Effect of Tantalum in Sublimation Growth of Aluminum Nitride

Sublimation growth of AlN on the SiC substrate was carried out using the graphite crucible. The single crystal AlN could be grown by placing the Ta sheet in the graphite crucible. However, if the Ta sheet was not placed, AlN could not be achieved and only the graphite layer was formed on the SiC substrate. It was confirmed by XPS that Ta combined with C and changed to TaC after the crystal growth. Since Ta acted as a carbon ge tter during the sublimation growth, single crystal AlN was achieved in the graphite crucible without graphit ization of the substrate. Introduction The group III nitride semiconductors such as gallium nitride (G aN), aluminum nitride (AlN) and its alloy have been expected and developed for applications of high f requency, high power and optical devices in the ultraviolet region with low loss. The la ttice mismatch between AlN (a=3.11Å) and GaN (a=3.18Å) is small, and AlN has high thermal conductivity (2.2 W/cm·K) and a high thermal stability up to 1400 °C. AlN is promising as the substr ate for the GaN based semiconductor devices. In the electrical communication region, AlN is used as a material for the surface acoustic wave (SAW) devices [1]. AlN wafers are needed to produce s uch devices, so that bulk crystal must be grown. Sublimation growth is a useful technique to prepare silicon carbide ( S C) bulk crystals, since it is easy to achieve high growth rates and high crystal quality in this technique [2]. In this study, bulk crystal growth of AlN in the graphite crucible was carrie d out by close space configuration of the sublimation method. While we have tried the sublimation growth of AlN, the measured grow th rate was only 5.0 μm/h [3]. In order to achieve the higher growth rate, it is e ffective to grow at high temperature up to 2200 °C [4]. However, if sublimation growth of AlN is carried out i n a graphite crucible at the high temperature, it is difficult to obtain single crystal A lN. Because carbon from the crucible reacts with nitrogen sublimated from the source powder [5]. With the exception of the graphite, the metal with the high melt ing point such as tungsten (W), rhenium (Re) and tantalum (Ta) was used as the crucible material [5, 6]. Howe ver, th se materials are hard to process and very expensive. Therefore, we are particular about the gr aphit c ucible. We have reported that Ta reacted as a carbon getter in the s ublimation growth of SiC and placing Ta in the graphite crucible was useful to improve crystal qu lity, surface morphology and impurity doping of the grown SiC [7]. In order to suppress the react ion of carbon with nitrogen and achieve the single crystal AlN in sublimation growth of AlN us ing the graphite crucible, Ta sheet was used in this experiment. In this study, the effect of Ta toward sublimation growth of AlN using the graphite crucible was investigated. Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 975-978 doi:10.4028/www.scientific. net/MSF.433-436.975 © 2003 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-12/03/20,10:57:11) Experiment The crucible to grow AlN was the same as that designed for SiC sublimation growth. The substrate and the source were placed in a graphite crucible, whic h as heated by RF generator at a frequency of 35 kHz. The substrate, the source and Ta sheet were pl aced in a graphite crucible as shown Fig. 1. The substrate was 4H-SiC (0001) Si-face with the off-axis ang le of 8.0° toward <11-20>. Size of the substrate was 6.0 mm × 6.0 mm. Effective growth area was 4.0 mm × 4.0 mm. The source was AlN powder which was commercially available. The distance between the substrate and the source was 2.0 mm. During sublimation growth, the temperature at bott om f the crucible, which was called the growth temperature, was kept at 1930 °C and the pres sure of the atmospheric gas was maintained at 1.0 atm. The dominant gas of the atmosphere was N 2 and a small amount of H 2 was added. H2 was added to increase the sublimation rate of the AlN source powder. Result and Discussion For precise identification of grown crystal, Raman scatterin g was used. Fig. 2 shows the Raman spectra of the substrate, (a), and the grown layers, (b): 5 hours growth, 60 μm thick and (c): 10 hours growth, 110μm thick. Both layers were grown with placing Ta. The peaks around 776 cm and 970 cm assigned to 4H-SiC E 2T and A1L, respectively [8]. The peaks around 660 cm and 890 cm are due to E 2 (high) and A1 (LO) of single crystal 2H-AlN [9]. Only 4H-SiC signals can be observed from the 4H-SiC substrate. Intensity of t he 2H-AlN signals from the grown layer increases as thickness of the grown layer incre asing. From this result, it was concluded that 2H-AlN was successfully grown on the 4H-SiC substrate by using Ta. Fig. 1 Schematic image of the graphite crucible Fig. 2 Raman spectra of (a) 4H -SiC substrate, (b) Grown layer, 5 hours growth and 60 μm thick and (c) Grown layer, 10 hours growth 110 μm thick. 976 Silicon Carbide and Related Materials 2002