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

Periodic Reporting for period 2 - NARCISO (NAtuRal instability of semiConductors thIn SOlid films for sensing and photonic applications)

Reporting period: 2020-03-01 to 2023-01-31

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 devices 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 in isolated islands in a time-frame independent from the sample size. The potential of the solid state dewetting is to form monocrystalline and facetted structures (atomically smooth) providing unprecedented size tuning, free from defects and the typical roughness produced by conventional top-down etching methods. The spontaneous dewetting can produce correlated disordered patterns over arbitrary scales that cannot be designed numerically. Via sol-gel deposition and nano-imprint lithography we transfer the dewetted structures on nano- and micro-architectures made of metal oxides. Finally we develop theoretical, continuum method able to simulate, understand and eventually control the shape instabilities in solids and mass flow during the dewetting process.
1) We achieve the ultimate control of Si dewetting forming ordered ~mm-long nano-wires that were then used as field-effect transistors. The perfect crystalline state and the smoothness of the surface of these wires allowed to reach state-of-the art performances of the transistors in terms of trans-conductance and electron mobility. Central to NARCISO was the achievement of spinodal-like solid state dewetting of SiGe featuring a disordered hyperuniform (dHU) character. A size (50 to 3000 nm) and shape (isolated islands and connected structures) control of SiGe dewetting was shown together with its dHU features. We demonstrated the control of Si and Ge dewetting over 200 mm wafers that represents the actual record for this method. This allows to form nm to um sized structures over large scales with a simple method.
2) Phase field simulations have been systematically exploited to model, analyse, assess and predict the features of dewetted structures. Anisotropic-phase field allowed to precisely predict the final morphology of SiGe nanostructures including the effect of preferential crystallographic directions for adatoms surface diffusion (e.g. faceting, final shape). We are now capable to fine tune the initial shape of the patch and its orientation with respect to the crystallographic directions targeting a specific final state of dewetting. This modelling approach is also an effective tool to predict the disordered hyperuniform character of a final structure obtained via SiGe dewetting, helping in targeting specific features and performance of a device. Other achievements related to PF simulations include the modelling of fluid-dynamics in disordered hyperunifom structures addressing the case of swimming (motile and non-motile) bacteria. The simulations have been used to benchmark the relevant aspects of fluid dynamics in porous materials, helping the understanding of the role of dead-end pores in bacteria colonization of porous media.
3) The disordered hyperuniform morphologies were exploited to fabricate micro-fluidic channels to study the transport of colloids and colonization strategies of bacteria. This study allowed to explore fundamental phenomena related to particles filtering, trapping, bacterial motility, chemotaxis and colonization of porous media with a particular focus on the role of dead-end pores.
4) NARCISO has been intensively working on NIL of metal oxides (MOx-NIL) by developing new materials and demonstrating new features and applicability of original structures. As an example of application of MOx-NIL we reported the fabrication of anti-reflection coatings (ARCs) made of SiO2 atop glass and fused silica wafers.
5) In NARCISO we developed a new hyper-spectral imaging method that allows to extract information on the angular pattern of specific Mie resonances in a single measurement and avoiding the use of polarisers. About the disordered hyperuniform photonic devices, we mention the important result of engineered photonic crystals to demonstrate the formation of high quality random modes and strong light localization at near infrared frequency. The transition from localization to diffusive light transport within a single disordered hyperunifom system was demonstrated by exploiting near-field mode imaging.
-For spontaneous dewetting of SiGe, we increased the maximal processable surface up to 200 mm wafers dewetting films with initial thickness from 5 nm to ~1 um. This results in textured surface with structures with a footprint spanning over three orders of magnitude, from ~10 nm up to ~10 um. These structures have been exploited in several contexts such as anti-reflection coatings, structural colour, and flexible photonic devices (e.g. removing the SiGe islands from the substrate and embedding them in flexible and stretchable membranes). The most promising application of these structures over large scales is their use as intermediate structures for nano-imprint lithography (e.g. of metal oxides). The wafer bearing dewetted particles are used as hard-master to create a flexible mould that is then applied on a sol-gel containing the precursor of a metal oxide (e.g. SiO2 and TiO2) to reproduce the initial pattern. This idea is currently being exploited by SOLNIL and OBDUCAT for anti-reflection coating on glass, fused silica, silicon and sapphire.
-A last advance in solid state dewetting was the fabrication of Ge islands by using a conventional rapid thermal processor instead of an ultra-high vacuum chamber (in a molecular beam epitaxy reactor).
-The dewetted SiGe architectures with spinodal like features were exploited as a model material to reproduce a porous medium to investigate the role of transmitting channels, where a fluid can flow, and dead-end pores, where the fluid does not flow. Our finding is important to develop new strategies for water filtering and detection of bacteria in a faster and more efficient way.
- NARCISO provided the ground to create SOLNIL, a spin-off of AMU at work on sol-gel coatings and nano-imprint lithography. Beyond the results mentioned in this report, all the knowledge produced in NARCISO can be exploited by SOLNIL for micro-fluidic (e.g. for DNA sequencing) and photonic devices (e.g. for anti-reflection coatings, photonic metasurfaces, meta-lenses). They are both produced by SOLNIL that has already attracted several customers that are paying to develop specific materials and devices deigns. Beyond these customers, SOLNIL is currently undergoing a fund-rising (pre-seed) that should be finalised by April 2023.
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