The objectives of the HESSILSIL Action were to:
-grow epitaxially ultra-thin films of FeSi2 in its semiconducting phase on silicon
-characterise and study these heterostructures by a number of in situ and ex situ techniques
-examine the bandgap engineering problem with ternary semiconducting silicides (both theoretically and experimentally)
-fabricate integrated photo-detectors.
New materials were sought for the fabrication of optoelectronic devices. Semiconducting silicides are alternative materials to compound semiconductors and organic materials. They could well provide better photodetectors, optical fibre links and on chip electrooptic interconnects. An attractive feature of the use of silicides is the wide range of possiblities for optical tuning inherent in the material.
During this research iron disilicade silicon heterostructure elaboration has received special attention. Iron disilicide in its beta semiconducting phase is a promising material because the expected energy gap is around 0.9 eV, and nicely fits the 1.5 micron optical transmission window of silica optical fibres. The following results have been obtained:
iron/silicon (III) and iron silicon (100) interface development at room temperature (these interfaces may give rise to precursor phases which control epitaxial growth at higher temperatures);
epitaxial growth of beta-iron disilide on silicon (III) and silicon (100) by various epitaxial techniques;
determination of the morphology of the films and its dependence on various elaboration parameters such as thickness and temperature;
determination of the band gap, which is about 0.9 eV;
determination of the interface structure between silicide and silicon by X-ray standing wave techniques;
determination of the theoretical ternary silicide phase diagram (iron(x) cobalt(y) silicon(z));
observation of thin beta-iron disilicide strained layers and a metallic semiconducting phase transition.
APPROACH AND METHODS
The silicides were grown on silicon substrates by solid phase epitaxy (SPE), co-evaporation techniques, chemical beam epitaxy (CBE) and molecular beam epitaxy (MBE). Both the structural and electronic properties of the silicide heterostructures were to bestudied. The structural properties were characterised by:
-glancing incidence X-ray diffraction, X-ray standing waves and extended X-ray absorption fine structures (EXAFS)
-low and high-energy electron diffraction
-electronic microscopy and micro-analysis
-scanning tunnelling microscopy (STM)
-electron energy loss fine structure (EELFS) and related techniques.
Electronic properties were characterised by:
-UV and X-ray photoemission
-Bremsstrahlung isochromat spectroscopy (BIS) and related techniques
-electrical and optical characterisation techniques.
PROGRESS AND RESULTS
During this Action, FeSi2-Si heterostructure elaboration has received special attention. Iron di-silicide (FeSi2) in its semiconducting phase is a promising material because the expected energy gap is around 0.9 eV, and nicely fits the 1.5 micron optica l transmission window of silica optical fibres. The following results have been obtained:
-Fe/Si(111) and (100) interface development at room temperature; these interfaces may give rise to precursor phases which control epitaxial growth at higher temperatures
-epitaxial growth of -FeSi2 on Si(111) and Si(100) by SPE, RDE, MBE and CBE techniques
-determination of the morphology of the films and its dependence on various elaboration parameters such as thickness and temperature
-determination of the band gap, which is about 0.9 eV
-determination of the interface structure between silicide and silicon by X-ray standing wave techniques
-determination of the theoretical ternary silicide phase diagram (FexCoySiz).
-observation of thin -FeSi2 strained layers and a metallic - semiconducting phase transition.
Successful exploitation of the tuning possibilities will lead to a breakthrough in heterostructures in silicon and related compounds, with the eventual production of better silicon-compatible photodetectors and optical fibre links. The wavelength can be tuned by using ternary silicides.
Since the silicide fabrication techniques are compatible with current silicon technologies, the know-how obtained in this Action can be transferred immediately to development and production.