The CVD of Non-Metallic Elements

This chapter is a review of the CVD of non-metallic elements and covers boron, silicon, and germanium. Silicon and germanium are borderline elements with metalloid characteristics. Both are important semiconductor materials, particularly silicon, which forms the backbone of the largest business in the world the electronic industry. All three materials are deposited by CVD on an industrial scale and a wide variety of CVD reactions are available. 

Boron is a light element produced by CVD in the form of coatings and fibers with limited industrial usage. 

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A work dealing with the simplest form of CVD, the growth of elemental metals, has been edited by Kodas and Hampden-Smith. The epitaxial preparation of compound semiconducting materials by CVD is presented in a book by O Brien and Jones. The present work covers the area of non-metallic materials. 

The second part is a review of the materials deposited by CVD, i.e., metals, non-metallic elements, ceramics and semiconductors, and the reactions used in their deposition. 

CVD processes have a greater flexibility of using a wide range of chemical precursors such as halides, hydrides, organo-metallic compounds and so forth which enable the deposition of a large spectrum of materials, including metals, non-metallic elements, carbides, nitrides, oxides, sulphides, as well as polymers. Up to now, around 70% of elements in the periodic table have been deposited by the CVD technique, some of which are in the form of the pure element, however, more often the compound materials. 

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

CVD also has a number of disadvantages. One of the primary disadvantages lies in the properties of the precursors. Ideally, the precursors need to be volatile at near-room temperatures. This is non-trivial for a number of elements in the periodic table, although the use of metal-organic precursors has eased this situation. CVD precursors can also be highly toxic (Ni(CO) ), explosive (B Hg), or corrosive (SiCl ). The byproducts of CVD reactions can also be hazardous (CO, H, or HP). Some of these precursors, especially the metal-organic precursors, can also be quite costly. The other major disadvantage is the fact that the films are usually deposited at elevated temperatures. This puts some restrictions on the kind of substrates that can be coated. More importantly, it leads to stresses in films deposited on materials with different thermal expansion coefficients, which can cause mechanical instabilities in the deposited films. 

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