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Emission of Light in Silicon

Obiettivo

The fabrication of silicon-based optoelectronic devices is of prime importance for the development of optical interconnects in microelectronics. The recent demonstration that microporous porous silicon (PS) layers are able to produce intense room temperature and visible photo- and electroluminescence (the latter under anodic oxidation conditions), is the first step toward the realisation of silicon emissive devices (light-emitting diodes, lasers, etc). However, the origin of the phenomenon is not yet totally understood. The main goal of EOLIS is to understand this effect through an extensive analysis of various PS layers, and fabrication by sophisticated methods of model structures in the form of dots and wires.
A study was made of the mechanisms that give rise to luminescence in silicon nanostructures and in particular in microporous porous silicon (PS) layers.
Sample processing:
The properties of PS processed by different recipes are being assessed. PS formed in polysilicon layers exhibits a much lower porosity (35%) than layers fabricated, in the same conditions, in monocrystalline substrates (85%) but presents a red luminescence which is much weaker than the latter. PS layers (55% porosity obtained from p-doped substrates), have been stabilized by oxidation at low temperature (300 to 400 C). The results indicate that this treatment does not modify the morphology of PS.

Modelling:
The electronic properties of thin silicon (111) layers embedded in a calcium fluoride host crystal have been investigated. The band gap is found to increase to the visible range for silicon layer thickness less than 4 double layers (dl). For 1 and 2 dl the bonding silicon-calcium state emerges from the silicon valence band and leads to an almost direct gap at finite wavevectors that could account for efficient luminescence in this system. Calcium fluoride growth on stepped (111) silicon surfaces has been investigated. Observations suggest that the step lattice is not influenced by the fluoride deposit.

Characterization:
Rapid thermal oxidation (RTO) of PS layers was studied. Aligned crystallites remain throughout oxidized luminescent layers with a wide distribution of size down to below 3 nm. For the highest RTO temperatures, negligible crystalline silicon remains in the layer in accord with the absence of 750 to 800 nm PL emission. In situ atomic force microscopy (AFM) in the contact mode has been carried out on highly porous PS layers. In situ coupled photomodulated infra red (IR) spectroscopy and PL measurements have been carried out on PS layers. Results show that the yellow green luminescence is not only a blue shift of the red luminescence and suggest that it exists 2 different regimes.
APPROACH AND METHODS

The prime objectives will be to determine the effective dimensionality and degree of quantum confinement in light-emitting porous silicon structures and the role of surface states, dangling bond density and silicon morphology on radiative efficiency. Luminescence measurements made on porous silicon will be compared with those made on epitaxially grown and polymeric silicon structures with well-defined dimensions, crystalline quality and surface states. When combined with theoretical calculations of band structure and oscillator strengths, a model will be built up for the processes giving rise to luminescence in these structures. A wide range of techniques will be required to characterise them.

Model dot and wire structures will be fabricated using a variety of techniques which include chemical synthesis, molecular beam epitaxy (MBE), chemical beam epitaxy (CBE) and electron beam and STM lithographies combined with anisotropic etching. The luminescence efficiency of the model structures will be investigated using ex situ and in situ techniques, and compared with theoretical predictions of similar structures.

Classical characterisation techniques will include transmission electron microscopy, photoluminescence, electron spin resonance, and atomic force microscopy. These will be combined with novel techniques such as flow microcalorimetry and in situ infrared absorption and photoluminescence to determine the relationship between the radiative efficiency and the nature of the quantum structures.

POTENTIAL

The know-how gained from these studies will give new insights into the mechanisms giving rise to efficient luminescence in Si material. The knowledge of these mechanisms will allow the fabrication of a new class of silicon devices (light-emitting diodes, lasers).

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CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
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Indirizzo
Campus de Luminy, Case 913
13288 MARSEILLE
Francia

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Ultimo aggiornamento: 17 Giugno 1994

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Permalink: https://cordis.europa.eu/project/id/7228/it

European Union, 2024