J.F.
Kelly, P. Barnes
Industrial
Materials Group, School of Crystallography, Birkbeck College, University of
London, Malet Street, London WC1E 7HX U.K.
Topography, Silicon Carbide, Polytypes
The current widespread interest in SiC as a high temperature, power semiconductor can be attributed to its wide band gap (Eg ~3eV) electronic properties. This potential has been handicapped by the presence of defects and the prolific tendency for SiC to form so many polytypic modifications. A major stumbling block appears to be a complete theoretical description of the existence of long period polytype structures and the coalescence of 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 detailing the relative location of different
polytypes in the same crystal have been reported (1). However with the advent of
synchrotron radiation source X-Ray diffraction edge topography(2) (SRS-XRDT) 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 and it is now possible to identify and confidently resolve thin
one-dimensionally disordered (1DD) layers (~5µm) (3) and regions of high defect density
as well as long period polytypes (LPP’s) (4).
This renewed focus in the interface between
polytypes in syntactic coalescence
has meant that this shortcoming has been properly addressed for the first time
by constructing morphologically accurate models of the layer-stacked SiC edges.
A unique database on
these adjoining polytype patterns has prompted the authors to propose a
non-degenerate polytype-polytype configuration termed a sandwich model. The most
common termed an American club
sandwich model (5).
1DD, LPP’s and the next nearest polytype
relationships are important clues to the growth scenario of Lely vapour grown
silicon carbide. These ubiquitous
features are illustrated here with several examples.
[1] Takei W.J., Francombe M.H. “ X-ray topographic observation of
polytype distributions in silicon carbide”, J. Appl. Phys. (1967), 18: 1589-1592.
[2} Fisher G.R., Barnes P. “Towards a unified view of
polytypism in silicon carbide”, Philos. Mag. (1990), B61, no. 2:
217-236.
[3] Barnes P., Kelly J.F., Fisher G.R.
“Observation of
fine one-dimensionally disordered layers in silicon carbide”, Philos. Mag.
Lett., (1991), 64, no. 1, 7-13.
[4] Kelly J.F., Barnes P., Fisher G.R.
“Long period
polytype boundaries in silicon carbide”, Ferroelectrics, (2001) 250,
187-190.
[5] Kelly J.F., Barnes P., Fisher G.R. “The use of synchrotron edge topography to study polytype nearest neighbour relationships in SiC”, Radiat. Phys. Chem., (1995), 45: 509-522.