Improved Heteroepitaxial MBE GaN Growth with a Ga Metal Buffer Layer
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Improved Heteroepitaxial MBE GaN Growth with a Ga Metal Buffer Layer Yihwan Kim1, Sudhir G. Subramanya1, Joachim Krueger1, Henrik Siegle1, Noad Shapiro1, Robert Armitage1, Henning Feick1, Eicke R. Weber1, Christian Kisielowski2, Yi Yang 3, and Franco Cerrina3 1 Department of Materials Science and Engineering, University of California at Berkeley, and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. 2 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. 3 Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, WI 53706, U.S.A. ABSTRACT We demonstrate that the use of pure gallium (Ga) as a buffer layer results in improved crystal quality of GaN epilayers grown by plasma-assisted molecular beam epitaxy on c-plane sapphire. The resulting epilayers show electron Hall mobilities as high as 400 cm2/Vs at a background carrier concentration of 4 x 1017 cm-3, an outstanding value for an MBE-grown GaN layer on sapphire. Structural properties are also improved; the asymmetric (101) X-ray rocking curve width is drastically reduced with respect to that of the reference GaN epilayer grown on a low-temperature GaN buffer layer. Nitrided Ga metal layers were investigated for different Ga deposition time. These layers can be regarded as templates for the subsequent Ga main layer growth. It was found that there is an optimum Ga metal layer deposition time for improving the electron mobility in the epilayer. Heating of the Ga metal layer to the epilayer growth temperature under nitrogen plasma is found to be sufficient to produce highly oriented GaN crystals. However, nonuniform surface morphology and incomplete surface coverage were observed after nitridation of comparatively thick Ga metal layers. This is shown to be the reason for the decreasing electron mobility of the epilayers as the Ga metal layer thickness exceeds the optimum value. INTRODUCTION Gallium nitride (GaN) and related compounds have attracted considerable academic and commercial interest. This is due to potential application in opto-electronics at short wavelengths, and high-frequency high-power electronics [1,2]. Due to the lack of large area bulk singlecrystalline GaN, epitaxial growth of GaN has to be done on foreign substrates such as sapphire or SiC. Heteroepitaxial growth on lattice- and thermal-mismatched substrates results in the formation of stress and defects in the GaN epilayer [3]. It is known that the crystal quality of the GaN epilayer can be improved by introducing a buffer layer between the GaN epilayer and the substrate [4,5]. However, the mechanisms by which the buffer layer relieves stress, and by which the stress relaxation affects defect formation, are not well understood. In a previous study, we found that the composition of the low-temperature GaN (LT-GaN) buffer layer is an important parameter to control stress and material properties of the GaN epilayer [6]. GaN epilayers grown on a Ga-rich LT-GaN buffer laye
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