Is there a finite limit to the number of polytypes in silicon carbide ?

J.F. Kelly

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

 

Abstract

It is timely now, that a century after the discovery of hexagonal platelets of silicon carbide (SiC) crystals 1 in a meteorite impact crater by Henri Moissan (1905), that this burgeoning wide band-gap (EG~3eV) semiconductor material is ultimately unlocking the secrets of its prolific tendency to exhibit polytypic behaviour.

Synchrotron radiation source (SRS) X-ray diffraction topography (XRDT) has proved to be a powerful tool in investigating the spatial extent of one-dimensional disorder (1DD), long-period polytypes (LPP’s) 2 and the boundaries of polytype layers in SiC. Two decades after the potential of SRS-XRDT was first shown to be useful for assessing polytype structures, neighbourhood coalescence models are now providing numerical values which can be used to prompt questions about the limits of polytypism and disorder in SiC

A simple mathematical relationship (t = kN n) 3 between the polytype thickness (t in microns) and the number of hexagonal layers (N) in the polytype stacking repeat of long-period polytypes (LPP’s) has been discovered. Values for k and n have been determined from plots of the data. It can be shown in the limit that:

 

N = (2co/k)1/n-1

These suggest that there is a natural limit, of approximately N = 2,600 layer repeats expressed in a crystal thickness t ~1 μm, where co is the SiC h.c.p. repeat distance (2.513 Å). Intriguingly, verification of this upper limit on the number N of polytype layers, lies just within the resolution range of current third generation synchrotron sources. 

 

 

References

1. Kelly J. "On the cubic zinc-blende form of meteoritic silicon carbide" cited in Philosophical Transactions of the Royal Society A, London Vol 359,no.1787, p1989 (2001). doi:10.1098/rsta.2001.0891

2. Kelly J.F., Barnes P., Fisher G.R., "Long period polytype boundaries in silicon carbide" Ferroelectrics, 250 187-190 (2001) Proceedings of Aperiodic 2000 Conference

3. Kelly J.F., Fisher G.R., Barnes P. doi:10.1016/j.materresbull.2004.10.008 "Correlation between layer thickness and periodicity of long polytypes in silicon carbide" Mat. Res. Bull. Vol. 40 Issue 2 p249-255 (2005) Elsevier