a) Chemical vapor deposition (CVD) involves the dissociation and/or chemical reactions of gas phase reactants in an activated (heat, light, plasma) environment, followed by the formation of a stable solid product.
b) Three basic nucleation modes:
1. Island or Volmer-Weber growth: adatominteractions are stronger than those of the adatom with the surface, leading to the formation of three-dimensional adatom clusters or islands. Growth of these clusters, along with coarsening, will cause rough multi-layer films to grow on the substrate surface.
2. Layer or Frank-van der Merwe growth: thin films grow epitaxially at a crystal surface or interface. It is also knows as 'layer-by-layer growth'.
3. Island-layer or Stranski-Krastonov growth: It follows a two-step process: initially, complete films of adsorbates, up to several monolayers thick, grow in a layer-by-layer fashion on a crystal substrate. Beyond a critical layer thickness, which depends on strain and the chemical potential of the deposited film, growth continues through the nucleation and coalescence of adsorbate 'islands'