Correlation between Vickers microhardness, porous layer thickness and porosity in p-type nanostructured silicon

Mechanical characteristics via Vickers microindentation technique of nanostructured silicon films have been studied. The samples were grown by the usual electrochemical etching process in presence of hydrofluoric acid in two distinct proportions and for several exposure times. The current density was kept constant for all runs. Porosity and porous layer thickness were measured by gravimetric and optical means, respectively. A linear relation between layer thickness and exposure time shows up, in accordance with previous studies. Correspondingly, a slight decrement in the Vickers hardness is observed. On the contrary, the porosity remains independent of exposure time. A possible explanation is therefore proposed and discussed.     

Self-affinity study of nanostructured porous silicon-crystalline silicon interfaces

Self-affinity was used to analyze the roughness at the porous silicon (PS)-crystalline Si (cSi) interfaces fabricated under different conditions. Using the variable bandwidth method, the self-affinity behavior was, qualitatively and quantitatively, analyzed from the cross-section micrographs of the PS samples obtained by field emission scanning electron microscope. The results show that correlation length is related with the average pore width. Roughness exponent is found to be correlated with the interface roughness. In addition, similar experimental roughness exponents were obtained for several interfaces grown by different methods, indicating the intrinsic fractal nature of the PS-cSi interfaces. The results were confirmed through the self-affinity analysis done on the atomic force microscopy profiles.     

Photoluminescence studies on porous silicon/polymer heterostructure

Hybrid devices formed by filling porous silicon with MEH-PPV or poly [2-methoxy-5(2-ethylhexyloxy-p-phenylenevinylene)] have been investigated in this work. Analyses of the structures by scanning electron microscopy (SEM) demonstrated that the porous silicon layer was filled by the polymer with no significant change of the structures except that the polymer was infiltrated in the pores. The photoluminescence (PL) of the structures at 300 K showed that the emission intensity was very high as compared with that of the MEH-PPV films on different substrates such as crystalline silicon (c-Si) and indium tin oxide (ITO). The PL peak in the MEH-PPV/porous silicon composite structure is found to be shifted towards higher energy in comparison with porous silicon PL. A number of possibilities are discussed to explain the observations.     

Coulomb blockade sensors based on nanostructured mesoporous silicon

Mesoporous silicon (mesoPS) is a nanosponge where Si nanocrystals are interconnected forming a disordered 3D array. The electronic characteristics of this material are particularly interesting, due to some intriguing effects, such as a huge increase of conductivity, reversible insulator-to-metal transition and n- or p-type doping of the nanocrystals, exhibited in presence of donor or acceptor molecules like NH₃ and NO₂. Here we report on the observation of a sharp conductance gap, which can be ascribed to Coulomb blockade phenomena. Moreover, we show that the width of the gap can be tuned by NO₂ molecules, so that the fabrication of highly sensitive threshold sensors is possible. Our results suggest that electrochemical etching of heavily doped Si can be used as a simple self-assembly technique for the production of Si nanocrystal arrays and for the fabrication of sensitive nanosensors.     

Properties of fluorescence based on the immobilization of graphene oxide quantum dots in nanostructured porous silicon films

The fluorescence of grapliene oxide quantum dots(GOQDs) that are infiltrated into porous silicon(PSi) is investigated. By dropping activated GOQDs solution onto silanized PSi samples. GOQDs are successfully in?filtrated into a PSi device. The results indicate that the intensity of the fluorescence of the GOQD-infiltrated multilayer with a high reflection band located at its fluorescence spectra scope is approximately double that of the single layer sample. This indicates that the multilayer GOQD-infiltrated PSi substrate is a suitable material for the preparation of sensitive photoluminescence biosensors.     

基于微结构参数建模的多孔硅绝热层热导率研究

多孔硅由于具有较低的热导率,因而可以将其作为半导体器件中的绝热层.与其他从边界散射等复杂微观热传导机制出发建模研究多孔硅的热导率不同,将多孔硅热导率影响机制更表观地归结到孔洞的存在和分布等结构因素上,把整个多孔硅视为由硅连续材料介质和孔洞连续介质通过串联和并联组合成的复合微结构,给予其低热导率一个更为易于理解和简化的解释.进一步把孔隙率对等效热导率的影响分解为两个不同的部分,即纵向部分和横向部分,半定量地给出不同的孔洞结构和分布下孔隙率与等效热导率的关系.与实验数据进行对比后验证了模型的有效性.继而从结构的角度说明了多孔硅热导率较低的原因.     

Study on structural properties of epitaxial silicon films on annealed double layer porous silicon

In this paper, epitaxial silicon films were grown on annealed double layer porous silicon by LPCVD. The evolvement of the double layer porous silicon before and after thermal annealing was investigated by scanning electron microscope. X-ray diffraction and Raman spectroscopy were used to investigate the structural properties of the epitaxial silicon thin films grown at different temperature and different pressure. The results show that the surface of the low-porosity layer becomes smooth and there are just few silicon-bridges connecting the porous layer and the substrate wafer. The qualities of the epitaxial silicon thin films become better along with increasing deposition temperature. All of the Raman peaks of silicon films with different deposition pressure are situated at 521 cm⁻¹ under the deposition temperature of 1100 °C, and the Raman intensity of the silicon film deposited at 100 Pa is much closer to that of the monocrystalline silicon wafer. The epitaxial silicon films are all (4 0 0)-oriented and (4 0 0) peak of silicon film deposited at 100 Pa is more symmetric.     

A new nanoporous material based on amorphous silicon dioxide

Processes for making nanoporous SiO₂ layers on Si via the irradiation of thermally oxidized silicon wafers with fast ions followed by chemical treatment in a solution or vapor of hydrofluoric acid are presented. It is shown that the density, shape, diameter, and length-to-diameter ratio of channels etched in silicon dioxide can be controlled by varying the regimes of fast ion irradiation or chemical treatment of SiO₂/Si structures. Track parameters calculated using the thermal spike model are compared with the chemical etching data.     

Sensor properties of structures with porous silicon layer

We have measured at room temperature current-voltage and noise characteristics of structures with a porous silicon (porosity 80%) layer at adsorption of gases ammonia, propane and butane mixture, and ethyl alcohol vapor. It was obtained that the largest change in CVC and low-frequency noise is observed under action of ammonia gas on the structure. Physical reasons of sensor properties of studied samples are discussed.     

SERS-active substrates based on n-type porous silicon

Porous silicon (PS) prepared from n-type Si crystal is proposed as a new material for the fabrication of sensitive substrates for surface-enhanced Raman scattering (SERS). The formation procedure for nanostructured silver films on the surface of PS was optimized. Maximum of SERS enhancement for rhodamine 6G probing molecule is observed for samples obtained by the immersion plating from the water solution of AgNO₃ with the 10 mM concentration during 5 min. The dependence of morphological parameters of PS and corresponding silvered surfaces on the anodization current density has been studied. It is shown that the most SERS activities possess substrates produced from PS with lower porosity. The optimum of the PS layer thickness for high Raman signal is about 5 μm. The detection limit for rhodamine 6G adsorbed on Ag-coated PS from the 100 pM solution is established to be comparable with that for p-type PS-based substrates. Thus, the n-type porous silicon is suitable material for the preparation of sensitive SERS-active substrates.     

Buffer layer influence on guiding properties of oxidized porous silicon waveguides

We studied the influence of the thickness and porosity of the buffer layer on the guiding properties of oxidized porous silicon waveguides (OPSWG). It is demonstrated how a modified anodization process acts on the porosity of the final oxidized porous silicon. In this way, it is possible to control the refractive index jump between the core of OPSWG made of compact silicon dioxide and the bottom buffer layer made of porous silicon dioxide. The adoption of a double-step anodization process decreases the propagation losses to 0.5 dB/cm against the 8 dB/cm measured for the waveguide realized using a single-step anodization. The main reason seems not to be the increase of the difference of refractive index values but the more homogeneous buffer layer obtained along the core of the waveguide. This homogeneous layer permits a better lateral confinement of the light as demonstrated by spatial refractive index profile measurement.     

Photothermal investigation study of porous silicon layer doped lithium

This paper presents a series of experimental photothermal deflection technique (PTD) spectra of porous silicon layers doped with lithium on crystalline silicon backing (PS/Li) and their numerical analysis. The aim of this work is to investigate the influence of Li doping on the opto-thermal properties (optical absorption, band-gap energy, thermal diffusivity and thermal conductivity). Also, we correlate these results with other evaluation studies such as I–V measurements and atomic force microscope analysis performed on the material. We observe a red shift of the gap, which can be related to the reduction of crystallite size. Moreover, we notice a decrease of thermal properties with the same behavior as electrical conductivity.     

Optimal tunability of waveguides based on silicon photonic crystals infiltrated with liquid crystals

In this work we study the optimization of the tunability range in waveguides based on two-dimensional silicon photonic crystal infiltrated with liquid crystal. The analyzed structure consists of a two-dimensional silicon photonic crystal with a triangular lattice of circular holes where a line of scatterers in the direction Γ–K has been replaced by a line of circular holes with different radius infiltrated by E7 liquid crystal. To this end, we use the plane-wave expansion method considering anisotropy and modelling supercells to account for the lattice defects that define the waveguide. Finally we study the field distributions of the guided modes in order to analyze their symmetries and confinement.     

Arresting photodegradation of porous silicon by a polymer coating

Light soaking in air rapidly decreases photoluminescence (PL) of porous silicon (PS) and increases electron spin resonance (ESR) signal. In vacuum, a short light exposure (<2700 s) increases PL and decreases ESR, but longer exposures again degrade the PL. We could arrest the light-induced degradation over long periods by applying a thin polymer coating, which resulted in constant PL and ESR intensities. The PL intensity of coated PS is comparable to the PL intensity of a fresh PS sample in air. FTIR spectrum suggests new bond formations at the PS/polymer interface that may be responsible for PL stability.     

Electro-tunable one-dimensional photonic crystal structures based on grooved silicon infiltrated with liquid crystal

The paper reports on composite periodic structures fabricated by means of wet anisotropic etching of (1 1 0)-oriented Si on a SOI platform and infiltrated with liquid crystal E7. The electro-optical effect under low voltages was registered for inter-digital structures by both optical microscopy and micro-Raman spectroscopy.     

Heterojunction solar cells produced by porous silicon layer transfer technology

In this paper, we present the result of heterojunction solar cells based on porous silicon layer transfer technology. a-Si/c-Si structured solar cells were prepared in which the c-Si was deposited on annealed double-layer porous silicon by low-pressure chemical vapor deposition. The structural properties and the evolvement of the double-layer porous silicon before and after thermal annealing were investigated by scanning electron microscopy. X-ray diffraction, Raman spectroscopy and a microwave photoconductive decay method were used to investigate the properties of the epitaxial silicon thin films deposited at different pressures. And, the influence of the deposition pressure on the properties of the c-silicon thin films was investigated. The spectral responses of the cells were studied by a quantum efficiency test. The results show that the epitaxial silicon thin film deposited at 100 Pa has better carrier lifetime and better spectral response. Furthermore, the Raman peak intensity of the silicon film prepared at 100 Pa is much closer to that of a monocrystalline silicon wafer. A simple solar cell structure without any light-trapping features showed an efficiency of up to 10.1 %.     

Effective in-device r33 of 735 pm/V on electro-optic polymer infiltrated silicon photonic crystal slot waveguides

We design and fabricate a 320?nm slot for an electro-optic (E-O) polymer infiltrated silicon photonic crystal waveguide. Because of the large slot width, the poling efficiency of the infiltrated E-O polymer (AJCKL1/amorphous polycarbonate) is significantly improved. When coupled with the slow light effect from the silicon photonic crystal waveguide, an effective in-device r(33) of 735?pm/V, which to our knowledge is a record high, is demonstrated, which is ten times higher than the E-O coefficient achieved in thin film material. Because of this ultrahigh E-O efficiency, the V(π)L of the device is only 0.44?V?mm, which is to our knowledge the best result of all E-O polymer modulators.     

Immunosensor application based on Raman spectroscopy of porous silicon

Porous silicon (PS) suitable for optical detection of immunoreaction is fabricated. The structure of immunosensor is prepared by the following steps: oxidization, silanization, glutaraldehyde cross-linker, and covalent binding of antibody. When antigen is added into the immunosensor, the Raman intensity is estimated to be linearly reduced according to the concentration of the surface protective antigen protein A (spaA) of below 4.0 μg ml-1. The ultimate detection limit is 1.412 × 102 pg ml-1. Controlled experiments are also presented with non-immune antigen of the spaA, and results show that the immunosensor has high specificity. Compared with the conventional enzyme-linked immuno sorbent assay (ELISA), this method is quick, inexpensive, and label-free.     

Surface wave photonic device based on porous silicon multilayers

Porous silicon is widely studied in the field of photonics due to its interesting optical properties. In this work, we present theoretical and first experimental studies of a new kind of porous silicon photonic device based on optical surface wave. A theoretical analysis of the device is presented using plane-wave approximation. The porous silicon multilayered structures are realized using electrochemical etching of p⁺-type silicon. Morphological and optical characterizations of the realized structures are reported.