Search

Machine learning is changing how we design and interpret experiments in materials science. In this work we show how unsupervised learning, combined with ab initio random structure searching, improves our understanding of structural metastability in multicomponent alloys. We focus on the case of Al-O-N alloys where the formation of aluminum vacancies in wurtzite AlN upon the incorporation of substitutional oxygen can be seen as a general mechanism of solids where crystal symmetry is reduced to stabilize defects. The ideal AlN wurtzite crystal structure occupation cannot be matched due to the presence of an aliovalent hetero-element into the structure. The traditional interpretation of the c-lattice shrinkage in sputter-deposited Al-O-N films from X-ray diffraction (XRD) experiments suggests the existence of a solubility limit at 8 at.% oxygen content. Here we show that such naive interpretation is misleading. We support XRD data with accurate ab initio modeling and dimensionality reduction on advanced structural descriptors to map structure-property relationships. No signs of a possible solubility limit are found. Instead, the presence of a wide range of non-equilibrium oxygen-rich defective structures emerging at increasing oxygen contents suggests that the formation of grain boundaries is the most plausible mechanism responsible for the lattice shrinkage measured in Al-O-N sputtered films. We further confirm our hypothesis using positron annihilation lifetime spectroscopy.

We show that a relatively simple top-down fabrication can be used to locally deform germanium in order to achieve uniaxial tensile strain of up to 4%. Such high strain values are theoretically predicted to transform germanium from an indirect to a direct gap semiconductor. These values of strain were obtained by control of the perimetral forces exerted by epitaxial SiGe nanostructures acting as stressors. These highly strained regions can be used to control the band structure of silicon-integrated germanium epilayers.

Germanium is known to become a direct band gap material when subject to a biaxial tensile strain of 2% (Vogl et al 1993 Phys. Scr. T49B 476) or uniaxial tensile strain of 4% (Aldaghri et al 2012 J. Appl. Phys. 111 053106). This makes it appealing for the integration of optoelectronics into current CMOS technology. It is known that the induced strain is highly dependent on the geometry and composition of the whole system (stressors and substrate), leaving a large number of variables to the experimenters willing to realize this transition and just a trial-and-error procedure. The study in this paper aims at reducing this freedom. We adopt a finite element approach to systematically study the elastic strain induced by different configurations of lithographically-created SiGe nanostructures on a Ge substrate, by focusing on their composition and geometries. We numerically investigate the role played by the Ge substrate by comparing the strain induced on a bulk or on a suspended membrane. These results and their interpretation can provide the community starting guidelines to choose the appropriate subset of parameters to achieve the desired strain. A case of a very large optically active area of a Ge membrane is reported. © 2014 IOP Publishing Ltd.

We report the observation of large scale self-assembly of long horizontal nanowires into orthogonally oriented bundles, during in situ annealing of a few monolayers of Ge on Si(001). Results are interpreted in terms of a collective wave-propagation mechanism, previously suggested for interpreting ripple faceting on Ge/Si(1110) surfaces. Quantitative agreement between experiments and theory is found. The onset of the mechanism, the number of wires in the bundles, and their total density can be controlled by carefully tuning the growth parameters.

Ge/Si(001) is a prototypical system for investigating three-dimensional island self-assembly owed to the Stranski-Krastanow growth mode. More than twenty years of research have produced an impressive amount of results, together with various theoretical interpretations. It is commonly believed that lattice-mismatch strain relief is the major driving force leading to the formation of these islands. However, a set of recent results on Si(001) and vicinals point out that, under suitable conditions, this is not the case. Indeed, we here review experimental and theoretical results dealing with nanostructures mainly determined by surface-energy minimization. Results are intriguing, as they reveal the existence of magic sizes, show the presence of very peculiar morphologies, such as micron-long wires, and distinguish among attempts to facet the wetting-layer and true SK islands.

Deposition of Ge upon Si substrates is the prototype of the mechanism called Stranski- Krastanov growth, i.e. the self-assembled formation of 3D islands, following the formation of a thin, 2D Wetting Layer. As is shown in Chapter 1, the nucleation of these islands is random and non-uniform when the deposition is performed upon the standard Si(001) substrate. Deposition of SiGe on different substrates of Si can lead, however, to a high degree of uniformity. Some examples are described in Chapters 3 and 4. The aim of this thesis is to supply a quantitative analysis for some peculiar phenomena concerning island nucleation that occurs on these non-standard substrates. This is performed through the evaluation of the internal energy of the island (including also effects on the substrate) as discussed at length in Chapter 2, where the elastic, surface and edge energy contributions are described, focusing particular attention to the second one, that turns out to be quite complicated. Particular attention is devoted to the island nucleation on stepped substrates of Si (Chapter 4). An in-depth analysis of the formation of faceting upon the substrate Si(1 1 10) is carried out in Chapter 5, whereas Chapter 6 deals with the transition from the faceting to three-dimensional islands on this peculiar system. Conclusions are drawn in Chapter 7. Appendices A and B contain some additional information regarding the elastic field and the description of surfaces. In Appendix C an analysis of the strained surface energy is carried out. In Appendix D some preliminary work performed during the PhD period on other impor- tant surfaces for the SiGe system is discussed. Finally at page 203 a Curriculum Vitae is reported.

SiGe heteroepitaxy on vicinal Si (1 1 10) is studied as a model system for one-dimensional (1D) to three-dimensional growth mode transitions. By in situ scanning tunneling microscopy it is shown that the 1D-3D transition proceeds smoothly from perfectly facetted 1D nanoripples to coarsened superripples, tadpoles, asymmetric domes, and barns without involving coalescence or agglomeration. By extension of the studies to a wide range of SiGe compositions, a 1D-3D growth phase diagram is obtained. Total energy calculations reveal that the observed critical transition volumes are fully consistent with thermodynamic driven strain relaxation.

Self-assembled Ge wires with a height of only 3 unit cells and a length of up to 2 micrometers were grown on Si(001) by means of a catalyst-free method based on molecular beam epitaxy. The wires grow horizontally along either the [100] or the [010] direction. On atomically flat surfaces, they exhibit a highly uniform, triangular cross section. A simple thermodynamic model accounts for the existence of a preferential base width for longitudinal expansion, in quantitative agreement with the experimental findings. Despite the absence of intentional doping, the first transistor-type devices made from single wires show low-resistive electrical contacts and single-hole transport at sub-Kelvin temperatures. 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.

Ge growth on high-indexed Si (1110) is shown to result in the spontaneous formation of a perfectly {105} faceted one-dimensional nanoripple structure. This evolution differs from the usual Stranski-Krastanow growth mode because from initial ripple seeds a faceted Ge layer is formed that extends down to the heterointerface. Ab initio calculations reveal that ripple formation is mainly driven by lowering of surface energy rather than by elastic strain relief and the onset is governed by the edge energy of the ripple facets. Wavelike ripple replication is identified as an effective kinetic pathway for the transformation process.

The shape of coherent SiGe islands epitaxially grown on pit-patterned Si(001) substrates displays very uniform collective oscillations with increasing Ge deposition, transforming cyclically between shallower "dome'' and steeper "barn'' morphologies. Correspondingly, the average Ge content in the alloyed islands also displays an oscillatory behavior, superimposed on a progressive Si enrichment with increasing size. We show that such a growth mode, remarkably different from the flat-substrate case, allows the islands to keep growing in size while avoiding plastic relaxation