Hydrofluoric Acid Solution Research Articles

ZnO/Si heterojunction solar cell fabricated by atomic layer deposition and hydrothermal methods

Abstract Currently, the first generation of silicon solar cells is dominating the photovoltaic market. Silicon cells are produced by various methods, which employ either crystalline or multi-crystalline substrates. However, both these manufacturing processes are expensive and potentially harmful to the environment and health. One example of this is that the surface is given its texture in a highly corrosive water solution of nitric and hydrofluoric acid. Additionally, both the diffusion and manufacturing of p-n junction and of metal contacts are associated with very high temperatures. This prompted us in our search for cheaper and more environmental friendly technologies. In this work, we discuss the possibility of producing components of photovoltaic cells by employing atomic layer deposition and hydrothermal technologies. This does not require the use of hazardous chemicals and high temperatures. The maximum efficiency of zinc oxide/silicon solar cells is 14% and 10% for textured and planar structures, respectively. A environmentally-friendly and simple procedure is thus being proposed, which, together with its relative efficiency, makes it an attractive alternative to the present procedure.

Negative differential resistance in partially fluorinated graphene films

Partially fluorinated graphene films were created by chemical functionalization of graphene layers in an aqueous solution of hydrofluoric acid. The formation of graphene islands or graphene quantum dots (GQDs) and a fluorinated graphene network is demonstrated in such films. Negative differential resistance (NDR) resulting from the formation of the potential barrier system in the films was observed for different fluorination degrees of suspension. The origin of the NDR varies with an increase in the fluorination degree of the suspension. Numerical calculations were performed to elucidate the tunneling between adjacent energy levels and creation of NDR. It was found that in the case of films with smaller flake and smaller GQD sizes, multi-peak NDR appears in the I–V curve. We predict that the NDR peak position shifts towards lower voltage with a decrease in the GQD size. Surprisingly, we observed a negative step-like valley for positive biases in the I-V curve of samples. Our findings with detailed analysis shed light on understanding the mechanisms of the NDR phenomenon in a partially fluorinated graphene system.

Nanoporous copper fabricated from Zr65Cu17.5Fe10Al7.5 amorphous alloy and its electrocatalytic oxidation performance

A series of bi-continuous and uniform nanoporous structures were fabricated successfully through chemical dealloying from quaternary Zr65Cu17.5Fe10Al7.5 amorphous ribbons in hydrofluoric acid (HF) solutions under a free corrosion condition. Nanoporous structures obtained from dealloying amorphous ribbons for 5 h in 0.02 M,0.035 M and 0.05 M HF solutions have mean pore sizes of 30–40 nm. A three-dimensional bi-continuous nanoporous copper (NPC) structure formed on Zr65Cu17.5Fe10Al7.5 ribbon alloys has pore sizes of 44.91 nm,52.60 nm and 57.37 nm, respectively, after immersion in 0.02 M HF solution for 15 h, 20 h, and 25 h. It shows that the pore size of NPC structure increases with increasing the dealloying time. The cyclic voltammetry (CV) results reveal that all NPC structures have the obvious electrocatalytic ability to the oxidation of (CH2OH)2 solution, compared with the initial amorphous ribbon without oxidation peak on CV curve. Zr-Cu-Fe-Al amorphous alloys can be new candidates for fabricating NPC structures.

Formation of cross-cutting structures with different porosity on thick silicon wafers

Three-layered pass-through structures of two types have been obtained on 500μm thick single crystal silicon wafers by electrochemical etching in a 48% solution of hydrofluoric acid without using additional single crystal layer removal operations. The first type of the pass-through structures comprises two outermost 220–247.5μm thick macroporous silicon layers with a pore diameter of 7–10μm and an intermediate 5–60μm thick mesoporous silica layer with a pore diameter of 100–150nm. For the formation of the first type structures we used two-stage etching without cell disassembly:–in a 48% water solution of hydrofluoric acid (H2O: HF = 1: 1 vol.) for 140–180min with a current density of 40mA/cm2;–in a 48% water/alcohol solution of hydrofluoric acid (H2O: HF: С2Н5ОН = 1: 1: 1 vol.) for 60–90min with a current density of 10mA/cm2.The second type pass-through structures include a macroporous silicon layer with a thickness of 250μm which interlock in the depth of the silicon wafer to form a cavity with a size of 4–8μm. For the formation of the second type structures we only used the first one of the abovementioned stages, the etching time being longer, i.e. 210min. All the etching procedures were carried out in a cooling chamber at 5°C. The developed technology will provided for easier and more reliable formation of the monolithic structures of membrane-electrode assembly micro fuel cells.

Control of Sidewall Profile in Dry Plasma Etching of Polyimide

Spin-on polyimide is an organic thin film often used as a sacrificial layer for surface micromachining due to its high thermal stability, ease of removal, and compatibility with many materials and processes used in the realization of microelectromechanical systems (MEMS). The incorporation of sloped sidewalls in polyimide for fabricating pedestal structures is crucial in order to provide strong anchors in free-standing MEMS devices, especially in cases having a high aspect ratio and/or where structural materials have limited deposition conformality. This paper demonstrates a reliable reactive ion etching (RIE) methodology for tuning the polyimide sidewall angle, ranging from a vertical sidewall up to an angle of about 25° from the vertical. The key modifications to the process parameter space include changes to the process temperature and chamber pressure. This paper also presents a novel lift-off process, which is based on the use of an interfacial polymer layer to facilitate removal of an overlying silicon oxide hard mask. This procedure allows polyimide sacrificial layers employing a silicon oxide hard mask to be used on samples that have exposed silicon oxide layers elsewhere on the chip that are required to remain intact during hard mask removal. Therefore, this lift-off process is applicable in situations where the silicon oxide hard mask removal cannot be accomplished by wet etching in hydrofluoric acid solutions.

Femtosecond laser assisted fabrication of networked semi-occlusive microfluidic channel on fused silica glass surface

Networked semi-occlusive microfluidic was demonstrated via fabricating channel on the surface of the silica substrate using femtosecond laser direct writing (FLDW) followed by chemical etching and covering a thin layer of polydimethylsiloxane (PDMS) film on substrate. The microfluidic channel formation mechanism is identified as femtosecond laser irradiation leading to substrate material modification, and modified areas acquiring an increased solubility to hydrofluoric (HF) acid solution. The morphologies of networked microfluidic were characterized by using optical microscopy and scanning electronic microscopy. Furthermore, in order to test flowing characteristics of the microfluidic, we conducted the experiment of liquid injection.

3D silicon shapes through bulk nano structuration by focused ion beam implantation and wet etching

The work presented in this paper concerns the synthesis of silicon (Si) 2D and 3D nanostructures using the delayed effect, caused by implanted Ga ions, on the dissolution of Si in aqueous solutions of tetramethylammonium hydroxide (TMAH). The crystalline silicon substrates (100) are first cleaned and then hydrogenated by immersion in an aqueous solution of hydrofluoric acid. The ion implantation is then carried out by a focused ion beam by varying the dose and the exposure time. Chemical etching in aqueous solutions of TMAH at 80 °C leads to the selective dissolution of the Si planes not exposed to the ions. The preliminary results obtained in the laboratory made it possible to optimize the experimental conditions for the synthesis of 2D and 3D nanoobjects of controlled shape and size. Analysis by transmission electron microscopy and energy dispersive x-ray showed the amorphous nature of the nanostructures obtained and the presence of 5%–20% Ga in these nanoobjects. The first experiments of recrystallization by rapid thermal annealing allowed to reconstitute the crystal structure of these nanoobjects.

An experimental study of solid source diffusion by spin on dopants and its application for minimal silicon-on-insulator CMOS fabrication

Solid source diffusions of phosphorus (P) and boron (B) into the half-inch (12.5 mm) minimal silicon (Si) wafers by spin on dopants (SOD) have been systematically investigated and the physical-vapor-deposited (PVD) titanium nitride (TiN) metal gate minimal silicon-on-insulator (SOI) complementary metal–oxide–semiconductor (CMOS) field-effect transistors (FETs) have successfully been fabricated using the developed SOD thermal diffusion technique. It was experimentally confirmed that a low temperature oxidation (LTO) process which depresses a boron silicide layer formation is effective way to remove boron-glass in a diluted hydrofluoric acid (DHF) solution. It was also found that top Si layer thickness of SOI wafers is reduced in the SOD thermal diffusion process because of its consumption by thermal oxidation owing to the oxygen atoms included in SOD films, which should be carefully considered in the ultrathin SOI device fabrication. Moreover, normal operations of the fabricated minimal PVD-TiN metal gate SOI-CMOS inverters, static random access memory (SRAM) cells and ring oscillators have been demonstrated. These circuit level results indicate that no remarkable particles and interface traps were introduced onto the minimal wafers during the device fabrication, and the developed solid source diffusion by SOD is useful for the fabrication of functional logic gate minimal SOI-CMOS integrated circuits.

Characterization of Ni Filled Luminescent Porous Silicon By Optical and Magnetic Methods

Porous silicon (PSi) gained by electrochemical etching of a silicon wafer can be fabricated with various morphologies ranging from pore diameters of a few nanometers to tens of micrometers. In the nanoporous regime the material offers a branched structure with pore diameters of 2 – 5 nm. Such microporous silicon (IUPAC notation) is known to emit light in the visible regime since 1990 and is still under investigation [1]. The aim of this work is to combine luminescent porous silicon with a ferromagnetic metal (Ni) to influence on the one hand the photoluminescence by the presence of metal deposits and on the other hand the optical properties by an external magnetic field. The porous silicon samples are fabricated by anodization of moderately doped p-type silicon in an aqueous hydrofluoric acid solution under a constant current density of 10 mA/cm2 for 20 minutes. The metal deposition within these small pores is challenging but succeeded. First the optical properties of the luminescent PSi are investigated, second Ni is electrochemically deposited in the porous silicon sample under cathodic conditions and subsequently the nanocomposite specimens are characterized optically and magnetically. Furthermore the structural characterization of the samples is carried out by SEM and TEM. Photoluminescence spectra of bare porous silicon show a maximum around 620 nm whereas in the case of Ni filled samples the peak is blue-shifted to around 580 nm and the luminescence intensity is increased. Concerning the magnetic properties of the nanocomposite the embedded Ni structures are superparamagnetic from the size of the pore diameters but due to the branched morphology the achieved deposits are interconnected and thus do not offer necessarily a superparamagnetic behavior. Field dependent magnetization measurements with the magnetic field applied perpendicular to the sample surface show a low coercivity of about 200 Oe and a squareness (magnetic remanence/saturation magnetization) of about 0.2 which is indicative for a film like behavior due to the interconnected Ni structures (figure 1). Temperature dependent magnetization measurements give no hint for a superparamagnetic behavior. The optical characterization of luminescent PSi with respect to photoluminescence intensity and decay times compared with Ni filled samples is discussed in detail as well as the corresponding magnetic properties of the nanocomposites. Furthermore the influence of a magnetic field on the optical properties is elucidated. The presented systems give not only rise to optoelectronics but also to magneto optical integrated devices. [1] J. Joo, T. Defforge, A. Loni, D. Kim, Z.Y. Li, M.J. Sailor, G. Gautier, L.T. Canham, Appl. Phys. Lett. 108, 153111 (2016). Figure 1: Field dependent magnetization curve of microporous silicon filled with Ni. Figure 1

Photoelectrochemical Properties and Photocatalytic Activity of Fluorine-Doped Plate-Like WO3 from Hydrothermal Radio-Frequency (RF) Sputtered Tungsten Thin Films

Fluorine-doped tungsten oxide (WO3) plate-like films were synthesized by hydrothermal radio-frequency (RF) sputtered tungsten (W) thin films in HF solution. The crystal structure, composition and morphology of nitrogen fluorine (F) doped WO3 were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), and UV-Vis diffuse reflectance spectra (DRS) techniques. The results indicate that fluorine can be doped successfully into WO3 plate-like films by hydrothermal synthesis in hydrofluoric acid (HF) solution. The F-doped WO3 samples show stronger absorption in the UV-Vis range and a red shift in the band gap transition. Incident photon to current efficiency (IPCE) measurements carried out on photoelectrochemical cell with F-doped WO3 plate-like films as anodes demonstrate a significant increase of photoresponse in the visible region compared with undoped WO3. The photocatalytic activity under visible light irradiation for newly synthesized F-doped WO3 plate-like films was investigated by degradation of methyl orange. The photocatalytic activity of F-doped WO3 plate-like films was 3-fold enhancement compared with pure WO3 samples.

Photoluminescence and enhanced chemical reactivity of amorphous SiO2 films irradiated with high fluencies of 133-MeV Xe ions

SiO2/Si structures have been irradiated with 133 MeV Xe+17 ions at fluencies of (1010–5 × 1014) cm−2. The structure transformation and light-emitting properties of irradiated SiO2 films were studied using RS, SEM, TEM and PL techniques as well as chemical etching in 4% solution of hydrofluoric acid (HF). An intensive photoluminescence in visible range was registered from the samples irradiated at a fluence of 1014 cm−2 and higher. Simultaneously, it was found a drastic increase of SiO2 etch velocity in HF solution for the irradiated samples. Annealing (1100 °C, 2 h) of irradiated samples resulted in PL quenching and etch velocity recovery practically to the value of non-irradiated SiO2. It was concluded that radiative oxygen deficient centers are responsible for the PL appearance. It was also shown that the etch velocity ratio of the irradiated and virgin SiO2 in 4%-HF (Virr/Vvirgin) can be used in order to estimate the radiation damage in SiO2 matrix irradiated with high fluencies of swift heavy ions.

1-kV vertical Ga2O3 field-plated Schottky barrier diodes

Ga2O3 field-plated Schottky barrier diodes (FP-SBDs) were fabricated on a Si-doped n−-Ga2O3 drift layer grown by halide vapor phase epitaxy on a Sn-doped n+-Ga2O3 (001) substrate. The specific on-resistance of the Ga2O3 FP-SBD was estimated to be 5.1 mΩ·cm2. Successful field-plate engineering resulted in a high breakdown voltage of 1076 V. A larger-than-expected effective barrier height of 1.46 eV, which was extracted from the temperature-dependent current–voltage characteristics, could be caused by the effect of fluorine atoms delivered in a hydrofluoric acid solution process.

Development of 126Sn separation method by means of anion exchange resin and gamma spectroscopy

This paper describes a method employing anion exchange resin for determination of 126Sn in radioactive waste. The method is suitable for the separation of 126Sn isotope from hydrochloric and hydrofluoric acid solution. The separation is based on precipitation of tin with ammonium sulfide in 0.5molL−1 HCl, dissolution of the precipitate in concentrated HCl, loading in 2molL−1 HCl onto anion exchange resin column and elution with 2molL−1 HNO3. 126Sn was measured by gamma spectrometry.

Obtaining and electrical properties of ceramics based on mixed niobium and tantalum pentoxide Nb2(1–y)Ta2y O5

A method is developed for the preparation of mixed pentoxide Nb2(1–y)Ta2y O5 by rapid coprecipitation of a mixture of hydroxides of niobium and tantalum from a solution of hydrofluoric acid by ammonia water, followed by purification and calcination of the precipitate. The proposed method allows obtaining a homogeneously mixed pentoxide of niobium and tantalum Nb2(1–y)Ta2y O5, in contrast to the method of obtaining Nb2(1–y)Ta2y O5 from a mechanical mixture of individual pentoxides of niobium and tantalum. The temperature dependences of the real part of the permittivity (e′) of ceramic samples of Nb2(1–y)Ta2y O5 at various frequencies were investigated and the magnitude of dielectric loss (tan(δ)) versus frequency measurement of the electric field and temperature was evaluated. On the basis of analysis of the data of impedance spectroscopy, the temperature dependence of the static conductivity σ dс of the ceramic Nb2(1–y)Ta2y O5 was determined. From the temperature dependence of σ dс , the enthalpies of activation of charge transport for different temperature ranges were evaluated.

Broadband gradient impedance matching using an acoustic metamaterial for ultrasonic transducers

High-quality broadband ultrasound transducers yield superior imaging performance in biomedical ultrasonography. However, proper design to perfectly bridge the energy between the active piezoelectric material and the target medium over the operating spectrum is still lacking. Here, we demonstrate a new anisotropic cone-structured acoustic metamaterial matching layer that acts as an inhomogeneous material with gradient acoustic impedance along the ultrasound propagation direction. When sandwiched between the piezoelectric material unit and the target medium, the acoustic metamaterial matching layer provides a broadband window to support extraordinary transmission of ultrasound over a wide frequency range. We fabricated the matching layer by etching the peeled silica optical fibre bundles with hydrofluoric acid solution. The experimental measurement of an ultrasound transducer equipped with this acoustic metamaterial matching layer shows that the corresponding −6 dB bandwidth is able to reach over 100%. This new material fully enables new high-end piezoelectric materials in the construction of high-performance ultrasound transducers and probes, leading to considerably improved resolutions in biomedical ultrasonography and compact harmonic imaging systems.

Surface nanocrystalline Si structure and its surface passivation for highly efficient black Si solar cells

Abstract19.5% conversion efficiency crystalline silicon (Si) solar cells having simple structure without antireflection coating have been fabricated using the surface structure chemical transfer method which produces a nanocrystalline Si layer simply by contacting catalytic platinum with Si wafers in hydrogen peroxide plus hydrofluoric acid solutions. The reflectivity becomes less than 3% after the surface structure chemical transfer method due to formation of black Si. Deposition of phosphosilicate glass and heat treatment at 925 °C performed for formation of pn‐junction effectively passivate the nanocrystalline Si surface. With this phosphosilicate glass passivation plus the hydrogen treatment at 400 °C, the internal quantum efficiency is greatly improved and reaches 81% at a wavelength of 400 nm. Analysis of ellipsometry data shows that incident light with wavelength shorter than 400 nm is almost completely absorbed by the nanocrystalline Si layer. The high internal quantum efficiency for short wavelength light is attributed to effective surface passivation and the nanocrystalline Si layer band‐gap energy which decreases with the distance from the top of the network structure of the nanocrystalline Si layer. Copyright © 2017 John Wiley & Sons, Ltd.

Synthesis of Flower-like Silver Nanostructures on Silicon and Their Application in Surface-enhanced Raman Scattering

To enhance the intensity of surface-enhanced Raman scattering (SERS), production of metal nanostructures with sharp points, lying side by side at the nanometer level plays an extremely important role. In this paper, we report on a manufacturing process in which the silver nanoparticles with the flower-like shape have been fabricated. Such silver nanoparticles have been fabricated by chemical deposition of silver particles on silicon wafers, using a solution of hydrofluoric acid (HF), silver nitrate (AgNO3) and ascorbic acid (AsA) in water, at room temperature. During the manufacturing we found that only when the concentrations of AgNO3 and AsA are appropriate, the flower-like silver nanoparticles will form. Note that while other authors mainly made flower-like silver nanoparticles in the form of suspensions, we have created flower-like silver nanoparticles with cabbage-shape on a silicon surface. The ensembles of flower-like silver nanoparticles above were used as SERS substrates to detect crystal violet (CV) in low concentrations and good results were obtained.

Covalent bonding of grafted polymer brushes of poly(poly(ethylene glycol) monomethacrylate) on surface of silicon quantum dots and the activation of the end hydroxyls

A technique of covalent bonding of grafted polymer brushes of poly(poly(ethylene glycol) monomethacrylate) (P(PEGMA)) on the surface of silicon quantum dots (SiQDs) and activation of the end hydroxyls with N-hydroxysuccinimide (NHS) esters were described in this work in detail. Firstly, a Si/SiQDs slice was disposed with hydrofluoric acid solution to produce surface Si-Hx (x=1, 2 or 3) species, which combined with vinyl group of 1-undecenol covalently gives terminal hydroxyls. Such hydroxyls were initialized and were involved in atom transfer radical polymerization to construct grafted polymer brushes of P(PEGMA). Hence the density of the surface hydroxyls was enhanced extremely on account of the end hydroxyls of the side chains of the polymer brushes. Then carboxyl groups were introduced via terminal esterification with succinic anhydride, which were activated subsequently with NHS giving NHS esters under rather mild conditions. The reactivity of the terminal NHS esters was demonstrated by SDS-PAGE gel electrophoresis and fluorescence imaging. The SiQDs were demonstrated by transmission electron microscopy. The stepwise modifications on SiQDs were characterized by infrared and X-ray photoelectron spectra. The photoluminescence of the SiQDs was remained during the chemical modifications, proved by the measurements of fluorescence spectra.

Anodes for Li-ion batteries based on p-Si with self-organized macropores

The practical application of microstructured anodes produced by the electrochemical etching of single-crystal silicon is limited by their cost. The proposed approach will make it possible to reduce the cost for several reasons: less stringent requirements to the quality of the initial material due to the replacement of n-Si with p-Si, the exclusion of operations that are aimed at the formation of nucleation centers, and the repeated use of a silicon substrate. The formation of random macropores in 10–20 Ω cm p-Si (100) in a 4% solution of hydrofluoric acid in dimethylformamide is studied and the role of chemical etching is revealed. The dependences on the current density are obtained for the etching rate, morphology of the porous layers, effective valence, pore density, and average pore diameter. The part played by chemical dissolution in the electrolyte is determined. A technology is developed for the deposition of 50-μ-thick porous layers. This technology combines successive detachments of several membranes from the same substrate and provides enhanced porosity (~70%). The electrochemical characteristics of anodes formed from these membranes are examined, and 120+ test cycles are performed in the mode with the charge capacity limited to 1000 mA h/g at a current of 0.2 A/g.

Boosted Photoactivity of Titania Nanotube Layers Doped with a Suspension of Gold Nanoparticles

In the present work, we report on the behavior of synthesized gold nano-particles suspension, incorporated in titanium dioxide nanotube layers (TiO2- NT) and fabricated by electrochemical anodization in 0.4 wt% hydrofluoric acid solution and we study its photocatalytic response. Gold nanoparticles were characterized using Transmission electron microscopy and X-ray diffraction. Scanning electron microscopy was used to study the morphology of TiO2 nanotube layers doped by gold nanoparticles. Boosted photocatalytic performances on the degradation of an azo dye were obtained by using TiO2 nanotube layers doped by gold nanoparticles (Au/TiO2-NT), compared to undoped TiO2 nanotube layer (TiO2-NT) catalysts. Under UV irradiation, this new nanomaterial, with noble metal-semi conductor heterojunction (Au/ TiO2-NT) exhibits a synergetic effect in accelerating the electron transfert, resulting in an enhanced photoactivity recorded in the kinetics of degradation of Acid Orange 7 (AO7). Chronoamperometry was used to highlight higher photocurrent produced by gold-titania interface submited to UV irradiation.