F. Muller

Electroluminescence in the visible range during anodic oxidation of porous silicon films

Porous silicon/silicon structures under anodic oxidation conditions give rise to an electroluminescence phenomenon in the visible range. Using an optical multichannel analyzer the spectral distribution of the emitted light was measured−in situ−during the anodic oxidation step. Recorded spectra show a maximum which shifts continuously from red‐orange at the beginning of the process towards the yellow range. The visible emission well above the band gap of bulk silicon is attributed to a quantum size effect in the very small size (5–20 A) silicon island which constitutes the porous silicon skeleton. The light emission is interrupted when the current flow stops due to the formation of a continuous oxide layer at the porous silicon/silicon interface.

Photoluminescence of high porosity and of electrochemically oxidized porous silicon layers

Abstract It is shown that strong visible photoluminescence can be obtained directly from as-formed high-porosity porous silicon samples, without need for subsequent chemical dissolution. Light emission at wavelengths as short as 560 nm can be observed after further thinning of the silicon pore walls, but this emission vanishes quite rapidly upon oxidation in air. Much more stable luminescence characteristics have been found for porous layers with silicon walls thinned by an electrochemical oxidation process. With this method, the wavelength of the maximum light emission is determined by the anodic current density used, thus allowing a good control of the luminescence characteristics.

Fourier transform IR monitoring of porous silicon passivation during post-treatments such as anodic oxidation and contact with organic solvents

Abstract The surface passivation of porous silicon is a determining factor in the emission efficiency of the material. The hydrogen surface coverage has been shown to provide very efficient passivation. In this work, we have performed Fourier transform IR (FTIR) measurements to monitor the Si-H surface coverage, which is readily obtained after the layer formation in HF, during different post-treatments (anodic oxidation and contact with organic solvent) and to relate it to the emission efficiency. FTIR studies, performed at different steps of the electrochemical oxidation, indicate that, during the anodic treatment, the hydrogen surface coverage is preserved and that the oxidation takes place on the back bonds of the surface silicon atoms. The importance of the hydrogen coverage is also shown by the analysis of porous layers treated in boiling CCl 4 after formation. This treatment provokes the desorption of the hydrogen atoms and results in a drastic decrease in the photoluminescence. When samples are immersed in boiling methanol after formation, FTIR analyses show that there is also a partial loss of the hydrogen coverage, but accompanied with an oxidation of the material, so that no significant changes in the emission efficiency can be observed.

Photoluminescence and electroluminescence from electrochemically oxidized porous silicon layers

Abstract In this paper we review the luminescence properties of porous silicon layers formed on p-type silicon substrates and subsequently oxidized by anodic polarization in an aqueous electrolyte. The electrochemical oxidation of the porous material leads to a large increase in the photoluminescence intensity, accompanied by a blue shift of the emitted spectra. A bright visible electroluminescence is also observed during anodic treatment, with characteristics showing similar trends to that of the photoluminescence. The features of the emission are analyzed using a model that expresses the energy dependence of the emitted intensity. The model is developed on the hypothesis that the visible light emission originates in the confinement of charge carriers in the quantum-sized crystallites which form the material, and that its efficiency is determined by nonradiative processes, which involve the carrier escape from the confined zone where they are created (or injected) through a tunneling mechanism. This model is shown to be well supported by the experimental results, and allows an understanding of the spectral shifts and the intensity variations of both photoluminescence and electroluminescence during electrochemical oxidation of the porous layers.

Visible light emission at room temperature from anodized plasma‐deposited silicon thin films

In situ boron‐doped hydrogenated silicon films plasma‐deposited on various conductive substrates (including transparent oxides on glass) have been anodized in hydrofluoric acid solutions and subsequently electrochemically oxidized in an aqueous electrolyte. At room temperature, the resulting layers yield visible photoluminescence and electroluminescence intensities and spectral shapes similar to those of p‐type crystalline porous silicon obtained in the same way. The results demonstrate the technological feasibility of light‐emitting devices by applying electrochemical processes to deposited silicon‐based films.

Origin of a parasitic surface film on p+ type porous silicon

The presence of a parasitic surface film of 80 nm thickness has been observed by x-ray reflectivity on the top of some p+ -type porous silicon layers, related to a contamination of the substrate. After testing several methods to clean the substrate and to avoid this film, it was found that a 300 °C thermal annealing of the substrate is sufficient to obtain a homogeneous porous layer. The thickness of the perturbed surface layer is determined by anodic oxidation experiments and the effect of the parasitic surface film on the porous silicon formation is studied by comparing porous layers formed on untreated and on annealed substrates. The hypothesis of a passivation of the boron doping atoms by hydrogen is discussed and we review the observations of nonhomogeneous porous layers which could be related to such a contamination problem.

Porous silicon as a material in microsensor technology

Abstract Silicon micromachining or localized insulation are technological steps in microsensor technology that can be achieved by using localized porous silicon formation. In this paper, the selectivity of porous silicon formation is studied in detail. It is shown that, because of the charge exchange mechanisms responsible for porous silicon formation, the local anodization potential under galvanostatic conditions strongly depends on the doping concentration of the silicon substrate, leading to a very high formation selectivity and allowing several dopant configurations in order to localize the formation of porous silicon. Some practical developments using porous silicon formation are presented.

Analysis of the electroluminescence observed during the anodic oxidation of porous layers formed on lightly p‐doped silicon

A detailed analysis of the different characteristics of the electroluminescence that is observed during the anodic oxidation of porous silicon layers formed on lightly p‐doped substrates is presented. It is shown that the emission presents characteristics very similar to that of the photoluminescence observed on the same porous layers, and that the same basic mechanisms are involved in the two phenomena. The emission intensity increase with the oxidation level is quantitatively explained by the passivation enhancement provided by the electrochemical oxidation. The spectral shift of the spectra during the oxidation is also discussed: It is shown to result from the decrease in the sizes of the largest emitting crystallites or/and from the significant improvement of the passivation of the smallest ones due to the oxide growth. The effect of the anodizing current density on the emission efficiency is also presented.

Bright visible light emission from electro-oxidized porous silicon: A quantum confinement effect

Among the various nanometer-sized silicon structures, high porosity anodically oxidized porous silicon has many interesting properties. Luminescence quantum efficiency as high as 3% at room temperature and luminescence decay rates as long as several hundreds of microseconds show that both radiative and nonradiative processes have low efficiencies. An analysis of the dependence of the nonradiative-decay rates on carrier confinement in terms of an escape of carriers from the confined zone by tunnelling through silicon oxide barriers accounts for our experimental results with an average barrier thickness of 3 nm. The same model is extended and explains the luminescence decay shapes and the electroluminescence signal.

Bright visible light emission from electro-oxidized porous silicon

Among the various nanometer-sized silicon structures, high porosity anodically oxidized porous silicon has many interesting properties. Luminescence quantum efficiency as high as 3% at room temperature and luminescence decay rates as long as several hundreds of microseconds show that both radiative and nonradiative processes have low efficiencies. An analysis of the dependence of the nonradiative-decay rates on carrier confinement in terms of an escape of carriers from the confined zone by tunnelling through silicon oxide barriers accounts for our experimental results with an average barrier thickness of 3 nm. The same model is extended and explains the luminescence decay shapes and the electroluminescence signal.