Real-time observations were made of the shape change from pyramids to domes during the growth of germanium-silicon islands on silicon (001). Small islands are pyramidal in shape, whereas larger islands are dome-shaped. During growth, the transition from pyramids to domes occurs through a series of asymmetric transition states with increasing numbers of highly inclined facets. Postgrowth annealing of pyramids results in a similar shape change process. The transition shapes are temperature dependent and transform reversibly to the final dome shape during cooling. These results are consistent with an anomalous coarsening model for island growth.

No abstract

We demonstrate that the nucleation sites of nanoscale, self-assembled Ge islands on Si(001) can be controlled by patterning the Si surface in situ with a focused ion beam. At low doses of 6000 Ga+ ions per <100 nm spot, the selective growth is achieved without modifying the initial surface topography. At larger doses, topographic effects produced by sputtering and redeposition control the selective nucleation sites. Islands grown on irradiated spots are smaller with higher aspect ratio than islands grown on clean Si(001), suggesting a strong surfactant effect of Ga.

We investigate the fundamental mechanism by which self-assembled Ge islands can be nucleated at specific sites on Si(001) using ultra-low-dose focused ion beam (FIB) pre-patterning. Island nucleation is controlled by a nanotopography that forms after the implantation of Ga ions during subsequent thermal annealing of the substrate. This nanotopography evolves during the annealing stage, changing from a nanoscale annular depression associated with each focused ion beam spot to a nanoscale pit, and eventually disappearing (planarizing). The correspondence of Ge quantum dot nucleation sites to the focused ion beam features requires a growth surface upon which the nanotopography is preserved. A further key observation is that the Ge wetting layer thickness is reduced in patterned regions, allowing the formation of islands on the templated regions without nucleation elsewhere. These results provide routes to the greatly enhanced design and control of quantum dot distributions and dimensions.

No abstract

Using low-energy electron microscopy we have found a new phase transition on the Si(001) surface at miscut angles smaller than -0.1°. The surface phase separates into facets with -300 A terrace width, and regions with much larger, wavy terraces. This wavy phase is stabilized by a reduction of surfacestress-induced strain energy. A theoretical study by TersofT and Pehlke compares favorably with our observations.PACS numbers: 6l.14. Hg, 6l.I6.Di, 68.35.Bs, 68.35.Rh The step structure of Si (001) surfaces intentionally miscut towards (ill) has been the subject of many recent studies [1][2][3][4][5][6][7]. This lively interest was sparked by experimental observations of a double-step predominance at larger miscut angles, with a phase transition to singleheight steps at smaller miscut angles. Much less work has been done on samples cut very close to the (001) direction. Diffraction techniques and scanning tunneling microscopy (STM), used with much success on the vicinal surfaces, are not very suitable for small miscuts, due to limited coherence length in the case of diffraction, and due to the limited field of view in STM. In this study we have used low-energy electron microscopy (LEEM) to show that in addition to the double-single-height phase transition at large miscut angles, the surface undergoes another phase transition at very small miscut angles, from straight single-height steps to a coexistence of uniquely spaced straight steps and large-terrace-width wavy steps.Step waviness allows for alternation of (1x2) and (2x1) domains not only normal to, but also parallel with, the global step edges, resulting in a reduction of surface-stress-induced strain energy. These observations are in good agreement with a theoretical study by Tersoff and Pehlke [8].The Si(001) surface exhibits a (2x1) dimer reconstruction. At a single-height atomic step the dimer orientation rotates 90° from (2x1) to (1x2) or vice versa. Experiments [9] and calculations [10] have shown that the surface stress tensor is anisotropic; tensile along the dimer direction, and compressive normal to the dimer. At small miscut angles an alternation of (2x1) and (1x2) domains separated by single-height atomic steps is favorable as it reduces the net stress in the surface. Because of the strong repulsion of S A steps (dimers on the upper terrace normal to the step edge) and SB steps (dimers parallel with the step edge) at close step separation [11], double-height steps become predominant at large miscut angles, although the energy gain is reduced by buildup of a net surface stress. The temperature-miscut phase diagram has been calculated theoretically [6,7], and appears to be in general agreement with experimental results. One of the unresolved issues, however, is the prediction that surface stress will be reduced at very small miscut angles by spontaneous generation of steps, in excess of those imposed by the miscut [5]. Such an effect has not been observed experimentally, and based on our observations we argue that this prediction results from the assu...

No abstract

We describe a new two-dimensional detector for the detection of ions scattered from a solid target, analyzed in energy and scattering angle by a toroidal electrostatic analyzer. The detector resolves the scattering angle with a resolution of 0.4° over a range of 25°, and the ion energy with a resolution of 120 eV over a range of 2000 eV, at 100 keV ion energy. The energy resolution of the spectrometer was improved with a factor 4 relative to its previous performance with a one-dimensional scattering angle detector, while−at the same time−the dose efficiency (count/μC) was improved by a factor 5–10.