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In this work high-temperature Raman spectra are used to compare temperature dependence of the lattice mode wavenumber of L-alanine, L-threonine and taurine crystals. Anharmonic effects observed are associated with intermolecular N-H· · ·O hydrogen bond that plays an important role in thermal decomposition process of these materials. Short and strong hydrogen bonds in L-alanine crystal were associated with anharmonic effects in lattice modes leading to low thermal stability compared to taurine crystals. Connection between thermal decomposition process and anharmonic effects is furnished for the first time.

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 Young's 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.

We have studied the stress in porous silicon films as a function of depth and porosity using micro-Raman spectroscopy. Raman spectra were measured at different points along a cross section cleaved normal to the layer planes. Each spectrum was fitted using the phonon confinement model with the bulk phonon wavenumber as a free parameter. From the variation of this parameter we get the stress using the known dependence of phonon frequency on stress for bulk silicon. We observe a compressive stress at the interface with the substrate due to the lattice mismatch between porous and bulk silicon. The maximum value of the stress increases with porosity. The results obtained by Raman micro-spectroscopy agree well with the lattice mismatch measured by X-ray diffraction reported in the literature.

Pressure-dependent Raman scattering studies in the range 0.0 -- 32 kbar were carried out in L-alanine in order to investigate its external mode phonon spectra in relation to the phase transitions in the crystal. A careful analysis of the spectra shows that the low-energy Raman modes exhibit variation both in frequency and in intensity and between 26 and 28 kbar it is observed a splitting of a external mode, indicating that the D_2 normal phase undergoes a transition. Pressure coefficients for external modes are also given., Comment: 11 pages, Latex, 2 figures

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.

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.

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.

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.

This paper is focused to study the variations of the photoluminescent properties of porous silicon when samples are immersed in deionized water or ethanol after etching. The emission peaks in the photoluminescence spectrum redshift and decreases with immersion time. We have observed that the photoluminescence disappears after about 20 h immersion in ethanol. The results are interpreted in terms of carbon contamination in the surface. This hypothesis is confirmed by Fourier transform infrared and X-ray photoelectron spectroscopies.

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.

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.

N-type and p-type Porous Silicon (PSi) have been observed by Scanning Electron Micrograph (SEM) and characterised by Photoluminescence (PL). The porous n-type silicon obtained under illumination (photoelectrochemical etching), consists of a layer of nanoporous silicon which covers a macroporous silicon layer with pores in the micron size range. Compared with the p-type PSi, the room temperature visible luminescence is many times reduced in the case of n-type PSi, and there is a photoluminescence peak shift to higher wavelengths. This shows that the luminescence from porous silicon is caused by quantum size effect.

The design of a practical electrochemical cell to fabricate porous silicon layers (PSLs) is described. This cell is useful to rapidly produce PSLs without the need to prepare new electrochemical solutions. The cell is also adapted for the formation of PSLs in the dark or under illumination, with nitrogen flow in the solution. The special design of the silicon wafer support guarantees a homogeneous electrical contact and no solution infiltration into it.

The existence of an outer cracked layer and an inner more structured porous layer has been observed in electrochemically obtained porous silicon when drying procedures are carried out. The changes in the charge transferred to the porous structure during voltammetric oxidation, the interference fringes obtained by Fourier‐transform infrared spectroscopic measurements, and scanning electron microscopy micrographs confirm the existence of this double layer. Also, drying procedures and voltammetric oxidations drastically affect the intensity and wavelength of the peak maximum in the photoluminescence spectrum. The evolution of the luminescent properties is explained by the introduction of nonradiative recombination centers.

X-ray photoelectron spectroscopy has been performed in photoluminescent porous Si layers after different treatments, namely, anodisation only in a HF-ethanol solution, rinsing in deionised water, ethanol immersion, and exposure to ambient conditions. The spectra recorded just after formation in the HF solution are the most representative of real porous Si layer structures. However, the pretreatment in water, exposure to ambient conditions for a long period, and immersion in ethanol degrades the porous Si layer surface to a greater or lesser extent. The growth of different oxide phases and the amorphisation of the structures after the pretreatments are also discussed. MST/3286

Electroluminiscence (EL) in liquid of porous silicon (PS) has been attributed to radiative recombination of electrochemically generated holes from the bulk of the semiconductor and electrons injected into the conduction band of PS by species adsorbed on the surface. An examination of the surface of PS submitted to oxidation by means of Fourier Transform Infrared Spectroscopy indicates that the concentration of Si-Hx bonds does not significantly vary during the time interval in which EL is recorded, and therefore that the electrons belonging to the Si-H bonds do not intervene in the generation of EL.

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.

A review is made in this paper on the recent advances of the photo‐ and electro‐luminescence properties of porous silicon (PS). We start by describing the electrochemical formation mechanisms of PS, followed by the most significant results obtained by different structural and chemical characterization techniques. Next we discuss the origin of the strong visible photoluminescence observed at room temperature in PS. Finally the possibility of using PS structures as electroluminescent devices and photodetectors is discussed. For this application the use of transparent conductive polymer films as electrical contacts to PS is considered.

We have measured the Raman spectrum of porous silicon layers for different light polarizations. For the polarizations where the first-order Raman peak is forbidden in bulk silicon, porous silicon shows a band. This band is almost as intense as the one in the allowed polarizations but its lineshape is very different. The ‘forbidden’ band is usually wider and shifted to lower energies with respect to the allowed band. Both bands are analyzed in terms of sample characteristics, layer depth, and excitation wavelength.

We report on the strong temperature-dependent thermal expansion, D, in CdS quantum dots (QDs) embedded in a glass template. We have performed a systematic study by using the temperature-dependent first-order Raman spectra, in CdS bulk and in dot samples, in order to assess the size dependence of D, and where the role of the compressive strain provoked by the glass host matrix on the dot response is discussed. We report the Gr¨ uneisen mode parameters and the anharmonic coupling constants for small CdS dots with mean radius R 2:0 nm. We found that parameters change, with respect to the bulk CdS, in a range between 20 and 50%, while the anharmonicity contribution from two-phonon decay channel becomes the most important process to the temperature-shift properties. (Some figures may appear in colour only in the online journal)

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.

3 pages, 3 figures., 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., The authors acknowledge financial support from the Spanish CICyT (Project Nos. MAT96-0395-CP and MAT97- 0725).

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.