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NAtuRal instability of semiConductors thIn SOlid films for sensing and photonic applications

Periodic Reporting for period 1 - NARCISO (NAtuRal instability of semiConductors thIn SOlid films for sensing and photonic applications)

Reporting period: 2019-03-01 to 2020-02-29

NARCISO is an interdisciplinary research project bringing together chemists and physicists aiming at developing novel chemical sensors, water filters and devices for light and electricity management. Based on non-conventional fabrication methods and device designs, the nano- and micro-metric architectures developed within NARCISO are implemented over ultra-large scales, with cost-effective technologies and are compatible with industrial processes.
At the core of NARCISO is the so-called solid-state dewetting, a natural shape instability occurring in thin solid films when heated at high temperature: it transforms a flat layer (e.g. of a polymer, a metal or a semiconductor) in isolated islands in a time-frame independent from the sample size. The potential of the solid state dewetting for applications based on silicon complex nano- and micro-architectures is still unexplored, despite the manifold advantages it offers:
a) it forms monocrystalline and facetted structures (atomically smooth) providing unprecedented size tuning, free from defects and the typical roughness produced by conventional top-down etching methods;
b) the islands are directly formed on an electrically and optically insulating substrate (SiO2);
c) templated dewetting (dewetting a thin silicon layer previously patterned) from simple patterns shape leads to more complex, monocrystalline and ordered architectures, with the additional advantage of reduced etching time with respect to conventional lithographic approaches;
d) spontaneous dewetting can produce complex patterns over arbitrary scales that cannot be designed numerically.
The second key-tool of NARCISO is the faithful replication via sol-gel deposition and nano-imprint lithography of the nano- and micro-architectures formed via solid-state dewetting in metal oxides (e.g. SiO2, TiO2). This technique offers a very broad control over tuning all the properties of the complex nano- and micro-architectures, such as composition, composition profile, porosity, wettability, integration of surfactants, colorant and light emitters as well as printing on a plethora of different substrates supporting the printed structures.
The third key-tool of NARCISO is the development of theoretical, continuum methods able to simulate, understand and eventually control the shape instabilities and mass flow during the dewetting process. This mainly builds on a so-called phase field model which is developed within the project, allowing for several qualitative predictions assessing the main physical contribution at play during the process such as role of surface-energy anisotropies, elasticity effects, interplay between initial geometries and local effects affecting mass transport.
The relevance of NARCISO activity for society falls into this framework: addressing crucial applications and providing a reliable answer to the need for implementing efficient devices over large scales with cost-effective and scalable methods. Thus, the main points addressed are how to extend solid-state dewetting to 30 cm wafers, replicate the dewetted structures via soft-NIL over the same size while controlling the size, morphology, density, composition, composition profile, porosity of the structures and targeting specific applications (nanostructured thin film coatings for light management, water filters, chemical, and bio-detectors).
One of the central aspects of NARCISO’s proposal was the implementation of spinodal structures featuring a disordered hyperuniform character. This emerging class of disordered materials was just a theoretical chimera since a few years ago. Recent efforts of several groups demonstrated the presence of these disordered systems in nature and eventually provided some examples of artificial synthesis of disordered hyperuniform structures via top-down methods. The importance of this class of material relies on exotic phenomena emerging from the hidden correlation characterizing them (e.g. topologically protected electronic states, glassy electronic quantum state transitions, Anderson localization of light, polarization selectivity, lasing and a full photonic band-gap for light propagation). We successfully demonstrated that solid-state dewetting is indeed an efficient way to produce disordered hyperuniform structures made of SiGe alloys.
These hyperuniform structures have been implemented within NARCISO over 2 cm x 2 cm wafers bearing structures having a size ranging from ~100 nm to 10000 nm.The corresponding structures have been used as a hard master to be replicated via soft-NIL in SiO2 and TiO2.
Besides the work on spinodal solid-state dewetting and, in turn, the realization and characterization of hyperuniform structures, specific work has been carried out to the optimization and engineering of conventional solid-state dewetting driven by initial patterning. In particular, we focused on the fabrication of monocrystalline, silicon-based nanowires and complex connected circuits exploiting low-resolution etching of thin silicon film and annealing.
NARCISO's network has also participated at several international conferences, by invitation or as oral presentations, in order to promote the results obtained during the first year of the project.
Our studies related to dewetting instability of semiconductor thin films have been also presented for 6 months at the exhibition “Broken Nature” (http://www.brokennature.org/).
Finally, dissemination towards the general public has been performed by participating to Meet Me Tonight, an event organized by the EU to put in contact EU citizens of all ages with the world of research.
The most important point of this first year is the clear-cut demonstration of the emerging hyperuniform character of the dewetted structures. This result provides the base for the implementation of photonic, electronic and micro-fluidic devices over large scales. They will be developed within NARCISO either directly in SiGe on SiO2 via dewetting and via sol-gel and nano-imprint lithography in metal oxides and polymers using the dewetted structures as masters.
The correlated disorder present in our hyperuniform devices is expected to provide enhanced functionalities in light, electricity and matter transport. Thus, in the second year of the project, we will assess the performances of the fabricated structures as anti-reflection coatings on silicon, with embedded light emitters (e.g. for random lasers), and for microfluidics (e.g for water filtration) to name a few possible applications. The potential impact is to provide a reliable and scalable method to produce performing devices over large scales. Very importantly, the fabrication of SiGe based structures directly provides the hard master for nano-imprint lithography. Thus, an important possible impact, is the low-cost fabrication of hard masters over 30 cm.
Finally, within the AMU node, a start-up has been created: METACERAM. This is actually in the incubator of AMU and will officially funded in April 2020 and includes two members of the NOVA team within the AMU node. The main goal of METACERAM is to produce photonic devices via sol-gel and nano-imprint lithography of metal oxides (examples are diffractive optics, anti-reflection coatings, structural color, etc. made of TiO2 and SiO2).
Meet Me Tonight event Milano Sept2019
Microscopical characterizations of the dewetted wires used as MOSFET
Microscopy image of the dewetting instability on flat Si film
"Panel exhibited at the Broken Nature exhibition ""From sand to nanotecnology"""
Microscopical image of the spinodal dewetted structures