Chemical Vapor Deposition

The acronym MOCVD designates a gas-phase process in which metal-organic compounds (precursor) are converted into a solid film by their decomposition on a hot substrate [383-386]. In addition to the four important parameters (i.e., temperature, deposition time, pressure, and surface specificity), the chemical nature of the precursor substance is of paramount importance in determining the quality of the film in terms of homogeneity, morphology, and contamination. The film growth involves the conversion of atoms and/or molecules in the gas phase into atoms or molecules at the surface of the film, which can then be incorporated into it [52, 387]. This process takes place by adsorption of the gas-phase species on the substrate surface. Depending upon the interactions of the adatoms with the surface, the surface adsorption can be physical (essentially van der Waals interaction) or chemical (covalent linkage between the molecule and the surface) in nature. The adsorbed molecules may wander on the surface and react with other surface species to form the solid deposit. The mobility of surface species is largest on metallic and semiconducting surfaces, where bonding is not very directional. In the case of dielectrics, the highly directional covalent bonds tend to hold the chemisorbed molecules, thereby limiting their mobility. The volatile by-products released in the gas-phase decomposition, collision, and surface reactions can be removed in vaccum and analyzed on-line by infrared spectroscopy or mass spectrom-etry to control the decomposition process as well as to elucidate the thermal decomposition of the precursor molecule. In a simplified view of the surface, it is assumed that there are a fixed total number of reactive sites upon which an incoming atom ("adatom") or molecule can adsorb. Once a site is occupied, a second molecule will not adsorb on it. This is equivalent to the assumption that the number of atoms adsorbing is equal to those desorbing, if no reaction is taking place. The number of atoms or molecules adsorbing on the surface is proportional to the concentration of the gasphase species or, in other words, equivalent to the partial pressure. Some of the fundamental processes occurring during the CVD of a metal-organic compound are depicted in Figure 11.

Since the conversion of an organometallic precursor to a useful thin film involves stripping most of the coordinated lig-ands, a well-understood decomposition chemistry is a prerequisite. The efficiency of the CVD process depends crucially on ligands that decompose by way of chemically productive pathways and with a low activation barrier. The precursors based on ligands with complex concomitant fragmentation reactions usually lead to the incorporation of heteroatoms such as C, B, N, O, or Si [383-386]. Furthermore, the intact vapor transport of the molecular precursor is highly essential to control over the process. The thermal instability or dissociative tendency of the precursor can initiate a cascade of reactions leading to nonideal geometry or undesirable deposits [305].

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