individual excellence of our team members, together with the strong record of collaboration will guarantee long-term success for this program.