Characterization of nanoporous silicon layer to reduce the optical losses of crystalline silicon solar cells

Reduction of optical losses in crystalline silicon solar cells by surface modification is one of the most important issues of silicon photovoltaics. Porous Si layers on the front surface of textured Si substrates have been investigated with the aim of improving the optical losses of the solar cells, because an anti-reflection coating and a surface passivation can be obtained simultaneously in one process. We have demonstrated the feasibility of a very efficient porous Si AR layer, prepared by a simple, cost effective, electrochemical etching method. Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in sodium carbonate solution. Then, the porous Si layers were formed by electrochemical etching in HF solutions. After that, the properties of porous Si in terms of morphology, structure and reflectance are summarized. The structure of porous Si layers was investigated using SEM. The formation of a nanoporous Si layer on the textured silicon wafer result in a reflectance lower than 5% in the wavelength region from 500 to 900 nm. Such a surface modification allows improving the Si solar cell characteristics. An efficiency of 13.4% is achieved on a monocrystalline silicon solar cell using the electrochemical technique.     

Effect of porous silicon stain etched on large area alkaline textured crystalline silicon solar cells

This work shows the effects of porous silicon stain etched on alkaline textured antireflection coatings of large area monocrystalline silicon solar cells. The texturization process has been produced by immersion of the silicon wafers in different carbonate-based solutions. The porous silicon layers were formed by stain etching in a HNO₃/HF aqueous solution before or after the texturization process. We study the effects of different alkaline and acidic solutions and the etching times on the solar cell parameters and the surface reflectance of the device. We have found that the average reflectance of the surface is lowered when the porous etching is combined with the texturization in the alkaline solution. However, the solar cell characteristics are not improved.     

Electrochemical deposition of zinc selenide and cadmium selenide onto porous silicon from aqueous acidic solutions

An electrochemical deposition process of ZnSe and CdSe compound semiconductors from aqueous acidic solutions onto silicon substrates with porous silicon layers formed on their surfaces was studied by the voltammetry method. The experimental data obtained were compared with the deposition data onto metal and silicon substrates, and the optimal conditions for the binary compound deposition onto porous silicon were determined. Semiconductor films deposited were studied by scanning electron microscopy, X-ray diffractometry, and X-ray microanalysis. The films are shown to have the crystalline structure and a nearly stoichiometric composition with a minor Se excess. Further annealing in air for 15 min allowed the Se concentration to be decreased.     

Photoluminescence studies of thermal impurity diffused porous silicon layers

Porous silicon layers were formed on diffused layers. Both boron and phosphorus impurities were thermally diffused using solid sources in n-Si, p-Si, n-epi/Si and p-epi/Si substrates of various resistivities. Porous silicon on these layers was formed by electrochemical and chemical etching under various etching conditions. Strong visible luminescence was observed from these porous silicon structures. Infrared absorption studies indicated that surface molecule identities are immaterial to the enhancement or degradation of photoluminescence. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enhanced photo luminescence from porous silicon on texturized surface

Porous silicon (PS) was formed on both polished and texturized single crystal silicon (100) by anodic etching. Photoluminescences (PL) from both of these silicon surfaces were measured and compared. A two-fold enhancement of PL from textured silicon surface was obtained. This enhancement could be ascribed to the geometry of the textured surface.     

Macro-/nanoporous silicon as a support for high-performance protein microarrays

The present work demonstrates the possibilities of using macroporous silicon as a substrate for highly sensitive protein chip applications. The formation of 3D porous silicon structures was performed by electrochemical dissolution of monocrystalline silicon. The fabricated macroporous silicon network has a rigid spongelike structure showing high uniformity and mechanical stability. The microfluidic properties of the substrates were found to be essential for a good bioassay performance. Small spot area, good spot reproducibility, and homogeneous spot profiles were demonstrated on the substrates for immobilized aRIgG. Water contact angles were measured on the porous surface and compared to that of planar silicon, silanized glass, and ordinary microscope glass slides. The effect of the porous surface on the performance of a model IgG-binding immunoassay is presented. aRIgG was microdispensed onto the chip surface forming a microarray of spots with high affinity for the target analyte. The dispensing was performed using an in-house-developed piezoelectric flow-through dispenser. Each spot was formed by a single droplet (100 pL) at each position. The macroporous silicon allowed a high-density microarraying with spot densities up to 4400 spots/cm2 in human plasma samples without cross-talk and consumption of only 0.6 pmol of antibodies/1-cm2 array. Antigen levels down to 70 pM were detected.     

Influence of substrates on the structural and morphological properties of RF sputtered ITO thin films for photovoltaic application

Polycrystalline ITO films were deposited by RF sputtering on three different substrates of glass, p-type (100) and multicrystalline textured silicon wafers. Properties of ITO films were analyzed by XRD, SEM, four point probe system and UV/VIS/IR spectrometer. The ITO film on mono and multi-Si crystallizes in a three-dimensional manner, and a granular crystalline structure is formed with a (222) XRD peak, while the ITO film on glass shows strong XRD (400) peak and grows in two dimensions and a domain structure is formed. Resistivity measurements reveal that resistivity of ITO on glass is minimum due to higher concentration of carriers.     

Morphological, structural and optical properties of GaN grown on porous silicon/Si(100) substrate

In the present paper, we report results of GaN layers grown at 800 °C by metal organic vapour phase epitaxy (MOVPE) on porous silicon (PS) formed on Si(100) substrates. The surface morphology and the crystallinity of the GaN films were characterized by scanning electron microscope and X-ray diffraction. It was shown that GaN grows on PS preferentially on hexagonal polycrystalline form. The SEM observation reveals roughly surface textured by disoriented GaN grains having different shapes and sizes. The surface coverage and the wetting of GaN to PS are improved when the thickness of GaN layer increases. The optical properties of GaN layers were examined by PL and CL at low and room temperatures. Besides, the near edge-band (BE) emission, shows yellow (YL) and deep localized excitons bands at approximately 2.2 and 3.3-3.36 eV respectively. The depth CL analysis shows a spatial variation of the dominating YL and BE emissions as the electron beam energy rises from 3 to 25 kV.     

电子束蒸发制备纳米硅锥及其场发射性能研究

应用超高真空电子束蒸发方法，以铁作为催化剂，在硅和多孔硅衬底上生长纳米Si锥阵列。采用原子力显微镜表征生长在不同衬底上纳米硅的形貌特征，测试和比较了不同衬底上纳米硅的电子场发射性能。实验结果表明用这种方法形成了高度为10—35nm的锥状纳米结构，并且这些纳米硅锥阵列的场发射性能良好。比较生长不同衬底上的纳米锥形貌与场发射性能，发现多孔硅衬底上更适合生长这种纳米硅锥。     

TEM study of the interface between HIPed silicon nitride and encapsulation borosilicate glass

A transmission electron microscope study has been made of a silicon nitride component with 6 w/o yttrium oxide as a sintering aid hot isostatically pressed (HIP) with an encapsulation glass of borosilicate. The TEM study concentrated on the interface region between ceramic and glass. Two different types of hexagonal boron nitride were formed near the interface. One, with a textured structure, seemed to nucleate heterogeneously on the surfaces of silicon oxynitride grains. The (001) planes of the crystals extended outwards, giving a thickness of approximately 0.5 microns. The other type formed as hexagonally shaped grains separate from the first type and appeared to have grown as several segments in different directions around a nucleus. In each segment BN layers are parallel to each other and perpendicular to their common [001]_BM direction. This second type of BN crystal was also detected a little further from the surface within the silicon nitride. The volume fraction of additive glassy phase tended to be lower in this surface region than in the bulk. Possible mechanisms of prevention of encapsulation glass penetration into the porous ceramic component during HIP were discussed.     

Atomic force and confocal microscopy for the study of cortical cells cultured on silicon wafers

The primary cortical cells were selected as a model to study the adherence and neural network development on chemically roughened silicon substrates without any coatings using confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM). The silicon substrates have a nano-range roughness (RMS) achieved by chemical etching using hydrofluoric (HF) acid. After 7 days of culturing, the neurons were observed to connect together and form dense neural networks. Furthermore, AFM results revealed that some porous structures at a few micrometer range existed between the neuron cells and the silicon substrates. It is suggested that the porous structures are made of extracellular matrix (ECM) components and play an important role in the neuronal adhesion and neurite outgrowth on the inert silicon wafers.     

Plasticity and Interfacial Dislocation Mechanisms in Epitaxial and Polycrystalline Al Films Constrained by Substrates

Stresses in epitaxial and textured Al films were determined by substrate-curvature measurements.It was found that in both cases the flow stresses increase with decreasing film thickness.The flow stresses in the epitaxial Al films are in agreement with a dislocation-based model,while the same model strongly underestimates the flow stresses of textured Al films.In-situ transmission electron microscopy studies indicate that dislocations channeling through epitxial Al films on single-crytalline(0001)α-Al2O3 substrates frequently deposit dislocation segments adjacent to the interface.Furthermore,the Al/α-Al2O3 interface of textured Al films on oxidized sillicon substrates is between the crystalline Al and the amorphous SiOx interlayer.It is speculated that the different nature of the interfaces changes dislocation mechanisms and thus influences the flow stresses.     

硅基含能材料爆炸性能研究

采用电化学双槽腐蚀法在P型单晶硅片表面生长多孔硅膜。通过扫描电镜（SEM）、能量色谱（EDS）对多孔硅结构参数以及多孔硅含能材料性能进行了分析,同时进行了爆炸性能测试。结果表明：采用电化学腐蚀法可以制备出20nm左右孔径的多孔硅膜;通过原位装药技术形成的多孔硅含能材料在开放空间以及热能、机械撞击、电能、激光能量刺激下发生猛烈爆炸作用。     

Superhydrophilic textured-surfaces on stainless steel substrates

Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) is used to produce micro/nano-textured surfaces on stainless steel substrates at low temperatures for altering the wetting property of the substrates. The micro/nano-textured surfaces were characterized using scanning electron microscopy, X-ray spectroscopy, and X-ray diffraction. The wetting properties of the textured surfaces were characterized by water contact angle measurements. It was found that AIC of a-Si changes the apparent contact angles of stainless steel substrates from 90° to about 0°, measured 0.5 s after a water droplet drops on the surfaces. The study also shows that a superhydrophilic textured surface can be converted to a highly hydrophobic surface with an apparent contact angle of 145° by coating the surface with a layer of octadecyltrichlorosilane.     

Experimental factors controlling analyte ion generation in laser desorption/ionization mass spectrometry on porous silicon

Desorption/ionization on porous silicon (DIOS) is a relatively new laser desorption/ionization technique for the direct mass spectrometric analysis of a wide variety of samples without the requirement of a matrix. Porous silicon substrates were fabricated using the recently developed nonelectrochemical H2O2-metal-HF etching as a versatile platform for investigating the effects of morphology and physical properties of porous silicon on DIOS-MS performance. In addition, laser wavelength, mode of ion detection, pH, and solvent contributions to the desorption/ionization process were studied. Other porous substrates such as GaAs and GaN, with similar surface characteristics but differing in thermal and optical properties from porous silicon, allowed the roles of surface area, optical absorption, and thermal conductivities in the desorption/ionization process to be investigated. Among the porous semiconductors studied, only porous silicon has the combination of large surface area, optical absorption, and thermal conductivity required for efficient analyte ion generation under the conditions studied. In addition to these substrate-related factors, surface wetting, determined by the interaction of deposition solvent with the surface, and charge state of the peptide were found to be important in determining ion generation efficiency.     

Production of a superhydrophilic surface by aluminum-induced crystallization of amorphous silicon

The wettability of glass substrates textured via aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) was studied. It was found that a stable superhydrophilic surface could be produced by AIC of a-Si. This research suggests that densely distributed micron-sized silicon islands with nanoscale silicon spikes produced by the technique of AIC of a-Si are responsible for the stable superhydrophilicity. The study also shows that these superhydrophilic surfaces can be easily converted to superhydrophobic ones by means of coating with C(4)F(8).     

Orientation-enhanced growth and optical properties of ZnO nanowires grown on porous silicon substrates

ZnO nanowires have been synthesized on porous silicon substrates with different porosities via the vapour-liquid-solid method. The texture coefficient analysed from the XRD spectra indicates that the nanowires are more highly orientated on the appropriate porosity of porous silicon substrate than on the smooth surface of silicon. The Raman spectrum reveals the high quality of the ZnO nanowires. From the temperature-dependent photoluminescence spectra, we deduced the activation energies of free and bound excitons.     

采用原电池法制备纳米多孔硅

利用原电池法在硅片表面制备了纳米多孔硅层;用扫描电镜SEM和原子力显微镜AFM观察了多孔硅表面形貌：原电池法与电化学法得到的多孔硅孔径均在10～20nm范围．研究结果表明：铂膜电极厚度的增大以及铂膜电极与暴露硅片面积比的增大,会导致多孔硅层的厚度增大．热学模拟结果表明：以纳米多孔硅作为绝热层可获得与悬浮结构相同的效果．     

化学刻蚀法制备多孔硅的表面形貌研究

应用各种表面形貌分析方法如SEM、TEM、AFM等,通过对用化学刻蚀法形成的多孔硅进行表面形貌分析,发现由于横向腐蚀比较严重而使多孔硅层在达到一定深度后就会自动脱落。腐蚀中由于大量H2的析出,对硅晶体产生巨大的应力,这些应力使硅在比较脆弱的晶粒边界或缺陷处产生微裂纹,多孔硅就从这些地方开始和生长。     

Preparation of NiO Buffer Layer by Direct Oxygenation of Ni Tape

Recently, Nickel-based alloys are widely used for textured substrates tapes. However, the deposition of oxide buffer layers on nickel tapes is required to stop the diffusion of nickel from the tape to superconducting layer and to improve the mismatch between the substrate and superconducting film. Biaxially textured NiO buffer layer is easily formed on cubic textured nickel tape by the technique called surface oxidation epitaxy. In this work, we developed a direct oxygenating method to make the NiO buffer layer on nickel tape. The nickel tape was directly oxygenated in different atmospheres. It is found that in inert atmosphere, a high quality NiO layer can be formed on the surface of the tape. The oxygenating conditions, including atmospheres and temperatures, and their influence on the structure of the NiO buffer layer, were studied in detail and discussed.