Theory, Simulation and Modelling of SiGe Nanostructures: from Nanoelectronics to Renewable Energy
CECAM-HQ-EPFL, Lausanne, Switzerland
The idea of combining the chemical and physical properties of Silicon (Si) and Germanium (Ge) represents a well-established strategy to engineer innovative materials and devices. SiGe systems have been experiencing tremendous development from the mid-1980’s, as high quality SiGe thin layers and superlattices became the subjects of an intense research and development.
On the other hand, in the last decade Nanostructures (NSs) started to be considered the key development for the next generation technology, due their ease of processing, unique properties and compatibility with the existent Si microelectronics. In this context nanowires (NWs) and nanocrystals (NCs) are the strongest candidate to provide the change of paradigm needed by the new generation of electron devices, either by replacing or, more realistically, integrating the existing CMOS technology. While Si, Ge and III-V NWs have been widely studied, only recently SiGe nanosystems are attracting an increasing interest for their multiple potentiality in many different fields, in particular in the field of energy, materials and information and communication technologies (ICT).
One-dimensional and zero-dimensional SiGe nanostructures (SiGe NSs) have acquired today a prominent role in several cutting-edge research topics in nanoscience, thanks to the latest relevant advances in synthesis, processing and characterization.
The key objective of this workshop is to promote the progress in the fundamental understanding (in the broad sense of theory combined with experiments) of the structural, electronic, optical and transport properties of Silicon-Germanium nanosystems. The aim is to offer the possibility to discuss hard questions and challenges, that could lead to future advanced technological applications. This will be done by facing the main role of theory and modeling in the characterization of these nano-materials, always with a look into experiments.
Since in these materials the size is, very often, at or below the characteristic length scale of some fundamental solid-state phenomena, their investigation requires clear and deep quantitative understanding of condensed matter at nanoscale. Indeed a significant uncertainty prevails in discerning the fundamental effect of size-dependent factors, such as quantum confinement, from other factors (shape, composition, local strain, interface states…) that affect the main physical properties.
In this context the role of theory, modeling and simulations in the description of SiGe NSs properties is clearly indispensable to not miss important challenges. This justifies the present workshop proposal. Indeed our purpose is to organize a workshop with primarily theoretical speakers and audience, while participation of some experimentalists will be strongly encouraged to guide the development of theory and simulations along the lines actually amenable to measurements.
What is particularly fascinating in these nanosystems is that by bringing together two similar elements –Si and Ge, neighbors in the periodic table–, a rich variety of new chemical and physical properties emerge, stimulating both fundamental and application-driven research in nanoscience. These materials present unique structural, electronic, optical and transport properties, which are intrinsically associated with their low dimensionality and with the quantum confinement effect.
Their physical properties are strictly related not only to the size of the system (like the corresponding pure Si and Ge NSs), but also to the relative composition of Si and Ge atoms, and to the geometry of Si/Ge interface. Substituting some of the atoms of a pure Si NS with Ge in random as well as ordered configurations of different compositions, strongly affects some fundamental properties such as band gap, effective mass, phonon and electron scattering processes and excitonic properties. As a consequence, SiGe nanosystems are the target of the most intriguing and exciting technological applications in the field of high performance nanoelectronics (such as FETs and interconnections), thermoelectrics, photovoltaics, biomedicine, superconductivity and spintronics. This points out the main motivation and novelty of this proposal: because of the rapid advance of experimental and device investigation in this area, the need of a precise atomic understanding of these materials is becoming more urgent, and its absence is increasingly a barrier to progress in the field quite generally. Indeed the major goal, today, is to produce SiGe NSs with tunable shape, composition, strain and doping. The investigation of such systems with experimental techniques is very often complicated by several factors not always well controlled (such as impurities, surface reconstructions, dislocations, etc.), which can hide the right comprehension of these physical quantities without the support of atomistic theoretical modeling.
Beyond to bring together world leading scientists and to assess which is the present state-of-the art in this specific research field, the vision of this workshop is to give a strong contribution into the development of robust computational tools for the quantitative understanding of growth, structure, electronic and dynamical processes in SiGe nanosystems in view of their applications in new generation technology. We do believe that an international workshop on this topic is timely and would attract a considerable interest.
We aim at identifying and covering both the achieved milestones and outlining research efforts in theory, modeling and simulation of these nanostructures. To reach this ambitious objective, depicting the state-of-the-art of all the new experimental techniques in SiGe NSs growth and characterization is a paramount and cannot be neglected. Furthermore in view of their enormous technological potential in several strategic fields - e.g. energy storage conversion, nanoelectronics, spintronics - the deep understanding of the basic physics governing these systems cannot be achieved without knowledge of their most advanced device applications.
A large number of fundamental issues in the description of these systems are far to be clarified and need a deep investigation. In particular in view of the need to fill different lacks of fundamental knowledge, three main hot fundamental topics will be considered:
i) Growth and characterization, to devise simulations and modeling of critical processes during the synthesis of SiGe NSs in order to reach a precisely controlled and tunable composition, structure, geometry.
ii) Electronic structure and Optical excitations, to simulate and describe with many-body accuracy fundamental topics, such as band offset, impurity activation energy and optical response.
iii) Transport Processes, to determine the basic physics governing the transport processes in these materials, including electronic, spin and thermal transport.
For each entry in the list it will be important to highlight which is the state-of-the-art and to clarify which are the research efforts done up to now and in which direction they should go in the future. As already mentioned before, the discussion will be mainly dedicated to theory and simulations, but at the same time we really feel that the participation of experimentalists is indispensable in this workshop to address the fundamental intellectual challenges in this field of research.
Indeed the study on SiGe NSs is one of the most rapidly developing area in materials science and in nanotechnology, but his future will heavily depend on how deeply the above mentioned issues will be investigated and exploited within fruitful exchanges of ideas and problematics among theoreticians and experimentalists. An international workshop on this research field would be extremely useful for the future development of nanoscience and nanotechnology. One of the main objectives of this workshop is to create a high-level interdisciplinary space for the critical consideration of these issues. A broad selection of world leading theoretical and experimental scientists will contribute to the workshop. In this way we will seek to examine the current limits and perspectives of SiGe NSs as categories of analysis and we will aim at addressing larger general discussion about the real applications of these nanomaterials in the current technology and also which is the relative role of theory and simulation. The workshop will serve as a space for participants to develop and share ideas, drawing upon the personal and academic experiences. Discussion of the state of the field and sharing knowledge and experience on an informal atmosphere will be crucial requirements to establish future directions for SiGe NSs research field and more in general to develop good science.
Michele Amato (Université Paris Sud) - Organiser
Stefano Ossicini (University of Modena and Reggio Emilia, Modena) - Organiser
MAURIZIA PALUMMO (university of rome Tor Vergata) - Organiser
Riccardo Rurali (ICMAB-CSIC) - Organiser