A. Bsiesy

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.

In situ electric field simulation in metal/insulator/metal capacitors

The authors report in this letter the effect of interface topography on metal/insulator/metal (MIM) capacitor electrical properties. This analysis was carried out by numerical simulations of the electric field established in a MIM structure with a 45nm thick Ta2O5 film. The metal/insulator interface profiles have been extracted from transmission electron microscopy micrographs of a fully integrated device. This in situ approach allows direct comparison between electrical properties and numerical simulations performed on the same device. Results show that the bottom electrode’s surface roughness induces a large electric field increase at the interface which could explain MIM capacitor’s asymmetrical electrical behavior.

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.

Method to assess the grain crystallographic orientation with a submicronic spatial resolution using Kelvin probe force microscope

In thin polycrystalline copper film, a direct correlation between the grain crystallographic orientation and the work function has been evidenced by comparing Kelvin probe force microscope (KFM) mapping and electron backscattered diffraction analysis performed over the same region. As a result, work function mapping provided by KFM technique can be used to assess the crystallographic properties of thin layers with a spatial resolution better than 100nm.

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.

Light-induced porous silicon photoluminescence quenching

The photoluminescence quenching of lightly doped p-type porous silicon in contact with aqueous acidic electrolytes is investigated under reverse-bias conditions. A complete and reversible quenching of the light emission is observed under infra-red illumination of the samples. This quenching is assigned to the injection into the porous layer of the electrons which are photogenerated in the substrate. The quenching features are studied as a function of the electron concentration in the porous layer, which is varied either by changing the intensity of the light excitation that generates the minority carriers in the silicon bulk or, at a given light intensity, by changing the electrolyte composition. In the latter case, the electron concentration is dependent on the electrochemical reactions which take place at the porous layer surface and which partly consume the injected electrons. It is shown that the amount of injected electrons directly determines the magnitude of the quenching and the associated spectral changes. It is concluded that the assumption of an enhanced charge carrier separation by the electric field as a possible quenching mechanism can be ruled out, and that the experimental results rather support the hypothesis that the quenching involves an Auger recombination process.