This paper describes the initial reaction kinetics of Ge deposition after exposure of Si(100)-2 x 1 to GeH -CVD system. The rate of Ge growth, (4) in a UHIV especially at the wetting layer stage, was investigated using in situ X-ray photoelectron spectroscopy to measure the Ge signal at the onset of deposition. A kinetic analysis of the initial growth of the Ge wetting layer at temperatures ranging from 698 to 823 K revealed an activation energy of 30.7 kcal/mol. Density functional theory calculations suggested that opening of the Si dimer-with a closely matching energy barrier of 29.7 kcal/mol, following hydrogen atom migration-was the rate controlling step for the incorporation of a GeH2 unit into the lattice to complete the growth of the Ge wetting layer after dissociative adsorption of GeH4.