In order for a film to grow, some process must convert atoms or molecules in the gas phase into atoms or molecules at the surface of the film, which can then be incorporated into it. This process is normally thought of as taking place in several steps. Once molecules are on the surface, they may wander around a bit and perhaps react with some other surface species, releasing products and forming film. The mobility of surface species is largest on metallic and semiconducting surfaces, where bonding is not very directional, and rather limited on dielectrics where highly directional covalent bonds tend to hold a molecule in one place once it is chemisorbed.
Surfaces are potentially complex beasts.
Unfortunately, most of this research is done in ultra-high vacuum on nearly ideal surfaces, and is thus not directly relevant to most CVD, which takes place at pressures from a few mTorr to an atmosphere. The first question we'll try to address is to establish a plausible form for the relationship between the concentration of a species in the gas phase, and the amount of species adsorbed on a surface. Once a site is occupied, a second molecule will not adsorb on it. The steady-state value results when the number of atoms adsorbing is equal to those desorbing, if no reaction is taking place. The number desorbing is proportional to the number of filled sites.