Work Area: Novel Concepts and New Materials for Optical Devices / All-Optical Computing
Keywords porous silicon, molecular beam epitaxy, electron beam lithography, scanning tunnelling microscopy, surface analysis
Start Date: 1st October 1992 / Duration: 36 months / Status: running
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Abstract EOLIS aims to elucidate the mechanisms giving rise to luminescence in silicon nanostructures and in particular in microporous porous silicon (PS) layers. Studies on PS structures are being compared with experimental and theoretical studies of model structures using classical and novel characterisation techniques.
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.
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.
In order to assess and compare the properties of PS processed by different recipes in the different laboratories, a round-robin study of PS generated in the different laboratories of the consortium, and UMIST (a SOLDES partner) is in progress. PS formed in poly-Si layers exhibits a much lower porosity (35%) than layers fabricated, in the same conditions (current density and HF concentration), in monocrystalline substrates (85%) but present a red luminescence which is much weaker than the latter. PS layers (55% porosity obtained from p-doped substrates), have been stabilised by oxidation at low temperature (300-400ĄC). The oxidation is very fast, then saturates. Once the oxide removed by HF the specific area is found to be very close to that of freshly anodised material which indicates that this treatment does not modify the morphology of PS.
We have investigated through first principle calculations the electronic properties of thin silicon (111) layers embedded in a CaF2 host crystal. The band gap is found to increase to the visible range for Si layer thickness less than 4 double layers (dl). For 1 and 2 dl the bonding Si-Ca state emerges from the Si valence band and leads to an almost direct gap at finite wavevectors that could account for efficient luminescence in this system. CaF2 growth on stepped (111) Si surfaces has been investigated. After the deposit LEED and RHEED observations suggest that the step lattice is not influenced by the fluoride deposit. Fabrication of Si (2 to 7 dl)/CaF2(+10) is in progress.
Rapid thermal oxidation (RTO) of PS layers were studied. Aligned crystallites remain throughout oxidised luminescent layers with a wide distribution of size down to below 3nm. The volume fraction of Si remaining is quite low and the vast majority of these crystallites are physically isolated from one another by amorphous oxide. For the highest RTO temperatures, essentially negligible crystalline Si remains in the layer in accord with the absence of 750-800 nm PL emission. In-situ AFM in the contact mode has been carried out on highly porous PS layers. This imaging technique is highly destructive and resonant. AFM imaging is under development. In-situ coupled photomodulated infra-red (IR) spectroscopy and PL measurements have been carried out on PS layers in both HF or ambient. In the two cases the absence of any correlation between IR adsorption of surface SiH and the PL as well as the lack of SiO or oxidised SiH vibrational contributions come against the polysilane or siloxene phypotheses. On the contrary the data recorded are consistent with a direct recombination of photoinduced free carriers when PS is in HF (yellow-green PL) and with a recombination mediated by localised states when PS is in ambient (red PL). These important results show that the yellow-green luminescence is not only a blue shift of the red luminescence and suggest that it exists two different regimes.
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).
A major part of this work has been presented at the NATO-ARW workshop Optical Properties of Low Dimensional Silicon Structures, 1-4 March 1993, Grenoble (France) and to be published in NATO ASI series, edited by Bensahel D, Canham L T, Ossicini S and Bomchil G.
As mentioned above, a NATO-ARW workshop organised by the members of the consortium was held in Grenoble (France) on March 93.
A second NATO workshop will be held in March-April 94 (the place and dates to be announced).
Two joint meetings have been organised with SOLDES (No. 7260).
All the partners participated actively in various international conferences and workshops.
CRMC-CNRS - F
Campus de Luminy
F - 13288 MARSEILLE CEDEX 09
KFA Jülich - D
CNET-CNS - F
Ecole Polytechnique - F
NCSR Demokritos - GR
Università degli studi di Modena - I
Defence Research Agency - UK
Prof. F. Arnaud d'Avitaya
tel +33/91 17 28 61
fax +33/91 41 89 16
EOLIS - 7228, August 1994
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html version of synopsis by Nick Cook