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Équipe MATISEN: Matériaux pour les technologies de l’information, les capteurs et la conversion d’énergie.

ANR EXOSIL

De Équipe MATISEN: Matériaux pour les technologies de l’information, les capteurs et la conversion d’énergie.
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Welcome to the page of the grant Exosil2.jpg "Exotic silicon : silicon clathrate films" Anr3.png

This project aims at developing a novel material for energy in the form of films, silicon clathrates that form cages with various size.

Representation of type II silicon clathrates by R. Vollondat et al.

Several forms of silicon are used in industry, mainly crystalline, multicrystalline and amorphous silicon. Here we propose to investigate a novel form of silicon films, namely silicon clathrates. They are similar to carbon fullerenes as they form hollow spheres. The electronic and optical properties of these clathrates are strongly different to the “standard” silicon (direct bandgap) and can pave the way for novel applications in electronics and optoelectronics, but also potentially in batteries and hydrogen storage. A large part of the project will be to modulate the properties of Si clathrate films by varying the fabrication processes. The fabricated Si clathrates films will need to be analyzed in terms of structural, optical, electrical, surface properties by a wide range of techniques. In particular, not only the size of the clathrates but also the presence of doping atoms can dramatically modify their properties.

Objectives and scope

Bulk silicon clathrates were discovered in 1965 (Na8Si46 and NaxSi136) and are, as in fullerenes, forming hollow spheres of various sizes. However, silicon clathrates films (SCF) are only just starting to be discovered but with technological, industrial and scientific bottlenecks that this project aims to solve. Clathrates are crystalline compounds based on a three-dimensional polyhedron of a species, here Si, enclosing a second species that is often Na. Most Na can be removed with appropriate annealing or chemical treatment. The properties of these clathrates depend strongly on their size and the impurity atoms inside the cage. The electronic and optical properties of these clathrates are strongly different to the “standard” silicon and can pave the way for novel applications. In particular, direct bandgaps can be obtained in some silicon clathrates and thus they are even more appropriate than “standard” silicon for direct solar conversion or emission. Therefore there are numerous applications for electronics and optoelectronics. Two of them are LEDs and photovoltaics. However, although some modelling has been reported, experimental work for producing working devices based on silicon clathrates has not been demonstrated yet. We aim to tackle these challenges in the EXOSIL project which would start from material synthesis to end with device fabrication.


Methods

The work is divided into five workpackages.
WP1 is dedicated to the synthesis of SCF on various starting material such as mono, poly-crystalline and amorphous silicon. The substrates used can be silicon, glass, or sapphire. The structure properties, phase purity will be studied, e.g. by x-ray diffraction (XRD), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), and transmission electron microscopy (TEM) available within the consortium.
WP2 The experimental parameters such as the sintering conditions will need to be tuned in order to obtain the right silicon phases, which will be assessed in WP2 together with the optical, electrical and surface characterizations. The optical properties will be studied by UV-visible spectrophotometry, spectroscopic ellipsometry, photoluminescence (PL) and PL lifetime.
WP3 will deal with tuning the electrical properties of SCF. This can be done either using doped starting material (such as phosphorus and boron doped silicon) or by doping SCF after fabrication by ion implantation or spin coating or dopant diffusion from a gas phase which are available within the consortium. The electrical and optoelectronic properties will be determined by ECV (electrochemical capacitance voltage), Hall effect, resistivity vs. temperature, surface photovoltage, quasi-steady-state photoconductance (QSSPC).
WP4 will be dedicated to the fabrication of early devices, such as first pn junction for LEDs, photodetectors, or solar cells. Material options for ohmic contact are a key problem and have to be investigated in this perspective (study of work function for different type of metallization). WP5 consisting in simulations will help evaluate the most interesting properties of the clathrates fabricated in view of applications and devices

Preliminary results: cross-sectional SEM image showing SCF on c-Si a) as fabricated b) after pressure annealing at 250°C. c) Surface Photovoltage Spectroscopy (SPS) of p-Si substrate and type I or II silicon clathrate films on p-Si.


Participants on 17/03/2023

Coordinator : Thomas Fix]
ICube : A. Bharwal, T. Fix, Y. Le Gall, D. Muller, S. Roques, A. Slaoui
IPCMS : A. Dinia, D. Ihiawakrim, D. Stoeffler
INL : C. Chevalier, A. Fave, E. Fourmond, C. Seassal

Publications

[3] Tunability of silicon clathrate film properties by controlled guest-occupation of their cages, R. Vollondat, D. Stoeffler, D. Preziosi, S. Roques, A. Slaoui, T. Fix, J. Chem. Phys. 158, 164709 (2023)
[2] Synthesis and characterization of silicon clathrates of type I Na8Si46 and type II NaxSi136 by thermal decomposition, R. Vollondat, S. Roques, C. Chevalier, J. Bartringer, J.-L. Rehspringer, A. Slaoui, T. Fix, Journal of Alloys and Compounds 903, 163967 (2022)
[1] Silicon Clathrate Films for Photovoltaic Applications, T. Fix, R. Vollondat, A. Ameur, S. Roques, J.-L. Rehspringer, C. Chevalier, D. Muller, and A. Slaoui, J. Phys. Chem. C 124, 28, 14972–14977 (2020)