Self - Elongating Nanowires
We report the self-assembled growth of Ge nanowires with a height of only 3 unit cells and a length of up to 2 micrometers by means of molecular beam epitaxy. Compared to nanowires grown by catalytic methods, the catalyst-free Ge nanowires we obtained exhibit an outstanding uniformity in their lateral size, they lie horizontally along well-defined crystallographic directions, and they are monolithically integrated into the silicon substrate. In view of their exceptionally small and self-defined cross section, these Ge wires hold promise for the realization of hole systems with exotic properties and provide a new development route for silicon-based nanoelectronics.
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How nanostructures healthily cope with stress...
Elastic stress is the main driving force for the “birth and fate” of self-assembled quantum dots (QDs). Some of them rapidly grow in size and need to find ways to release the increasing amount of stress. Eventually, beyond a certain critical size, the QD usually relaxes by crystal defect introduction, which is analogous to its “death”. By guiding the formation of QDs on periodically patterned substrates, the QDs are able to cope with the increasing stress by cyclically incorporating large amounts of material from the substrate and corresponding changes of their shape. This allows them to keep growing in size while delaying relaxation mediated by defects.
Lateral Si quantum dot molecules
Site-controlled SiGe islands which have both large sizes and high Ge fraction are successfully obtained. Finite element method calculations of the strain distribution with the realistic structure reveal that (i) the Si spacer between a pair of islands can act as a lateral quantum dot molecule made of four nearby dots for electrons and (ii) the tensile strain in a Si cap deposited on top of the stack is significantly enhanced with respect to a single layer. This work was carried out in collaboration with the Institute of Semiconductor and Solid State Physics, University Linz.
Advanced quantum dot configurations
We have presented an overview on approaches currently employed to fabricate advanced quantum dot configurations by epitaxial growth. Absolute position control of self-assembled quantum dots, so-called 'seeded' quantum dot crystals, is achieved by the combination of bottom–up and top–down methods. A promising way to realize quantum dot crystals with controlled spatial and optical properties is described.
Quantum dot crystal defects
A quantum dot crystal is a periodic arrangement of quantum dots in one, two, or three dimensions, and is often referred to as an artificial crystal. In analogy to a conventional crystal we have introduced the terminology and formation of vacancy and interstitial defects in artificial crystals. The images show experimental and simulation results of a perfect artificial crystal on the left side and an artificial crystal with vacancies (marked by yellow circles) on the right hand side.
Nanogrooves attract quantum dots
Accurate positioning of nanoobjects on a substrate is a long-standing problem. Here we demonstrate that even gentle grooves on a Si substrate "attract" self-assembled SiGe quantum dots (or islands) and provide a way to control their position on the substrate. Energy minimization is the driving force responsible for the attraction between grooves and dots.
Absolute positioning of quantum dots for large scale integration
One of the major difficulties in the fabrication of single quantum dot devices is the placement of the single dot at a pre-determined position in the device structure. However, we can control the nucleation site of a quantum dot by a small pit patterned in the substrate. Since we use conventional electron beam lithography to define these pits, this method can be easily incorporated with the definition of larger scale alignment markers. These can then be used to locate the site-controlled dot array post-growth for further device processing. We show here luminescence from such an array of widely spaced site-controlled dots surrounded by micron-wide alignment markers.
Three-dimensional (3D) composition profiles of quantum dots
Self-assembled quantum dots with excellent optical and electronic properties are readily fabricated by strained epitaxial growth. The dots generally consist of an inhomogeneous alloy of deposited and substrate materials. The composition profiles determine the electronic structure of the dots and have been probed by several techniques. However, none of them was able to reconstruct the full 3D profiles of single dots. We have now reached this goal by using a simple “nanotomography” approach based on scanning probe microscopy and selective wet chemical etching. By imaging the same sample area after several etching steps and by employing proper software algorithms, we have unveiled the Ge concentration in SiGe/Si(001) dots. The results can be used to model the properties of electronic devices based on such nanostructures.
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Photonics Spectra 42 (2008) URL
Lateral alignment of epitaxial quantum dots
Quantum dots constitute a natural template to construct refined artificial matter, such as artificial atoms, molecules and possibly artificial crystals with entirely new electronic and optical properties. However, the full advantage of their unique properties can be exploited, only, if a controlled positioning of the quantum dots inside a more complex device structure can be achieved. The prime task of this book is to review recent techniques, which allow the controlled positioning and lateral alignment of quantum dots on standard substrate surfaces.
Oliver G. Schmidt (Ed.), Lateral alignment of epitaxial quantum dots (Springer, Berlin, 2007) URL
From dots to rods
Periodic metal patterns on a silicon surface provide unprecedented control over the morphology of highly ordered Ge islands. Island shapes such as nanorods and truncated pyramids are set by the metal species and substrate orientation while investigations of island composition reveal the importance of Si-Ge intermixing in island evolution. Our results might be exploited to tailor the functionality of island arrays in heteroepitaxial systems.