Silicon carbide sublimation

W.A. de Heer, C. Berger, M. Ruan, M. Sprinkle, X. Li, Y. Hu, et al., Large area and structured epitaxial graphene produced by confinement controlled sublimation of silicon carbide, Proceedings of the National Academy of Sciences of the United States of America, 108 (2011) 16900-16905. 

Silicon carbide is a very hard snbstance with a Young s modulus of 424 GPa [1]. It is chemically inert and reacts poorly (if at all) with any known material at room temperature. The only known efficient etch at moderate temperatures is molten KOH at 400-600°C. It is practically impossible to diffuse anything into SiC. Dopants need to be implanted or grown into the material. Eurthermore, it lacks a liqnid phase and instead sublimes at temperatures above 1,800°C. The vapor constituents during sublimation are mainly Si, SqC, and SiC in specific ratios, depending on the temperature. 

Silicon carbide, SiC covalent-network covalent bonds 2,700°C (sublimes)... 

Thermal oxidation of the two most common forms of single-crystal silicon carbide with potential for semiconductor electronics applications is discussed 3C-SiC formed by heteroepitaxial growth by chemical vapour deposition on silicon, and 6H-SiC wafers grown in bulk by vacuum sublimation or the Lely method. SiC is also an important ceramic ana abrasive that exists in many different forms. Its oxidation has been studied under a wide variety of conditions. Thermal oxidation of SiC for semiconductor electronic applications is discussed in the following section. Insulating layers on SiC, other than thermal oxide, are discussed in Section C, and the electrical properties of the thermal oxide and metal-oxide-semiconductor capacitors formed on SiC are discussed in Section D. 

Sublimation is one of the main methods of growing silicon carbide. This method is employed for growth of the material for abrasive applications as well as for the growth of single crystals and epitaxial layers for use in semiconductor electronics. The idea of the method is fairly simple, and is based on material transport from a hot source of material to a substrate which rests at a somewhat lower temperature. The transport is performed by the intrinsic vapour of the material at a high temperature, usually in the range 1600-2700 °C. 

The high efficiency of the method is evidenced by the fact that the first sublimation method of growing the crystalline material, the Acherson method, was proposed at the beginning of the 20th century and it is used today with only small variations. The Acherson process yields material for abrasive use and the rate of production is really very high, more than half a million tons per year [1]. No other technique of growing silicon carbide can be compared with sublimation in its productivity and efficiency. 

A new approach to the directed crystallisation of silicon carbide has been proposed by Vodakov and Mokhov [6]. Their idea was to exclude the conditions which could permit any uncontrolled nucleation. They employ a nearly flat source positioned close to the substrate and perform the growth under near-equilibrium conditions. This has ensured a high quality of the grown material. The method was named the sublimation sandwich method and it appeared to be very effective. 

The studies of growth by the sandwich method have provided a better understanding of the sublimation growth peculiarities and they have formed the basis of the new approach to the bulk crystal growth of silicon carbide. The first successful results in this direction were reported by Tairov and Tsvetkov [7,8]. Currently, similar studies are being performed by a number of research groups and rather impressive progress has been achieved thus far see Datareview 8.1. 

B EQUIPMENT AND METHODS OF GROWING SILICON CARBIDE BY SUBLIMATION... 

Aluminium contamination is seldom observed for low temperature vacuum sublimation. Aluminium has a low capture coefficient at low temperatures and it does not form refractory carbides with a low vapour pressure. Therefore, traces of aluminium can be easily removed by annealing the furnace in vacuum even if contamination occurs. However, if the material source is insufficiently pure, it can result in noticeable aluminium contamination, especially at elevated growth temperatures. For the bulk crystal growth, aluminium contamination is always observed when abrasive silicon carbide is used as source material [20,22]. The abrasive material usually is highly contaminated [1,22]. 

The most diverse data exist about the best-known compound silicon carbide (SiC), perhaps because oxidation (in air), dissociation, sublimation, and phase change obscure the picture. No aluminum silicide has been described. 

Sublimation method with spontaneous nucleation (mass crystallization). Crystal growth is carried out in cylindrical graphite crucibles (Fig. 14). Powder silicon carbide is used as a source material. Crucibles are heated in cylindrical furnaces with resistive or inductive heating. Growth proceeds in an inert atmosphere (argon, helium) at a temperature of 2500-... 

If silicon atoms are substituted for half the carbon atoms in this structure, the resulting structure is that of silicon carbide (carborundum). Both diamond and silicon carbide are extremely hard, and this accounts for their extensive use as abrasives. In fact, diamond is the hardest substance known. To scratch or break diamond or silicon carbide crystals, covalent bonds must be broken. These two materials are also nonconductors of electricity and do not melt or sublime except at very high temperatures. SiC sublimes at 2700 °C, and diamond melts above 3500 °C. 

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