In CBE processes, the hydrides are cracked in a high temperature injector before they reach the substrate. The temperatures are typically 100-150 °C lower than they are in a similar MOCVD or MOVPE. There is also no boundary layer (such as the one in MOCVD) and molecular collisions are minimal due to the low pressure. The group V alkyls are usually supplied in excess, and the group III alkyl molecules impinge directly onto the heated substrate as in conventional MBE. The group III alkyl molecule has two options when this happens. The first option is to dissociate its three alkyl radicals by acquiring thermal energy from the surface, and leaving behind the elemental group III atoms on the surface. The second option is to re-evaporate partially or completely undissociated. Thus, the growth rate is determined by the arrival rate of the group III alkyls at a higher substrate temperature, and by the surface pyrolysis rate at lower temperatures.