R. Guerrero-Lemus

Depth-resolved microspectroscopy of porous silicon multilayers

We have measured depth-resolved microphotoluminescence (PL) and micro-Raman spectra on the cross section of porous silicon multilayers to sample different layer depths. The PL emission band gets stronger, blueshifts, and narrows at the high porosity layers. On the contrary, the Raman band weakens and broadens. This band is fitted to the phonon confinement model. With the bulk silicon phonon frequency and its linewidth as free parameters, we obtain crystallite size, temperature, and stress as a function of depth. Sizes are larger than those estimated from PL. Laser power was reduced to eliminate heating effects. Compressive stresses in excess of 10 kbar are found in the deepest layer due to the lattice mismatch with the substrate.

A galvanostatic study of the electrodeposition of polypyrrole into porous silicon

Polypyrrole has been electrodeposited in the interior of the pores that form the porous silicon structure, and a very significant increase of the electrical conductivity of the samples has been observed. Micro-Raman spectroscopy experiments have allowed us to measure the amount of polymer as a function of the distance from the outer porous silicon surface. The degree of filling by the polymer has been found to be highly dependent on the electropolymerization conditions, particularly the current density applied.

Influence of wavelength on the Raman line shape in porous silicon

The line shape of the one phonon Raman peak has been used extensively in the literature to estimate the crystallite size in porous silicon. However it has been shown that the line shape obtained on top surface experiments depends on the excitation wavelength. Because the porosity depends on depth, previous results are masked by the change in penetration depth. In this communication we report depth-resolved micro-Raman spectra at 514.5 and 632.8 nm. The spectra were measured at different points along a cross section of porous silicon films. We show that even when the same layer and, therefore, the same porosity is probed the Raman peak is broader at shorter wavelengths. To explain the results we suggest a contribution of indirect gaps to the resonant Raman cross section induced by quantum confinement.

Determination of the spectral behaviour of porous silicon based photodiodes

Abstract Porous silicon (PS) based photodiodes were formed by depositing gold (Au) contacts onto the PS surface. PS was formed from p-silicon substrates under different formation parameters (current density and time of anodization), so PS layers with different porosities and thicknesses were obtained. It was determined the responsivity and the quantum efficiency of these structures in the 200–2500 nm wavelength range, from which it has been observed a different behaviour of those diodes depending on the porosity and thickness of the PS layer. It has also been studied the spectral response from different diodes in which semitransparent conducting films (gold and indium tin oxide) have been deposited onto the PS layer, obtaining a significant improvement in the photoelectronic properties in the visible and near infrared parts of the spectrum.

Ageing of aluminum electrical contacts to porous silicon

Electrical contacts to porous silicon (PS) were formed by depositing aluminum onto its surface. The corresponding Al/PS/Si structures show a rectifying behavior, even after prolonged times in contact to the atmosphere. The series resistance and the ideality factor as a function of time of storage in ambient air have been determined by employing a variation of the Norde method. We have also studied the influence of the electrolytic formation parameters in the process of aging of the Al/PS/Si structures. The results show that longer anodization times and higher formation current densities of the PS layer lead to a faster diminution of the current flowing through the Al/PS/Si structure as a result of its exposition to the atmosphere. However, when the surface of the PS layer is chemically etched, the diminution of the current is significantly slower than in the case of untreated samples.

Influence of oxidation and carbon-containing contamination in the stabilization of the luminescence in porous silicon

Abstract This paper is focused to the study of the stabilization of the photoluminescent properties of porous silicon (PS). For this purpose, as-formed PS samples were subjected to different surface treatments. Photoluminescent excitation spectra were used to obtain the indirect band gaps, yielding practically the same values for PS films having undergone different surface annealing and post-etch treatments. However, the excitation spectra show a significant blueshift with aging for the different samples, which has been related to the initial amount of oxygen present in the porous surface as detected by XPS and FTIR. The variation of the intensity of the excitation spectra with aging has also been studied and can be associated to carbon contamination. The results are interpreted in terms of quantum size effects in PS and the influence of the surface composition.

Development and characterization of porous silicon based photodiodes

Abstract Porous silicon (PS) based photodiodes have been developed from multicrystalline n + /p junctions, since this material can efficiently be employed to reduce optical reflection and to enhance absorption. These devices show a strong rectifying characteristic, in which the reverse and short-circuit current strongly depends on the presence of light. According to this, their use as solar sensors is considered as a practical application of PS-based photodiodes. The electrical performance (forward and reversed biased) of the PS-based devices under standard conditions of illumination and temperature has been determined. The angular dependence of the electrical response as a function of incidence light has also been established showing a cosine-type behavior. The degradation of these devices after extended photon irradiation as well as after long periods of atmospheric exposure has been found to be small. Finally, the variation of the electrical response under standard illumination conditions of PS-based photodetectors has also been estimated by varying the device working temperature from 10 up to 80°C.

Anodic oxidation of porous silicon bilayers

This paper focuses on the study of the effect of anodic oxidation in porous silicon bilayers composed of two porous layers of different porosities. The order of the two types of layers has been alternated, and the thicknesses and refractive indices have been optically characterized by Fourier transform infrared spectroscopy. The results show that the refractive index of anodic oxidized porous silicon is reduced significantly with respect to just formed porous silicon. It is also observed that the quality of the oxidation is related to the porosity of the inner porous layer of the silicon bilayer structure. This effect is interpreted in terms of quantum size effects.

Deposition of Polypyrrole into Porous Silicon

Polypyrrole has been electrocleposited in the interior of the pores that form the Porous Silicon structure, and a very significant increase of the electrical conductivity of the samples has been observed. The degree of filling by the polymer has been found to be highly dependent on the electropolymerization conditions. Micro-Raman Spectroscopy experiments have allowed us to measure the amount of polymer as a function to the distance from the outer PS surface.

Time evolution of surface porous silicon composition under different post-etch treatments

Abstract Diffuse reflectance Fourier transform infrared spectroscopy and photoluminescence measurements have been performed on porous silicon layers (PSLs) after different post-etch treatments: immediate drying under flowing N 2 , rinsing in deionized water, and ethanol immersion. Layer composition and luminescence were also subsequently monitored during lengthy storage in ambient conditions. This study reveals a different surface composition and morphology arising from each post-anodization procedure, and no clear relation between composition and photoluminescence evolution of PSLs.