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Topography, Silicon Carbide, Polytypes
Silicon carbide originated
before the birth of our solar system, formed in red giant carbon stars and
became trapped as interstellar grains in primitive meteorites roaming through
the Milky Way for billions of years.
Isotopic analysis of meteoritic silicon carbide(1) is thus
offering a new and exciting tool for exploring the structure and evolution of
our galaxy. Artificial carborundum on the other hand manufactured as a
substitute for diamonds initially has proved more useful for the abrasives
industry, the main commercial use of SiC today, although interest in Moissanite
gemstone production is reviving interest in this burgeoning material (2).
The current widespread
interest in SiC as a high temperature, power semiconductor can be attributed to
its wide band gap (Eg ~3eV) electronic properties. Despite this
potential device development 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 (3). However
renewed interest 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, the most common
termed an American club sandwich
model (4).
With the advent of
synchrotron radiation source x-ray diffraction edge topography (SRS-XRDT) (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 and it is now possible to identify and confidently resolve thin
one-dimensionally disordered layers (~5 µm) (6) and
regions of high defect density as well as long period polytypes (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. A unique database on these adjoining polytype patterns has prompted
the authors to propose a non-degenerate polytype-polytype configuration termed
a sandwich model. These ubiquitous features are illustrated here with several examples.
[1]
Anders E., Zinner E., Meteoritics (1993), 28: 490-514.
[2] Muller S.G., Glass R.C., Hobgood H.M., Tsvetkov V.F., Brady M.,
Henshall D., Jenny J.R., Malta D., Carter C.H., J. Cryst. Growth (2000), 211: 325-332.
[3] Takei W.J., Francombe M.H., J. Appl. Phys. (1967), 18: 1589-1592.
[4] Kelly J.F., Barnes P., Fisher G.R., Radiat. Phys.
Chem., (1995), 45: 509-522.
[5} Fisher G.R.,
Barnes P., Philos. Mag. (1990), B61, no. 2: 217-236.
[6] Barnes P.,
Kelly J.F., Fisher G.R., Philos. Mag. Lett., (1991), 64, no. 1, 7-13.
[7] Kelly J.F., Barnes P., Fisher G.R., Ferroelectrics (2001), 250,
187-190.