POLYTYPISM AND ONE-DIMENSIONAL DISORDER IN SILICON CARBIDE

J.F. Kelly ¹, P. Barnes ¹, G.R. Fisher ²

1 School of Crystallography, Birkbeck College, Malet Street, London WC1E 7HX

2 Electronic Materials Inc. Mailzone 6, 501 Pearl Drive, P.O. Box 8, St. Peter’s,

MO63376, U.S.A.

The phenomenon of polytypism first observed in silicon carbide (SiC) nearly ninety years ago [1], has been studied extensively both empirically and theoretically and a full explanation for its existence still remains elusive today [2]. The problem is essentially that the one-dimensional ordering arrangement in SiC has produced over 126 different layer periodicities based on the simple ABA… ABC… stacking sequences found in close packed structures [3]. This arises from the large number of possible repeat sequences, the largest reported spacing in SiC being 3015 Å [4]. Besides these long period ordered structures one-dimensional disorder, when there is no finite lattice repeat, is also a prevalent feature in silicon carbide.

Recently there has been renewed interest in the interface between polytypes in syntactic coalescence. With the advent of synchrotron radiation source x-ray diffraction edge topography (SRS-XRDET) [5] and the improved resolution available from second-generation machines, finer features have been revealed at polytype boundaries. Diffraction contrast is provided from the edges rather than the more substantial faces of the hexagonal crystals [6] and it is now possible to identify and confidently resolve one-dimensionally disordered layers and regions of high defect density as thin as 5 µm [7].

These ubiquitous features and the next nearest polytype relationships between the common 6H, 4H and 15R polytypes are important clues to the growth scenario of Lely vapour grown silicon carbide and are illustrated here with some examples. A unique database on these adjoining polytype patterns has prompted the authors to propose a non-degenerate polytype-polytype configuration termed a sandwich model [8].

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[3] G.C. Trigunayat, G.K. Chada (1971), Phys. Stat. Sol. A, 4, 9-42.

[4] R.S. Mitchell (1954), J. Chem. Phys., 22, 1977-1983.

[5] G.R. Fisher, P. Barnes (1984), J. Appl. Cryst., 17, 231-237.

[6] G.R. Fisher, P. Barnes (1990), Philos. Mag. B, 61, no. 2, 217-236.

[7] P. Barnes, J.F. Kelly, G.R. Fisher (1991), Philos. Mag. Lett., 64, no. 1, 7-13.

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