Physical Properties of SiC
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MRS BULLETIN/MARCH 1997
Polytypism of SiC Polytypism is one of the most unique features of SiC. If we designate a Si-C atom pair in an A-plane in close packing as Aa, and in the B-plane as Bb, and in the C-plane as Cc, then we can generate a series of structures by the variation of stacking along the principal crystal axis. For AaBbCcAaBbCc... stacking, we generate the 3C-SiC zinc blende lattice, and for AaBbAaBb... stacking, we generate the 2H-SiC wurtzite lattice. Other stacking sequences, such as AaBbAaCcAaBbAaCc..., will generate 4H-SiC; and more complicated polytypes with unit cells of ever increasing length along the principal axis (c-axis) have been found. In Figure 1, we have reduced the notation Aa, Bb, Cc to just A, B, C, and have listed a
3C
15R
number of important properties for five of the most common SiC polytypes. The number of atoms per unit cell varies from polytype to polytype, significantly affecting the number of electronic energy bands and vibrational branches possible for a given polytype. This diversity of electronic and vibrational band structures could and does profoundly affect the physical properties of different polytypes. Another important feature is the number of inequivalent sites in different polytypes of SiC. The two polytypes of greatest interest, 4H- and 6H-SiC, have two and three inequivalent sites, respectively. Therefore, 4H-SiC has the possibility of two donors or two acceptors for a particular substitutional impurity, while 6H-SiC may have three donors or three acceptors. Why is this so? In Figure 2, we designate C atoms on a particular plane perpendicular to the c-axis with a small dot and the associated Si atoms with a larger empty circle. The figure shows the zigzag of the Si and C atoms in the (1120) plane. A point h designates a lattice position in which the C or associated Si atom finds itself in a quasihexagonal stacking environment with respect to its neighboring stacking planes. Similarly, we see that k, and k2 represent lattice points in which the C and associated Si atoms find them-
6H
4H
2H
15.079
10.050
5.048
> C 15
A
a o = 4.349A F43m
|| 37.700
R3m c3v5
P63IT1C
P63IT1C
P63IT1C
c Rl , 4
1
C 0 IN A ATOMS PER UNIT CELL SPACE GROUPS INEQUIVALENT SITES
2.390
2.986
3.023
3.265
3.330
(eV)
Figure 1. Stacking sequences and selected physical properties of the five primary SiC polytypes.
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Physical Properties of SiC
selves in quasicubic stacking environments. In addition, as the inset shows, if we substitute a nitrogen atom at the h, ki, or k2 sites, the atomic distances for the substitutional Ni nitrogen to the nearest carbonlike or siliconlike planes distinctly differ from those for the substitutional N 2 and N 3 nitrogen atoms. The N2 and N3 nitrogen atoms sense different environments. This shows why the hexagonal site, and the two cubic sites ki and k2 in 6H-SiC, are expected to have, and do have, slightly different electronic properties. Band Structure The 1990s represent the decade in which large bandgap semiconductors (diamond, the I
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