LONG PERIOD POLYTYPE SANDWICH MODELS IN SILICON CARBIDE

J.F. Kelly, P. Barnes

Industrial Materials Group, School of Crystallography, Birkbeck College, University of London, Malet Street, London WC1E 7HX.

Silicon carbide is older than our solar system having wandered, trapped as micron sized grains in meteorites, through the Milky Way for billions of years as stardust originating in the atmospheres of red giant stars (Clayton 1997 [1]). Acheson as early as 1892 synthesized the first artificial carborundum as a substitute for diamonds (Moissanite gemstones [2] are reviving interest in this burgeoning material). This process paved the way for the large-scale production for the abrasives industry, the main commercial use of SiC today. The current widespread interest in SiC as a high temperature, power semiconductor can be attributed to its wide band gap (E_g ~3eV) electronic properties. Despite this potential, device development has been handicapped by wafer "micropipe" defects.

A major stumbling block appears to be a complete theoretical description of the prolific tendency for SiC to form so many polytypic modifications including long period structures (Kelly et al. 2001 [3]) and the equilibrium phases 6H, 15R and 4H. A significant gap in our understanding of polytypism exists, caused in part by the lack of experimental data on the spatial distribution of polytype coalescence and also in part by knowledge of the regions between adjoining polytypes. Few observations, Takei & Francombe 1967 [4] apart, detailing the relative location of different polytypes in the same crystal have been reported. This shortcoming has been properly addressed for the first time by constructing morphologically accurate models of the layer-stacked edges, the most common termed an American club sandwich model (Kelly et al. 1995 [5]).

A phenomenological description of the observed boundaries between polytypes, precise position of one-dimensional disorder (1DD) with respect to these and the existence of long period polytypes (LPP’s) has been made possible by the technique of synchrotron radiation source (SRS) X-ray diffraction topography (XRDT). These ubiquitous features are illustrated here with several examples.

[1] D.D. Clayton (1997), Astrophysical Journal, 484, L67-L70.

[2] S.G. Muller, R.C. Glass, H.M. Hobgood, V.F. Tsvetkov, M. Brady, D. Henshall, J.R. Jenny, D. Malta, C.H. Carter (2000), J. Cryst. Growth, 211, 325-332.

[3] J.F. Kelly, P. Barnes, G.R. Fisher (2001),Ferroelectrics, 250, 187-190.

[4] W.J. Takei, M.H. Francombe (1967), J. Appl. Phys., 18, 1589-1592

[5] J.F. Kelly, P. Barnes, G.R. Fisher (1995), Radiat. Phys. Chem., 45, 509-522.