Growth species

Depending on the nature of surface chain growth species, on the other hand, one is confronted mainly with the alkyl mechanism,6 based on the insertion of a methylene species C CHj) into the metal-alkyl bond, or with the alkenyl mechanism,2 wherein a surface vinyl species ( CH=CH2) reacts with a surface methylene ( CII2) to form an allyl species ( CH2CH=CH2). 

The concentration of precursor gases will decrease with respect to the flow direction over the susceptor due to the consumption of growth species, which results in a tapered layer thickness. This effect is known as depletion. To compensate for the depletion it is common to taper the susceptor such that the velocity of the gases increases along the flow direction over the susceptor and thus the boundary layer will be pushed downward, resulting in a shorter diffusion for the active species to the substrate. 

Generally speaking, the left-hand side of Eq. 17.5 represents availability of species k to the system, while the right-hand side represents losses from the system. If the species is a desirable one, say H atoms in a diamond reactor, then the left-hand side contributions are beneficial. The Sk term on the right-hand side is desirable in that it represents utilization of a gas-phase species at the growth surface. The gas-phase-destruction and radial-loss terms, however, represent undesirable losses from the system. For example, if a growth species like CH3 is either introduced through the inlet boundary or created by gas-phase reaction, then any flux of that species from the system (except by reaction at the surface) represents an inefficiency. 

FIGURE 20.1 Schematic illustrating three basic modes of initial nucleation in film growth. Island growth occurs when the growth species are more strongly bonded to each other than to the substrate. 

It should be noted that the aforementioned nucleation models and mechanisms are applicable to the formation of single-crystal, polycrystaUine, and amorphons deposits, and of inorganic, organic, and hybrid deposits. Whether the deposit is single crystalline, polycrystalline, or amorphous depends on the growth conditions and the substrate. Deposition temperature and the impinging rate of growth species are the two most important factors and are briefly summarized below ... 

Growth of single-crystal films is most difficult and reqnires (1) a singlecrystal snbstrate with a close lattice match, (2) a clean snbstrate snrface so as to avoid possible secondary nncleation, (3) high growth temperature so as to ensure sufficient mobility of the growth species, and (4) low... 

An impinging growth species onto the growth surface can be described in terms of the residence time and/or diffusion distance before escaping back to the vapor phase. The residence time, for a growth species on the surface is described by... 

Concerning growth species, there are two main areas for controversy (i) whether the growth units are solution species or particulates, and (ii) whether the formation of a particular structure requires the presence of specific building units. 

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