Porous Silicon Thin Films Research Articles

Engineering porous silicon nanostructures as tunable carriers for mitoxantrone dihydrochloride

Nanostructured porous silicon (PSi) thin films, fabricated by the electrochemical anodization of single crystalline Si wafers, are studied as delivery systems for the anticancer drug mitoxantrone dihydrochloride (MTX). The surface chemistry of the PSi carriers was tailored by surface alkylation using thermal hydrosilylation of 1-dodecene and undecylenic acid, followed by physical adsorption or covalent attachment of MTX to the Si scaffold. The nanostructure and the physiochemical properties of the different carriers were characterized by attenuated total reflectance Fourier transform infrared spectroscopy, nitrogen adsorption–desorption and contact angle measurements, demonstrating that surface alkylation results in a pronounced effect on the hydrophobicity/hydrophilicity of the scaffolds and a volumetric gain in pore wall, which in turn results in a decrease in pore diameter (>23%) and available porous volume (>40%). The effect of these key parameters on MTX loading efficacy, release profile, Si scaffold erosion kinetics and in vitro cytotoxicity on human breast carcinoma (MDA-MB-231) cells was studied and compared to the behavior of neat PSi carriers. We show that the chemically modified PSi carriers exhibit sustained release for several days to weeks with minimal to no burst effect, while for the native PSi MTX release was completed within 5h with a substantial burst release of ∼40%. Moreover, our in vitro cytotoxicity experiments have clearly demonstrated that the MTX released from all PSi carriers maintained its cytotoxic effect towards MDA-MB-231 cells, in comparison to the low toxicity of the PSi carriers.

Luminescence properties of Bi-doped oxidized porous silicon thin films

Luminescence properties of Bi-doped oxidized porous silicon (OPS) thin films were studied. It was found that this material shows two broad luminescence bands centered at 845nm with the FWHM of 120nm and at 1410nm with that of 220nm under 488nm excitation. A detailed analysis of the 3D plot of PL intensities versus excitation and emission wavelengths revealed that these luminescence bands arise from at least two different kinds of Bi luminescence centers. The broad luminescence covering the whole telecommunication window (1.2–1.6μm) suggests that Bi-doped OPS thin films can be a candidate material for a broadband waveguide-type optical amplifier at optical telecommunication wavelengths.

Biofunctionalization of free-standing porous silicon films for self-assembly of photonic devices

Here we test chemical strategies for the fabrication of free-standing porous silicon thin films that are covalently modified on one of their faces with biorecognition elements to guide the vertical self-assembly of optical structures. Chemical modification viahydrosilylation of alkenes followed by coupling of biomolecules is carried out after the porous silicon film is lifted off from the silicon wafer to avoid damage to the organic monolayer during the lift-off step. Micron-sized biotinylated particles produced from asymmetrically modified porous silicon films are deposited via biorecognition in the correct orientation onto a substrate modified with avidin to form optical resonant microcavities.

Fabrication of Gradient Optical Filter Containing Anisotropic Bragg Nanostructure

New gradient optical filters containing asymmetric Bragg structure were prepared from the distributed Bragg reflector (DBR) porous silicon (PSi). Anisotropic DBR PSi displaying a rainbow-colored reflection was generated by using an asymmetric etching configuration. Flexible anisotropic DBR PSi composite films were obtained by casting of polymer solution onto anisotropic DBR PSi thin films. The surface and cross-sectional images images of anisotropic DBR PSi composite films obtained with cold field emission scanning electron microscope indicated that the average pore size and the thickness of porous layer decreased as the lateral distance increased. As lateral distance increased, the reflection resonance shifted to shorter wavelength.

XPS study of palladium sensitized nano porous silicon thin film

Nano porous silicon (PS) was formed on p-type monocrystalline silicon of 2–5 Ω cm resistivity and (100) orientation by electrochemical anodization method using HF and ethanol as the electrolytes. High density of surface states, arising due to its nano structure, is responsible for the uncontrolled oxidation in air and for the deterioration of the PS surface with time. To stabilize the material PS surface was modified by a simple and low cost chemical method using PdCl2 solution at room temperature. X-ray photoelectron spectroscopy (XPS) was performed to reveal the chemical composition and the relative concentration of palladium on the nanoporous silicon thin films. An increase of SiO2 formation was observed after PdCl2 treatment and presence of palladium was also detected on the modified surface. I–V characteristics of Al/PS junction were studied using two lateral Al contacts and a linear relationship was obtained for Pd modified PS surface. Stability of the contact was studied for a time period of around 30 days and no significant ageing effect could be observed.

Electrochemically Deposited Aluminum in Template of Porous Silicon Film

In this article, we report on the observations that in the aqueous electrolyte of aluminum nitrate, the thin metallic conducting films on both internal and external surface of porous silicon (PS) thin films that emit visible photoluminescence at room temperature prior to electrochemical deposition have been obtained under electrochemical deposition condition. Add to this high surface-to-volume ratio and these make it a good candidate for the catalyst supporter. We have investigated the surface morphology of PS after the interval of about 30 hours of electrochemically deposited aluminum by means of scanning electron microscopy (SEM). It has been shown from SEM images that not only micrometer-sized pores are smoothed by deposition of aluminum microcrystal, but also the presences of semi-sphere aluminum microcrystal which rooted in the tip of micrometer-sized pores are observed. On the one hand, this extremely interesting phenomenon which the micrometer-sized pores are smoothed may be explained in terms of principle of electrochemical deposition; on the other hand, we have laid the formation mechanism of semi-spherical aluminum microcrystal at the door of Gibbs free energy.

Applications of porous silicon thin films in solar cells and biosensors

An overview of the applications of porous silicon (PS) thin films, as antireflection coatings (ARC) in silicon solar cells and transducers in biosensors, is presented. The reflectance spectra of PS films have been compared with other conventional ARCs (such as SiNx TiO2/MgF2 and ZnS), and optimal PS ARC with minimum reflectance has been obtained. The implementation of PS into an industrially compatible screen-printed (SP) solar cell by both the electrochemical etching (ECE) and chemical etching (CE) methods are reviewed. Porous silicon films, formed via ECE for short anodization times, on textured n+ emitter ofc-Si solar cell having SP front and back contacts, lead to improvements in the performance of solar cells and demonstrate their viability in industrial applications.

N[sub 2]-Based Thermal Passivation of Porous Silicon to Achieve Long-Term Optical Stability

A technique is presented to create porous silicon (PS) thin films with long-term optical stability when exposed to normal room conditions. This technique requires the thin films to undergo a rapid thermal annealing treatment at a relatively low temperature of 600°C in N 2 atmosphere for 6 min. Annealing at such a low temperature enables the surface passivation of PS via nitridation without causing the excessive roughness and sintering widely observed at higher annealing temperatures. A mechanism similar to the Haber process is proposed to explain the surface nitridation, in which the nitrogen atmosphere interacts with the as-anodized hydrogen surface species.

Porous silicon and polymer materials for optical chemical sensors

The optical characteristics of porous materials depend on their structural properties (porosity, pore size, and pore distribution). Specifically in materials with air-filled pores, the effective refractive index is a weighted average of the refractive indices of the relevant material and air, which is thus directly related to the material’s porosity. Mesoporous materials, such as porous silicon and porous polymers, have therefore been exploited to create photonic devices, from simple interference filters to exotic photonic-bandgap (PBG) structures.1, 2 In addition, the high internal surface area of porous materials provides an excellent host medium to immobilize analyte-specific recognition elements through sequestration.3 Interactions of the analyte molecules with the sequestered recognition elements often alter the effective index or other optical properties. These optical variations can be employed as a simple and straightforward transduction approach in optical sensing. Typically, porous silicon is produced by electrochemical etching in a hydrogen fluoride-based liquid etchant. A major drawback of this approach is the drying process, because the pressure gradient across the gas/liquid interface during evaporation produces high capillary stress in the porous structure that can lead to systematic cracking and destruction of the pores. Supercritical drying is often employed to avoid this problem. We recently developed a dry-etching method that overcomes the significant challenges of producing porous silicon by wet-etching chemistry. Our method relies on gaseous xenondifluoride to etch and produce a thin layer of macroporous silicon on the surface of bulk silicon. This layer produces an optical interference pattern whose reflectance spectra can be Figure 1. (a) Cross-sectional scanning-electron-microscope (SEM) image and (b) reflectance of a dry-etched porous-silicon thin film at different angles of incidence ( i ). The differences in the reflectance spectra are caused by optical interference.

Pulsed Anodization for Control of Porosity Gradients and Interface Roughness in Porous Silicon

We present a technique involving the use of pulsed anodization for porous silicon (PS) thin-film fabrication on low doped substrates, for which the interface roughness and porosity gradients usually observed in such films can be eliminated. The work presented includes a detailed characterization of the effects of duty cycle and frequency during pulsed anodization. The study spans pulsing frequencies of 0.1–1000 Hz and duty cycles of 5–50%. The combination of low frequency (0.1 Hz) and low duty cycle (5%) for the pulse train used for anodization produces PS thin films, displaying no measurable interface roughness or porosity gradient. The mechanisms behind the wide variation in available PS thin-film properties with pulsed anodization parameters are analyzed using a galvanostatic technique. The analysis indicates that the inhomogeneity and roughness observed in PS films fabricated on low doped starting wafers are both due to the unstable multistep dissolution kinetics of silicon during PS film formation.

Low temperature N 2-based passivation technique for porous silicon thin films

A technique is presented for the passivation of porous silicon (PS) thin films via nitrogen based annealing at the lowest temperature ever reported. Annealing freshly anodized PS thin films at temperatures as low as 520 ∘C under N 2 flow in a rapid thermal annealer produces films that show no change in refractive index when exposed to ambient conditions over 60 days. These films also exhibited chemical resistance by surviving a brief dip in both concentrated KOH and buffered HF. Unlike most other PS surface passivation methods, this technique causes negligible reduction in refractive index of the annealed PS thin films. Passivation only occurs when dangling bonds and mono-hydrides populate the PS surface, providing a path for thermal interactions with the N 2 gas.

Nanocrystal characterization by ellipsometry in porous silicon using model dielectric function

Porous silicon layers were prepared by electrochemical etching of p-type single-crystal Si (c-Si) of varying dopant concentration resulting in gradually changing morphology and nanocrystal (wall) sizes in the range of 2–25nm. We used the model dielectric function (MDF) of Adachi to characterize these porous silicon thin films of systematically changing nanocrystal size. In the optical model both the surface and interface roughnesses have to be taken into account, and the E0, E1, and E2 critical point (CP) features are all described by a combination of several lineshapes (two-dimensional CP, excitonic, damped harmonic oscillator). This results in using numerous parameters, so the number of fitted parameters were reduced by parameter coupling and neglecting insensitive parameters. Because of the large number of fitted parameters, cross correlations have to be investigated thoroughly. The broadening parameters of the interband transitions in the measured photon energy range correlate with the long-range order in the crystal. The advantage of this method over the robust and simple effective medium approximation (EMA) using a composition of voids and c-Si with a nanocrystalline Si reference [Petrik et al., Appl. Surf. Sci. 253, 200 (2006)] is that the combined EMA+MDF multilayer method of this work provides a more detailed description of the material and layer structure.

Porous silicon thin film as CO sensor

This work presents the electric behavior of porous silicon (PS) thin films when the material's surface is exposed to carbon monoxide. PS thin films were fabricated by the electrochemical anodization method of Si–c (100) substrates with resistivity 0.01Ωcm. The samples were prepared at 20min anodization time and 5mA/cm2 anodization current. Aluminum electrodes were deposited on the surface of the material by high vacuum evaporation, such that the electric conduction was parallel to the substrate's surface. The detector was placed in vacuum during 1h and then CO was allowed into the vacuum chamber. Measurements of the I–V characteristic were carried out at atmospheric pressure, in vacuum and with CO. Changes in the resistance of the material, of about MΩ, were observed in the different samples, indicating that the material is sensitive to the presence of CO and therefore suitable as gas sensor.

AC impedance spectroscopy of porous silicon thin films containing metallic cations

Thin porous silicon (PS) films were prepared by HF/HNO 3 vapor etching on silicon wafers. The infiltration of metallic cations inside the porous silicon matrix followed by slow heating in air leads to an interesting electrical and optical physical phenomena. Al 3+, Cu +, K +, Li + metallic cations as chloride or as nitrate solutions are infiltrated inside the silicon porous matrix. After annealing in air at 500 °C during 2 h a structure was achieved in order to measure the Nyquist diagram corresponding to the cations embedded in PS/silicon. The real and imaginary parts of the whole structure depend on the voltage bias and on the frequency. Those signify that the junction is a Schotky-barrier junction. From the variation of Ln( σT) versus absolute temperature T, where σ is the conductivity, we have deduced the activation energy of the metallic impurities in the [100 °C–400 °C] temperature range. We have found that the activation energies are of about 0.19 eV, 0.25 eV, 0.49 eV and 0.71 eV for Cu +, K +, Al 3+ and Li + cations respectively. These kinds of structures are suitable for gas sensing, optical sensing or for the fabrication of fuel cell membranes.

Ellipsometry and ab initio approaches to the refractive index of porous silicon

Spectroscopic ellipsometry is used to determine the complex refractive index(n−ik), porosity, and thickness of porous silicon (PSi) films. These films are obtained by anodizingp-type crystalline silicon in a hydrofluoric acid bath. After etching, PSi samples are heated to750 °C in a controlled oxygen environment. A detailed analysis of the ellipsometry data is performed inorder to determine the complex refractive index of PSi thin film. This frequency dependence ofn and k is compared with the results of ab initio quantum mechanical calculations carried out bymeans of CASTEP codes within the density functional theory. The theoretical results showa diminution of the lattice constant as the oxygen content grows, in contrast to thehydrogen-saturated surface case.

Optoelectronic study in porous silicon thin films

A photoconductivity (PC) study in as deposited porous silicon (PS) thin films is presented in this work. PS thin films were produced by the electrochemical anodizing method at different anodizing times. The films surfaces were characterized by SEM and porosity was determined by gravimetric methods. Photoluminescence and PC measurements were taken at room temperature. The maximum of the photoluminescence spectra are located around 650nm, whereas those of PC are placed around 400nm. The maximum of the photoluminescence signal shifts toward short wavelengths as the quantum dimension of the material skeleton diminishes, while any spectral displacement of the photocurrent signal as the porosity of the material increases is not observed. The spectral position of the PC signal does not change because it is strongly affected by the large quantity of defects present in the sample surface which diminishes the mean free path of the carriers to reach the electrodes. In all the samples photocurrent is small around 10−1μA and the intensity of the signal goes down as the porosity increases. Two mechanisms exist that compete with one another, the carrier generation and recombination through light emission centers which diminish the photocurrent.

Continuous planar phospholipid bilayer supported on porous silicon thin film reflector

Reconstituting artificial membranes for in vitro studies of cell barrier mechanisms and properties is of major interest in biology. Here, artificial membranes supported on porous silicon photonic crystal reflectors are prepared and investigated. The materials are of interest for label-free probing of supported membrane events such as protein binding, molecular recognition, and transport. The porous silicon substrates are prepared as multilayered films consisting of a periodically varying porosity, with pore dimensions of a few nanometers in size. Planar phospholipid bilayers are deposited on the topmost surface of the oxidized hydrophilic mesoporous silicon films. Atomic force microscopy provides evidence of continuous bilayer deposition at the surface, and optical measurements indicate that the lipids do not significantly infiltrate the porous region. The presence of the supported bilayer does not obstruct the optical spectrum from the porous silicon layer, suggesting that the composite structures can act as effective optical biosensors.

Residual Stress Measurement of Porous Silicon Thin Film by Substrate Curvature Method

Extensive research has been done on porous silicon (PS) and its applications in optoelectronics since the discovery of its light emitting properties. Porous silicon technology is also used for silicon micro machining. However, porous films can be seriously strained and this often causes mechanical curling, fracture and device failures. In the present study an optical apparatus based on substrate curvature method was developed for intrinsic stress measurement of thin films, which offered a lot of advantages as overall field, non-contact, high precision, nondestructive, easy operation and quick response. Using the apparatus, the residual stress in porous silicon layers prepared by electrochemical etching was obtained. The residual stresses in the films were determined by measuring the curvature of the Si substrate before and after etching. It is found that the residual tensile stress tends to increase with the porosity increasing and the doping concentration of the silicon wafer increasing. The results show that there is a deep connection between the microstructure PS and the residual stress distribution.

Influence of the current density and resistivity on the optical properties of p-type porous silicon thin films fabricated by the electrochemical anodizing method

Porous Silicon thin films were produced in this work by the electrochemical anodizing method. The samples were fabricated anodizing p type Si substrates with different resistivities in hydrofluoric acid. Samples were prepared at different current densities. The films were characterized through reflectance and room photoluminescence (PL) measurements in the visible region. A simple theoretical model is proposed to calculate the Optical constants of the material using a theoretical simulation of the reflectance spectra. The maximum positions of PL signals are shifted to long wavelengths as the porosity increase in samples prepared with substrates of resistivities of 0.1 and 1 W-cm, while an opposite behaviour is observed in samples prepared on more doped substrates. The optical constants obtained through the calculation are the spectral variation of the refractive index and extinction coefficient, also are obtained the sample thickness and surface roughness.

Optical properties of porphyrin molecules immobilized in nano-porous silicon

The paper aims at studying optical properties of porous silicon powders and thin films which were impregnated with different porphyrin molecules. It has been shown that introducing porphyrins into porous silicon matrix results in quenching of luminescence from porous silicon, while luminescence of porphyrins survives, though its structure changes. At the same time, porphyrins in porous silicon matrix which was preliminarily oxidized does not alter luminescence from porphyrins. Generation of singlet oxygen by illuminated porphyrin/porous silicon composite is confirmed by additional oxidation of porous silicon and by the observation of characteristic 1270 nm luminescence band.