Mechanisms of formation and topological analysis of porous silicon — computational modeling

Porous silicon formation under anodization in HF solution is studied by means of computer simulation, using the model, which takes into account the Si dissolution, thermal generation, diffusion and drift of holes and quantum confinement. Structures of porous layer, obtained in computational experiments at various doping level of initial Si, temperatures, HF concentrations, anode current densities, are shown and analyzed. The porosity and fractal dimension of the obtained porous structures are also analyzed.     

AC impedance spectroscopy of porous silicon thin films containing metallic cations

Thin porous silicon (PS) films were prepared by HF/HNO₃ 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. Al³⁺, 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⁺, Al³⁺ and Li⁺ cations respectively. These kinds of structures are suitable for gas sensing, optical sensing or for the fabrication of fuel cell membranes.     

Heavy ion radiation damage in double-sided silicon strip detectors

A ²⁵²Cf fission fragment source was used to produce heavy-ion radiation damage in a double-sided silicon strip detector. It was found that a good quality fission fragment spectrum (as determined by the peak to valley ration N_L/N_V) could not be achieved for radiation incident on the p⁺ face of the detector. However, for radiation incident on the n⁺ face, the ratio N_L/N_V remained adequate up to an accumulated dose of 4×10⁶ fragments mm⁻². For the measurement of alphas, typical resolution deteriorated from an initial 30 keV FWHM to 50 keV FWHM at a dose of 8×10⁶ fragments mm⁻² for incident on the n⁺ face, and 6×10⁶ for radiation incident on the p⁺ face. The interstrip resistance in one region of the n⁺ face broke down completely after a relatively small radiation doses incident on that face. Further investigation of this is still required.     

适用于微型燃料电池扩散层的多孔硅的可控制备工艺研究

多孔硅以其多孔结构及大的表面体积比等特点被认为是可在基于MEMS技术的微型燃料电池中代替碳 纸、碳布作为扩散层的材料. 本文考虑传统碳纸、碳布作为扩散层的要求, 并结合多孔硅材料的特点, 选用n<100>(0.04～0.15 Ω·cm)单晶硅进行了多孔硅制备工艺的研究; 考察了腐蚀液的浓度、电流密度和氧化处理时间对多孔硅的孔隙率、孔深度及氧化速率 的影响, 实现了多孔硅的可控制备. 孔隙率为40%、孔径为350～700nm、厚度为60μm的多孔硅膜电极经循环伏安测定, 在0.5mol/L H₂SO₄溶液中, 表现出与碳纸相近的电活性, 显示了潜在的应用价值.     

制备温度对多孔硅光学特性的影响(英文)

本文不同的温度下制备多孔硅.通过荧光光谱、光吸收谱、X射线光电子谱研究了多孔硅的光和结构特性.研究结果表明存在着一个制备临界温度343 K,当制备温度从临界温度之下提高到临界温度之上时,多孔硅的荧光和光吸收从红移转向蓝移,同时硅2p电子结合能也从减小转向增大.     

The dependence of hydrofluoric acid concentration during anodization on photoluminescence of porous silicon

The mixture of hydrofluoric (HF) acid and ethanol is used as an electrolyte during anodization of silicon. We investigated the effect of the ratio of HF acid to ethanol on photoluminescence. It is concluded that porous silicon anodized with the electrolyte containing 35 or 40% HF acid provides strong photoluminescence. The fact implies the existence of a chemical reaction including ethanol during anodization other than electrochemical reaction.     

近规整多孔硅电化学制备与表征

电化学阳极氧化条件对多孔硅孔排列的规整度有着显著的影响.提出了一种不需阳极氧化铝模板或预图案化而直接制备近规整多孔硅的电化学方法,分析了氧化时间、电解液组成、HF浓度对多孔硅形态的影响.结果表明,随着阳极氧化时间的增加,多孔硅孔的深度逐渐加大,孔径则呈先增大后稳定的趋势.当氢氟酸(40%)与N-N-二甲基甲酰胺(DMF...     

The dependence of hydrofluoric acid concentration during anodization on photoluminescence of porous silicon

The mixture of hydrofluoric (HF) acid and ethanol is used as an electrolyte during anodization of silicon. We investigated the effect of the ratio of HF acid to ethanol on photoluminescence. It is concluded that porous silicon anodized with the electrolyte containing 35 or 40% HF acid provides strong photoluminescence. The fact implies the existence of a chemical reaction including ethanol during anodization other than electrochemical reaction.     

Effects of porous silicon morphology on ballistic electron emission

Porous silicon ballistic electron emission source with a structure of metal/porous silicon/Si/metal is obtained by anodization, rapid thermal oxidation, and sputtering. The microstructures of porous silicon layers are characterized by means of scanning electron microscope. The results show that disordered pores are formed at anodization current densities of 15 mA/cm2, 30 mA/cm2, and 45 mA/cm2 for 5 min, respectively. However, straight pores are formed at anodization current densities of 60 mA/cm2, and 75 mA/cm2 for 5 min, respectively. The electron emission characteristic of porous silicon ballistic electron emission sources is measured in vacuum. The results show that electrons emitted into the vacuum from the porous silicon samples with disordered pores. Under a bias condition, injected electrons from the substrate are accelerated by the strong electric field on the surfaces of the Si nanocrystallites in disordered pores, and then emitted into the vacuum through Pt film. However, no electron emission is observed in porous silicon samples with straight pores. It attributes to the lack of Si nanocrystallites in straight pores. So there is not accelerating tunnels enough for electrons. According to disordered or straight pores, we can estimate whether PS samples emit electrons or not.     

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.     

Demonstration of the formation of porous silicon films with superior mechanical properties,morphology and stability

Highly stable and mechanically strong thick porous silicon (PS) films have been obtained on textured silicon substrates. Porous silicon formed on textured substrates exhibits higher porosity, better mechanical strength, non-fractured surface morphology and lower stress compared to porous silicon formed on polished silicon substrates at the same current density, time of anodization and method of drying. The improved properties are attributed to the formation of localized highly porous macroscopic plastic regions.     

直流电化学腐蚀法制备多孔硅的表面形貌研究

采用正交实验，直流电化学腐蚀法制备多孔硅。用原子力显微镜对表面进行观察，研究电化学腐蚀参数对其表面形貌的影响。氢氟酸浓度（CHF）升高，使临界电流密度（JPS）增大，有利于多孔硅的形成。电流密度（J）增大，多孔硅的孔隙率和孔径随之变大，而其纳米粒径将变小。腐蚀时间（t）越长，孔径越大，孔越深。     

Determination of Vickers microhardness on porous silicon surfaces

The Vickers microhardness values of two different sets of porous silicon layers were determined at applied load of 98 mN. The sets consisted of Boron-doped substrates anodized at diverse current densities for two different amounts of hydrofluoric acid (HF) in the etching solution. We found that the microhardness of the samples with lower content of HF at the anodization process showed higher values, whereas the Vickers parameter diminishes consistently for higher current densities. A possible explanation of this behavior is proposed.     

Investigation of porous silicon formation during anodic treatment in aqueous HF

The formation of porous silicon (PS) during the anodic treatment of monocrystalline silicon in 24% aqueous HF has been investigated. Three specific regions were identified in the electrode potential curves. Furthermore, it was observed that the density of the PS formed decreases with depth. The thickness of the PS increases according to a linear law. Taking into account these data and using the known chemical reactions at the silicon-electrode interface, a model of the formation of PS was developed.     

Computer modelling of porous silicon formation

Porous silicon formation has been simulated by the finite diffusion length (FDL) model. This considers a dynamic isoconcentration profile from which the aggregating particles begin their random walks. In this paper we report on the isoconcentration profile non-uniformities which increase as the finite diffusion length is increased. The implementation of the FDL model with zero diffusion length generates non-fractal structures with a fractal dimension close to 1. It is found that Eden clusters cannot be generated at zero diffusion length, due to the problem of sinking isoconcentration profile. We conclude that these are limitations that should be considered in the FDL model for improving the understanding of physical phenomena such as formation and morphology of porous silicon.     

Pore morphology influence on catalytic turn-over for enzyme activated porous silicon matrices

The enzyme glucose oxidase (GOx) was coupled to porous silicon of different morphologies and the catalytic turn-over of glucose was recorded for the samples. The recorded catalytic turn-over of the samples clearly indicated the influence of morphology, with respect to dopant concentration and current density, of the porous silicon carrier matrix. The highest rise in catalytic turn-over (350 times), when compared to a non-porous surface, was recorded for a sample with an n-type epilayer on an n⁺-type substrate anodised at 100 mA/cm². A storage and operational stability measurement was performed on the sample showing the highest catalytic efficiency. After 5 months of refrigerated storage a 2% loss of activity was noted, and after 4 days of constant glucose load (0.5 mM) a 56% loss of activity was recorded. A BET (Brunauer, Emmet, and Teller) nitrogen adsorption analysis was performed on one of the substrate types, p⁺-type (0.001–0.025 Ω cm). In spite of the fine porous morphology with a high surface area the recorded enzyme activities were moderate. The pore morphology achieved on this substrate most likely comprised too small pores in a too dense porous matrix giving poor diffusion conditions to give efficient access for the enzyme during the coupling procedure and for the reactant transport during operation to fully utilise the surface enlargement of the porous layer.     

Capacitance of silicon pixels

Capacitance measurements have been made on silicon pixel sensors of types n⁺ on n, p⁺on n, and n⁺ on p. The arrays test a variety of implant and gap widths, and the n⁺ on n devices test several p-stop designs. The measurements examine inter-pixel and backplane contributions and include studies of temperature dependence. Measurements were made before and after irradiation with fluences relevant to LHC experiments and Fermilab Tevatron Run 2.     

Surface and optical characterization of the porous silicon textured surface

In the present studies, the structural and optical properties of the electrochemically etched PS layers are presented. The formation conditions under constant anodization current density was varied to get a variety of PS samples to analyze the structural and optical characteristics of the porous silicon layers and, then to correlate the resultant surface morphology with the etching process. The low-porosity PS layers thus formed on the silicon substrate have a refractive index value (n_ps = 1.9), which is an intermediate value between bulk silicon substrate (n_Si = 3.4) and air (n_air = 1.0). The results of diffused reflectance, surface morphology by atomic force microscopy (AFM), and Raman scattering measurements show that the resultant surface morphology of the PS layers consist of irregular and randomly distributed nanocrystalline Si structures. The reduction in reflection of the low porosity porous silicon layers is due to light scattering and light trapping of the incoming light by total randomization of the incoming light within the PS structure. The Fourier transform infrared (FTIR) measurements on the PS layer on Si substrate show that PS surface is characterized by chemical species like Si—H and Si—O etc., co-existing on the surface. The presence of hydrogen-related species on the PS layer can provide to some extent a surface passivation effect.     

Anodically oxidized porous silicon as a substrate for EIS sensors

We study the electrochemical response of a field effect capacitor composed by a porous silicon (PS)/silicon dioxide (SiO₂) structure as transducer's surface and p-tert-butylcalix[6]arene molecules as a recognizing agent towards nickel ions. Silicon samples were electrochemically anodized in a hydrofluoric acid (HF) electrolyte leading to PS formation. SiO₂ layers were obtained by anodic oxidation (AO) of PS in aqueous solution. Electrochemical measurements of the sensor with an Electrolyte/Insulator/Semiconductor (EIS) structure have been performed in the Capacitance/Voltage (C/V) mode. A comparative study of sensor responses depending on AO solutions is presented. We have observed a closer Nernstian response, of the coated and oxidized PS, to the Ni²⁺ ions that were anodically oxidized in a KNO₃ (1 M) solution.     

Energy and spatial response of silicon strip detectors to X-rays

This paper describes an experimental investigation of the energy and spatial response of silicon strip detectors used for X-ray measurements. The measurements of single strip amplitude distributions have been performed for a p⁺–n silicon strip detector irradiated with X-rays for different detector bias voltages and for two measurements geometries (with the detector irradiated from either the strip side or from the ohmic contact side). The measured amplitude distributions have been compared with those obtained from simulations using the developed simulation package. The spatial response of the detector has been measured by scanning an edge across the strips and measuring the corresponding strip count rate. The measured spatial response has been compared with that obtained from simulations.