Reactant selection and design

An additional consideration in the development of a CVD process is design of the reactants such that a desirable growth behavior is obtained. Some of the issues in design and selection of reactants are the phase of the source material under standard conditions the ability to produce a vapor from the source material the stability of 

Source materials may be sohds, liquids or gases under standard laboratory conditions. Solids and liquids are most easily handled, stored, and transported. It is also relatively easy to purify sohds and hquids and to keep them pure prior to use, so they are desirable. However, CVD requires gaseous source materials in the reactor. Therefore it is necessary to convert sohds and hquids to gases to make use of them. 

Solid and low vapor pressure liquid (e.g. Ga) sources may be used in CVD systems by reaction with an etching compound (usuaUy a gas) resulting in a gas phase product. Two examples of etching reachons are  

In both cases a vapor containing the initiaUy sohd/liquid element is produced, usually upstream from the substrate in the CVD reactor itself These reactions can run in reverse to redeposit the vaporized species. In some cases it is possible to simply evaporate a high vapor pressure compormd such as CdTe in the CVD reactor, although this yields an evaporation-like process more than a CVD process. This case is referred to as closed-space vapor transport and is generally distinct from evaporation as described in Chapter 11 because the substrate and source are in equilibrium with a relatively dense vapor of the source material in a closed environment. The case above for Ga requires an etchant because Ga has a naturally low vapor pressure. In the case of liquids with high vapor pressures a different method can be used. 

Purity of the source materials is an important consideration. It is much more difficult to purify relatively delicate metalorganic compounds than hydrides. It is easier still to purify molecular gases such as H2 and CI2 from which, for example, high purity HCl can be produced. Etching purified As or Ga with HCl thereby produces very high purity metal trichlorides. The sensitivity to impurities is also dependent upon the material to be deposited. Thus, deposition of A1 compounds is more sensitive to oxygen-containing impurities than for materials less inclined to form oxides. 

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