R. Romestain

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...

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.

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.

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.

Relation between porous silicon photoluminescence and its voltage‐tunable electroluminescence

The voltage‐tunable electroluminescence (VTEL) observed on porous silicon‐electrolyte system is investigated in relation with the material photoluminescence (PL). It is shown that the PL line is the envelope of all the emitted EL spectra obtained upon the bias variation. Consequently, a blueshift of the (PL) line leads to a similar shift of all the corresponding EL lines. This strongly suggests a common origin of these two phenomenon. Moreover, this study seems to indicate that the VTEL of porous silicon is related to the size and efficiency distributions of the silicon nanocrystallites associated with an electrically induced selective carrier injection.

Direct determination of the absorption of porous silicon by photocurrent measurement at low temperature

Abstract Transmission of porous silicon (PS) has been measured using a new and direct method based on photo-induced current modulation in the silicon substrate. A current is induced in bulk silicon by electrical polarization on the back of the substrate. When the PS layer is illuminated, the light not absorbed in the layer and therefore transmitted to the silicon substrate modifies this current. In this configuration, the porous silicon layer is electrically passive. It is shown that the photo-induced current modulation strongly depends on the Al/Si contact used to polarize the silicon substrate. Al/Si contacts are responsible for the non-linear response of the system, which was overcome by working at low temperature and by using CW illumination on the back of the sample. Important advantages of this technique lie in the fact that removal of the PS layer is not necessary and that scattering of light at the interfaces does not affect the measurement. Using this method, absorption coefficients of low- and high-porosity PS layers have been measured in an energy range starting from the near IR to the UV. The absorption spectra show an exponential behavior at low energy, characteristic of amorphous material, but at higher energy a saturation is observed as in the case of bulk silicon.

Voltage-induced modifications of porous silicon luminescence

Abstract The remarkable voltage-tunable electroluminescence (VTEL) observed on porous silicon—electrolyte junctions is investigated in relation to material morphology and electrolysis parameters. The electroluminescence (EL) is obtained upon cathodic polarization of n-type porous silicon in contact with aqueous solutions containing the persulphate ion. The observed long-lived EL shows a reversible spectral shift as large as 300 nm for an external bias variation of about 0.6 V. The study of the EL behaviour as a function of the external voltage and the persulphate ion concentration shows that while the amplitude of the EL is proportional to the intensity of the exchanged current, the spectral position is only determined by the applied voltage. A qualitative model, taking into account the voltage dependence of the charge injection probability into the size-distributed silicon crystallites, gives a good description of the observed VTEL behaviour. In a similar manner, cathodic polarization induces a dramatic change in the porous silicon photoluminescence. It leads to a reversible, highly contrasted and energy-selective quenching of the photoluminescence (QPL) for a polarization variation of only about 500 mV. A spectral blue shift, along with a significant narrowing of the PL line accompanies the observed strong QPL. This results from selective quenching starting at the low luminescence energy and reaching progressively the high luminescence energy as the cathodic polarization is increased. Just as for VTEL, this selective character of the QPL can be explained by a voltage-induced enhancement of charge injection into the size-distributed silicon nanocrystallites.