Growth kinetics of Ge islands during Ga-surfactant-mediated ultrahigh vacuum chemical vapor deposition on Si(001)

The surfactant effect of Ga pre-deposited prior to ultrahigh vacuum (UHV) chemical vapor deposition (CVD) of Ge on Si(001) is investigated using in situ transmission electron microscopy. Island shape, nucleation and growth kinetics, and the influence of growth temperature and pressure are studied. When 1 monolayer of Ga covers the surface, Ge island nucleation occurs after a 5--10× longer growth time than without Ga for the same growth temperature and pressure, and the island density then varies with time in a more complex way than for Ga-free growth. The islands formed have a different shape and aspect ratio compared to conventional hut and dome clusters, and the critical lateral size for dislocation nucleation is reduced. Ex situ medium energy ion scattering and x-ray photoelectron scattering measurements of the total amount of Ge deposited before island nucleation demonstrate that the Ga layer decreases the surface reactivity and inhibits Ge growth. We discuss a simple model in which the surfactant-mediated growth kinetics are principally driven by the balance between Ge deposition and Ga desorption. We show that Ga surfactant-mediated UHV CVD can be used to produce Ge quantum dots with a higher surface density and a thinner wetting layer than is possible using conventional CVD.