Effect of ultraviolet illumination and ambient gases on the photoluminescence and electrical properties of nanoporous silicon layer for organic vapor sensor

The purpose of this research, the nanoporous silicon layer were fabricated and investigated the physical properties such as photoluminescence and the electrical properties in order to develop organic vapor sensor by using nanoporous silicon. The Changes in the photoluminescence intensity of nanoporous silicon samples are studied during ultraviolet illumination in various ambient gases such as nitrogen, oxigen and vacuum. In this paper, the nanoporous silicon layer was used as organic vapor adsorption and sensing element. The advantage of this device are simple process compatible in silicon technology and usable in room temperature. The structure of this device consists of nanoporous silicon layer which is formed by anodization of silicon wafer in hydrofluoric acid solution and aluminum electrode which deposited on the top of nanoporous silicon layer by evaporator. The nanoporous silicon sensors were placed in a gas chamber with various organic vapor such as ethanol, methanol and isopropyl alcohol. From studying on electrical characteristics of this device, it is found that the nanoporous silicon layer can detect the different organic vapor. Therefore, the nanoporous silicon is important material for organic vapor sensor and it can develop to other applications about gas sensors in the future.     

A technique for the fabrication of various multiparametric porous silicon samples on a single substrate

Porous silicon (PSi) samples generally have a uniform thickness and pore size according to specific anodization conditions, as the Si wafer is entirely immersed into hydrofluoric (HF) acid during the anodization process. In contrast, multiparametric (MP) PSi, as described in the present work, is fabricated by inserting a Si wafer gradually (or by stages) into a HF solution during the anodization process. Consequently, MP-PSi allows single layer fabrication with a pore-size and layer thickness gradient or various multilayers, on a single substrate. Therefore, MP-PSi can be readily used in sensor application areas to determine optimized detection conditions for various materials, such as gas, liquid, and bio-materials. MP-PSi layer with a lateral pore gradient distribution can also be used as size-exclusion matrix. In addition, the MP-PSi multilayer array is expected to open up application areas involving optical electronic nose systems.     

A carpet cloak for visible light

We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.     

High-performance and broadband photodetection of bicrystalline(GaN)1-x(ZnO)x solid solution nanowires via crystal defect engineering

Crystal defect engineering is widely used as an effective approach to regulate the optical and opto-electronic properties of semiconductor nanostructures.However,photogenerated electron-hole pair recombination centers caused by structural defects usually lead to the reduction of optoelectronic perfor-mance.In this work,a high-performance photodetector based on(GaN)1-x(ZnO)x solid solution nanowire with bicrystal structure is fabricated and it shows excellent photoresponse to ultraviolet and visible light.The highest responsivity of the photodetector is as high as 60,86 and 43 A/W under the irradiation of 365 nm,532 nm and 650 nm,respectively.The corresponding response time is as fast as 170,320 and 160 ms.Such wide spectral responses can be attributed to various intermediate energy levels induced by the introduction of various structural defects and dopants in the solid solution nanowire.Moreover,the peculiar bicrystal boundary along the axial direction of the nanowire provides two parallel and fast trans-mission channels for photo-generated carriers,reducing the recombination of photo-generated carriers.Our findings provide a valued example using crystal defect engineering to broaden the photoresponse range and improve the photodetector performance and thus can be extended to other material systems for various optoelectronic applications.     

Vertically Oriented Titania Nanotubes Prepared by Anodic Oxidation on Si Substrates

Vertically oriented titania nanotube arrays were fabricated by anodization of titanium film deposited on silicon substrates under different processing conditions. The anodic formation of nanoporous titania on silicon substrate was investigated in aqueous solutions mixed with highly corrosive Na₂SO₄/NaF/citric acid. In the result of the anodization of titanium film deposited at room temperature, a very thin layer of ~70 nm having a worm-like structure was grown on the top of the porous layer. But, in the case of titanium film deposited at 500deg, vertically oriented TiO₂ nanotube arrays were formed. The average tube outer diameter of the nanotube was 74 nm to 100 nm. The longest nanotube of 681 mum was obtained at 15 V and 30 min. The current density transient curve recorded during anodization under a constant voltage showed a typical behavior for self-organized pore formation.     

Significant improvement of luminance and stability of a red electroluminescent device using nanocrystalline silicon

An electroluminescent (EL) device using nanocrystalline silicon (nc-Si) was fabricated by annealing after cosputtering of Si chips and silicon dioxide target and subsequent hydrofluoric acid solution treatment. The device emitted a red light with a peak at 670 nm by applying a low direct current (DC) operating voltage of 4.5 V. The external quantum efficiency (EQE) of red luminescence at 4.5 V was 0.35%. Moreover, the intensity of red luminescence was very stable for an operating time of 15000 min. These results are a strong indication that the HF-treated device can be adapted to future light-related devices.     

GaN-UV photodetector integrated with asymmetric metal semiconductor metal structure for enhanced responsivity

Fabrication of very thin GaN ultraviolet photodetectors on Si (111) substrate integrated with asymmetric (Pt–Ag, Pt–Cr) metal–semiconductor–metal (MSM) structure have been illustrated. Designed GaN photodetection device displays significant enhancement in responsivity for asymmetric (Pt–Ag) MSM structure (280 mA/W) in comparison to symmetric (Pt–Pt) MSM structure (126 mA/W) at 10 V bias. The fabricated asymmetric and symmetric devices also exhibit fast response time in the range of 30–59 ms. The enhancement in responsivity using asymmetric MSM structure ascribed to large difference in work function which lead to change in Schottky barrier height of the metal semiconductor junction. Additionally, power dependent photoresponse analysis of GaN asymmetric (Pt–Ag) ultraviolet photodetector was showing a responsivity of 116 mA/W at low optical power of 1 mW. Such GaN asymmetric MSM ultraviolet photodetectors having high responsivity can extensively be used for low power, high speed ultraviolet photo detection applications.     

Vertically aligned and ordered ZnO/CdS nanowire arrays for self-powered UV–visible photosensing

Photodetectors based on photoconductivity effect are usually driven by an external power source. A self-powered photodetector can be powered by incident light using the photovoltaic effect. Here, photoelectrochemical cells with periodically aligned ZnO/CdS nanowire arrays as photoanodes were fabricated and investigated for detecting UV and visible light. At zero bias, this self-powered UV–visible photodetector showed high responsivities of 35.4 and 23.2 mA/W for UV and visible light, a fast rise time of 0.18 s, and a decay time of 0.32 s. The spectral responses of the self-powered photodetectors based on ZnO/CdS nanowire arrays exhibited superior photoresponse in both UV and visible regions in comparison with ZnO nanowire film and ZnO nanowire arrays. The high photosensing performance originates from the excellent light trapping ability at broadband wavelengths and the high charge collection efficiency of the highly ordered ZnO/CdS nanowire arrays. The results indicate that the ZnO/CdS heterojunctions with periodic nanostructures provide a facile frame for UV–visible detecting applications.     

Fabrication of highly efficient full-color electroluminescent device composed of nanocrystalline silicon

We fabricated a highly-efficient full-color electroluminescent device composed of nanocrystalline silicon (nc-Si). High luminance red, green and blue luminescence from the device was achieved by using hydrofluoric acid solution and oxidation techniques, because these techniques lead to reduction of both nc-Si size and P(b)-center on the surface, which is related closely to luminescent color and luminance, respectively. Moreover, direct current (DC) operating voltage on red/green/blue light emission of the device was realized at a relative low value below 10.0 V by controlling the thickness of the oxidized layer on the nc-Si surface. These results are a strong indication that the device developed in this study can be adapted to future flat panel display and illumination fields.     

Absorption enhancement of near infrared in Te doped nanoporous silicon

In this paper, the optical properties of Te doped nanoporous silicon have been studied. The nanoporous silicon was fabricated by using alkaline etching and electrochemical anodization. The etched nanoporous silicon was injected with Te atoms by ion implantation. These nanostructures formed in electrochemical anodization directly affect the optical properties of nanoporous silicon such as reflectance, transmittance and absorptance. According to the optical measurement, the absorptance of the Te doped nanoporous silicon is over 80 % in the wavelength range from 250 to 1,100 nm. The absorptance of Te doped nanoporous silicon at wavelength longer than 1,100 nm is almost four times of that of untreated silicon, indicating that the ion implantation of Te element increases the NIR absorption of nanoporous silicon considerably.     

In Situ Fabrication of Vertical Multilayered MoS2/Si Homotype Heterojunction for High‐Speed Visible–Near‐Infrared Photodetectors

c2D transition metal dichalcogenides (TMDCs)‐based heterostructures have been demonstrated to achieve superior light absorption and photovoltaic effects theoretically and experimentally, making them extremely attractive for realizing optoelectronic devices. In this work, a vertical multilayered n‐MoS₂/n‐silicon homotype heterojunction is fabricated, which takes advantage of multilayered MoS₂ grown in situ directly on plane silicon. Electrical characterization reveals that the resultant device exhibits high sensitivity to visible–near‐infrared light with responsivity up to 11.9 A W^–1. Notably, the photodetector shows high‐speed response time of ≈30.5 µs/71.6 µs and capability to work under higher pulsed light irradiation approaching 100 kHz. The high response speed could be attributed to a good quality of the multilayer MoS₂, as well as in situ device fabrication process. These findings suggest that the multilayered MoS₂/Si homotype heterojunction have great potential application in the field of visible–near‐infrared detection and might be used as elements for construction of high‐speed integrated optoelectronic sensor circuitry.     

Characterization of Nanoporous Silicon-Based DNA Biosensor for the Detection of Salmonella Enteritidis

A biosensor for the detection of food-borne pathogens (Salmonella Enteritidis) was fabricated based on nanoporous silicon (NPS). P-type silicon wafers (100, 0.01 ) were anodized electrochemically in an electrochemical Teflon cell, containing ethanoic hydrofluoric acid solution to produce the porous layer on the silicon surface. The porous silicon surface was functionalized with DNA probes specific to the insertion element (Iel) gene of Salmonella Enteritidis. A biotin-streptavidin system was utilized to characterize the availability of the nanopores and the specificity of the DNA probe. Based on the electrical property of DNA, redox indicators and cyclic voltammetry were used for the characterization of the biosensor. Results showed that the DNA probe was specific to the target DNA, and the porous silicon-based biosensor had more active surface area and higher sensitivity (1 ng/mL) than the planar silicon-based biosensor. This simple, label-free porous silicon-based biosensor has potential applications in high-throughput detection of pathogens.     

Acoustic investigation of porous silicon layers

Porous silicon (PS) layers are formed on p⁺ -type silicon wafers by electrochemical anodization in hydrofluoric acid solutions. Microechography and acoustic signature, V(z), have been performed at 1.5 GHz and 600 MHz, respectively, in order to study the elastic properties of PS layers. The thicknesses of PS layers were measured and longitudinal, shear and Rayleigh velocities and Young's modulus were obtained as a function of porosity. Equations showing the porosity dependence of bulk wave velocities and Young's modulus have also been proposed.     

Lattice distortion of porous Si by Li absorption using two-dimensional photoelectron diffraction

Lithiation and delithiation of porous silicon were studied using reflection high energy electron diffraction (RHEED), two-dimensional photoelectron diffraction, and a stereo atom-scope, which is realized by the combination of a display-type spherical mirror analyzer and circularly polarized soft X-ray. A nanosized porous silicon layer was prepared by electrochemical etching of p-type silicon (001) wafer in ethanolic solutions containing hydrofluoric acid. The morphology of the as-grown porous silicon as observed using SEM was filled with about 9 nm holes. This porous silicon also retains the crystallographic orientation of the wafer from which it was etched and is optically active with visible photoluminescence. The measured RHEED pattern and 2π steradian Si 2p photoelectron diffraction pattern from Si (001) surface showed an increase in lattice constant by lithiation, and that change in lattice constant was restored to its original values by delithiation.     

Micropore x-ray optics using anisotropic wet etching of (110) silicon wafers

To develop x-ray mirrors for micropore optics, smooth silicon (111) sidewalls obtained after anisotropic wet etching of a silicon (110) wafer were studied. A sample device with 19 microm wide (111) sidewalls was fabricated using a 220 microm thick silicon (110) wafer and potassium hydroxide solution. For what we believe to be the first time, x-ray reflection on the (111) sidewalls was detected in the angular response measurement. Compared to ray-tracing simulations, the surface roughness of the sidewalls was estimated to be 3-5 nm, which is consistent with the atomic force microscope and the surface profiler measurements.     

Effect of electroless etching parameters on the growth and reflection properties of silicon nanowires

Vertically aligned silicon nanowire (Si NW) arrays have been fabricated over large areas using an electroless etching (EE) method, which involves etching of silicon wafers in a silver nitrate and hydrofluoric acid based solution. A detailed parametric study determining the relationship between nanowire morphology and time, temperature, solution concentration and starting wafer characteristics (doping type, resistivity, crystallographic orientation) is presented. The as-fabricated Si NW arrays were analyzed by field emission scanning electron microscope (FE-SEM) and a linear dependency of nanowire length to both temperature and time was obtained and the change in the growth rate of Si NWs at increased etching durations was shown. Furthermore, the effects of EE parameters on the optical reflectivity of the Si NWs were investigated in this study. Reflectivity measurements show that the 42.8% reflectivity of the starting silicon wafer drops to 1.3%, recorded for 10 μm long Si NW arrays. The remarkable decrease in optical reflectivity indicates that Si NWs have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection.     

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。     

影响多孔硅孔隙率的因素

研究了在制备多孔硅过程中影响其孔隙率的各种因素 ,给出了氢氟酸浓度、腐蚀时间、阳极腐蚀电流、温度及光照度与多孔硅孔隙率的关系 ,同时研究了多孔硅的晶格常数随其孔隙率变化的规律 ,并对以上各项结果作出了初步解释。     

A micro-system based on glass-nanoporous silicon for optical sensing of organic solvent vapor

We present a recent experimental study on the application of nanoporous silicon (np-Si) to an optical vapor sensor. We fabricated the micro-system based on a glass-nanoporous silicon layer on a p(+)-type silicon wafer. To check the selectivity and sensitivity of the np-Si layer to organic vapors, we prepared three types of np-Si layer samples--a single layer, distributed Bragg reflector (DBR) layer, and microcavity layer--and investigated its reflectance spectra upon exposure to different concentrations of various organic vapors. When the np-Si layer samples were exposed to the organic vapors, a red-shift occurred in the reflectance spectrum, and we determined that this red-shift can be attributed to the changes in the refractive index induced by the capillary condensation of the organic vapor within the pores of the np-Si layer. The np-Si layer samples showed excellent sensing ability to different types and concentrations of organic vapors. After removing the organic vapors, the reflectance spectrum immediately returned to its original state.     

Fabrication of highly ordered nanoscale poly (lactic acid) surface based on self-assembled silica microspheres

A simple process was developed to fabricate poly (lactic acid) (PLA) film possessing a highly ordered nanoscale surface. For the first step, an array of silica microspheres was prepared by self-assembly on a completely hydrophilic silicon wafer. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images showed that a highly ordered array was formed, and then this array was used as the template for fabricating polymer film with highly ordered nanoscale surface. Next, a PLA solution was spin coated on the template. After solidifying, silica microspheres were embedded in the thin layer of PLA, maintaining their highly organized structures. Finally, silica microspheres were etched away by hydrofluoric acid, and only the PLA film with a close-packed hexagonally pattern structure was left on the silicon wafer substrate.