What Are the Polytypes of Silicon Carbide Single Crystal?

What Are the Polytypes of Silicon Carbide Single Crystal?

Silicon carbide has a chemical formula of SiC and a molecular weight of 40.1. Although the chemical formula is simple, it has a wide range of applications, which is determined by the silicon carbide polytypes.

Structure={components, relationship between components}

Silicon carbide is a simple substance, and the components are carbon atoms and silicon atoms. Silicon carbide crystals are composed of carbon atoms and silicon atoms in an orderly arrangement. Both carbon and silicon belong to the second period elements, and the atomic radii are not very different. The stacking method can be considered from the closest stacking direction of the equal diameter sphere.

Choose carbon atoms (or silicon) to form the most densely packed layer, called the A layer. At this time, there will be two positions to place the next layer of silicon atoms: the B position of the upper triangle or the C position of the lower triangle. If it is filled in position B, the next layer is called layer B; if it is filled in position C, the next layer is called layer C. This is only a simple way of analyzing the formation of the silicon carbide polytypes to fix the viewing direction, and the most accurate is the space group.

As a result, there are countless ways of accumulation, the common silicon carbide polytypes are as follows:

Type AB corresponding to 2H-SiC: AB AB……

ABC type corresponding to 3C-SiC: ABC ABC……

The ABAC type corresponding to 4H-SiC: ABAC ABAC……

ABCACB type corresponding to 6H-SiC: ABCACB ABCACB……

15R-SiC corresponding ABACBCACBABCBAC type: ABACBCACBABCBAC ABACBCACBABCBAC……

Stacking method of silicon carbide polytypes

Originally, the SiC polytypes crystal structure is represented by the space group symbol. In order to distinguish the single crystal silicon carbide of the same space group, a simpler symbol can be used: the crystal form symbol is represented by numbers + letters. Among them, the number represents the number of carbon-silicon diatomic layers along the (001) direction of a unit cell, C represents cubic crystal system (Cubic), H represents hexagonal crystal system (Hexagonal), and R represents Trigonal crystal system (Rhombohedral). F-43m silicon carbide crystal is written as 3C-SiC. P63mc, Z=4 silicon carbide crystal is written as 4H-SiC; P63mc, Z=6 silicon carbide crystal is written as 6H-SiC.

The period of the image is reflected on the (110) (11-20) plane, which corresponds to the two writing methods (hkl) (hkil) of the crystal plane, as shown in the figure below:

Schematic diagram of the connection

The connection of the carbon-silicon diatomic layer on the 2H-SiC(a), 4H-SiC(b), 6H-SiC(c), 15R-SiC(d), 3C-SiC(e) polytype (11-20) planes

In the 4H-SiC silicon carbide polytype crystal, there is obviously a stacking fault of 2H and 6H.

Electron micrograph of 4H-SiC

Electron micrograph of 4H-SiC

It is worth noting that due to the four-coordination requirement of carbon-silicon, there will be a silicon layer and a carbon layer with repeated positions; two of the three layers must be at the same position. In other words, the A layer of carbon will connect a layer of A layer of silicon and a layer of B/C densely packed silicon layer.

Different stacking methods have caused great differences in some performances.

Just talk about density:

Properties V/Z A3 Density g/cm3
2H-SiC 20.74 3.210
3C-SiC 20.70 3.216
4H-SiC 20.68 3.219
6H-SiC 20.72 3.213
15R-SiC 20.55 3.240

* The polytypes of SiC wafers PAM-XIAMEN can offer are 4H-SiC and 6H-SiC. More information please visit https://www.powerwaywafer.com/sic-wafer/sic-wafer-substrate.html.

The close arrangement of sic polytypes also brings great hardness and refractive index.

In the gem world, silicon carbide is also called “Moissanite”. Mohs hardness is 9.2-9.8 (diamond is 10); refractive index is 2.654 (diamond is 2.417); dispersion value is 0.104 (diamond is 0.044), and fire color is 2.5 times that of diamonds.

These distinctive properties make silicon carbide have application advantages, and single crystal growth has technical requirements.

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