1
Cubic Silicon Carbide: Growth, Properties, and Electrochemical Applications

Nianjun Yang and Xin Jiang

Institute of Materials Engineering, University of Siegen, Paul-Bonatz-Str. 9-11, , 57076 Siegen, Germany

1.1 General Overview of Silicon Carbide

It is well known that carbon and silicon atoms form similar, covalently bonded and giant structures, as shown schematically in Figure 1.1a. They are thus called carbon diamond and silicon diamond. In both diamond structures, each atom is covalently bonded to four other atoms located at the corner of a tetrahedron. Another diamond-like compound is silicon carbide (), building up with silicon and carbon atoms. In this crystal, each atom is sp3-hybridized and forms four bonds to four other atoms of the opposite kind. The tetrahedral arrangement of atoms encountered in the pure carbon and silicon diamond structures is preserved in SiC (Figure 1.1a).

Figure 1.1 Chemical structures of carbon diamond, silicon diamond, SiC (a), 3C(ß)-SiC (b), 4H-SiC (c), and 6H(a)-SiC (d) using ball-stick models.

The existence of a compound containing SiC bonds was proposed in 1824 for the first time by Jöns Jacob Berzelius, a Swedish chemist [1]. In 1905, Henri Moissan, a French chemist and the Nobel laureate, discovered SiC in nature [2]. In mineralogy, SiC is therefore known as moissanite [3]. In nature, moissanite SiC is very rare and only found in certain types of meteorite. The most commonly encountered SiC material is actually man-made.

SiC exists in about 250 crystalline forms, as variations of the same chemical compound that are identical in two dimensions but differ in the third. They can be viewed as layers stacked in a certain sequence. Different stacking sequences of C-Si double layers lead to different crystalline structures, or so-called polytypes [4]. Therefore, more than 250 polytypes have been predicted [4,5]. Of these polytypes, only a few of them have been studied in detail. In principle, only three are of major importance: cubic (3C, or ß)-SiC, 4H-SiC, and 6H(a)-SiC, which are shown schematically in Figure 1.1b-d, respectively. The most commonly encountered polymorph is 6H(a)-SiC, which forms at temperatures higher than 1700°C and has a hexagonal crystal structure (similar to wurtzite) (Figure 1.1c). Cubic 3C(ß)-SiC (Figure 1. 1b) is formed at temperatures below 1700°C and has a zincblende (ZnS) crystal structure, similar to diamond [6].

1.1.1 SiC Properties

SiC is a fascinating material, although it has quite complicated polytypes. This is because the type of SiC polytype implies a corresponding set of relevant physical properties. As examples, some important physical properties of 4H-, 6H-, and 3C-SiC are listed in Table 1.1, compared with those of diamond and silicon.

Table 1.1 Basic properties of three kinds of SiC, Si, and diamond.

SiC has been known for decades to be a semiconductor, based on the very first electroluminescence (yellowish light) from SiC crystals when subjected to electricity in 1907 [7]. More interestingly, its indirect bandgap is tunable in the range of 2.36-3.23?eV, determined by the polytype of SiC films. For instance, the bandgaps for 3C-, 4H-, and 6H-SiC are 2.36, 3.23, and 3.05?eV, respectively. However, SiC can be varied from insulating, semiconductive, to metallic-like in its properties when the dopants (n- or p-type) and the doping levels are altered. For example, SiC films can be doped with either n-type dopants (e.g. nitrogen, phosphorus) or p-type dopants (e.g. beryllium, boron, aluminum, gallium). Metallic conductivities of SiC films have been achieved by their heavy doping with boron, aluminum, or nitrogen. For example, at the same temperature of 1.5?K, superconductivity has been detected in 3C-SiC films doped with aluminum and boron as well as in 6H-SiC films doped with boron.

In comparison with Si, SiC has a higher thermal conductivity, electric field breakdown strength, and current density. It features a very low coefficient of thermal expansion (4.0?×?10-6 K-1) and experiences no phase transitions that cause discontinuities in thermal expansion. The sublimation temperature of SiC is very high (approximately 2700°C), which makes it useful for bearings and furnace parts. SiC does not melt at any known temperature.

SiC is transparent to visible light. Pure SiC is colorless. The brown to black color of industrial SiC products results from iron impurities. The rainbow-like lusters of SiC crystals are caused by the passivation layers of SiO2 that form on the SiC surface.

SiC is a very hard material. Taking Mohs hardness scale as an example, the value of talc is given by 1 and diamond is given by 10: SiC has the value of 9.3 [8].

SiC is chemically inert. For example, it is resistive to radiation and many chemicals. This is because the electron bonds between the silicon and carbon atoms inside SiC are extremely strong. More importantly, SiC has shown superior biocompatibility and is non-toxic in both in vitro and in vivo tests. In addition, SiC is multifunctional, originating from the possibility of adopting both silicon and carbon chemistry on its surface.

In conclusion, SiC is a material with exceptional physical properties (e.g. a low density, a high strength, a high thermal conductivity, high stability at high temperatures, a high resistance to shocks, low thermal expansion, a high refractive index, a wide but tunable bandgap) and chemical features. They present multiple options for smart devices through their electrical, chemical, and optical properties [59-15].

1.1.2 SiC Applications

Thanks to its unique physical properties (e.g. electrical, thermal properties), SiC has found wide and varied applications where high blocking voltages or high switching frequencies are required [59-15]. Shockley thus predicted in the 1950s that SiC would quickly replace Si. SiC-based power electronics can greatly reduce the power losses of electrical energy in most generators and distribution systems. The higher frequency, smaller dimensions, reduced cooling requirements, and greater efficiency obtained with SiC power electronics will give more efficient systems in any application where AC-DC, DC-AC, or DC-DC conversion is required. One example application of SiC is for compact power supply units with extremely low losses, which also keep the power supply network free of electric smog (the unwanted interference frequencies resulting from the use of computers) [5,15].

SiC is also suited for space-saving control units and for variable-speed drives, which are generally mounted directly on the mortars. For these applications, homoepitaxial SiC films are generally required. However, the typical growth rate for homoepitaxial SiC layers is 5-10?µm?h-1. Thus, the epitaxial growth of SiC layers is very time-consuming, making them very expensive for most devices. The long production time and high cost of these...