Analysis of Electron Transport in a High-Mobility Freestanding GaN Substrate Grown by Hydride Vapor-Phase Epitaxy
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Analysis of Electron Transport in a High-Mobility Freestanding GaN Substrate Grown by Hydride Vapor-Phase Epitaxy F. Yun1∗, H. Morkoç1, D. L. Rode2, K. T. Tsen3, L. Farina4, Ç. Kurdak4, S. S. Park5, and K. Y. Lee5 1
Dept. of Electrical Engineering and Physics, Virginia Commonwealth University, Richmond, VA 23284 Washington University, Department of Electrical Engineering, St. Louis, Missouri 63130 3 Department of Physics, Arizona State University, Tempe, AZ 85287 4 Physics Department, University of Michigan, Ann Arbor, MI 48109 5 Samsung Advanced Institute of Technology, P.O. Box 111, Suwon, Korea 440-600 2
ABSTRACT Semiconductor nitrides grown on substrates with a large lattice mismatch typically contain extended and point defects that prevent the full potential of this material system from being attained. Among all the substrate options explored so far, freestanding GaN templates appear ideal for homoepitaxial growth of GaN films. To this end, hydride vapor-phase epitaxial (HVPE) grown GaN templates with a thickness of more than 200 µm were thermally lifted off from the sapphire substrate and mechanically polished. The defect density of such a template is expected to be non-uniform in the growth direction, especially near the back surface which was in close vicinity of the sapphire substrate. We, therefore, studied the transport properties of this template before and after the removal of a 30 µm region from the back-side. For as-prepared GaN, Hall mobilities of 1100 cm2/V-s and 6800 cm2/V-s were obtained at 295 K and 50 K, respectively. A simultaneous fitting of mobility and carrier concentration was used to quantify the contribution of different scattering mechanisms. When the backside was etched by ~30 µm, Hall mobilities improved to 1200 cm2/V-s at 295 K and 7385 cm2/V-s at 48 K, respectively. A numerical solution of the Boltzmann transport equation (BTE) that deals with the inelastic nature of electron scattering by polar optical mode was employed to determine the acceptor concentration. Raman spectroscopy was employed to obtain LO and TO phonon energies, which were then used in the above-mentioned calculations. The best fittings of the mobility and carrier concentration data yield an average acceptor concentration of 4.9x1015 cm-3 and a donor concentration of 2.1x1016 cm-3 for the as-prepared GaN. The average acceptor concentration decreased to 2.4x1015 cm-3 after etching of the backside, which confirms that the etched-away region contained higher density of defects. The donor activation energy is derived to be 25.2 meV. Our analysis demonstrated high quality of the freestanding GaN substrate with the highest reported electron mobility for wurtzite GaN. INTRODUCTION An important issue in the study of GaN is to understand the formation of extended and point defects, and to find solutions to reduce or eliminate them for high-temperature, high-power electronics and optoelectronics device applications.1,2 Variable-temperature Hall Effect measurements, and their quantitative analysis are powerful in shedding lig
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