High-Quality GaN Grown by Molecular Beam Epitaxy on Ge(001)
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W. AGER Ifl**, C. KISLELOWSKI***, E. R. WEBER* *Department of Materials Science and Mineral Engineering, UC Berkeley, Berkeley, CA 94720 ** Lawrence Berkeley National Laboratory, Materials Science Division, Berkeley, CA 94720 "*** Lawrence Berkeley National Laboratory, National Center for Electron Microscopy, Berkeley, California 94720
ABSTRACT We report on growth of GaN on Germanium as an alternative substrate material. The GaN films were deposited on Ge(001) substrates by plasma-assisted molecular beam epitaxy. Atomic force microscopy, x-ray diffraction, photoluminescence, and Raman spectroscopy were used to characterize the structural and optical properties of the films. We observed that the Ga/N ratio plays a crucial role in determining the phase purity and crystal quality. Under N-rich conditions the films were phase-mixed, containing cubic and hexagonal GaN, while in the Ga-rich regime they were purily hexagonal. The latter samples show bandedge luminescence with linewidths as small as 31 meV at low temperatures. INTRODUCTION The availability of suitable substrate materials is a key requirement for the epitaxial growth of GaN thin films. The lack of GaN substrates of appropriate size and structural perfection has motivated the search for alternative substrates for heteroepitaxial growth. The most common substrates, sapphire, SiC and GaAs, exhibit large differences in lattice constants and thermal expansion coefficients to GaN, resulting in large amounts of stress and high defect concentrations in the epilayers. [1,2] Ge in contrast possess the same thermal expansion coefficient as GaN. [3] Since most of the stress in heteroepitaxial GaN layers arises from the ill-matched expansion coefficients, growth on Ge would lower the amount of stress in the GaN epilayers and thus improve their structural, electrical, and optical quality. Besides, Ge can easily be doped and therefore could serve as a backside/bottom contact.
451 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
We demonstrate that Ge provide an alternative substrate material for GaN growth. The GaN layers were deposited on Ge(001) substrates by plasma-assisted molecular beam epitaxy (MBE). Their structural and optical properties were characterized by atomic force microscopy, x-ray diffraction, Raman and Photoluminescence spectroscopy. EXPERIMENT A Riber 1000 MBE system was used for the epitaxial growth. An effusion cell provided elemental gallium by evaporation and activated nitrogen was produced by a constricted glow discharge (CGD) plasma source with pure nitrogen gas (99.9995%). Details of the plasma source are given elsewhere. [4] Intrinsic Ge (001) wafers with miscuts of 6 degree toward served as substrates. Prior growth they were degreased by boiling in acetone and isopropyl alcohol and were finally rinsed in deionized water to remove Ge oxides. The substrates were then heated up to 680'C in ultra high vacuum for thermal desorption of surface contaminants. In contrast to growth on sapphire, the Ge substrates were not nitridat
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