Biological functionalization and patterning of porous silicon prepared by Pt-assisted chemical etching

Porous silicon fabricated via Pt-assisted chemical etching of p-type Si (1 0 0) in 1:1:1 EtOH/HF/H₂O₂ solution possesses a longer durability in air and in aqueous media than anodized one, which is advantageous for biomedical applications. Its surface SiHx (x = 1 and 2) species can react with 10-undecylenic acid completely under microwave irradiation, and subsequent derivatizations of the end carboxylic acid result in affinity capture of proteins. We applied two approaches to produce protein microarrays: photolithography and spotting. The former provides a homogeneous microarray with a very low fluorescence background, while the latter presents an inhomogeneous microarray with a high noise background.     

Surface and bulk structural properties of nanostructured porous silicon prepared by electrochemical etching at different etching time

Nanostructured porous silicon (NPSi) is versatile nanomaterials, and attractive area in device application after visible luminescence was observed from NPSi by Canham (1990). NPSi has been prepared by electrochemical techniques with silicon wafer as a based material. The electrolyte solution consists of ethanol and hydrofluoric acid at volume ratio of 1:1. The etching time was varied while other preparation parameters were fixed to produce different porosity of NPSi samples. The structural properties of samples were measured using field emission scanning electron microscope and Raman spectrometer. The surface structural study has shown the surface roughness increase at inertial stage but decrease gradually with longer etching time. However, nanostructured surface was decreased with increasing of etching time. From side view measurement, the nanopillar of NPSi becomes smaller size while increase of etching time. The crystallinity of PSi is observed by Raman scattering varied with different etching time. The photoluminescence measurement will be carried out to study the correlation between optical and structural properties.     

Optical properties of one-dimensional photonic crystals fabricated by photo-electrochemical etching of silicon

The optical properties of one-dimensional photonic crystals (1D PCs), fabricated by electrochemical etching of periodic wall arrays on n-type (100) Si substrates, are investigated in this study. Several 1D PCs were fabricated with lattice periods varying from 4 to 7 μm and with trench depths in the range 160–210 μm. In-plane reflection spectra of the photonic structures at different depths were registered over a wide spectral range of 1.5–15 μm using Fourier Transform Infra-Red (FTIR) micro-spectroscopy. Some of the features observed in the reflection spectra of the structures investigated are believed to be as a result of interface roughness. A corrugated side-wall surface, an artifact of the fabrication technique, results in the degradation of optical reflection characteristics, principally mainly in the near IR spectral range, and the emergence of optical anisotropy. As a result of the periodicity, modulation of the reflection spectra, that is, the difference between the maxima and minima of the interference fringes, reached a value of 95% in the mid-infrared. The optical properties of the structures investigated indicate that they show promise for microphotonics applications.     

Effect of different electrolytes on porous GaN using photo-electrochemical etching

This article reports the properties and the behavior of GaN during the photoelectrochemical etching process using four different electrolytes. The measurements show that the porosity strongly depends on the electrolyte and highly affects the surface morphology of etched samples, which has been revealed by scanning electron microscopy (SEM) images. Peak intensity of the photoluminescence (PL) spectra of the porous GaN samples was observed to be enhanced and strongly depend on the electrolytes. Among the samples, there is a little difference in the peak position indicating that the change of porosity has little influence on the PL peak shift, while it highly affecting the peak intensity. Raman spectra of porous GaN under four different solution exhibit phonon mode E₂ (high), A₁ (LO), A₁ (TO) and E₂ (low). There was a red shift in E₂ (high) in all samples, indicating a relaxation of stress in the porous GaN surface with respect to the underlying single crystalline epitaxial GaN. Raman and PL intensities were high for samples etched in H₂SO₄:H₂O₂ and KOH followed by the samples etched in HF:HNO₃ and in HF:C₂H₅OH.     

Characterization of porous silicon prepared by powder technology

In this study, we have proposed the powder technology as new method for preparation of bulk porous silicon. Formation of porous silicon by high-energy ball milling followed by pressing and sintering was studied by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). A crystalline wafer with (1 1 1) orientation was extensively ball milled up to 72 h leading to a decrease in average crystallite size up to 15 nm. The most significant reduction of crystallite size was observed after milling process for about 24 h. The nanopowders were then pressed into pellets at a pressure up to 400 MPa and sintered at 1173 K for 60 min in a high purity argon atmosphere. Results showed that after sintering the material became porous with uniform porosity in whole volume, independently of the sinter size. It is not possible to prepare such porous materials using the conventional electrochemical etching, where the porous structure depth usually does not exceed tens of micrometers. Core-level XPS studies showed very good agreement between peak positions of the sintered porous silicon and in-situ prepared polycrystalline 20 nm-Si thin film or single-crystalline Si (1 1 1) wafer. Furthermore, the valence band spectra measured for sintered samples are broader compared to those measured for the Si (1 1 1) wafer or polycrystalline Si thin film. On the other hand, the shape and broadening of the valence bands measured for the sintered samples are in very good agreement with those reported for electrochemically prepared porous silicon.     

The effect of etching time of porous silicon on solar cell performance

Porous silicon (PS) layers based on crystalline silicon (c-Si) n-type wafers with (1 0 0) orientation were prepared using electrochemical etching process at different etching times. The optimal etching time for fabricating the PS layers is 20 min. Nanopores were produced on the PS layer with an average diameter of 5.7 nm. These increased the porosity to 91%. The reduction in the average crystallite size was confirmed by an increase in the broadening of the FWHM as estimated from XRD measurements. The photoluminescence (PL) peaks intensities increased with increasing porosity and showed a greater blue shift in luminescence. Stronger Raman spectral intensity was observed, which shifted and broadened to a lower wave numbers of 514.5 cm⁻¹ as a function of etching time. The lowest effective reflectance of the PS layers was obtained at 20 min etching time. The PS exhibited excellent light-trapping at wavelengths ranging from 400 to 1000 nm. The fabrication of the solar cells based on the PS anti-reflection coating (ARC) layers achieved its highest efficiency at 15.50% at 20 min etching time. The I-V characteristics were studied under 100 mW/cm² illumination conditions.     

Nanostructural and photoluminescence features of nanoporous silicon prepared by anodic etching

The nanostructural and photoluminescence (PL) features of nanoporous Si (NPS) were investigated in terms of various process parameters such as current density, etching time and oxidation conditions. The NPS was prepared by electrochemical anodic etching of p-type (0 0 1) Si wafers of 4 Ω cm resistivity in HF solution. The pores are of polygon-type columns with 5, 6 and 7 side walls. The average diameter of the column-shaped pores is critically determined by the current density, while the etching time plays an important role on the pore depth; in particular, when the current densities of 30 and 100 mA/cm² were applied, the pore diameters were 9 nm and 3.3 μm, respectively. The variation in the PL characteristics of the NPS with oxidation condition and etching current density was measured and then related with their structural changes. The aging and thermal treatments produce oxidation and lattice distortion in the NPS. The degree of deviation from the as-prepared NPS during aging or thermal treatment seems to depend on the nanostructure as well as morphology of the NPS. It is found in this study that etching current density plays an important role on such structural features of the NPS.     

Structural and optical features of nanoporous silicon prepared by electrochemical anodic etching

Nanoporous silicon (NPS) samples were prepared by electrochemical anodic etching of p-type (0 0 1) silicon wafers in HF solution, and some of them were aged in air. The nanostructural, optical and chemical features of the NPS were investigated in terms of etching and aging conditions. The surface of the porous Si exhibits an etched layer with a thickness of 30–40 nm; this layer appears to consist of aggregates of 5–10 nm size nano-crystallites. The NPS exhibited broad photoluminescence (PL) spectra with its peak in the red light region (740 nm). After aging the porous samples for 4 weeks in air, we observed the PL intensity became approximately a fifth of that of the as-prepared one, along with a blue shift. It is very likely that the blue shift of the PL peak was caused by the shrinkage of the Si nano-crystallites due to the oxidation in the surface of the nano-crystallites.     

Stiffness properties of porous silicon nanowires fabricated by electrochemical and laser-induced etching

Nanowires with dimensions of few nanometers were formed on the whole etched surface. The optical analysis of silicon nanostructures was studied. Blue shift luminescence was observed at 660 nm for PS produced by electrochemical etching, and at 629 nm for laser-induced etching. PS produced a blue shift at 622 nm using both etching procedures simultaneously. X-ray diffraction (XRD) was used to investigate the crystallites size of PS as well as to provide an estimate of the degree of crystallinty of the etched sample. Refractive index, optical dielectric constant, bulk modulus and elasticity are calculated to investigate the optical and stiffness properties of PS nanowires, respectively. The elastic constants and the short-range force constants of PS are investigated.     

Porous silicon layers prepared by electrochemical etching for application in silicon thin film solar cells

In this paper, multilayer structures of porous silicon were fabricated by using electrochemical etching and characterized for its optical properties and surface morphology. Samples of monolayer of porous silicon were grown to study the characteristics of porous layer formation with respect to applied current density, etching time and hydrofluoric acid concentrations. Photoluminescence peaks of red emission at wavelength 695 and 650 nm were observed from multilayer porous silicon structures. By atomic force microscopy measurement, hillocks like surface were clearly observed within the host material, which confirmed the formation of pores.     

Black Silicon nanostructures on silicon thin films prepared by reactive ion etching

In this letter, the application of dry etching to thin films on glass is described. The utilized （ICP-RIE） of SF6 and 02 is discussed and a demonstrated. prepare Black Silicon nanostructures on crystalline silicon reactive ion etching with an inductively coupled plasma remarkable increase in light absorption of about 70% is demonstrated.     

Patterning of porous silicon by metal-assisted chemical etching under open circuit potential conditions

Porous silicon is the most studied Si-based light-emitting material. The potential for the application of porous silicon in optoelectronics and also for chemical or biochemical sensing is high. Therefore, the successful patterning of porous silicon on Si wafers is of great interest. HF-based aqueous solutions containing H₂O₂ as oxidizing agent, in combination with appropriate metal deposition, can supply the necessary current in order to sustain the electrochemical etching of single crystalline Si under no external anodic bias. The H₂O₂ concentration can tune the etching rate of the Si wafers as well as the observed photoluminescence intensity and photon energy. We demonstrate that porous silicon growth can be preferentially initiated at sites where metal (Pt) has been deposited and effectively be confined there, in order to form a well-defined pattern of desired geometry. Conventional DC sputtering using stainless-steel masks was applied in order to test various patterning geometries and lengthscales. Photoluminescence spectroscopy, atomic force and optical microscopy were used in order to characterize the produced porous silicon patterns. This method could be a simple, cost-effective way for the production of porous silicon patterns on Si wafers, which could be used in various fields of application.     

Steady-state and time-resolved spectroscopy of porous silicon

We present experimental data on steady-state properties, time-resolved properties and on polarization characteristics of porous silicon photoluminescence and models for the decay processes of the red-orange band. The manifold manifestation of inhomogeneous broadening of this band in emission, excitation, polarization, kinetics and degradation supports the model in which porous silicon is treated as a network of crystallites connected via an oxide interface. Spectral inhomogeneties of the red-orange band can be described in terms of varying shape and size of silicon clusters. The polarization of emission is explained by coexistence of dot-like and wire-like entities, i.e. spherical and non-spherical clusters. The relative weight of these species determines the polarization degree, whereas the kinetics are controlled by the transport of excitations among the clusters. The decay is modeled by a modified stretched exponential function with the local lifetime, the migration lifetime, and a scaling factor. The latter is determined by the dimensionality of the space available for migration which was found to be close to but less than unity. On the nanosecond range two distinct bands in the blue-green region are evaluated that need further studies for interpretation. Generally, arguments are proposed in favor of a quantum confinement origin of the red-orange band and a bridge between quantum-wire and quantum-dot models is provided.     

Low beam current density Auger spectroscopy and surface analysis

Auger and secondary electron spectroscopy become a more and more routine technique in surface characterization. Even with primary electron beam current density as low as 10^?2 or 10^?3 A cm^?2 beam damage were reported in both Auger and LEED experiments. So we developed and compared counting method, brightness modulation and Harris' modulation techniques in terms of signal to noise ratio. The two first methods offer the advantage of a primary beam current density decreasing about 10⁴ times. So various mechanisms of beam damage were identified as thermal, chemical and electrical. The advantage of the method is shown with hydrocarbons adsorption layer; the beam cracking of the organic chain produces a chemical shift of the C_KLL maximum Auger line about 5 eV. This progressive shift is observed with current densities of 10^?5 A cm^?2 order of magnitude. The reproducibility of this low current density Auger spectroscopy allowed the study of the background and the true secondary electron yield modifications when adsorbed layers are built up.     

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.     

Structural and spectroscopic characterization of porous silicon carbide formed by Pt-assisted electroless chemical etching

A novel electroless method of producing porous silicon carbide (PSiC) is presented. Unlike anodic methods of producing PSiC, the electroless process does not require electrical contact during etching. Rather, platinum metal deposited on the wafer before etching serves as a catalyst for the reduction of a chemical oxidant, which combined with UV illumination injects holes into the valence band, the holes subsequently participating in the oxidation and dissolution of the substrate. The etchant is composed of HF and K₂S₂O₈ in water. Various porous morphologies are presented as a function of etchant concentration, time of etching, and SiC polytype. Wafer quality is of the utmost concern when utilizing the electroless wet etchant, since defects such as stacking faults, dislocations, and micropipes have a large impact on the resulting porous structure. Results of imaging and spectroscopic characterization indicate that the porous morphologies produced in this manner should be useful in producing sensors and porous substrates for overgrowth of low dislocation density epitaxial material.     

The effects of chemical etching of porous silicon on Raman spectra

We have investigated the effects of chemical etching on Raman spectra of porous silicon. The as-anodized porous silicon consisted mainly of crystalline silicon, as indicated by the Raman spectra. The background in the spectrum was strong, indicating that the porous silicon surface was rough due to the presence of pores. When chemical etching was performed five times, the Raman spectrum revealed the presence of spherically shaped nanocrystalline silicon whose diameter was around 3.5 nm. Further chemical etching, however, extinguished the nanocrystallites, in addition to smoothing the surface morphology.