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SiC Polytypes

Silicon carbide occurs in many different crystal structures, called polytypes. Let us define three SiC bilayer structures (that is 3 atoms with two bonds in between in the pictures 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, as is shown above on example of the hexagonal polytypes 2H, 4H and 6H. 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 twice longer, and the second half is twisted compared to 2H-SiC, forming the ABCB stacking. The 6H-SiC cell is triple that of 2H, and the stacking sequence is ABCACB. The cubic 3C-SiC (not shown) has ABC stacking.

Despite the fact that all SiC polytypes chemically consist of 50% carbon atoms covalently bonded with 50% silicon atoms,each SiC polytype has its own distinct set of electrical semiconductor properties. While there are over 100 known polytypes of SiC, only a few are commonly grown in a reproducible form acceptable for use as an electronic semiconductor. The most common polytypes of SiC presently being developed for electronics are 3C-SiC, 4H-SiC, and 6H-SiC. The different polytypes of SiC are actually composed of different stacking sequences of Si–C bilayers (also called Si–C double layers), where each single Si–C bilayer is denoted by the dotted boxes. Each atom within a bilayer has three covalent chemical bonds with other atoms in the same (its own) bilayer, and only one bond to an atom in an adjacent bilayer.

4H-SiC polytype requires four Si–C bilayers to define the unit cell repeat distance along the c-axis stacking direction (denoted by <0 0 0 1> Miller indices). Similarly,the 6H-SiC polytype repeats its stacking sequence every six bilayers throughout the crystal along the stacking direction. The <1-1 0 0> direction is often referred to as one of (along with<1  1 -2 0> ) the a-axis directions. SiC is a polar semiconductor across the c-axis, in that one surface normal to the c-axis is terminated with silicon atoms while the opposite normal c-axis surface is terminated with carbon atoms,these surfaces are typically referred to as “silicon face” and “carbon face” surfaces, respectively. Atoms along the left-or right-side edge would reside on <1 -1 0 0> “a-face” crystal surface plane normal to the <1 -1 0 0> direction.3C-SiC, also referred to as ß-SiC, is the only form of SiC with a cubic crystal lattice structure. The noncubic polytypes of SiC are sometimes ambiguously referred to as á-SiC. 4H-SiC and 6H-SiC are only two of the many possible SiC polytypes with hexagonal crystal structure. Similarly, 15R-SiC is the most common of the many possible SiC polytypes with a rhombohedral crystal structure.

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