Surface reaction mechanisms are investigated to determine the activation energies of pure boron (PureB) layer deposition at temperatures from 350°C to 850°C when using chemical-vapor deposition from diborane in a commercial Si/SiGe epitaxial reactor with either hydrogen or nitrogen as carrier gas. Three distinguishable regions are identified to be related to the dominance of specific chemical reaction mechanisms. Activation energies in H 2 are found to be 28 kcal/mol below 400°C and 6.5 kcal/mol from 400°C to 700°C. In N 2, the value decreases to 2.1 kcal/mol for all temperatures below 700°C. The rate of hydrogen desorption is decisive for this behavior.