Many compound materials exhibit polymorphism, allowing them to exist in different structural forms called polymorphs. Notably, silicon carbide (SiC) has over 250 identified polymorphs, including some with a lattice constant up to 301.5 nm, significantly larger than typical SiC lattice spacings. SiC polymorphs encompass various amorphous phases and a family of crystalline polytypes, which differ only in the stacking sequence of atomic layers arranged in configurations A, B, or C. This layered stacking determines the crystal structure’s unit cell and is a characteristic shared with other binary tetrahedral compounds like zinc oxide and cadmium sulfide.
Categorizing the polytypes
A shorthand has been developed to catalogue the vast number of possible polytype crystal structures: Let us define three SiC bilayer structures (that is 3 atoms with two bonds in between in the illustrations below) and label them as A, B and C. Elements A and B do not change the orientation of the bilayer (except for possible rotation by 120°, which does not change the lattice and is ignored hereafter); the only difference between A and B is shift of the lattice. Element C, however, twists the lattice by 60°.
Using those A,B,C elements, we can construct any SiC polytype. Shown above are examples of the hexagonal polytypes 2H, 4H and 6H as they would be written in the Ramsdell notation where the number indicates the layer and the letter indicates the Bravais lattice.4 The 2H-SiC structure is equivalent to that of wurtzite and is composed of only elements A and B stacked as ABABAB. The 4H-SiC unit cell is two times longer, and the second half is twisted compared to 2H-SiC, resulting in ABCB stacking. The 6H-SiC cell is three times longer than that of 2H, and the stacking sequence is ABCACB. The cubic 3C-SiC, also called β-SiC, has ABC stacking.5
Physical properties
Main article: silicon carbide
The different polytypes have widely ranging physical properties. 3C-SiC has the highest electron mobility and saturation velocity because of reduced phonon scattering resulting from the higher symmetry. The band gaps differ widely among the polytypes ranging from 2.3 eV for 3C-SiC to 3 eV in 6H SiC to 3.3 eV for 2H-SiC. In general, the greater the wurtzite component, the larger the band gap. Among the SiC polytypes, 6H is most easily prepared and best studied, while the 3C and 4H polytypes are attracting more attention for their superior electronic properties. The polytypism of SiC makes it nontrivial to grow single-phase material, but it also offers some potential advantages - if crystal growth methods can be developed sufficiently then heterojunctions of different SiC polytypes can be prepared and applied in electronic devices.6
Summary of polytypes
All symbols in the SiC structures have a specific meaning: The number 3 in 3C-SiC refers to the three-bilayer periodicity of the stacking (ABC) and the letter C denotes the cubic symmetry of the crystal. 3C-SiC is the only possible cubic polytype. The wurtzite ABAB... stacking sequence is denoted as 2H-SiC, indicating its two-bilayer stacking periodicity and hexagonal symmetry. This periodicity doubles and triples in 4H- and 6H-SiC polytypes. The family of rhombohedral polytypes is labeled R, for example, 15R-SiC.
Properties of major SiC polytypes7891011 (Z is number of atoms per unit cell)| Poly-type | Space group | Z | Pearsonsymbol | lattice constants | Bandgap(eV) | Hexago-nality (%) | ||
|---|---|---|---|---|---|---|---|---|
| Name | No | a (Å) | c (Å) | |||||
| 3C | T d 2 - F 43 m {\displaystyle \mathrm {T_{d}^{2}{\mbox{-}}F43m} } | 216 | 2 | cF8 | 4.3596 | 4.3596 | 2.3 | 0 |
| 2H | C 6 v 4 - P 6 3 m c {\displaystyle \mathrm {C_{6v}^{4}{\mbox{-}}P6_{3}mc} } | 186 | 4 | hP4 | 3.0730 | 5.0480 | 3.3 | 100 |
| 4H | 8 | hP8 | 10.053 | 3.2 | 50 | |||
| 6H | 12 | hP12 | 15.11 | 3.0 | 33.3 | |||
| 8H | 16 | hP16 | 20.147 | 2.86 | 25 | |||
| 10H | P 3 m 1 {\displaystyle \mathrm {P3m1} } | 156 | 10 | hP20 | 3.0730 | 25.184 | 2.8 | 20 |
| 19H | 19 | hP38 | 47.8495 | |||||
| 21H | 21 | hP42 | 52.87 | |||||
| 27H | 27 | hP54 | 67.996 | |||||
| 36H | 36 | hP72 | 90.65 | |||||
| 9R12 | C 3 v 5 - R 3 m {\displaystyle \mathrm {C_{3v}^{5}{\mbox{-}}R3m} } | 160 | 9 | hR18 | 3.037 | 22.603 | 3.233 | 66.6 |
| 15R | 15 | hR30 | 3.0730 | 37.7 | 3.0 | 40 | ||
| 21R | 21 | hR42 | 52.89 | 2.85 | 28.5 | |||
| 24R | 24 | hR48 | 60.49 | 2.73 | 25 | |||
| 27R | 27 | hR54 | 67.996 | 2.73 | 44 | |||
| 33R | 33 | hR66 | 83.11 | 36.3 | ||||
| 45R | 45 | hR90 | 113.33 | 40 | ||||
| 51R | 51 | hR102 | 128.437 | 35.3 | ||||
| 57R | 57 | hR114 | 143.526 | |||||
| 66R | 66 | hR132 | 166.188 | 36.4 | ||||
| 75R | 75 | hR150 | 188.88 | |||||
| 84R | 84 | hR168 | 211.544 | |||||
| 87R | 87 | hR174 | 219.1 | |||||
| 93R | 93 | hR186 | 234.17 | |||||
| 105R | 105 | hR210 | 264.39 | |||||
| 111R | 111 | hR222 | 279.5 | |||||
| 120R | 120 | hR240 | 302.4 | |||||
| 141R | 141 | hR282 | 355.049 | |||||
| 189R | 189 | hR378 | 476.28 | |||||
| 393R | 393 | hR786 | 987.60 | |||||
See also
External links
- A Brief History of Silicon Carbide Dr J F Kelly, University of London
- Material Safety Data Sheet for Silicon Carbide
References
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