J. D. Moreno

Lattice-mismatch induced-stress in porous silicon films

Abstract We have studied the stress in porous silicon films with different porosities at the interface with the substrate. Micro-Raman spectra were measured along a cleaved cross-section to sample different layer depths. Each spectrum was fit to the phonon confinement model, with the bulk phonon frequency as a free parameter to remove phonon confinement effects. At the interface this frequency increases sharply, indicating a compressive stress on the porous silicon pillars. The stress is due to the lattice mismatch, measured by X-ray diffraction, between the porous film and the bulk silicon substrate. For porosities between 50 and 85% the stress and the lattice mismatch vary, respectively, between 4 and 10 kbar, and between 2.9×10 −3 and 3.5×10 −3 . Finally, from the dependence of stress on the lattice mismatch we obtain a microscopic Youngs modulus of 156 GPa. This magnitude, mostly dependent on atomic bonding, is close to the bulk silicon value and is much larger than the macroscopic modulus, strongly dependent on the porous structure, reported in the literature.

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.

DEPTH-RESOLVED MICRO-RAMAN STUDY OF POROUS SILICON AT DIFFERENT OXIDATION STATES

Photoluminescence (PL) and Raman spectra were measured along a cross section of porous silicon films at different oxidation times after application of anodic current transients. The average crystallite size was determined from the Raman spectra with the standard phonon confinement model. Before oxidation, the PL emission energy and crystallite size were found to be independent of the layer depth. Also, the integrated PL emission was larger for the middle layers. The effect of oxidation was a blueshift of the PL band and a decrease in the integrated emission for the outer layers. The crystallite size increases for all layers, particularly the outer ones.

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.

On the electroluminescence and photoluminescence of porous silicon layers submitted to electrooxidation

Photoluminescent and electroluminescent properties of porous silicon layers were studied at different oxidation times after application of anodic current transients. Changes in the wavelength and intensity of the emitted photoluminescence could be explained as the consequence of a gradual electrooxidation of silicon structures. Also, the time delay associated with the recording of electroluminescence was interpreted as being due to the oxidation of bulk silicon by holes injected by the power supply. Once bulk silicon becomes oxidized, holes can reach the valence band of silicon microstructures where quantum size effects lead to visible light emission.