Le microscopie a scansione di sonda

Le Nanoscienze e lo studio della materia sulla scala nano, tramite moderne tecniche di microscopia, dal punto di vista didattico formativo rappresentano uno strumento estremamente utile e potente per introdurre concetti della meccanica quantistica. Abbiamo illustrato i principi base delle microscopie a scansione di sonda più utilizzate nei laboratori di ricerca e dalle industrie: la microscopia ad effetto tunnel e quella a forza atomica, entrambe in grado di misurare le proprietà fisiche ed elettroniche della materia su scala atomica ed in tempo reale. Interessante il dibattito con i docenti

Ordering of Ge quantum dots on silicon surfaces via bottom-up and top-down approaches

The nanoscale ordering of inorganic semiconductor quantum dots (QDs) is crucial to obtain reliable structures for novel nanotechnological applications such as nanomemories, nanolasers and nanoelectronic devices. We have directly grown Ge QDs by physical vapour deposition (PVD) on Si(111), Si(100) and some of its vicinal surfaces and studied innovative bottom up techniques to order such nanostructures. Specifically, we harnessed naturally occurring instabilities due to reconstruction and intrinsic anisotropic diffusion in Si bare surfaces, such as step bunching and natural steps occurring in silicon vicinal surfaces, to order the QDs both in one dimension and in the plane. We have also shown the use of controlled quantities of surfactants, like Sb, dramatically improves the desired ordering. Moreover, we have assisted these self-assembling processes using top-down approaches like Focused Ion Beam (FIB) milling and STM nanoindentation to control the nucleation sites and the density of the Ge QDs. Real-time study of growth and self-assembly has been accomplished using Scanning Tunneling Microscopy imaging in UHV. An explanation of the occurring processes is given, and a software routine is used to quantify the ordering of the QDs both in pre-patterned and bare surfaces. Applications, mainly in the field of Nanocrystal Nonvolatile Memories, are discussed

A study of the pair distribution function of self-organized Ge quantum dots

We explore the use of the pair distribution function to study the self-organization process of Gequantum dots on both nanopatterned and nonpatterned oxidized Si(001) surfaces.Dots formation and ordering upon annealing of a Ge thin film are analyzed. The method we use is not limited to this case study. We show how it can be applied to determine short and long range self-ordering of nanostructures. We support our results by applying a software routine to simulate patterns of dots to finally spot the relevant physical aspects of Ge islands self-assembly

Modified strain and elastic energy behavior of Ge islands formed on high-miscut Si(0 0 1) substrates

Abstract We investigate here the influence of Si substrate miscut on the strain and elastic energy of Ge islands. We show how the morphology, composition and the elastic energy for 4 and 13 monolayers (ML) Ge islands grown at 600 °C and 730 °C on vicinal Si(0 0 1) surfaces change with miscut angles ranging between 0° and 10°. Scanning Tunneling Microscopy is used to determine the island morphology. Resonant x-ray diffraction near the Ge-K absorption edge allows the determination of the Ge concentration as well as the elastic energy stored on such structures from their dependency on the lattice parameter. Simulations using the Finite Elements Method indicate that the enlargement of the SiGe broad peak retrieved from the x-ray diffraction measurements is actually caused by the asymmetrical faceting induced by large miscut angles. Such faceting has a strong effect on island density and elastic energy, producing differences that are proportional to those observed in conditions with distinct SiGe content

Quantum dots: substrate nanopatterning as a path towards the applications

Nanotechnology aims at exploiting the remarkable size effects that arise when materials are reduced to nanoscale dimensions. Exploiting such effects will lead to new applications in different areas of human endeavour. The self assembly of three-dimensional islands is one of the most promising paths towards the fabrication of artificial atoms, or quantum dots (QDs) devoted to nanoelectronic and nanophotonic applications. In order to exploit the unique electronic properties of semiconductor quantum dots in novel quantum effect devices, lateral dimensions of these structures have to be reduced to the order of tens of nm’s, the range of De Broglie wavelength of electrons inside these materials. Moreover, millions of quantum dots should be orderly packed in dense arrays to achieve the necessary active volume. So far, the most promising quantum structures have been fabricated using techniques based on self assembling, but their ordering is possible only by appropriate substrate nanopatterning. In this paper we will explore different ways of patterning a substrate and how they affect the growth and ordering of the quantum dots

Influence of Patterning on the Nucleation of Ge Islands on Si and SiO2 Surfaces

Surface patterning is expected to influence the nucleation site of deposited nanostructures. In the present study, clean Si and SiO2 surfaces were patterned by a nanolithographic process using a Focused Ion Beam (FIB). Ge was evaporated in ultra high vacuum at 873 K on these substrates, resulting in the formation of island arrays. Based on scanning tunneling microscopy and atomic force microscopy images, a statistical analysis was performed in order to highlight the effect of patterning on the size distribution of islands compared to a non-patterned surface. We find that the self-organization mechanism on patterned substrates results in a very good arrangement and positioning of Ge nanostructures, depending on growth conditions and holes distance, both on Si and SiO2 surfaces