High Temperature Chemical Vapor Deposition

In 1995, a new technique, called HTCVD, was presented for the growth of SiC boules. This technique uses gases instead of a powder as source material. It was first presented at ICSCRM 95 in Kyoto, Japan, but the first publication of HTCVD in a journal was in 1996 [37]. More recent publications are available where the technique is better described [38]. 

Silane and ethylene are present at very high concentrations so that homogeneous nucleation dominates the process. As the gases enter into the hot part of the injector, the silane will decompose and form small Si liquid droplets or solid microcrystals, depending on the temperature. The ethylene will also take part in the reaction, forming microparticles of Si C. It has been noted [39] that even a small addition of hydrocarbons converts the Si droplets to stable particles of Si C (or non-stoichiometric SiC). The stability may, in a hand-waving circumstantial way, be intuitively understood from a solubility point of view. The solubility of carbon in silicon is very low, thus, when carbon is added to the Si droplets, the phase will be solid rather than liquid. 

The process can work without additions of a hydrocarbon, in which case the carbon is supplied through a reaction between the hot silicon and the graphite walls. This is usually not the preferred growth mode and additions of hydrocarbons are needed to obtain a high growth rate. 

There are similarities between seeded sublimation growth and HTCVD in that solid particles sublimate in the reactor and the vapor condenses on a seed crystal maintained at a lower temperature. However, the differences are quite dramatic and the outcome even more so. Take, for instance, the dynamics governing the growth 

there are several ways to improve the growth rate, as can be seen. But in the end it all comes down to the size of the particles that need to be sublimed in the sublimation zone. Typical maximum growth rates are in the order of 0.8 mm/hr-1 mm/hr. 

---

Another even more interesting development was occurring at the same time. This was the development of a new growth technique, called the High Temperature Chemical Vapor Deposition (HTCVD) technique [34], that produced crystals that were intrinsically semi-insulating. In a paper by Ellison et al. [34], the authors reported on a defect with an activation energy of 1.15 eV yielding an extrapolated room temperature resistivity in excess of 10 il-cm. 

Tube-type carbons are obtained when organic compounds are carbonized in a thin-film state on the template pore walls [6], The tube-type carbon can be obtained even after the entire volume of pores is filled with carbon source, if the excess carbon source is removed before the carbonization is completed. For example, cylindrical pores are generated along the center of the carbon frameworks due to the systematic volume decrease when furfiiryl alcohol is pyrolyzed under vacuum after the initial polymerization. Alternatively, the tube-type carbons can be synthesized as follows carbonization can be controlled to occur partially by catalyst at the pore walls at moderate temperatures. The remaining carbon source is removed by evacuation, and the carbonization is completed by pyrolysis at high temperature. Chemical vapor deposition on the pore walls can also be used to produce the tube-type carbon [11] as well as the aforementioned rod-type carbons [12]. The structure of the resultant carbon depends on the thickness of the carbon deposition. 

O Kordina, C Hallin, AS Bakin. I Ivanov, A Henry, M Tuominen, R Yakimova, A Vehanen, E Janzen. High temperature chemical vapor deposition. Technical Digest of International Conference on SiC and Related Materials, Kyoto, 1995, p 609. 

Z. Yuan, R.J. Puddephatt, M. Slayer, Low-temperature chemical vapor deposition of ruthenium dioxide from ruthenium tetroxide A simple approach to high-purity RUO2 films. Chem. Mat. 5, 908-910 (1993)... 

High process temperatures generally not achievable by other means are possible when induction heating of a graphite susceptor is combined with the use of low conductivity high temperature insulation such as flake carbon interposed between the coil and the susceptor. Temperatures of 3000°C are routine for both batch or continuous production. Processes include purification, graphitization, chemical vapor deposition, or carbon vapor deposition to produce components for the aircraft and defense industry. Figure 7 illustrates a furnace suitable for the production of aerospace brake components in a batch operation. 

Titanium carbide may also be made by the reaction at high temperature of titanium with carbon titanium tetrachloride with organic compounds such as methane, chloroform, or poly(vinyl chloride) titanium disulfide [12039-13-3] with carbon organotitanates with carbon precursor polymers (31) and titanium tetrachloride with hydrogen and carbon monoxide. Much of this work is directed toward the production of ultrafine (<1 jim) powders. The reaction of titanium tetrachloride with a hydrocarbon-hydrogen mixture at ca 1000°C is used for the chemical vapor deposition (CVD) of thin carbide films used in wear-resistant coatings. 

Carbon Composites. In this class of materials, carbon or graphite fibers are embedded in a carbon or graphite matrix. The matrix can be formed by two methods chemical vapor deposition (CVD) and coking. In the case of chemical vapor deposition (see Film deposition techniques) a hydrocarbon gas is introduced into a reaction chamber in which carbon formed from the decomposition of the gas condenses on the surface of carbon fibers. An alternative method is to mold a carbon fiber—resin mixture into shape and coke the resin precursor at high temperatures and then foUow with CVD. In both methods the process has to be repeated until a desired density is obtained. 

Sihcon carbide is also a prime candidate material for high temperature fibers (qv). These fibers are produced by three main approaches polymer pyrolysis, chemical vapor deposition (CVD), and sintering. Whereas fiber from the former two approaches are already available as commercial products, the sintered SiC fiber is still under development. Because of its relatively simple process, the sintered a-SiC fiber approach offers the potential of high performance and extreme temperature stabiUty at a relatively low cost. A comparison of the manufacturing methods and properties of various SiC fibers is presented in Table 4 (121,122). 

Of the many forms of carbon and graphite produced commercially, only pyrolytic graphite (8,9) is produced from the gas phase via the pyrolysis of hydrocarbons. The process for making pyrolytic graphite is referred to as the chemical vapor deposition (CVD) process. Deposition occurs on some suitable substrate, usually graphite, that is heated at high temperatures, usually in excess of 1000°C, in the presence of a hydrocarbon, eg, methane, propane, acetjiene, or benzene. 

Zinc sulfide, with its wide band gap of 3.66 eV, has been considered as an excellent electroluminescent (EL) material. The electroluminescence of ZnS has been used as a probe for unraveling the energetics at the ZnS/electrolyte interface and for possible application to display devices. Fan and Bard [127] examined the effect of temperature on EL of Al-doped self-activated ZnS single crystals in a persulfate-butyronitrile solution, as well as the time-resolved photoluminescence (PL) of the compound. Further [128], they investigated the PL and EL from single-crystal Mn-doped ZnS (ZnS Mn) centered at 580 nm. The PL was quenched by surface modification with U-treated poly(vinylferrocene). The effect of pH and temperature on the EL of ZnS Mn in aqueous and butyronitrile solutions upon reduction of per-oxydisulfate ion was also studied. EL of polycrystalline chemical vapor deposited (CVD) ZnS doped with Al, Cu-Al, and Mn was also observed with peaks at 430, 475, and 565 nm, respectively. High EL efficiency, comparable to that of singlecrystal ZnS, was found for the doped CVD polycrystalline ZnS. In all cases, the EL efficiency was about 0.2-0.3%. 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

---