Kinetics and thermodynamics of thin-film deposition by molecular beam methods. I. Adsorption and clustering of cadmium on germanium single crystals

The use of mass-spectrometric molecular beam techniques allowed a detailed study to be made of the kinetics and thermodynamics of thin-film deposition. Thermal accommodation, adsorption, nucleation, growth, and evaporation were measured for a single system. The deposits were characterized by their coverage-dependent steady-state evaporation rate, which is directly related to the thermodynamic potential of the evaporant in the film. The methods used and results obtained are exemplified by the present study of all stages of the ultrahigh-vacuum (UHV) deposition of cadmium on the (100), (110), (111), (211), and (331) faces of germanium. In this paper, the initial stages of adsorption and clustering are described. The thermal accommodation coefficient was found to be between 0.8 and 1.0. On (111) substrates, adsorption of one monolayer of immobile cadmium atoms precedes nucleation of bulk cadmium. On the other substrates, 40% to 70% of a monolayer is adsorbed prior to nucleation. The extent of this adsorption is predictable from the structure of the germanium surfaces. Adsorption energies range from 25 to 32 kcal/g atom, compared with a heat of sublimation of 26.7 kcal/g atom. The kinetics of the desorption of cadmium during deposition is in agreement with an evaporation mechanism in which desorption of mobile single cadmium adatoms is the rate-determining step. The desorption energy of mobile adatoms is approximately 12 kcal/g atom in the nucleation stage. It is shown that the stability of the cadmium deposit increases only slowly after nucleation. Electrically active dopants in the germanium increase the extent of cadmium adsorption which precedes nucleation.