22 nm silicon nanowire gas sensor fabricated by trilayer nanoimprint and wet etching

In this work, we demonstrate the fabrication of silicon nanowires down to 22 nm wide using trilayer nanoimprint lithography and wet etching. Using the same template prepared by E-beam lithography (EBL), nanowires with top width of 22 nm and 75 nm are fabricated on boron-doped top silicon layer of SOI substrate. The two samples are tested in 250 ppm NO₂ ambient for gas detection. The 22 nm wide one shows a much higher relative sensitivity than the 75 nm wide one. The simulation which calculates the carrier density by solving Poisson equation was carried out and the results well explain the sensitivity disparity between the two samples.     

Room temperature NO2 gas sensing properties of DBSA doped PPy–WO3 hybrid nanocomposite sensor

We demonstrate the chemiresistive NO₂ gas sensor based on DBSA doped PPy–WO₃ hybrid nanocomposites operating at room temperature. The sensor was fabricated on glass substrate using simple and cost effective drop casting method. The gas sensing performance of sensor was studied for various toxic/flammable analytes like NO₂, C₂H₅OH, CH₃OH, H₂S and NH₃. The sensor shows higher selectivity towards NO₂ gas with 72% response at 100 ppm. Also the sensor can successfully detect low concentration of NO₂ gas upto 5 ppm with reasonable response of 12%. Structural, morphological and compositional analyses evidenced the successful formation of DBSA doped PPy–WO₃ hybrid nanocomposite with uniform dispersion of DBSA into PPy–WO₃ hybrid nanocomposite and enhance the gas sensing behavior. We demonstrated that DBSA doped PPy–WO₃ hybrid nanocomposite sensor films shows excellent reproducibility, high stability, moderate response and recovery time for NO₂ gas in the concentration range of 5–100 ppm. A gas sensing mechanism based on the formation of random nano p–n junctions distributed over the surface of the sensor film has been proposed. In addition modulation of depletion width takes place in sensor on interaction with the target NO₂ gas has been depicted on the basis of schematic energy band diagram. Impedance spectroscopy was employed to study bulk, grain boundary resistance and capacitance before and after exposure of NO₂ gas. The structural and intermolecular interaction within the hybrid nanocomposites were explored by Raman and X-ray photoelectron spectroscopy (XPS), while field emission scanning electron microscopy (FESEM) was used to characterize surface morphology. The present method can be extended to fabricate other organic dopent-conducting polymer–metal oxide hybrid nanocomposite materials and could find better application in the gas sensing.     

Nitrogen dioxide (NO2) sensing performance of p-polypyrrole/n-tungsten oxide hybrid nanocomposites at room temperature

Polypyrrole (PPy)–tungsten oxide (WO₃) hybrid nanocomposite have been successfully synthesized using different weight percentages of tungsten oxide (10–50%) dispersed in polypyrrole matrix by solid state synthesis method. The sensor based on PPy–WO₃ was fabricated on glass substrate using cost effective spin coating method for detection of NO₂ gas in the low concentration range of 5–100 ppm. The gas sensing performance of hybrid material was studied and compared with those of pure PPy and WO₃. It was found that PPy–WO₃ hybrid nanocomposite sensor can complement the drawbacks of pure PPy and WO₃. The structure, morphology and surface composition properties of PPy–WO₃ hybrid nanocomposites were employed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The presence of WO₃ in PPy matrix and their interaction was confirmed using XRD, FTIR techniques. The porous surface morphology was observed with addition of WO₃ in PPy matrix which is useful morphology for gas sensing applications. TEM image of PPy–WO₃ hybrid nanocomposites shows the average diameter of 80–90 nm. X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical composition of nanocomposites. It was observed that 50% WO₃ loaded PPy sensor operating at room temperature exhibit maximum response of 61% towards 100 ppm of NO₂ gas and able to detect low concentration of 5 ppm NO₂ gas with reasonable response of 8%. The hybrid sensor shows better sensitivity, selectivity, reproducibility and stability compared to pure PPy and WO₃. The proposed sensing mechanism of hybrid nanocomposite in presence of air and NO₂ atmosphere was discussed with the help of energy band diagram. Furthermore, the interaction of NO₂ gas with PPy–WO₃ hybrid nanocomposites sensor was studied by cole–cole plot using impedance spectroscopy.     

New insights into acidic wet chemical silicon etching by HF/H2O–NOHSO4–H2SO4 solutions

Acidic wet chemical etching of crystalline silicon has been examined by utilization of HF–NOHSO₄–H₂SO₄ mixtures. In light of our previous studies the effects of nitrosyl ion concentrations on etching rates were studied time- and temperature resolved. The reactivity of crystalline silicon surfaces in HF/H₂SO₄ solutions is determined by NO⁺-ion concentrations at the silicon/electrolyte interface, measured by ion chromatography. Quantitative solution analysis proofed accumulation of ammonium ions and indicated the conversion of NO⁺ as limiting for the overall etching process. Direct participation in the rate-limiting step was confirmed by calculation of activation energies. Increasing NO⁺-ion contents cause transition from reaction (E_A=55 kJ mol^?1) to diffusion controlled (E_A=10 kJ mol^?1) etching procedures. In combination with time and concentration dependent studies of produced structures a convenient regime for selective texturing or polishing polycrystalline silicon surfaces is reported. Qualitative analysis by ¹⁹F-NMR and Raman spectroscopy identified SiF₅^?/HF₂^? complexes as well as elementary hydrogen (H₂) as hitherto unknown products of silicon dissolution reactions in HF–NOHSO₄–H₂SO₄ mixtures. Based on our findings a strategy for fundamental investigations of relevant reaction pathways is presented and discussed with regard to reported mechanistic concepts.     

Phase transformation-dependent sensing performance of multi-walled carbon nanotube–alumina nanocomposite-based gas sensors

Alumina (Al₂O₃) exists in three different phases having different physical properties. In view of this fact, a systematic study has been carried out for the first time to investigate how its various phases influence the sensing performance of a MWCNTs–alumina nanocomposite based trace level gas sensor. A series of composite sensing film were prepared by dispersing MWCNTs in alumina solution followed by a sol–gel process, where the phase of alumina is controlled by specific temperatures set for an annealing process. The analysis revealed that porosity as well as the surface area varies from phase to phase in the composite film and it is the key factor which governs the sensing performance. Brunaur, Emmet and Teller (BET) analysis showed the significant increase in specific surface area of the composite film when boehmite (β-phase) was transformed into γ-phase. X-ray diffraction (XRD) results confirmed the presence of γ-, mixed δ- θ- and α-alumina phases when the annealing temperature of the composite film raised from room temperature to 450 °C, 800 °C and 1000 °C respectively. Field emission scanning electron microscopy (FESEM), BET and Atomic force microscopy (AFM) techniques were employed to examine the resultant porous structure and surface area of the annealed composite films in various phases. The composite having γ-alumina phase (annealed at 450 °C) was found to have maximum response, where the composite having α-alumina phase (annealed at 1000 °C) had the least.     

High responsivity MSM black silicon photodetector

The effect of annealing temperature on photoelectric properties of metal–semiconductor–metal (MSM) black silicon photodetector has been studied. The black silicon was fabricated by alkaline etching and metal assisted etching. The nanopores and micro-columns formed by the etching process enhance spectral absorptance significantly at wavelength from 250 nm to 1100 nm. The MSM black silicon photodetectors were annealed at different temperatures in N₂ ambient with a rapid thermal annealing (RTA) process. The fast ramp-up and cool-down rate of RTA is a key factor that eliminates the tensile stress and point defects in Si nanoparticle made from metal assisted wet etching, leading to significant increase of mobility, conductivity and carrier concentration. In addition, the photocurrent and spectral responsivities of detectors increase with annealing temperature. At the wavelength of 600 nm, the responsivity (76.8 A/W) at 673 K is almost three orders of magnitude greater than that of the unannealed sample.     

Response time and mechanism of Pd modified TiO2 gas sensor

In this work, gas response properties of Pd modified TiO₂ sensing films are discussed when exposed to H₂ and O₂. TiO₂ films are surface modified in PdCl₂-containing solution by the dipping method and treated for different treatment times to get different surface states. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Kröger–Vink defect theory are used to characterize the sensing films. The gas response properties indicate that the sensor response time which related to the rate of change of sensor resistance is affected by the activation energy (E). In particular, the sensor treated at 900 °C for 2 h exhibits a response time of about 20–240 ms when exposed to H₂ and 40–130 ms when exposed to O₂ at 500–800 °C.     

Studies on non-thermal atmospheric pressure plasma process conditions for groove formation on silicon nitride for silicon solar cells

Fabrications of narrow electrode grooves for front electrodes on single crystalline silicon solar cells were examined using surface discharges, in which the electrode grooves were formed by etching a silicon nitride (SiN) film on substrates. The surface discharge could effectively etch the SiN film within 10 s and that a high etching rate more than 1800 nm/min was obtained. An optimum ratio of Ar gas, which was enough to maintain the formation of innumerable surface streamers, was 2.3 times larger than that of etching gas, and a short-term etching with the high discharge voltage was effective to narrow groove width.     

Synthesis,characterization, and supercapacitor studies of manganese (IV) oxide nanowires

One-dimensional manganese (IV) oxide (MnO₂) (～20 nm in average diameter) were synthesized by cathodic electrodeposition and heat treatment. The mechanism of electrodeposition and nanowire formation were discussed. The product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR). Nanowires with varying lengths and diameters were found in TEM and SEM images of the sample. The results of N₂ adsorption–desorption analysis indicated that the BET surface area of the MnO₂ nanowires was 157 m² g^?1 and the pore size distributions were 2.5 and 4.5 nm. The electrochemical performances of the prepared MnO₂ as an electrode material for supercapacitors were evaluated by cyclic voltammetry and galvanostatic charge/discharge measurements in a solution of 0.5 M Na₂SO₄. The higher specific capacitance of 318 F g^?1 and good capacity retention of 86% were achieved after 1000 charge–discharge cycles had been observed for the MnO₂ nanowires electrode.     

Characteristics of germanium dry etching using inductively coupled SF6 plasma

Inductively coupled SF₆ plasma etching of germanium (Ge) was investigated at different inductively coupled plasma (ICP) power levels, the SF₆ flow rate, and the working pressure. The etch rate of Ge increases from 1007 to 2447 nm/min as the SF₆ flow rate increases from 10 to 60 sccm. Also, the etch rate of Ge increases from 265 to 1007 nm/min as the ICP power level increases from 100 to 400 W whereas the etch rate of Ge decreases from 552 to 295 nm/min as the working pressure increases from 5 to 20 mTorr. The etch profile is isotropic. As SF₆ flow, ICP power and working pressure decrease the surface roughness decreases. Optical emission spectroscopy was used to examine the gas phase species in the plasma, and emission from excited atomic S and F has been identified. Composition of the surface due to SF₆ plasmas has been obtained using X-ray photoelectron spectroscopy. Reaction layers on germanium due to inductively coupled SF₆ plasma etching are found to be a thin, layer with of G–-S, Ge–F and Ge–O bonded species.     

Phonon–plasmon coupling in Si doped GaN nanowires

The vibrational properties of silicon doped GaN nanowires with diameters comprised between 40 and 100 nm are studied by Raman spectroscopy through excitation with two different wavelengths: 532 and 405 nm. Excitation at 532 nm does not allow the observation of the coupled phonon–plasmon upper mode for the intentionally doped samples. Yet, excitation at 405 nm results in the appearance of a narrow peak at frequencies close to that of the uncoupled A₁(LO) mode for all samples. This behavior points to phonon–plasmon scattering mediated by large phonon wave-vector in these thin and highly doped nanowires.     

Vertically-tapered silicon nanowire arrays prepared by plasma enhanced chemical vapor deposition: Synthesis,structural characterization and photoluminescence

Vertically aligned silicon nanowires (SiNWs) have been successfully synthesized using pure silane gas as a precursor by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method. The effect of the growth temperature on the morphology, structure and photoluminescence properties of SiNWs has been studied. The SiNWs were needle-liked materials with the length of a few microns having the diameters of tens of nanometers near the bottom and a few nanometers at the top. Thinner nanowires have been obtained at the higher growth temperature process. High resolution transmission electron microscopy confirms that the nanowires are composed of a crystalline silicon core with an oxide shell. The PL spectrum of the Si nanoneedles have shown two emission bands around 450 nm and ~750, which originate from the defects related to oxygen fault in the oxide shell and interfaces between the crystalline Si core and the oxide shell, respectively.     

Effects of reaction time on the morphological,structural, and gas sensing properties of ZnO nanostructures

Morphological transformation was achieved from ZnO hexagonal needle-like rods to hexagonal flower-like rods by varying the reaction growth time using the hydrothermal method. Optical bandgap energies were calculated from the absorption spectra using UV‐vis spectroscopy. Gas sensing properties of flower-like hexagonal ZnO structures at 50 ppm for ethanol (C₂H₅OH) and nitrogen dioxide (NO₂) at different temperatures were analyzed. The sensor showed a higher response toward C₂H₅OH than NO₂ gas at 350 °C.     

Controlling the diameter of silicon nanowires grown using a tin catalyst

Silicon nanowires were grown on ITO-coated glass substrates via a pulsed plasma enhanced chemical vapor deposition method, using tin as a catalyst. The thin films of catalyst, with different thicknesses in the range 10–100 nm, were deposited on the substrates by a thermal evaporation method. The effect of the thickness of the thin film catalyst on the morphology of the silicon nanowires was investigated. The scanning/transmission electron microscopy images showed that the wire diameter increased as the thickness of the thin film catalyst increased. The nanowires grown using a thin film thickness of 10 nm were inhomogeneous in diameter, whereas the other thicknesses led to an increase in the homogeneity of the diameters of the nanowires. The dominant wire diameter of the grown silicon nanowires ranged from 70 to 80 nm with 10 nm catalyst thin film thickness, and increased to a range of 190–200 nm with 100 nm catalyst thin film thickness.     

Texturization of silicon wafers with Na2CO3 and Na2CO3/NaHCO3 solutions for heterojunction solar-cell applications

The formation of pyramidal structures by anisotropic etching of 〈1 0 0〉-oriented monocrystalline silicon wafer surfaces is an effective method to reduce reflection losses originating on the front side of conventional silicon solar cells and silicon-heterojunction (SHJ) solar cells. One of the most common methods of texturization used in the solar-cell industry is based on aqueous solutions of NaOH or KOH and isopropyl alcohol (IPA). However, IPA is toxic and relatively expensive, so efforts are being made to replace it. Among the potential alternatives, solutions based on Na₂CO₃ and Na₂CO₃/NaHCO₃ mixtures have been proposed. In the present study, solutions of Na₂CO₃ and Na₂CO₃/NaHCO₃ mixtures were prepared in order to form pyramidal structures on silicon wafer surfaces. It was not possible to obtain uniform and completely textured surfaces by using aqueous solutions consisting only of Na₂CO₃. NaHCO₃ must be added in order to achieve uniform textured surfaces with low hemispherical reflectance suitable for SHJ solar-cell applications. Textured surfaces with good uniformity and low average hemispherical reflectance (15.4%) were prepared from 〈1 0 0〉 silicon substrates with relatively low etching times (25 min). Good surface passivation (lifetime >600 μs and implicit open-circuit voltage of 690±10 mV) on these p-type textured wafers were achieved.     

Simulation and fabrication of HF microelectromechanical bandpass filter

A high-frequency (HF) micromechanical bandpass filter fabricated using the commercial 0.35 μm complementary metal oxide semiconductor (CMOS) process and the post-process has been investigated in this study. The area of the filter is about 150×200 μm². The filter is composed of two resonators, which are joined by a coupling beam. Each resonator contains a membrane, four supported beams and two fixed electrodes, and the membrane is supported by four supported beams. The filter requires a post-process to etch the sacrificial layer, and to release the suspended structures. The post-process needs only one wet etching to etch silicon dioxide layer. The filter contains a sensing part and a driving part. When applying a driving voltage to the driving part, the sensing part generates a change in capacitance. The capacitance variation of the sensing part is converted into the output voltage by a sensing circuitry. Experiments show that the filter has a center frequency of about 39.6 MHz and a bandwidth of 330 kHz.     

The enhancement of electrical and optical properties of PEDOT:PSS using one-step dynamic etching for flexible application

The conductivity enhancement of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by dynamic etching process was investigated to introduce the outstanding and simplest method for soft electronics. Four different samples which were pristine PEDOT:PSS, PEDOT:PSS doped with 5 wt.% DMSO, PEDOT:PSS with dipping process, and PEDOT:PSS with dynamic etching process were prepared to compare the properties such as conductivity, morphology, relative atomic percentage, and topography. All samples were characterized by four point probe, current atomic force microscopy (C-AFM), X-ray photoelectron spectroscopy (XPS), and UV–visible spectroscopy. The conductivity of the sample with dynamic etching process showed the highest value as 1299 S/cm among four samples. We proved that the dynamic etching process is superior to remove PSS phase from PEDOT:PSS film, to flow strong current through entire surface of PEDOT:PSS, and to show the smoothest surface (RMS 2.28 nm). XPS analysis was conducted for accurate chemical and structural surface environments of four samples and the relative atomic percentage of PEDOT in the sample with dynamic etching was the highest as 29.5%. The device performance of the sample with the dynamic etching process was outstanding as 10.31 mA/cm² of J_sc, 0.75 eV of V_oc, 0.46 of FF, and 3.53% of PCE. All properties and the device performance for PEDOT:PSS film by dynamic etching process were the most excellent among the samples.     

Electron beam induced current microscopy investigation of GaN nanowire arrays grown on Si substrates

We report on the electron beam induced current (EBIC) investigation of GaN nanowires grown on n-doped Si (111) substrates. The objective of this study is to acquire information about the modifications of the substrate properties induced by the wire growth. We show that the growth procedure using deposition of an ultra-thin AlN layer prior to the nanowire growth step leads to the formation of a p-n junction in the Si substrate with a high surface conductivity. The induced p-n junction exhibits a photoresponse over the spectral range from 360 nm to 1100 nm. The properties of the induced p-n junction are investigated on the cross section and in a top view configuration with EBIC microscopy. For a localized contact of the GaN nanowires, the collection range in Si extends over a few millimeters. The treatment of the surface using reactive ion etching with a CHF₃ plasma leads to the inhibition of the surface conductivity and to the appearance of an S-shape in the current-voltage characteristics under illumination. The conversion efficiency of the plasma-treated sample under AM1.5G solar spectrum is estimated to be in the 2.1–2.7% range.     

Synthesis of Sn doped ZnO/TiO2 nanocomposite film and their application to H2 gas sensing properties

Tin doped Zinc oxide/Titanium oxide nanocomposite (TZO/TiO₂) was prepared by two methods: TiO₂ nanotube (Nt) arrays are grown by anodic oxidation of titanium foil and TZO films was deposited on the TiO₂ Nt obtained by hydrothermal process. The morphological characteristics and structures of ZnO/TiO₂ and TZO/TiO₂ were examined by (scanning elecron miscroscopy) SEM, (X rays diffraction) XRD and (energy dispersive spectroscopy) EDS analysis. The diameter of TiO₂ Nts was ranged from 40 nm to 90 nm with wall thicknesses of approximately 10 nm. The anatase structure of Titania, the hexagonal Zincite crystal of zinc oxide and tetragonal structure of tin oxide were identified by XRD. EDS analysis revealed the presence of O, Zn, Ti and Sn elements in the obtained deposits.These nanocomposites have been used as active layer in hydrogen gas sensing application. The hydrogen sensing characteristics of the sensor was analyzed by measuring the sensor responses in the temperature of 100 °C and 160 °C. The highest gas response is approximately 1.48 at 160 °C.The sensing mechanism of the nanocomposite sensor was explained in terms of H₂ chimisorption on the highly active nanotube surface.