Atomic-Scale Pathway of the Pyramid-to-Dome Transition during Ge Growth on Si(001)

Abstract

By high resolution scanning tunneling microscopy, we investigate the morphological transition from pyramid to dome islands during the growth of Ge on Si(001). We show that pyramids grow from top to bottom and that, from a critical size on, incomplete facets are formed. We demonstrate that the bunching of the steps delimiting these facets evolves into the steeper dome facets. Based on first principles and Tersoff-potential calculations, we develop a microscopic model for the onset of the morphological transition, able to reproduce closely the experimentally observed behavior. DOI: 10.1103/PhysRevLett.93.216102 PACS numbers: 68.55.Ac, 68.35.Md, 68.37.Ef, 81.10.Aj Three-dimensional Ge islands coherently grown on Si(001) at high temperature are well known to show a bimodal behavior, with small, shallow f105g-faceted pyramids and larger domes, exhibiting steeper facets [1,2]. In the past few years some interpretations of the bimodal behavior have been provided, mostly relying on thermodynamic arguments. Based on volumetric strain relief, surface energies, and edge contributions [3], one explanation is that pyramids and domes correspond to two minima in the energy per atom, with an activated transition from one to the other morphology [1]. The second interpretation is grounded on the chemical potential of the island, which is argued to undergo an abrupt change at a certain critical volume, corresponding to the crossover between the energy per atom for a dome and the corresponding one for a pyramid [4][5][6]. Therefore, no energy minima are required to explain the bimodal behavior in this picture.When Ge is grown on Si(001) at relatively low temperature, only elongated {105}-faceted islands with narrow size distribution are observed. An interpretation of this phenomenon comes from kinetic models [7,8], where a self-limiting growth is explained in terms of kinetic slowing down occurring with increasing volume. This, in turn, should be provided by a size-dependent activation energy for adding a new monolayer to the f105g facets. In particular, Jesson et al. [7] suppose the additional layer to nucleate at a lower corner of the facet, where the strain is larger. Similarly, Kästner and Voigtländer [8] assume that the layer grows from bottom to top, founding their kinetic model on the stepped nature of the f105g facets. However, both the thermodynamic and kinetic models do not explain how the shape transition is microscopically accomplished. Recently, a description of how the shape transition occurs has been provided by Seifert and co-workers [9,10], suggesting a variation of the Jesson et al. model [7]. Here, at a critical pyramid size, new, steeper facets are supposed to nucleate close to the pyramid apex, where the lattice parameter is more relaxed. Yet, this hypothesis was based on qualitative arguments, with no modeling.In this Letter, we use high resolution scanning tunneling microscopy (STM), to investigate Ge islands grown on Si(001) with two different techniques. We show that the growth proceeds from top to bo...

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