The ability to precisely control the thermal conductivity (kappa) of a material is fundamental in the development of on-chip heat management or energy conversion applications. Nanostructuring permits a marked reduction of kappa of single-crystalline materials, as recently demonstrated for silicon nanowires. However, silicon-based nanostructured materials with extremely low kappa are not limited to nanowires. By engineering a set of individual phonon-scattering nanodot barriers we have accurately tailored the thermal conductivity of a single-crystalline SiGe material in spatially defined regions as short as approximately 15 nm. Single-barrier thermal resistances between 2 and 4 x 10(-9) m(2) K W(-1) were attained, resulting in a room-temperature kappa down to about 0.9 W m(-1) K(-1), in multilayered structures with as little as five barriers. Such low thermal conductivity is compatible with a totally diffuse mismatch model for the barriers, and it is well below the amorphous limit. The results are in agreement with atomistic Green's function simulations.

We apply a selective etching procedure to probe the lateral composition profile of self-assembled SiGe pyramids on a Si(001) substrate surface. We find that the pyramids consist of highly Si intermixed corners, whereas the edges, the apex, and the center of the pyramids remain Ge rich. Our results cannot be explained by existing growth models that minimize strain energy. We use a model that includes surface interdiffusion during island growth, underlining the paramount importance of surface processes during the formation of self-assembled quantum dot heterostructures in many different material systems.

SiGe islands move laterally on a Si(001) substrate during in situ postgrowth annealing. This surprising behavior is revealed by an analysis of the substrate surface morphology after island removal using wet chemical etching. We explain the island motion by asymmetric surface-mediated alloying. Material leaves one side of the island by surface diffusion, and mixes with additional Si from the surrounding surface as it redeposits on the other side. Thus the island moves laterally while becoming larger and more dilute.

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...