A Comparison of Magnesium and Beryllium Acceptors in GaN Grown by rf-Plasma Assisted Molecular Beam Epitaxy
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A Comparison of Magnesium and Beryllium Acceptors in GaN Grown by rf-Plasma Assisted Molecular Beam Epitaxy A.J. Ptak1, T.H. Myers1, Lijun Wang1, N.C. Giles1, M. Moldovan2, C.R. Da Cunha2, L.A. Hornak2, C. Tian3, R. A. Hockett3, S. Mitha3 and P. Van Lierde3 1 Department of Physics, 2Department of Electrical Engineering, West Virginia University, Morgantown, WV 26506, 3 Evans Analytical Group, Charles Evans and Associates, Sunnyvale, CA 94086 ABSTRACT Step-doped structures of both magnesium and beryllium were grown in GaN and analyzed using secondary ion mass spectrometry. Dopant incorporation was studied as a function of substrate temperature and dopant flux for Ga-polarity and N-polarity GaN. Incorporation is different for each polarity, with Mg incorporating by up to a factor of 20 times more (30 times more with atomic hydrogen) on the Ga-face, while Be incorporates more readily on the N-face. The effect of atomic hydrogen on the incorporation kinetics of both Mg and Be is also discussed. Mg and Be both undergo surface segregation during growth. Photoluminescence measurements suggest that Be is a p-type dopant with an optical activation energy of approximately 100 meV. INTRODUCTION While magnesium is currently the most technologically important p-type dopant for GaN, Be also shows promise. Although Mg, and Be have been studied in the last several years, incorporation mechanisms during growth by molecular beam epitaxy (MBE) remain unclear. For example, previous studies have presented results that indicate Mg incorporation is independent of Mg flux over a wide flux range but is quite sensitive to substrate temperature,[1] and there is evidence for significant surface accumulation of Mg.[2] We present the results of a study of both Mg and Be incorporation for both N- and Ga-polarity GaN that strongly indicates surface accumulation occurs for both dopants under Ga-rich growth conditions, and that surface polarity can have a dramatic influence on incorporation. DETAILS OF GROWTH Mg-doped GaN layers were grown by rf plasma-assisted MBE using an EPI Unibulb nitrogen plasma source. Conventional effusion cells were used for Ga and Mg while Be was evaporated from a dopant effusion cell (EPI-5-D). N-polarity GaN was obtained by nucleating GaN buffer layers directly on sapphire under heavily Ga-rich conditions. Incorporation in Gapolarity GaN was studied by growth on MOCVD GaN templates on (0001) sapphire substrates. The doped layers were grown at a rate of 0.25 µm/hr, which corresponded to a nitrogen flow rate of 0.85 sccm and rf power of 200 W. The samples were grown under Ga-stable conditions (Ga/N flux ratio >1) which results in high quality GaN growth.[3,4,5] Step-doped structures were produced by opening and closing the dopant shutter. In-situ growth rates were monitored using laser interferometry. All changes in oven and substrate temperatures occurred with the dopant shutter closed. Atomic hydrogen was produced using a thermal cracker (EPI-AHS, EPI Vacuum Products, Inc). Secondary ion mass spectrometry (SIMS) measurem
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