Defect structure in single crystal titanium carbide
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J. Wu, A. K. Chaddha, H. S. Chen, and J. D. Parsons Department of Electrical Engineering and Applied Physics, Oregon Graduate Inst. of Science and Technology, 20000 NW Walker Road, P.O. Box 91000, Portland, Oregon 97291-1000
D. Downham Department of Materials Science and Engineering, Oregon Graduate Inst. of Science and Technology, 20000 NW Walker Road, P.O. Box 91000, Portland, Oregon 97291-1000 (Received 27 December 1993; accepted 22 April 1994)
The defects and crystallinity of as-grown and annealed TiQ (grown by the high-pressure float-zone) were examined by TEM and double crystal x-ray rocking curves. Three types of subboundaries and planar defects within subgrains were observed in as-grown TiQ. Subboundaries are classified by structure as (i) wide-extended, fault-like defects (WEFLD's), (ii) edge dislocation arrays, and (iii) dislocation networks. Planar defects were observed at dislocation nodes of subboundaries and also within subgrains; this is the first reported observation of planar defects within TiQ subgrains. The misorientation and/or density of subgrains in T i Q was reduced significantly by annealing at 2300 °C in contact with graphite.
I. INTRODUCTION Several properties of single crystal titanium carbide (TiC*) make its use as a substrate for nucleation and growth of y8-SiC epitaxial layers potentially advantageous.1"3 These advantages are due to its crystal structure, lattice parameter, thermal expansion coefficient, and chemical stability. Titanium carbide has a rock salt crystal structure which can exist, as a single phase, within an extraordinarily wide composition range.4 The minimum lattice parameter (a) mismatch between TiC* and /3-SiC (a = 4.358 A) 5 occurs at x = 0.86 (a TiC = 4.331 A). 4 The thermal expansion coefficient of TiCx is greater than that of /3-SiC; thus, /3-SiC epilayers grown on T i Q are under compressive strain, which reduces the chances of formation of microcracks and pinholes in /3-SiC. The melting temperature of TiC* is about 3100 °C, and it does not decompose or undergo phase transformations in any temperature range of interest for epitaxial growth of /3-SiC. The predominant defects in single crystal TiC* are subboundaries. The subboundaries are thought to be due to thermal stress caused by nonuniform temperature gradients at the solid-liquid interface and across the solidified crystal during high pressure float zone (HPFZ) growth.6 These subboundaries in T i Q can be reproduced in /3-SiC epilayers, which will provide electrical shorting paths for current leakage. Therefore, they are a major obstacle to TiC* as a substrate for 2096
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 8, Aug 1994
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epitaxial growth of device quality /3-SiC. Development of TiCx single crystals that can serve as substrates for epitaxial growth of device quality /3-SiC requires a more thorough understanding of defect type and cause in bulk grown TiC* crystals. Planar defects observed in T i Q are thought to be due primarily to impurities, such as boron.6"13
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