Exotic silicon: silicon clathrate films – EXOSIL

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. 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. It comprises the ab initio calculation of the electronic structure, and simulations with software such as AFORS-HET, SCAPS, to forecast the efficiency of solar cells based on these materials.

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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 hyrdrogen 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.