Surface Control of ZrB 2 (0001) Substrate for Molecular-Beam Epitaxy of GaN
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Y10.65.1
Surface Control of ZrB2 (0001) Substrate for Molecular-Beam Epitaxy of GaN Jun Suda, Hiroyasu Yamashita, Robert Armitage, Tsunenobu Kimoto and Hiroyuki Matsunami Department of Electronic Science and Engineering, Kyoto University, Kyoto University Katsura Campus, Nishikyo-ku, Kyoto 615-8510, Japan ABSTRACT Zirconium diboride (ZrB2) is a promising lattice-matched substrate for GaN-based materials. A key issue to realize high-quality heteroepitaxial growth is preparation of the substrate surface. The ZrB2 surface was studied by x-ray photoemission spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). XPS results indicated the presence of both ZrO2 and ZrB2 on the as-received substrate surface. Thermal cleaning at 1000°C in ultra-high vacuum, Ar+ ion sputtering, and wet chemical treatments were examined as surface preparation methods. After treatment with HF acid, the O peak intensity was much reduced. The combination of HF treatment and thermal cleaning resulted in sharp and intense RHEED from the ZrB2 surface. GaN grown on the surface by molecular-beam epitaxy exhibited intense photoluminescence, suggesting that this treatment is effective to obtain high-quality GaN on ZrB2 substrates.
INTRODUCTION Gallium nitride-based group-III nitrides are the most promising materials for short wavelength light-emitting devices, high-frequency transistors, and power switching devices owing to their superior material properties. Electrically and thermally conductive substrates are desirable for III-N devices with high-density vertical current flow, such as laser diodes and power switching devices. Zirconium diboride (ZrB2) has excellent electrical conductivity (several µΩ-cm[1], semi-metallic[2]) as well as good thermal conductivity (1.4 W/cm-K[3], comparable to silicon). This compound has a non-polar hexagonal crystal structure (AlB2 structure, P6/mmm), whose lattice constants are a = 3.169Å and c = 3.530 Å[4]. The in-plane lattice constant is nearly lattice-matched to GaN (a = 3.189 Å, 0.6% mismatch) and perfectly matched to Al0.26Ga0.74N. This is the main advantage of ZrB2 versus another electrically and thermally conductive substrate, SiC; exactly latticed-matched AlGaN layers cannot be grown on SiC since its lattice constant is 0.9% smaller than that of AlN. Recent progress in the floating-zone (FZ) method has yielded ZrB2 bulk crystals suitable for use as substrates [5-7]. However, the III-N growth process on ZrB2 must still be optimized to fully exploit the benefits of this novel substrate.
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We previously reported on the growth of GaN and AlN on ZrB2 (0001) substrates by molecular-beam epitaxy (MBE) using elemental Al and Ga and rf-plasma excited nitrogen.[8] From X-ray diffraction pole-figure measurement, it was revealed that GaN grows epitaxially on ZrB2, i.e., [0001]GaN // [0001]ZrB2, [11-20]GaN // [11-20]ZrB2. However, the epilayer quality was insufficient for device applications. A major reason for this is thought to be the presence of contaminants on the ZrB2 surface prior to
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