I. Mihalcescu

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.

Fabrication and optical properties of Si/CaF2(111) multi‐quantum wells

We have synthesized, by molecular beam epitaxy, Si/CaF2(111) multi‐quantum wells which are photoluminescent at room temperature after ageing in air. In this article, we report on the structural properties and on a detailed optical study of these heterostructures. The photoluminescence spectra for various confinements and the temperature dependence of the lifetimes as a function of emission wavelength are described in comparison with the corresponding characteristics of porous silicon and hydrogenated amorphous silicon. A model based on quantum confinement is proposed to explain the experimental data.

Porous silicon anisotropy investigated by guided light

Abstract We have studied light propagation in p-type waveguiding porous silicon layer. The planar waveguides were made by etching two layers with different porosities, the smaller one located on the top of the structure being the guiding layer. Light (a He–Ne laser beam λ =633 nm) is injected in the guide by a prism coupler. Analysis of the different TE and TM guided modes allows to determine with high precision the refractive index of the two porous layers, for both electric field polarisations. For as-formed structures, self-supported or not, we measured a relatively strong index anisotropy which diminishes when the structure is oxidised (anodically and thermally). The origin of the dielectric constant anisotropy is attributed to a morphological anisotropy. We present a model which describes the porous layer as preferentially oriented cylinders immersed in an isotropic medium composed of silicon spheres. The important result of this oversimplified model is that a relatively small proportion of silicon, less than 2% in columns, can determine the optical anisotropy for a 65% porosity sample. The same model also permits to explain the optical anisotropy decrease when oxidised.

Carrier localization in porous silicon investigated by time‐resolved luminescence analysis

We analyzed the photoluminescence (PL) mechanisms of porous silicon, and in particular, the origin of the PL high quantum efficiency (QE) at room temperature. For this we used postformation treatments, anodic oxidation, and hydrofluoric acid (HF) etching (known for their strong QE enhancement effect) correlated with a PL time resolved analysis. A third parameter was the temperature which, for heating above room temperature, gave a reversible quenching of the PL. All three parameters give a similar evolution of the PL decay shape, which we consider to originate from the same evolution of the carrier dynamics. Porous silicon is described as an undulating wire. The high QE at room temperature is attributed to carrier localization inside minima of the fluctuating potential along the wire; these considerations are extended to another porous material: amorphous porous silicon. Anodic oxidation and HF dissolution diminish the wire size, giving a reduction of the localization length of the carriers and progressiv...

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.

Electrically induced selective quenching of porous silicon photoluminescence

Abstract Visible photoluminescence (PL) of porous silicon is found to be completely quenched by the application of a cathodic polarization. Cathodically biased lightly doped n-type porous layers in contact with an aqueous solution exhibit reversible, complete and energy-selective quenching for a polarization variation of only about 500 m V. A spectral blueshift along with a narrowing of the line width accompanies the observed strong PL quenching. It results from a selective quenching starting by the low luminescence energy and reaching progressively the high luminescence energy as the cathodic polarization is increased.

Visible light emission from electro-oxidized porous silicon

The luminescent properties of porous silicon films are reviewed and discussed in relation with the mechanisms proposed for light emission. It is shown that a good control of the light emission is obtained by the use of electrochemically oxidized porous layers. The transient behavior of the photoluminescence is characterized, leading to propose a model for the recombination mechanisms involved in the light emission phenomena. Finally, the electroluminescence which appears during the anodic oxidation of porous silicon and the first reports of EL from solid devices are described.