Si Hx Research Articles

Optimizing the performance of extended-gate field-effect transistor (EGFET) pH sensor by regulating the structural properties of the nanostructured porous silicon layer

We report the performance of extended-gate field-effect transistor (EGFET) pH sensors based on the properties of nanostructured porous silicon layer (nano PSi). Four different forms of nano PSi layers with different characteristic features are used as a sensor for determining the pH level. The synthesized nano PSi layer was prepared by laser-assisted etching of n-type (100) by using different illumination intensities 15, 30, 45 and 60 mW/cm2 of 530 nm laser wavelength. The structural and morphological properties of nano PSi were investigated through exploring scanning electron microscopy, X-ray diffraction (XRD) and gravimetrical measurements. Nano PSi with pore-like structures of various pore shapes and sizes are as follows: fine circular pores, star full pore, circular pores and rectangular pores; star full pore shape was prepared through varying the laser illumination intensity. The Fourier transform infrared spectroscopy, current–voltage measurements and specific surface area of were employed to describe the density of the hydrogenated dangling bonds (Si–Hx) of the nano PSi layer. The synthesized nano PSi pH sensors were tested and dry out in the pH range from 3 to 11 at room temperature. The performance of nano PSi pH sensors was comparatively analyzed based on the density of (Si–Hx), specific surface area, porosity and layer thickness of nano PSi. Concerning EGFET pH sensor showed noticeably enhanced pH sensitivity and linearity with increasing the density of (Si–Hx) and specific surface area. Higher sensitivity with excellent linearity was obtained for pore-like structures of circular pore shapes when the laser illumination intensity was about 45 mW/cm2.

Room temperature oxidation of Si nanocrystals at dry and wet air

Oxidation of HF vapor-etched nanocrystalline silicon films, prepared by drop coating from nanocrystalline Si sol in acetonitrile, was studied. Oxidation of nanocrystalline silicon at room temperature in air with 5% and 86% relative humidity was observed by means of infrared spectroscopy for 2 days. The change in film mass after 15 h of oxidation was determined using quartz crystal microbalance. In dry air, film mass and integral intensity of bands attributed to vibrations in Si3 − x–Si–Hx and Si–O–Si groups changed linearly with time. In humid air, intensity of in Si3 − x–Si–Hx band decays exponentially and intensity of Si–O–Si band increases as a square root of oxidation time. Film mass gain after 15 h of oxidation corresponds to an average oxide layer thickness of 0.02 nm in dry air and 0.51 nm in wet air.

Research on surface nano-texturation and wet-chemical passivation of multi-crystalline silicon wafer

Surfaces nano-texturing has triggered off much attention for trapping sunlight to improve the efficiency of solar cells. Silicon nanowire (SiNWs) arrays, with excellent antireflection performance, will hopefully improve photoelectric conversion of solar cells, however, the deteriorated effective carrier lifetime seriously limits efficiency enhancement of solar devices. Until now, the effect of SiNWs structure on effective carrier lifetime remains unexplored. Herein, the effects of fabrication parameters on the morphology structure and effective carrier lifetime of textured mc-Si were studied in detail. We also firstly discover that the relationship of SiNWs arrays length and effective carrier lifetime shows the negative exponential relation. Moreover, the effects of ethanolic iodine (I–E) concentration, immersion time, and surface pre-conditioning (with and without native oxide) on surface passivation of SiNWs arrays were investigated. It is found that more effective surface passivation could be achieved for the SiNWs arrays with shorter length. Meanwhile, the HF dipping pretreatment is conducive for SiNWs passivation, and which is attributed to Si–Hx termination with lower dissociation energy.

Electronic Properties of Si–Hx Vibrational Modes at Si Waveguide Interface

Attenuated total reflectance (ATR) and X-ray photoelectron spectroscopy in suite with Kelvin probe were conjugated to explore the electronic properties of Si-Hx vibrational modes by developing Si waveguide with large dynamic detection range compared with conventional IR. The Si 2p emission and work-function related to the formation and elimination of Si-Hx bonds at Si surfaces are monitored based on the detection of vibrational mode frequencies. A transition between various Si-Hx bonds and thus related vibrational modes is monitored for which effective momentum transfer could be demonstrated. The combination of the aforementioned methods provides for results that permit a model for the kinetics of hydrogen termination of Si surfaces with time and advanced surface characterizing of hybrid-terminated semiconducting solids.

On the quality of hydrogenated amorphous silicon deposited by sputtering

Amorphous hydrogenated silicon (a-Si:H) is well-known material in the global semiconductor industry. The quality of the a-Si:H films is generally decided by silicon and hydrogen bonding configuration (Si–Hx, x=1,2) and hydrogen concentration (CH). These quality aspects are correlated with the plasma parameters like ion density (Ni) and electron temperature (Te) of DC, Pulsed DC (PDC) and RF plasmas during the sputter-deposition of a-Si:H thin films. It was found that the Ni and Te play a major role in deciding Si–Hx bonding configuration and the CH value in a-Si:H films. We observed a trend in the variation of Si–H and Si–H2 bonding configurations, and CH in the films deposited by DC, Pulsed DC and RF reactive sputtering techniques. Ion density and electron energy are higher in RF plasma followed by PDC and DC plasma. Electrons with two different energies were observed in all the plasmas. At a particular hydrogen partial pressure, RF deposited films have higher CH followed by PDC and then DC deposited films. The maximum energy that can be acquired by the ions was found to be higher in RF plasma. Floating potential (Vf) is more negative in DC plasma, whereas, plasma potential (Vp) is found to be more positive in RF plasma.

Super-hydrophobic coatings with nano-size roughness prepared with simple PECVD method

A simple and conventional method to synthesize nearly flat super-hydrophobic coatings was studied. Conventional plasma enhanced chemical vapour deposition (PECVD) was adopted to synthesize hydrophobic coatings on plastic and glass substrates at room temperature. Hexamethyldisilane was used as a precursor, and hydrogen gas was added to modulate the surface roughness and passivate defects, such as dangling bond and electrically uncovered polar sites rendering non-hydrophobicity. The static water contact angle (WCA) was controlled in the range 120°–160° by adjusting process parameters, especially the hydrogen flow rate and power. AFM showed that the film with a WCA of 145° has as small as 2.5 nm roughness in rms value. In the resistance test of salt water and cosmetics, this film showed excellent results owing to super-hydrophobicity and defect passivation which keeps the surface isolated from external agents. In order to exploit these results, Rare gas analysis was used to examine the process plasma and Fourier transform infrared (FTIR) was used to analyse the chemical structures of the super-hydrophobic films. In the FTIR results, the remarkable increase in the modes of Si–Hx and Si–C bonds as well as Si–CH2–Si in the film was observed indicating the defect passivation and closely packed dense film structure.

The effect of differently sized Ag catalysts on the fabrication of a silicon nanowire array using Ag-assisted electroless etching

A simple and low cost method to generate single-crystalline, well-aligned silicon nanowires (SiNWs) of large area, using Ag-assisted electroless etching, is presented and the effect of differently sized Ag catalysts on the fabrication of SiNWs arrays is investigated. The experimental results show that the size of the Ag catalysts can be controlled by adjusting the pre-deposition time in the AgNO3/HF solution. The optimum pre-deposition time for the fabrication of a SiNWs array is 3 min (about 162.04 ± 38.53 nm Ag catalyst size). Ag catalysts with smaller sizes were formed in a shorter pre-deposition time (0.5 min), which induced the formation of silicon holes. In contrast, a large amount of Ag dendrites were formed on the silicon substrate, after a longer pre-deposition time (4 min). The existence of these Ag dendrites is disadvantageous to the fabrication of SiNWs. Therefore, a proper pre-deposition time for the Ag catalyst is beneficial to the formation of SiNWs.SiNWs were synthesized in the H2O2/HF solution system for different periods of time, using Ag-assisted electroless etching (pre-deposition of the Ag catalyst for 3 min). The length of the SiNWs increases linearly with immersion time. From TEM, SAED and HRTEM analysis, the axial orientation of the SiNWs is identified to be along the [001] direction, which is the same as that of the initial Si wafer. The use of HF may induce Si–Hx bonds onto the SiNW array surface. Overall, the Ag-assisted electroless etching technique has advantages, such as low temperature, operation without the need for high energy and the lack of a need for catalysts or dopants.

Study on the role of filament temperature on growth of indium-catalyzed silicon nanowires by the hot-wire chemical vapor deposition technique

Silicon nanowires (SiNWs) were synthesized from indium catalysts on the Si(111) substrate using the hot-wire chemical vapor deposition technique. A tungsten filament with purity of 99.95% was employed for both the evaporation of an indium wire as catalyst and the decomposition of the precursor gas silane diluted in hydrogen. In this study, we investigated the role of the filament temperature (Tf) on the growth and structural properties of the SiNWs. A threshold Tf for the successive growth of the SiNWs via a vapor-liquid-solid process was observed at Tf between 1400 and 1500 °C. For Tf of 1400 °C and below, only a layer of Si shell cladding was formed on the indium core. An increase in Tf above the threshold resulted in a significant increase in the number density and the aspect ratio of the SiNWs. X-ray diffraction and micro-Raman measurements indicated an enhancement in crystallinity of the SiNWs with the increase in Tf. Fourier transform infrared analysis showed an enhancement in the presence of Si–O and Si–H related bonds with the increase in Tf. The Si–O bond is mostly originated from the native oxide layer of SiNWs, while Si–H bond suggests that Si–Hx species were responsible for the growth.

Constructing Metal-Based Structures on Nanopatterned Etched Silicon

Silicon surfaces with nanoscale etched patterns were obtained using polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer films as templates, followed by brief immersion in HF(aq). The resulting interfaces were comprised of pseudohexagonal arrays of pits on the silicon, whose shapes depended upon the chosen silicon orientation. The top unetched face of silicon remains capped by the native oxide, and the pit interiors are terminated by Si-H(x). Selective chemical functionalization via these two chemical handles was demonstrated to be a viable approach toward building nanostructured metal oxide and metal features within these silicon pits and on the top face. Using a series of interfacial chemical reactions, including oxidation (of Si-H(x)-terminated regions), hydrosilylation, and alkoxysilane-based chemistry on silicon oxide, the growth of metal-based structures can be spatially controlled. In the first approach, titania nanobowls were grown within the etch pits, and in the second, galvanic displacement was used to produce gold nanoparticles either within the etch pits, on the top silicon face, or both.

Correlated photoluminescence spectroscopy investigation of grain boundaries and diffusion processes in nanocrystalline and amorphous silicon (nc-Si:H) mixtures

ABSTRACTPhotoluminescence (PL) spectra obtained with correlated set of experiments investigating grain boundary characteristics and diffusion processes in nanocrystalline silicon alloys (nc-Si:H), provide insight regarding formation and passivation of electronic defects in these regions. Based upon current results and previous works we believe thermally driven processes induce a PL band centered at 0.7 eV upon thermal annealing, and most likely involve diffusion of hydrogen and oxygen near interfaces. A nc-Si:H sample set with varied crystal volume fraction, Xc, was subject to thermal annealing treatments at different temperatures – each exceeding the deposition temperature. Fourier-transform photoluminescence (FTPL) and Fourier-transform infrared absorption spectroscopy (FTIR), were employed to correlate the relative 0.7 eV defect band emergence with compositional changes indicative of Si–Hx and Si–O species, for each sample, at each temperature, respectively. Hydrogen effusion data provide additional perspective.We find the Xc to strongly affect susceptibility of nc-Si:H to oxygen related effects. The higher the Xc, the more readily oxygen penetrates the nc-Si:H network. We attribute this relationship to elevated diffusivity of oxygen in highly crystalline nc-Si:H materials, owing to their abundance of gain boundaries and interfaces, which serve as pathways for impurity migration. These findings corroborate the expectation that oxygen impurities and diffusion processes contribute to development of microstructural features giving rise to radiative recombination through deep defects in nc-Si:H.

Properties of a-Si:H films deposited by RF magnetron sputtering at 95 °C

In this work we have investigated the dependence of optical and electrical properties of RF sputtered undoped a-Si:H films and B or P doped a-Si:H films on hydrogen flow rate (FH). Low deposition temperature of 95°C was used, a process compatible with low-cost plastic substrates. FTIR spectroscopy and ESR measurements were used for the investigation of Si–Hx bonding configurations, and concentrations of hydrogen and dangling bonds. We found that there is a strong correlation between the total hydrogen concentration, the dangling bonds density and the optoelectronic properties of the films. The best photosensitivity value was found to be 1.4×104 for the undoped films. The dark conductivity (σD) of the doped layers varied from 5.9×10−8 to 6.5×10−6 (Ωcm)−1 for different ratios FAr/FH. These variations are attributed to both the different B and P concentrations in the films (according to SIMS measurements) and the enhanced disorder of the films introduced by the large number of inactive impurities. The B doping efficiency is lower compared to the P one. A small photovoltaic effect is also observed in n–i–p solar cells fabricated on polyimide (PI) substrates having ITO as antireflective coating, with an efficiency of 1.54%.

Loading and release of a model protein from porous silicon powders

AbstractPorous silicon (p‐Si) has been investigated as a novel delivery system for protein therapeutics. The loading of a model hydrophilic protein, Papain into anodised and stain etched p‐Si powders has been investigated using X‐ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FTIR) and correlations made with the release kinetics. Variation in specific Papain (Amide I/II) and p‐Si (Si–Hx ) functional group absorbances with Papain loading level was characterised using FTIR, while the surface chemical distribution was assessed using XPS. A combination of burst release and sustained release of Papain was observed from p‐Si powders; this was dependent on p‐Si type and the Papain loading level. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sum‐frequency generation from surface species in porous silicon

AbstractFor the first time, the sum‐frequency generation (SFG) spectra of surface species have been measured in porous silicon (PS) layers. The range of the vibrations 2000–2300 cm–1 for the native species Si–Hx has been explored. Furthermore for the studied mesoporous materials, it has been found that, as opposed to the infrared absorption, the SFG signal increases with porosity. Finally, the angular dependence and the enhancement of the SFG signal were measured in a one‐dimensional PS photonic crystal structure with a defect producing an energy level in the photonic band gap. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of UV irradiations on the structural and optical features of porous silicon: application in silicon solar cells

The aim of this paper is to investigate the structural and optical stability of porous silicon layers (PSLs) planned to be used in silicon solar cells technology. The PSLs were prepared by a HNO3/HF vapor etching (VE) based method. Fourier transform infrared (FT-IR) spectroscopy shows that fresh VE-based PSLs contain NH and SiF bonds related to a ammonium hexafluorosilicate (NH4)2SiF6 minor phase, and conventional SiHx and SiOx bonds. Free air exposures of PSLs without and with UV irradiation lead to oxidation or photo-oxidation of the porous layer, respectively. FT-IR characterisation of the PSLs shows that UV irradiations modify the transformation kinetics replacing instable SiHx by SiOx or SiOH bonds. When fresh PSLs undergo free air oxidation within 7 days, the surface reflectivity decreases from 10 to about 8%, while it drops to about 4% when a 10min free air UV irradiation is applied. Long periods of free air oxidation do not ensure the reflectivity to be stable, whereas it becomes stable after only 10min of UV irradiation. This behaviour was explained taking into account the kinetic differences between oxidation with and without UV irradiation. Fresh VE-based PSLs were found to improve efficiently the photovoltaic (PV) characteristics of crystalline silicon solar cells. The passivating action of VE-based PSLs was discussed. An improvement of the PV performances was observed solely for stable oxidized porous silicon (PS) structures obtained from UV irradiations.

Formation of carbonized porous silicon surfaces by thermal and optically induced reaction with acetylene

Formation of a carbonized porous silicon surface by thermal and photo-assisted reaction with gas phase acetylene is reported. The porous silicon samples subjected to flowing acetylene either in a heated quartz reactor or under optical illumination show strong quenching of the photo- luminescence (PL) followed by a recovery on further exposure to acetylene. In thermally treated samples, a significant blue shift of 80 nm was observed. The recovered PL signal does not exhibit quenching on further exposure to laser illumination. Fourier transform infrared spectroscopy investigations confirm absence of Si–Hx absorption and no enhancement of the oxygen back bonded Si–H bonding. It indicates the formation of a practically stable carbonized porous silicon surface.

Photoluminescence stabilization of anodically-oxidized porous silicon layers by chemical functionalization

Electrochemical oxidation of porous silicon (PSi) produces a surface that is covered with native silicon–hydrogen (Si–Hx) bonds and regions with oxidized Si–Si back-bonds (OSi–Hx). Such anodically oxidized PSi layers were chemically modified using 1-decene under thermal conditions. The hydrosilylation reaction consumes mainly the nonoxidized Si–Hx bonds and yields a surface with oxidized and alkylated regions that were characterized using transmission IR and Raman spectroscopies. The brightest photoluminescence (PL) was obtained when the PSi sample was anodized in 1 M sulfuric acid (H2SO4) at 3 mA/cm2 for 5 min. The chemical process preserves the PL and the physical properties of the porous layer. The derivatized PSi surfaces are stable in boiling CCl4 and in water.

Infrared spectroscopic study of dimethylaluminum-hydride adsorption on oxidized, hydrogen-terminated, and reconstructed Si surfaces

Adsorption and decomposition of dimethylaluminum-hydride (DMAH) on various Si(111) and Si(100) surfaces [chemically oxidized, hydrogen-terminated, reconstructed (7×7) and (2×1)] have been investigated at 90 K with a multiple reflection infrared (IR) attenuated total reflection (ATR) method. On the chemically oxidized Si(111) surface, IR absorption due to DMAH increased linearly with DMAH exposure, indicating that DMAH was merely condensed on the surface. Upon DMAH exposure to the hydrogen-terminated Si(111) surface, the stretch band of the surface monohydride (Si–H) diminished immediately, revealing that DMAH reacts with the terminated hydrogens. In contrast, DMAH exposure to the hydrogen terminated Si(100) surface leads to a slight intensity decrease in the bands due to surface hydrides (Si–Hx:x=1–3). On the reconstructed Si surfaces [Si(100)(2×1) and Si(111)(7×7)], IR bands due to DMAH could not be observed during the early stage of DMAH exposure, suggesting that the reactivity of DMAH with the reconstr...

Properties of amorphous carbon–silicon alloys deposited by a high plasma density source

The addition of silicon to hydrogenated amorphous carbon can have the advantageous effect of lowering the compressive stress, improving the thermal stability of its hydrogen, and maintaining a low friction coefficient up to high humidity. Most experiments to date have been on hydrogenated amorphous carbon–silicon alloys (a-C1−xSix:H) deposited by rf plasma enhanced chemical vapor deposition. This method gives alloys with sizeable hydrogen content and only moderate hardness. Here we use a high plasma density source known as the electron cyclotron wave resonance source to prepare films with higher sp3 content and lower hydrogen content. The composition and bonding in the alloys is determined by x-ray photoelectron spectroscopy, Rutherford backscattering, elastic recoil detection analysis, visible and ultraviolet (UV) Raman spectroscopy, infrared spectroscopy, and x-ray reflectivity. We find that it is possible to produce relatively hard, low stress, low friction, almost humidity insensitive a-C1−xSix:H alloys with a good optical transparency and a band gap well over 2.5 eV. The friction behavior and friction mechanism of these alloys are studied and compared with that of a-C:H, ta-C:H, and ta-C. We show how UV Raman spectroscopy allows the direct detection of Si–C, Si–Hx, and C–Hx vibrations, not seen in visible Raman spectra.

Annihilation of nonradiative defects on hydrogenated silicon surfaces under pulsed-laser irradiation

Annihilation of nonradiative recombination defects on hydrogenated Si surfaces has been found after irradiation with XeCl laser pulses of energies below the melting threshold. A decrease of the total numbers of the stretched Si–Hx (x=1,2,3) bonds and an increase of the relative part of the Si–H bonds of the surface coverage have been established simultaneously by Fourier transform infrared spectroscopy. The drop of the number of surface nonradiative defects despite the efficient hydrogen desorption is explained by the mechanism of bonding of Si dangling bonds under the high electronic excitation induced by the XeCl laser pulse.

Unusual stabilization of Si surfaces during roughening in fluoride solution

Surface recombination, Si–Hx bonds and initial formation of luminescing porous Si are investigated in situ by infrared and photoluminescence spectroscopy during electrochemical roughening of Si in fluoride solution. The concentration of nonradiative surface defects remains unchanged even during the formation of a rough hydrogenated Si surface. The SiH/SiH2 ratio of such a surface is constant and independent of type of doping, surface orientation, and formation of porous Si. This effect is explained by a decrease of the polarizability of Si backbonds in a disordered Si–Hx surface layer.