5-2-2-1 SiC Crystallography: Important Polytypes and Definitions
Silicon carbide occurs in many different crystal structures, called polytypes. A more comprehensive
introduction to SiC crystallography and polytypism can be found in Reference 9. 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 atomic crystal structure of the two most common
polytypes is shown in the schematic cross section in Figure 5.1. As discussed much more thoroughly in
References 9 and 10, 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 in Figure 5.1. 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. Figure 5.1a shows the
bilayer of the stacking sequence of 4H-SiC polytype, which requires four Si–C bilayers to define the unit
cell repeat distance along the c-axis stacking direction (denoted byMiller indices). Similarly,
the 6H-SiC polytype illustrated in Figure 5.1b repeats its stacking sequence every six bilayers throughout
the crystal along the stacking direction. The
direction depicted in Figure 5.1 is often referred to as one of
(along with )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. As shown in Figure 5.1a, these surfaces are typically referred to as
“silicon face” and “carbon face” surfaces, respectively. Atoms along the left-or right-side edge of Figure 5.1a
would reside on “a-face” crystal surface
plane normal to the direction. 3C-SiC,
also referred to as β-SiC, is the only form of SiC with a cubic crystal lattice structure. The noncubic polytypes of
SiC are sometimes ambiguously referred to as α-SiC. 4H-SiC and 6H-SiC are only two of the many.
FIGURE 5.1 Schematic cross-sectional depictions of (a) 4H-SiC and (b) 6H-SiC atomic crystal structure, showing
important crystallographic directions and surfaces.
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.