Bulk AlN Crystal Growth on SiC Seeds and Defects Study
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Bulk AlN Crystal Growth on SiC Seeds and Defects Study P Lu1, J H Edgar1, C Cao1, K Hohn1, R Dalmau2, R Schlesser3, and Z Sitar2,3 1 Department of Chemical Engineering, Kansas State University, Manhattan, KS, 66506 2 Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC, 27695 3 HexaTech, Inc, Morrisville, NC, 27560 ABSTRACT Sublimation growth of AlN was performed on Si-face, 8 ° off-axis 4H-SiC (0001) and 3.5 º off-axis 6H-SiC (0001) seeds. AlN layers 500 - 900 µm thick and 20 mm in diameter were grown at 1830 ºC by consecutive growths and continuous growth. The c-axis growth rate was approximately 8 - 18 µm/hr. On both the 8 º and 3.5° off-axis SiC substrates, “step” features formed on the AlN surface with uniform terrace width and step density. The step heights and terrace widths increased as the AlN grew thicker. In addition, a single facet of 9 mm × 5 mm formed on the top of the layer grown on the 3.5 º off-axis SiC. High resolution x-ray diffraction showed the AlN (00.2) 2θ value shifted from pure AlN toward SiC. Approximately 3 – 4 at% of SiC was detected at the surface of the AlN by XPS. Molten KOH/NaOH etching revealed that both samples had Al-polar surface with dislocation densities on the order of 106-107 cm-2. The cross-section etching showed the re-nucleation layer and void defects at the interfaces of the consecutive growths. INTRODUCTION Aluminum nitride (AlN) is a direct wide band gap semiconductor (Eg = 6.2 eV) with high thermal conductivity and high thermal stability. Due to its small lattice mismatch and small differences in thermal expansion coefficient with GaN, AlN is an ideal substrate for GaN based devices such as high-power UV lasers, UV photodetectors and blue light emitting diodes. The most successful growth of bulk AlN is by the sublimation-recondensation method, as demonstrated by Slack and McNelly [1,2], Bickermann [3], and others. SiC crystals were first employed as seed crystals for bulk AlN growth by Campbell and Chang in 1967 [4]. SiC is a suitable substrate for AlN bulk crystal growth since it has a small a-lattice mismatch (0.96% for 6H-SiC and 1.2% 4H-SiC) [5] with AlN. Dalmau et al. [6] grew 3 mm thick single crystal AlN layers on both on-axis 6H-SiC (0001) and off-axis 4H-SiC (0001). On Si-face of SiC, the AlN layers were primarily Al-polarity. Mokhov et al. [7] successfully grew a 10-12 mm thick single crystal AlN layer on SiC substrates. The nucleation of AlN on SiC has been studied in the author’s previous paper [8]. Initial growth for 15 minutes and extend growth for 2 hours suggested that 1800 – 1850 ºC was the optimum temperature range of AlN crystal growth. In this present work, long time growth, (20 – 50 hours), was performed to deposit thick AlN layer on SiC substrates.
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EXPERIMENT AlN crystals were grown by sublimation in a resistively heated graphite furnace. The source material was AlN powder originally containing less than 1 wt % oxygen and 0.06 wt % carbon. Prior to growth, the AlN powers were purified by ba
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