Many compound materials exhibit polymorphism, that is they can exist in different structures called polymorphs. Silicon carbide (SiC) is unique in this regard as more than 250 polymorphs of silicon carbide had been identified by 2006,[1] with some of them having a lattice constant as long as 301.5 nm, about one thousand times the usual SiC lattice spacings.[2]
The polymorphs of SiC include various amorphous phases observed in thin films and fibers,[3] as well as a large family of similar crystalline structures called polytypes. They are variations of the same chemical compound that are identical in two dimensions and differ in the third. Thus, they can be viewed as layers stacked in a certain sequence. The atoms of those layers can be arranged in three configurations, A, B or C, to achieve closest packing. The stacking sequence of those configurations defines the crystal structure, where the unit cell is the shortest periodically repeated sequence of the stacking sequence. This description is not unique to SiC, but also applies to other binary tetrahedral materials, such as zinc oxide and cadmium sulfide.
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°.
Structure of major SiC polytypes.
2H-SiC
4H-SiC
6H-SiC
3C structure
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]
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.[5]
Summary of polytypesedit
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 polytypes[6][7][8][9][10] "Z" is number of atoms per unit cell, "SgNo" is space group number, a and c are lattice constants
^Rebecca Cheung (2006). Silicon carbide microelectromechanical systems for harsh environments. Imperial College Press. p. 3. ISBN1-86094-624-0.
^J.F. Kelly; et al. (2005). "Correlation between layer thickness and periodicity of long polytypes in silicon carbide" (PDF). Materials Research Bulletin. 40 (2): 249–255. doi:10.1016/j.materresbull.2004.10.008.
^Laine, Richard M. (1993). "Preceramic polymer routes to silicon carbide". Chemistry of Materials. 5 (3): 260–279. doi:10.1021/cm00027a007.
^Ramsdell, L.S., Studies on Silicon Carbide, Am. Mineral. 32, (1945), p. 64–82
^ abMorkoç, H. (1994). "Large-band-gap SiC, III-V nitride, and II-VI ZnSe-based semiconductor device technologies". Journal of Applied Physics. 76 (3): 1363–1398. Bibcode:1994JAP....76.1363M. doi:10.1063/1.358463.
^"Properties of Silicon Carbide (SiC)". Ioffe Institute. Retrieved 2009-06-06.
^Yoon-Soo Park, Willardson, Eicke R Weber (1998). SiC materials and devices. Academic Press. pp. 1–18. ISBN0-12-752160-7.{{cite book}}: CS1 maint: multiple names: authors list (link)
^S. Adachi (1999). Optical Constants of Crystalline and Amorphous Semiconductors: Numerical Data and Graphical Information. Springer. ISBN0-7923-8567-5.
^W. J. Choyke; Hiroyuki Matsunami; Gerhard Pensl (2003). Silicon carbide: recent major advances. Springer. p. 430. ISBN3-540-40458-9.
^Nakashima, S (1991). "Raman intensity profiles and the stacking structure in SiC polytypes". Solid State Communications. 80 (1): 21–24. Bibcode:1991SSCom..80...21N. doi:10.1016/0038-1098(91)90590-R.
External linksedit
Dr J F Kelly, University of London
for Silicon Carbide
January 01, 1970
polymorphs, silicon, carbide, many, compound, materials, exhibit, polymorphism, that, they, exist, different, structures, called, polymorphs, silicon, carbide, unique, this, regard, more, than, polymorphs, silicon, carbide, been, identified, 2006, with, some, . Many compound materials exhibit polymorphism that is they can exist in different structures called polymorphs Silicon carbide SiC is unique in this regard as more than 250 polymorphs of silicon carbide had been identified by 2006 1 with some of them having a lattice constant as long as 301 5 nm about one thousand times the usual SiC lattice spacings 2 The polymorphs of SiC include various amorphous phases observed in thin films and fibers 3 as well as a large family of similar crystalline structures called polytypes They are variations of the same chemical compound that are identical in two dimensions and differ in the third Thus they can be viewed as layers stacked in a certain sequence The atoms of those layers can be arranged in three configurations A B or C to achieve closest packing The stacking sequence of those configurations defines the crystal structure where the unit cell is the shortest periodically repeated sequence of the stacking sequence This description is not unique to SiC but also applies to other binary tetrahedral materials such as zinc oxide and cadmium sulfide Contents 1 Categorizing the polytypes 2 Physical properties 3 Summary of polytypes 4 See also 5 References 6 External linksCategorizing the polytypes edit nbsp 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 Structure of major SiC polytypes nbsp 2H SiC nbsp 4H SiC nbsp 6H SiC nbsp 3C structure 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 b SiC has ABC stacking 5 Physical properties editMain 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 5 Summary of polytypes editAll 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 polytypes 6 7 8 9 10 Z is number of atoms per unit cell SgNo is space group number a and c are lattice constants Polytype Space group Z Pearson symbol SgNo a A c A Bandgap eV Hexagonality 3C T2d F43m 2 cF8 216 4 3596 4 3596 2 3 0 2H C46v P63mc 4 hP4 186 3 0730 5 0480 3 3 100 4H C46v P63mc 8 hP8 186 3 0730 10 053 3 2 50 6H C46v P63mc 12 hP12 186 3 0730 15 11 3 0 33 3 8H C46v P63mc 16 hP16 186 3 0730 20 147 2 86 25 10H P3m1 10 hP20 156 3 0730 25 184 2 8 20 19H P3m1 19 hP38 156 3 0730 47 8495 21H P3m1 21 hP42 156 3 0730 52 87 27H P3m1 27 hP54 156 3 0730 67 996 36H P3m1 36 hP72 156 3 0730 90 65 9R not found 9 hR18 160 3 073 66 6 15R C53v R3m 15 hR30 160 3 073 37 7 3 0 40 21R C53v R3m 21 hR42 160 3 073 52 89 2 85 28 5 24R C53v R3m 24 hR48 160 3 073 60 49 2 73 25 27R C53v R3m 27 hR54 160 3 073 67 996 2 73 44 33R C53v R3m 33 hR66 160 3 073 83 11 36 3 45R C53v R3m 45 hR90 160 3 073 113 33 40 51R C53v R3m 51 hR102 160 3 073 128 437 35 3 57R C53v R3m 57 hR114 160 3 073 143 526 66R C53v R3m 66 hR132 160 3 073 166 188 36 4 75R C53v R3m 75 hR150 160 3 073 188 88 84R C53v R3m 84 hR168 160 3 073 211 544 87R C53v R3m 87 hR174 160 3 073 219 1 93R C53v R3m 93 hR186 160 3 073 234 17 105R C53v R3m 105 hR210 160 3 073 264 39 111R C53v R3m 111 hR222 160 3 073 279 5 120R C53v R3m 120 hR240 160 3 073 302 4 141R C53v R3m 141 hR282 160 3 073 355 049 189R C53v R3m 189 hR378 160 3 073 476 28 393R C53v R3m 393 hR786 160 3 073 987 60See also editSilicon carbide fibersReferences edit Rebecca Cheung 2006 Silicon carbide microelectromechanical systems for harsh environments Imperial College Press p 3 ISBN 1 86094 624 0 J F Kelly et al 2005 Correlation between layer thickness and periodicity of long polytypes in silicon carbide PDF Materials Research Bulletin 40 2 249 255 doi 10 1016 j materresbull 2004 10 008 Laine Richard M 1993 Preceramic polymer routes to silicon carbide Chemistry of Materials 5 3 260 279 doi 10 1021 cm00027a007 Ramsdell L S Studies on Silicon Carbide Am Mineral 32 1945 p 64 82 a b Morkoc H 1994 Large band gap SiC III V nitride and II VI ZnSe based semiconductor device technologies Journal of Applied Physics 76 3 1363 1398 Bibcode 1994JAP 76 1363M doi 10 1063 1 358463 Properties of Silicon Carbide SiC Ioffe Institute Retrieved 2009 06 06 Yoon Soo Park Willardson Eicke R Weber 1998 SiC materials and devices Academic Press pp 1 18 ISBN 0 12 752160 7 a href Template Cite book html title Template Cite book cite book a CS1 maint multiple names authors list link S Adachi 1999 Optical Constants of Crystalline and Amorphous Semiconductors Numerical Data and Graphical Information Springer ISBN 0 7923 8567 5 W J Choyke Hiroyuki Matsunami Gerhard Pensl 2003 Silicon carbide recent major advances Springer p 430 ISBN 3 540 40458 9 Nakashima S 1991 Raman intensity profiles and the stacking structure in SiC polytypes Solid State Communications 80 1 21 24 Bibcode 1991SSCom 80 21N doi 10 1016 0038 1098 91 90590 R External links editA Brief History of Silicon Carbide Dr J F Kelly University of London Material Safety Data Sheet for Silicon Carbide Retrieved from https en wikipedia org w index php title Polymorphs of silicon carbide amp oldid 1177522097, wikipedia, wiki, book, books, library,
