The Effect of a Porous Layer on I-V Characterization of a Polysilicon p-n Junction

In this study, we investigate the effect of an etching process on the rectification property of a p-n junction. To achieve this goal, electrochemical etching (ECE) was employed, using a HF based solution. The morphological properties of the porous samples were characterized by Field Emission Scanning Electron Microscopy (FESEM). The images showed distributed pores in the range of several nanometers. Also, the distribution showed improvement in the size of meso- to macropores. The Current-Voltage measurement of porous and pristine poly silicon p-n junctions was done in dark and illumination conditions. I-V properties in illumination revealed intensive rectification in the proposed junction. Under dark condition, the ideality factor of the porous poly silicon p-n junction was approximately 3.9, which compared to 4.2 for the pristine sample demonstrates 7% improvement. Also, the light sensitivity was increased in the porous one. Furthermore, the light response (Δφ _BP) of the porous sample gave the value of 0.02. In conclusion, the rectification mode could be enhanced through relevant porosity.     

Structuring of Macroporous Silicon for Applications as Photonic Crystals

Uniformity and high refractive index contrast make macroporous silicon an ideal two-dimensional photonic crystal, that can be tailored over a wide range of frequencies. For optical transmission measurements the porous silicon has to be structured further. Light has to be coupled in perpendicular to the pore axis and to traverse a well defined number of pore layers. For this purpose a lateral structuring technique has been developed that allows to remove the porous silicon with a precision of less than one pore lattice constant. Bars of macroporous silicon which are 100 m high, 2–200 m wide and several mm long have been prepared. These bars have been aligned with designed defect structures like linear or bent waveguides in the porous silicon. The achieved samples are well suited to investigate the optical properties of these defects with light traveling perpendicular to the pore axis.     

Small Angle Neutron Scattering in P+-Doped Porous Silicon

On the surface of single crystal silicon wafers, porous layers can be formed by electrochemical etching and their structural properties are determined by the doping type and concentration of the substrate. In p⁺-type doped, (1 0 0) oriented wafers the porous structure consists of tube-like voids and column-like remains of the silicon matrix, all perpendicular with respect to the wafer surface. In small angle neutron scattering experiments the micrometer long and nanometer diameter elongated scattering elements, i.e., tubes and columns can be well represented and approximated by cylindrical form factors. In an earlier experiment the diameter and scattering length distribution of the cylindrical scattering elements were measured in a p⁺-type (1 0 0) oriented porous silicon wafer [G. Kádár, G. Káli, Cs. Dücsõ, and E.B. Vázsonyi, Physica B 234–236, 1014 (1997)] and the tube diameters were seen to vary in the range from about 10 to 24 nm. In this paper the continuing small angle neutron scattering study of porous silicon layers will be presented. The evaluation conditions and method for the measured neutron intensity distributions will be discussed. The pore diameter distribution data calculated from the neutron scattering intensity curves are collected and compared in various samples of (1 0 0) oriented p⁺-doped wafers prepared with different porosity and different layer depth. The structural results and data obtained by small angle neutron scattering experiments may help in understanding the practically useful chemical, electronic and other properties of porous silicon.     

Catalytic Activity of Noble Metals for Metal-Assisted Chemical Etching of Silicon

ABSTRACT: Metal-assisted chemical etching of silicon is an electroless method that can produce porous silicon by immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias. We have been studying the metal-assisted hydrofluoric acid etching of silicon using dissolved oxygen as an oxidizing agent. Three major factors control the etching reaction and the porous silicon structure: photoillumination during etching, oxidizing agents, and metal particles. In this study, the influence of noble metal particles, silver, gold, platinum, and rhodium, on this etching is investigated under dark conditions: the absence of photogenerated charges in the silicon. The silicon dissolution is localized under the particles, and nanopores are formed whose diameters resemble the size of the metal nanoparticles. The etching rate of the silicon and the catalytic activity of the metals for the cathodic reduction of oxygen in the hydrofluoric acid solution increase in the order of silver, gold, platinum and rhodium.     

Porous Silicon Micromachining to Position Optical Fibres in Silicon Integrated Optical Circuits

Low-loss optical fibre connections require deep grooves etched in silicon substrate for accurate fibre positioning. As shown in this paper these grooves can be obtained by using localised formation of porous silicon on patterned substrates. Cr-Au masking layer with a duration in HF solution longer than 30 min is used to fabricate grooves with a depth higher than 75 m. N⁺-type silicon provides grooves with a pseudo-V shape which is compatible with accurate fibre alignment. By using this technology, arrays of optical fibres are positioned with an accuracy higher than 1 m.     

Current improvement of porous silicon photovoltaic devices by using double layer porous silicon structure: applicable in porous silicon solar cells

The photoluminescence (PL) phenomena of porous silicon (PS) samples with different etching times were examined to find out a relationship between PL emission energy (experimental value of PS band gap energy) and the etching time for fabrication of double (two) layer porous silicon sample on one silicon substrate. The dependence of PL Peak energy with etching time was discussed. A double layer PS structure was formed by using two electrochemical reactions with different etching times of 20 and 10 min, respectively. The photovoltaic (PV) properties of mono layer and double layer porous silicon PV devices were examined and compared. The main result is the enhanced short-circuit current (I_sc) of double layer PS structure compared to monolayer ones.     

发光多孔硅的光谱及电化学研究

The light emitting properties of porous silicon formed by electrochemical etching process and the potential distribution at the Si/electrolyte (HF) interface have been studied. The results show that porous silicon photoluminescent properties are sensitive to the formation conditions. During anodic oxidation, selective dissolution of silicon that leads to porous structure formation has significant influence on the interface capacitance characteristics.     

Formation of porous silicon at elevated temperatures

The features of electrochemical formation process of porous silicon (PS) at the temperatures above the room temperature have been studied. It was found that besides electrochemical dissolution, chemical etching takes part in the formation process of PS even for concentrated HF electrolyte. The role of chemical etching increases with temperature causing an increase of the porosity and the crater depth. The temperature dependence of chemical etching rate has been established. Obtained results enable to conclude that OH⁻ ions play a major role in the chemical etching. Electrochemical etching allows to fabricate PS with good surface quality at the temperatures at least below 65 °C provided that HF electrolyte is concentrated.     

Experimental study of macropore formation in p-type silicon in a fluoride solution and the transition between macropore formation and electropolishing

Anodic dissolution of p-Si is studied in diluted fluoride solution (HF 0.05 M + NH₄F 0.05 M, pH 3), with special focus on the physico-chemical parameters which govern the morphology of pore formation (crystallographic orientation, applied potential, and etching time). The effect of potential has been investigated in the transition region between macropore formation and electropolishing. Upon increasing the anodization potential, the pore cross-section changes from circular to square shape, and the bottom of the pores changes from a rounded to a V-shaped profile. Prolonged etching of the contour of (1 1 0) p-Si disks in the regime of porous silicon formation allows for a comparison of the etching characteristics of the orientations. SEM observation indicates indeed different morphologies as a function of the crystal orientation, and the formation of fractal-like structures is obtained for some orientations. In the same geometry and at a potential just above the onset of the electropolishing regime, prolonged anodization allows for a direct measurement of the Si thickness removed as a function of the crystallographic orientation. We clearly observe the etching anisotropy, with etch depth τ(1 1 1) < τ(1 1 0) < τ(1 0 0). This sequence, similar to that observed for current density in more concentrated HF, differs from that observed for the chemical etching of Si in an alkaline solution.     

Plasma-deposited fluoropolymer film mask for local porous silicon formation

ABSTRACT: The study of an innovative fluoropolymer masking layer for silicon anodization is proposed. Due to its high chemical resistance to hydrofluoric acid even under anodic bias, this thin film deposited by plasma has allowed the formation of deep porous silicon regions patterned on the silicon wafer. Unlike most of other masks, fluoropolymer removal after electrochemical etching is rapid and does not alter the porous layer. Local porous regions were thus fabricated both in p+-type and low-doped n-type silicon substrates.     

Selective formation of porous layer on n-type InP by anodic etching combined with scratching

The selective formation of porous layer on n-type InP (0 0 1) surface was investigated by using scratching with a diamond scriber followed by anodic etching in deaerated 0.5 M HCl. Since the InP specimen was highly doped, the anodic etching proceeded in the dark. The potentiodynamic polarization showed the anodic current shoulder in the potential region between 0.8 and 1.3 V (SHE) for the scratched area in addition to the anodic current peak at 1.7 V (SHE) for the intact area. The selective formation of porous layer on the scratched are was brought by the anodic etching at a constant potential between 1.0 and 1.2 V (SHE) for a certain time. The nucleation and growth of etch pits on intact area, however, took place when the time passed the critical value.The cross section of porous layer on the scratched area perpendicular to the or [1 1 0] scratching direction had a V-shape, while the cross section of porous layer on the scratched area parallel to the or [1 1 0] scratching direction had a band structure with stripes oriented to the or direction. Moreover, nano-scratching at a constant normal force in the micro-Newton range followed by anodic etching showed the possibility for selective formation of porous wire with a nano-meter width.     

Released micromachined beams utilizing laterally uniform porosity porous silicon

Suspended micromachined porous silicon beams with laterally uniform porosity are reported, which have been fabricated using standard photolithography processes designed for compatibility with complementary metal-oxide-semiconductor (CMOS) processes. Anodization, annealing, reactive ion etching, repeated photolithography, lift off and electropolishing processes were used to release patterned porous silicon microbeams on a Si substrate. This is the first time that micromachined, suspended PS microbeams have been demonstrated with laterally uniform porosity, well-defined anchors and flat surfaces.

PACS

81.16.-c; 81.16.Nd; 81.16.Rf     

New EXAFS Measurements by XEOL and TEY on Porous Silicon

Results of an EXAFS investigation on porous Silicon carried out by X-ray Excited Optical Luminescence (XEOL) and Total Electron Yield (TEY) techniques, at the Si K absorption edge, are reported. For the first time XEOL spectra of porous silicon have been recorded in a wide energy range (1800–2500 eV) and EXAFS signals have been singled out from them. Simultaneous TEY and XEOL measurements yield to different results: in particular TEY-EXAFS is sensitive up to the third coordination shell of Si, while XEOL-EXAFS reveals only the contributions of the first two coordination shells; moreover they show a different dependence on changes of the etching parameters. This evidences the sensitivity of XEOL technique to the local structure of the quantum confined luminescent sites. The dependence of the light emission properties on the main preparation parameters and their influence on the short-range structure of red and yellow porous silicon samples are also investigated.     

Synthesis and Photoluminescence Properties of Porous Silicon Nanowire Arrays

Herein, we prepare vertical and single crystalline porous silicon nanowires (SiNWs) via a two-step metal-assisted electroless etching method. The porosity of the nanowires is restricted by etchant concentration, etching time and doping lever of the silicon wafer. The diffusion of silver ions could lead to the nucleation of silver nanoparticles on the nanowires and open new etching ways. Like porous silicon (PS), these porous nanowires also show excellent photoluminescence (PL) properties. The PL intensity increases with porosity, with an enhancement of about 100 times observed in our condition experiments. A “red-shift” of the PL peak is also found. Further studies prove that the PL spectrum should be decomposed into two elementary PL bands. The peak at 850 nm is the emission of the localized excitation in the nanoporous structure, while the 750-nm peak should be attributed to the surface-oxidized nanostructure. It could be confirmed from the Fourier transform infrared spectroscopy analyses. These porous SiNW arrays may be useful as the nanoscale optoelectronic devices.     

Crystallization of amorphous silicon thin films deposited by PECVD on nickel-metalized porous silicon

ABSTRACT: Porous silicon layers were elaborated by electrochemical etching of heavily doped p-type silicon substrates. Metallization of porous silicon was carried out by immersion of substrates in diluted aqueous solution of nickel. Amorphous silicon thin films were deposited by plasma-enhanced chemical vapor deposition on metalized porous layers. Deposited amorphous thin films were crystallized under vacuum at 750 [DEGREE SIGN]C. Obtained results from structural, optical, and electrical characterizations show that thermal annealing of amorphous silicon deposited on Ni-metalized porous silicon leads to an enhancement in the crystalline quality and physical properties of the silicon thin films. The improvement in the quality of the film is due to the crystallization of the amorphous film during annealing. This simple and easy method can be used to produce silicon thin films with high quality suitable for thin film solar cell applications.     

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.

PACS

81.05.Rm; 81.16.Dn; 83.80Kn; 42.79.Pw     

石墨烯量子点修饰柔性硅纳米线阵列用于NO2检测

将化学刻蚀法与金属辅助刻蚀法相结合,制备了形貌统一、分布均匀的高质量柔性硅纳米线阵列结构,用石墨烯量子点对其表面进行修饰,得到了表面稳定且具有强载流子传输能力的柔性石墨烯量子点/硅纳米线核?壳结构阵列,用其制备气敏设备检测NO2. 结果表明,基于该阵列的电阻式气敏设备对NO2的检测灵敏性及可重复性极高,检测浓度极限达20 mg/m3;不同弯曲度的柔性石墨烯量子点/硅纳米线阵列的气敏特性未大幅度降低,弯曲90o时响应电流峰值为未弯曲时的70%.     

Permeated Porous Silicon Suspended Membrane as Sub-ppm Benzene Sensor for Air Quality Monitoring

To realise advanced microsensors for a reliable monitoring of very low concentrations of pollutant species such as NO_x, SO₂, CO, O₃ and aromatic hydrocarbons, the use of porous silicon (PS) permeated with semiconducting oxides has been explored. To reduce the power consumption and to make feasible the device to operate in a fast pulsed temperature mode, a novel sensor architecture has been designed. The main feature of the device is represented by a permeated suspended macroporous Si membrane, few tens of microns thick. In this paper the porous silicon formation through a suspended silicon membrane and the morphological characterization of the PS layer are reported. Moreover, the performance of a C₆H₆ gas sensor based on the suspended macroporous Si membrane (30 m thick), permeated with the chemical precursor of Sn oxide is presented. The results have demonstrated the feasibility to realize a macroporous silicon suspended membrane with high specific surface area, efficient electrical insulation and negligible warpage. Furthermore, the permeation of the oxidized macroporous silicon membrane with SnO₂ has been proved to be a valuable approach to fabricate gas sensors suitable to detect aromatic hydrocarbons in a sub-ppm range.     

Luminescence in Porous Silicon—Surface vs. Bulk

The mechanism of luminescence in porous silicon still remains poorly understood. The main point of controversy is whether the luminescence is due to recombination in the quantum size structures that constitute porous silicon or whether it is dominated by surface recombination. In this paper, we present evidence that emphasises the role that surface recombination plays in the luminescence of porous silicon. In this framework, we also attempt to reconcile the resonant luminescence data (which argues for bulk recombination) with our results.     

A Systematic Study of Synchrotron Light Induced Luminescence from Porous Silicon: Implications to Morphology and Chemical Effect Dependent Electronic Behaviour

Using synchrotron as a tunable excitation source, we have carried out a study on the photoluminescence systematics from a series of porous silicon samples prepared under different conditions, Luminescence spectra were recorded with excitation photon energies tuned to the Si L₃,2 absorption edge (100 eV). The luminescence yield was in turn used to monitor the Si L₃,2-edge absorption characteristics of porous silicon. A trend of luminescence wavelength and intensity as a function of preparation conditions emerges. Other related observations are also noted.