In MOCVD, group III alkyls are already partially dissociated in the gas stream. These diffuse through a stagnant boundary layer that exists over the heated substrate, after which they dissociate into the atomic group III elements. These atoms then migrate to the appropriate lattice site and deposit epitaxially by associating with a group V atom that was derived from the thermal decomposition of the hydrides. The growth rate here is usually limited by the diffusion rate of the group III alkyls through the boundary layer. Gas phase reactions between the reactants have also been observed in this process.