Porous Silicon Monolayers Research Articles

Plasmonic Nanohole Arrays on Top of Porous Silicon Sensors: A Win-Win Situation.

Label-free optical sensors are attractive candidates, for example, for detecting toxic substances and monitoring biomolecular interactions. Their performance can be pushed by the design of the sensor through clever material choices and integration of components. In this work, two porous materials, namely, porous silicon and plasmonic nanohole arrays, are combined in order to obtain increased sensitivity and dual-mode sensing capabilities. For this purpose, porous silicon monolayers are prepared by electrochemical etching and plasmonic nanohole arrays are obtained using a bottom-up strategy. Hybrid sensors of these two materials are realized by transferring the plasmonic nanohole array on top of the porous silicon. Reflectance spectra of the hybrid sensors are characterized by a fringe pattern resulting from the Fabry-Pérot interference at the porous silicon borders, which is overlaid with a broad dip based on surface plasmon resonance in the plasmonic nanohole array. In addition, the hybrid sensor shows a significant higher reflectance in comparison to the porous silicon monolayer. The sensitivities of the hybrid sensor to refractive index changes are separately determined for both components. A significant increase in sensitivity from 213 ± 12 to 386 ± 5 nm/RIU is determined for the transfer of the plasmonic nanohole array sensors from solid glass substrates to porous silicon monolayers. In contrast, the spectral position of the interference pattern of porous silicon monolayers in different media is not affected by the presence of the plasmonic nanohole array. However, the changes in fringe pattern reflectance of the hybrid sensor are increased 3.7-fold after being covered with plasmonic nanohole arrays and could be used for high-sensitivity sensing. Finally, the capability of the hybrid sensor for simultaneous and independent dual-mode sensing is demonstrated.

Alternating current impedance spectroscopic investigation of an a-Si:H/c-Si heterojunction with porous silicon multilayers

In this paper, we study structures contained thin hydrogenated amorphous silicon (a-Si:H) films deposited on porous silicon multilayers (Distributed Bragg Reflector DBR) formed by electrochemical etching of crystalline silicon (c-Si) wafers. The a-Si:H thin films were deposited by the Direct-current magnetron sputtering technique in a mixture of argon and hydrogen atmosphere. It is shown in our previous work that the Bragg mirrors formed on crystalline silicon play an important role in the light trapping in a-Si:H. The structural features of porous silicon, in terms of porosity, thickness and surface roughness, were determined using spectroscopic ellipsometry. The elaborated structures were analysed by impedance spectroscopy. The fit of data obtained from the impedance spectroscopy allowed us to determine the electrical equivalent circuit. This permits the identification of the electronic behaviour of the structures and allows us to explain the role of their different components, including the interfaces. For this purpose, we have proceeded successively. First, by the characterisation of porous silicon monolayers, afterward structures formed by porous silicon multilayers as DBR and structures of a-Si:H/c-Si and a-Si:H/PSi(DBR)/c-Si are characterised. A negative capacitance was found for the heterojunction with DBR, which was attributed to the existence of defect states at the interfaces between porous and amorphous silicon.

One Spot—Two Sensors: Porous Silicon Interferometers in Combination With Gold Nanostructures Showing Localized Surface Plasmon Resonance

Sensors composed of a porous silicon monolayer covered with a film of nanostructured gold layer, which provide two optical signal transduction methods, are fabricated and thoroughly characterized concerning their sensing performance. For this purpose, silicon substrates were electrochemically etched in order to obtain porous silicon monolayers, which were subsequently immersed in gold salt solution facilitating the formation of a porous gold nanoparticle layer on top of the porous silicon. The deposition process was monitored by reflectance spectroscopy, and the appearance of a dip in the interference pattern of the porous silicon layer was observed. This dip can be assigned to the absorption of light by the deposited gold nanostructures leading to localized surface plasmon resonance. The bulk sensitivity of these sensors was determined by recording reflectance spectra in media having different refractive indices and compared to sensors exclusively based on porous silicon or gold nanostructures. A thorough analysis of resulting shifts of the different optical signals in the reflectance spectra on the wavelength scale indicated that the optical response of the porous silicon sensor is not influenced by the presence of a gold nanostructure on top. Moreover, the adsorption of thiol-terminated polystyrene to the sensor surface was solely detected by changes in the position of the dip in the reflectance spectrum, which is assigned to localized surface plasmon resonance in the gold nanostructures. The interference pattern resulting from the porous silicon layer is not shifted to longer wavelengths by the adsorption indicating the independence of the optical response of the two nanostructures, namely porous silicon and nanostructured gold layer, to refractive index changes and pointing to the successful realization of two sensors in one spot.

UV distributed Bragg reflectors build from porous silicon multilayers

UV Distributed Bragg reflectors were fabricated by a two-step thermal oxidation process over porous silicon multilayers (PS-ML), which were prepared by room-temperature electrochemical anodization of silicon wafers. The optical behavior of the PS-ML before and after oxidation was studied by reflectance measurements. It was observed an UV shift from 430 to 300 nm in the peak of the reflectance spectrum after oxidation of the PS-ML. This was attributed to the presence of silicon oxide over the surface of the silicon filaments. Such oxide also reduced the refractive index of each porous silicon monolayer. The bandgap of the PS-ML was calculated by the Kubelka-Munk approximation, which showed an increase in the bandgap from 3.11 to 4.36 eV after the thermal oxidation process. It was suggested that the observed optical response could opens the possibility of fabrication of UV optoelectronic devices based entirely in the silicon technology.

Optical characterization of porous silicon monolayers decorated with hydrogel microspheres

The optical response of porous silicon (pSi) films, covered with a quasi-hexagonal array of hydrogel microspheres, to immersion in ethanol/water mixtures was investigated. For this study, pSi monolayers were fabricated by electrochemical etching, stabilized by thermal oxidation, and decorated with hydrogel microspheres using spin coating. Reflectance spectra of pSi samples with and without deposited hydrogel microspheres were taken at normal incidence. The employed hydrogel microspheres, composed of poly-N-isopropylacrylamide (polyNIPAM), are stimuli-responsive and change their size as well as their refractive index upon exposure to alcohol/water mixtures. Hence, distinct differences in the interference pattern of bare pSi films and pSi layers covered with polyNIPAM spheres could be observed upon their immersion in the respective solutions using reflective interferometric Fourier transform spectroscopy (RIFTS). Here, the amount of reflected light (fast Fourier transform (FFT) amplitude), which corresponds to the refractive index contrast and light scattering at the pSi film interfaces, showed distinct differences for the two fabricated samples. Whereas the FFT amplitude of the bare porous silicon film followed the changes in the refractive index of the surrounding medium, the FFT amplitude of the pSi/polyNIPAM structure depended on the swelling/shrinking of the attached hydrogel spheres and exhibited a minimum in ethanol-water mixtures with 20 wt% ethanol. At this value, the polyNIPAM microgel is collapsed to its minimum size. In contrast, the effective optical thickness, which reflects the effective refractive index of the porous layer, was not influenced by the attached hydrogel spheres.PACS81.05.Rm; 81.16.Dn; 83.80Kn; 42.79.Pw

Photoluminescent logic gate controlled by the optical Kerr effect exhibited by porous silicon

Abstract The magnitude of the third order optical susceptibility exhibited by porous silicon monolayers was measured by a non-degenerated vectorial two-wave interaction. Optical irradiations at 488 nm and 532 nm wavelengths were employed to carry out the nonlinear optical experiments. Compared to bulk silicon material, a noticeable enhancement in the third order nonlinear optical response was identified. Photoluminescence and photoconductive properties were evaluated for the two studied wavelengths. The photoluminescent logic gate function AND was experimentally demonstrated using as a control a reflective optical Kerr gate configuration. A perceptible contribution for the third order optical nonlinearities seems to be related to the optical Kerr effect originated by excited states population. A two-level model was considered in order to describe the observed optical behavior.

Applications of porous silicon formed by electrochemical etching using an electrolyte based on HF:formaldehyde

In this work, we report the experimental results on the formation of porous silicon (PSi) monolayers by electrochemical etching using a formaldehyde based electrolyte. The results were compared with PSi monolayers obtained with the traditional electrolyte (HF:ethanol). Both electrolytes facilitate the removal of H2 generated as a subproduct during the electrochemical etching process in the surface of the c-Si substrate. Formaldehyde presents a good affinity to surfaces and interfaces and the excess of water in the electrolyte reduces the pore sizes of PSi samples. The porosity and etching rate values are similar than those obtained using HF:et solutions. The refractive index values are the same in both cases at the same porosity in the visible range. The results have shown that the chemical characteristics of the ethanol and formaldehyde can give some different advantages to the PSi process and its applications.

Fabrication and characterization of porous silicon layers for applications in optoelectronics

In the present paper, several samples of porous silicon monolayers and multilayers were prepared at different anodization conditions with fixed HF concentration. The room temperature photoluminescence wavelength observed to be increased with increased etching time and current density respectively. By Raman measurement it has been observed that as the size of silicon crystallites decreased with increased etching time, the silicon optical phonon line shifted somewhat to lower frequency from 520.5 cm−1 and became broader asymmetrically. The surface roughness and pyramid like hillocks surface was confirmed by AFM measurement. In SEM images, the porous silicon layers were clearly observed by white and black strips. It was also observed that the reflectivity increased as the number of porous silicon layers was increased.

Confocal imaging of protein distributions in porous silicon optical structures

The performances of porous silicon optical biosensors depend strongly on the arrangementof the biological probes into their sponge-like structures: it is well known that in this casethe sensing species do not fill the pores but instead cover their internal surface. In thispaper, the direct imaging of labelled proteins into different porous silicon structures byusing a confocal laser microscope is reported. The distribution of the biological matter inthe nanostructured material follows a Gaussian behaviour which is typical of the diffusionprocess in the porous media but with substantial differences between a porous siliconmonolayer and a multilayer such as a Bragg mirror. Even if semi-quantitative, the resultscan be very useful in the design of the porous silicon based biosensing devices.

Positive and Negative Pulse Etching Method of Porous SiliconFabrication

We present a new method in which both positive and negative pulses areused to etch silicon for fabrication of porous silicon (PS) monolayer.The optical thickness and morphology of PS monolayer fabricated withdifferent negative pulse voltages are investigated by means ofreflectance spectra, scanning electron microscopy and photoluminescencespectra. It is found that with this method the PS monolayer is thickerand more uniform. The micropores also appear to be more regular thanthose made by common positive pulse etching. This phenomenon isattributed to the vertical etching effect of the PS monolayer beingstrengthened while lateral etching process is restrained. Theexplanation we propose is that negative pulse can help the hydrogencations (H+) in the electrolyte move into the micropores of PSmonolayer. These H+ ions combine with the Si atoms on the wallof new-formed micropores, leading to formation of Si–H bonds. Theformation of Si–H bonds results in a hole depletion layer near themicropore wall surface, which decreases hole density on the surface,preventing the micropore wall from being eroded laterally by F−anions. Therefore during the positive pulse period the etching reactionoccurs exclusively only at the bottom of the micropores where lots ofholes are provided by the anode.

Fabrication of UV filters from porous silicon at low temperatures

AbstractIn the present work we study the influence of the anodization temperature on the morphology and optical properties of porous silicon monolayers and multilayers. UV filters for the wavelength range of 200– 400 nm were fabricated at low temperatures and their optical response was compared with the metallic mirrors as well as with the filters made at room temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A new approach based on transfer matrix formalism to characterize porous silicon layers by reflectometry

AbstractWe use reflectometry coupled to transfer matrix formalism in order to investigate the comparative effect of surface (localized) and volume (distributed) losses inside a porous silicon monolayer. Both are modeled as fictive absorption. Surface losses are described as a Dirac‐like singularity of permittivity localized at an interface whereas volume losses are described trough the imaginary part of the porous silicon complex permittivity. A good agreement with experimental data is determined by this formalism. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of the microstructure during annealing of porous silicon multilayers

We investigate the structural changes in porous silicon multilayers during annealing. Porous silicon multilayers play an important role in layer transfer technologies, where a thin crystalline silicon layer is separated from a wafer and transferred to a foreign substrate. High processing temperatures during epitaxial growth on top of the porous silicon layer lead to a restructuring of the porous layer. With transmission electron microscopy, we evaluate porous silicon monolayers and double-layers. The pore shape changes from open channel-like pores to closed facetted pores during anneal. In double-layers we observe a strong interaction between the two layers leading to an enhanced porosity in the high porosity layer. The observed microstructural evolution is discussed by means of the classical theory of sintering.

Structural Analysis of Porous Silicon Multilayer using X-Ray Diffraction

A porous silicon monolayer (PSL) and four types of porous silicon multilayers (PSMLs) with 2-, 3-, 7-, and 50-layers were studied using the X-ray double crystal method. PSMLs were formed by controlling the anodization current density. Only three distinct peaks were observed in each rocking curve for these PSMLs, which showed two kinds of porous silicon layers with good crystallinity. It was also shown that the PSL and PSMLs were slightly different in the state of lattice strain. To observe the structure and depth-distribution of lattice strain of PSMLs, a new X-ray cross-sectional topographic technique (XCST) has been proposed. Applying XCST to a three-layer porous silicon consisting of 5 µm, 10 µm, and 5 µm layer thicknesses resulted in a clear X-ray cross-sectional topograph. From the topograph and the corresponding rocking curves, the precise distribution of the lattice strain was revealed. The XCST is useful for evaluating not only the PSML, but also general crystal multilayer structures such as superlattice hetero structures.