A novel method of fabricating porous silicon material: ultrasonically enhanced anodic electrochemical etching

Ultrasonically enhanced anodic electrochemical etching is developed to fabricate luminescent porous silicon (PS) material. The samples prepared by the new etching method exhibit superior characteristics to those prepared by conventional direct current etching. By applying ultrasonically enhanced etching, PS microcavities with much higher quality factors can be fabricated. The improved quality induced by ultrasonic etching can be ascribed to increased rates of escape of hydrogen bubbles and other etched chemical species from the porous silicon pillars' surface. This process will cause the reaction between the etchant and the silicon wafer to proceed more rapidly along the vertical direction in the silicon pores than laterally.     

Dendritic structures by glass electrolysis: Studies and SERS capability

We have grown silver dendrites by electrolysis of silver-to-sodium ion-exchanged silicate glass slides. The DC voltage of 150 and 300 V was applied to thin film coplanar electrodes deposited on the slide surface. Exposing the silver structures with weak etchant resulted in some increase in optical absorption and in the essential enhancement of Raman scattering, by the factor of ~10⁶-10⁷. The solid state nature of the glass-dendrite system provided stability of the formed structures, and the prepared SERS substrates did not require special conditions for storage, but a short etching before use.     

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.     

Fabrication of nanostructured silicon surface using selective chemical etching

A two-stage process based on selective chemical etching induced by metal nanoclusters is used to fabricate nanostructured surfaces of silicon plates with a relatively low reflectance. At silicon surfaces covered with silver nanoclusters, the SERS effect is observed for rhodamine concentrations of about 10^–12 M. At certain technological parameters, the depth of the nanostructured layer weakly depends on the conditions for the two-stage etching, in particular, etching time. Under otherwise equal conditions for etching, the rate of the formation of textured layer in the p-type silicon is two times greater than the formation rate in the n-type silicon.     

Silicon Porosification: State of the Art

This review is devoted to the analysis of the problems related to fabrication of the Si porous layers. The review was motivated by a great interest to Si-based porous materials from nano- to macro-scale for various applications in electronics, optoelectronics, photonics, chemical sensors, biosensors, etc. The peculiarities of the silicon porosification and the principles of preparing porous layers are considered in the present article. Various methods used for Si porosification such as chemical stain etching, chemical vapor etching, laser-induced etching, metal-assisted etching, spark processing and reactive ion (plasma) etching were analyzed. However, the main attention was focused on electrochemical porosification of Si. The review discusses in detail the influence of parameters such as electrolyte composition and pH, current density, etching time, temperature, wafer doping and orientation, lighting, magnetic field, and ultrasonic agitation on the process of Si porosification. It was shown that the structure of porous silicon strongly depends on both technological parameters of electrochemical etching and the parameters of the semiconductor subject to treatment. This review also addresses the main properties of porous silicon, porous multilayer and 3D structure formation, oxidation of porous Si, release of the porous layer, drying, storage, etching, filling and surface functionalizing of porous Si. Features of III-V compound porosification are also briefly analyzed.     

A novel anti-shock silicon etching apparatus for solving diaphragm release problems

This paper presents a novel anti-shock bulk silicon etching apparatus for solving a universal problem which occurs when releasing the diaphragm (e.g.\ SiNx), that the diaphragm tends to be probably cracked by the impact of heating-induced bubbles, the swirling of heating-induced etchant, dithering of the hand and imbalanced etchant pressure during the wafer being taken out. Through finite element methods, the causes of the diaphragm cracking are analysed. The impact of heating-induced bubbles could be the main factor which results in the failure stress of the SiNx diaphragm and the rupture of it. In order to reduce the four potential effects on the cracking of the released diaphragm, an anti-shock bulk silicon etching apparatus is proposed for using during the last etching process of the diaphragm release. That is, the silicon wafer is first put into the regular constant temperature etching apparatus or ultrasonic plus, and when the residual bulk silicon to be etched reaches near the interface of the silicon and SiNx diaphragm, within a distance of 50--80~\mu m (the exact value is determined by the thickness, surface area and intensity of the released diaphragm), the wafer is taken out carefully and put into the said anti-shock silicon etching apparatus. The wafer's position is at the geometrical centre, also the centre of gravity of the etching vessel. An etchant outlet is built at the bottom. The wafer is etched continuously, and at the same time the etchant flows out of the vessel. Optionally, two symmetrically placed low-power heating resistors are put in the anti-shock silicon etching apparatus to quicken the etching process. The heating resistors' power should be low enough to avoid the swirling of the heating-induced etchant and the impact of the heating-induced bubbles on the released diaphragm. According to the experimental results, the released SiNx diaphragm thus treated is unbroken, which proves the practicality of the said anti-shock bulk silicon etching apparatus.     

A comparative study of surface‐enhanced Raman scattering from silver‐coated anodic aluminum oxide and porous silicon

Three types of Ag‐coated arrays from porous anodic aluminum oxide (AAO) were prepared and studied as substrates for surface‐enhanced Raman scattering (SERS). They were compared with Ag‐coated porous silicon (PSi) samples. AAO‐based substrates were prepared by the vapor deposition of silver directly onto the surface of porous AAO with different morphologies of the pores, whereas SERS‐active island films on the PSi were prepared by immersion plating. The resulting metallic nanostructures were characterized by UV‐vis absorption spectroscopy and scanning electron microscopy (SEM). Thermal evaporation leads to the formation of granular arrays of Ag nanoparticles on the surface of AAO. SERS activity of the substrates was tested using water‐soluble cationic Zn(II)‐tetrakis (4‐N‐methylpyridyl) porphyrin (ZnTMPyP4) as a probe molecule. The results indicate that all AAO‐based substrates studied here exhibit some degree of SERS activity. Noteworthy, for excitation at 532 nm, signals from AAO‐based substrates were comparable with those from the PSi‐based ones, whereas for 441.6 nm excitation they were about twice higher. The strongest SERS‐enhancement at 441.6 nm excitationwas provided by the AAO substrates with silver deposited on the monolith (originally nonporous) side of AAO. Preferential SERS‐enhancement of the bands ascribed to the vibrations of the N‐methylpyridinium group of ZnTMPyP4 when going to blue excitation was found. Copyright © 2010 John Wiley & Sons, Ltd.     

银纳米颗粒/多孔硅复合材料的制备与气敏性能研究

采用双槽电化学腐蚀法以电阻率为10-15 Ω·cm的p型<100>晶向的单晶硅片制备了孔径约为1.5 μm, 孔深约为15-20 μm的p型多孔硅, 并以此多孔硅作为基底采用无电沉积法通过调控沉积时间在其表面沉积了不同厚度的银纳米颗粒薄膜. 采用扫描电子显微镜和X 射线衍射仪表征了银纳米颗粒/多孔硅复合材料的形貌和微观结构, 结果表明银纳米颗粒较均匀的分布于多孔硅的表面上且沉积时间对产物的形貌有重要影响. 采用静态配气法在室温下研究了银纳米颗粒/多孔硅复合材料对NH₃的气敏性能. 气敏测试结果表明沉积时间对产物的气敏性能影响较大. 当沉积时间较短时, 适量银纳米颗粒掺杂的多孔硅复合材料由于其较高的比表面积以及特殊的形貌和结构, 对NH₃气体表现出较高的灵敏度、优良的响应/恢复性能. 室温下, 其对50 ppm 的NH₃气体的气敏灵敏度可以达到5.8左右.     

The effects of surface terminal bonds and microstructure of SiO2 aerogel films on dry etching

The effects of microstructure and surface terminal bonds of SiO₂ aerogel films on dry etching were investigated using Ar, SF₆, and C₂F₆ plasma gases. With Ar plasma etching, physical effect of ion bombardment on porous film was found. In residue-free SF₆ plasma etching, reactive etchant transport and high-mass ion bombardment were observed. With C₂F₆ plasma etching, fluorocarbon residue layer was revealed to maintain surface morphology as acting a barrier to radical transport and ion bombardment. An etching of 450°C-annealed SiO₂ aerogel showed that a dense surface induced the decrease in reaction area, inhibition of etchant transport, and then uniform etching.     

Coupled chemical reactions in dynamic nanometric confinement: Ag2O membrane formation during ion track etching

In this study, continuous swift heavy ion tracks in thin polymer foils were etched from both sides to create two conical nanopores opposing each other. Shortly before both cones merged, one of the nanopores was filled with a silver salt solution, whereas etching of the other cone continued. At the moment of track breakthrough, the etchant reacted with the silver salt solution by forming an impermeable and insulating membrane. Continued etching around the thus-created obstacle led to repetitive {etchant – silver salt solution} interactions. The coupling of the two chemical reactions, {etchant – polymer} and {etchant – silver salt solution}, within the confinement of etched tracks, with continuously changing shapes, showed a highly dynamic nature as recorded by measuring both the electrical current and the optical transmission across the foils. At low etching speeds, a central membrane that grew in radius and thickness with time until, at a critical thickness, the membrane became rather impermeable was formed. However, at high etching speeds, the emerging reaction products exhibited a sponge-like consistency, which allowed for their infinite growth. This precipitation was accompanied by a pronounced current spike formation. A simple theoretical model explains, at a minimum, the basic features.     

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.     

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.     

Glass‐embedded silver nanoparticle patterns by masked ion‐exchange process for surface‐enhanced Raman scattering

Glass‐embedded silver nanoparticle patterns were fabricated by masked silver–sodium ion‐exchange process followed by etching to reveal the particles for surface‐enhanced Raman scattering (SERS). The intensity of the enhanced Raman signal is comparable to that of the fluorescence, and the detection limit of 1 nM for Rhodamine 6G has been achieved. Raman images at different etching depths and corresponding morphological images are compared to find optimal SERS signal. Our results demonstrate that silver nanoparticle patterns embedded in glass can be used as SERS‐active substrates. Nanoparticles can be formed in a glass of high optical quality and have potential to be integrated with optical waveguides for a sensor chip. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of pre-surface treatment on the morphology of silicon nanowires fabricated by metal-assisted etching

Herein we demonstrate an improved metal-assisted etching method to achieve highly dense and uniform silicon nanowire arrays. A pre-surface treatment was applied on a silicon wafer before the process of metal-assisted etching in silver nitrate and hydrogen fluoride solution. The treatment made silver ion continuously reduce on silver nuclei adherence on the silicon surface, leading to formation of dense silver nanoparticles. Silver nanoparticles acting as local redox centers cause the formation of dense silicon nanowire arrays. In contrast, an H-terminated silicon surface made silver ion reduce uniformly on the silicon surface to form silver flakes. The silicon nanowires fabricated with a pre-surface treatment reveals higher density than those fabricated without a pre-surface treatment. The volume fraction improves from 18 to 38%. This improvement reduces the solar-weighted reflectance to as low as 3.3% for silicon nanowires with a length of only 0.87 μm. In comparison, the silicon nanowires fabricated without a pre-surface treatment have to be as long as 1.812 μm to achieve the same reflectance.     

银纳米有序阵列SERS活性基底的制备及在膀胱肿瘤细胞检测中的应用

以多孔阳极氧化铝(porous anodic alumina,PAA)膜为模板,采用真空电子束蒸镀技术,分别在PAA多孔层以及阻挡层表面形成了银纳米孔和银纳米帽有序阵列表面增强拉曼散射(surface-enhancedRaman scattering,SERS)活性基底,并以膀胱肿瘤细胞作为分子探针,测试和分析了这两种SERS活性基底的表面增强拉曼光谱的特性。结果表明,两种SERS活性基底对膀胱肿瘤细胞的拉曼散射信号均有很好的增强作用。银纳米帽有序阵列SERS活性基底不仅具有较高的SERS增强和荧光猝灭效应,而且不存在与PAA膜中草酸根杂质相关的干扰峰,可获得膀胱肿瘤细胞拉曼散射光谱的更多细节信息。     

Raman of indigo on a silver surface. Raman and theoretical characterization of indigo deposited on silicon dioxide-coated and uncoated silver nanoparticles

Raman, surface-enhanced Raman scattering, and shell isolated nanoparticles-enhanced Raman scattering techniques were used to study the indigo–nanoparticle interaction nature. Silver nanoparticles were employed with and without a silicon dioxide spacer inert layer. The SERS spectral profile, obtained using silver nanoparticles, is different from the Raman one, which led to the proposition that the indigo–silver interaction is in the range of intermolecular interactions. SERS spectral reproducibility suggests identical organization and orientation of the analyte on the metal surface. The shell isolated nanoparticles enhanced Raman scattering spectrum of indigo, obtained by using silicon dioxide coated silver nanoparticles resulted similar to its Raman spectrum. This result indicates that the indigo structure is chemically unmodified by the silicon dioxide-coated silver surface. From the shell-isolated nanoparticles-enhanced Raman scattering experiments, the electromagnetic mechanism is proposed as the reason for the spectral enhancement. Theoretical calculations allow one to infer both the indigo–silver surface interaction nature and the orientation of indigo on the surface.     

多孔硅光致发光峰半峰全宽的压缩

硅发光对于在单一硅片上实现光电集成是至关重要的.目前已有的使硅产生发光的方法有:掺杂深能级杂质、掺稀土离子、多孔硅、纳米硅以及Si/SiO₂超晶格.声空化所引发的特殊的物理、化学环境为制备光致发光多孔硅薄膜提供了一条重要的途径.实验表明,声化学处理对于改善多孔硅的微结构,提高发光效率和发光稳定性都是一项非常有效的技术.超声波加强阳极电化学腐蚀制备发光多孔硅薄膜,比目前通用的常规方法制备的样品显示出更优良的性质.这种超声波的化学效应源于声空化,即腐蚀液中气泡的形成、生长和急剧崩溃.在多孔硅的腐蚀过程中,由于超声波的作用增加了孔中氢气泡的逸出比率和塌缩,有利于孔沿垂直方向的腐蚀,使多孔硅光致发光峰的半峰全宽压缩到了3.8nm.     

硅中位错与蚀斑的对应关系

通过实验肯定了硅单晶的化学侵蚀定向方法,找出抛光液的最佳配比及抛光时间。确定了所选定的位错侵蚀剂的侵蚀规范;此侵蚀剂对晶面无选择性,能显示出刃型和螺型位错,以及“新”、“旧”位错。通过长时间侵蚀、逐层侵蚀、劈裂面蚀斑的对应、小角晶界上蚀斑的观察、形变硅单晶中蚀斑排列以及弯曲形变样品中蚀斑密度与曲率半径间的关系的研究等方法,证明了用此侵蚀剂所得的蚀斑确实与位错一一对应。     

树枝状银微米材料的制备及其表面增强拉曼光谱研究

为使表面增量拉曼散射(SERS)衬底的制备方法简单快速且提高的基底增强效果,采用置换反应的制备方法,用锌片和硝酸银反应制备出微米银结构SERS活性基底,其具有稳定性好,易保存,制备方法简单,过程快速等特点。用扫描电子显微镜观察得银微米材料表面形貌呈均匀对称的树枝状结构。实验中控制置换反应的时间分别为40,50,60 s时,得到的树枝状银微米材料的长度分别为3,5,10 μm左右,分支分别为700 nm,2 μm,3 μm,发现随着置换反应的时间的增长,微米银树枝及分枝的长度越长,且树枝分枝上逐渐长出纳米级“树叶”结构, 使得微米级银树枝表面具有纳米结构。并且将微米银材料置于硅片上作为SERS衬底,并以罗丹明6G为探针分子,用激发波长为1 064 nm的傅里叶变换拉曼光谱仪检测,研究其在表面增强拉曼光谱中的应用,结果表明树枝状银微米材料有很好的SERS特性,其中置换反应时间为40 s时制备的微米银树枝的增强效果最佳,其增强因子可达到103左右,并且采用用表面活性剂PVP处理硅片的方法后,保持其他条件不变,微米银衬底的SERS增强效果得到进一步加强,增强因子达到104左右。此外,将树枝状银微米材料用水可封存数月,且实验结果的重复性较好。     

罗丹明6G在银纳米线阵列上的SERS光谱研究

制备出有序、均匀的活性衬底一直足表面增强拉曼散射(SERS)研究中的关键.阳极氧化法制备的多孔氧化铝膜的结构有序、均匀,为纳米金属SERS基底的制备提供了模板.以沉积了银的多孔氧化铝组装体为衬底,研究了罗丹明6G(Rh6G)分子的表面增强拉曼散射光谱.结果表明,沉积了银的多孔氧化铝模板是很好的SERS衬底,Rh6G分子在此衬底上的SERS谱强度与银纳米线在表面的显露高度有关,而其拉曼频移未受表面状态的影响,而PO43-离子的存在使SERS强度得到很大提高.