Multiscale modeling of Ge islands on Si(001): growth mode and morphological transition

Deposition of Ge on Si(001) induces the formation of nanometric Ge pyramids bounded by {105} facets. Pyramids grow self-similarly until, at a critical size, they transform into dome-shaped islands. A very detailed, microscopic explanation of the evolution is given, based on high-resolution STM images and extensive theoretical modeling [1]. It is shown by ab initio calculations that Ge atoms are able to easily climb the {105} facets, reaching the island top, where nucleation is favored. The new pyramid layer is then completed by a fast step-flow process. When pyramids reach a critical size, however, steps start to group at the top, bunching together and eventually evolving into steep new facets, typical of the dome geometry. The appearance of multistepped pyramids, clearly imaged by STM, is explained by a simple, local-thermodynamic model based on the above described results and on further Tersoff-potential simulations of realistically-sized islands. [1] F. Montalenti et al., Phys. Rev. Lett. 93, 216102 (2004)