Bias Voltage Vbias Research Articles

On the Feasibility of an LCD-Based Real-Time Converter for Ionizing Radiation Imaging.

Here we present the cascade converter (CC), which provides real-time imaging of ionizing radiation (IoR) distribution. It was designed and manufactured with the simplest architecture, utilizing liquid crystal display (LCD) technology. Based on two merged substrates with transparent electrodes, armed with functional layers, with the cell filled with nematic liquid crystal, a display-like, IoR-stimulated CC was achieved. The CC comprises low-absorbing polymer substrates (made of polyethylene terephthalate-PET) armed with a transparent ITO electrode covered with a thin semipermeable membrane of polymer (biphenylperfluorocyclobutyl: BP-PFCB) doped with functional nanoparticles (NPs) of Lu2O3:Eu. This stack was covered with a photoconductive layer of α-Se and finally with a thin polyimide (PI) layer for liquid crystal alignment. The opposite substrate was made of LCD-type glass with ITO and polyimide aligning layers. Both substrates form a cell with a twisted structure of nematic liquid crystal (TN) driven with an effective electric field Eeff. An effective electric field driving TN structure is generated with a sum of (1) a bias voltage VBIAS applied to ITO transparent electrodes and (2) the photogenerated additional voltage VXray induced between ITO and α-Se layers with a NPs-doped BP-PFCB polymer layer in-between. The IoR (here, X-ray) conversion into real imaging of the IoR distribution was achieved in the following stages: (1) conversion of IoR distribution into non-ionizing red light emitted with functional NPs, (2) transformation of red light into an electric charge distributed in a layer of the photoconductive α-Se, which is what results in the generation of distributed voltage VXray, and (3) a voltage-mediated, distributed switching of the TN structure observed with the naked eye. The presented imaging device is characterized by a simple structure and a simple manufacturing process, with the potential for use as a portable element of IoR detection and as a dosimeter.

Characterization of nano-bio silicon carbide

Plasma-enhanced chemical vapor deposition, reactive magnetron sputtering, hot-wire chemical vapor deposition and radio frequency plasma-enhanced chemical vapor deposition were used to develop technology for preparation of nano-bio silicon carbide coating of ceramic materials for dental applications. The effect of the bias voltage applied to the ceramic prostheses and dental crowns on the crystallization processes have been recognized. The optimal bias voltage applied to conductive substrate was –200 V, whereas for dielectric substrate the bias voltage Vbias did not affect the properties of SiC coating. The analysis of CVCs and spectroscopic diagnostics as the methods for studying the mechanism of interfacial rearrangements to investigate SiC phase transition in nano silicon carbide coatings were used. The conductivity of the SiC coating coincided with the conductivity on the dielectric (µn0 = 1012…1013 сm–1·s–1·V–1). The conductive substrate had a significant effect on the properties of the coating and thus depended on the bias voltage Vbias. The conductivity increased by three-four orders of magnitude (µn0 = 3·1017 сm–1·s–1·V–1), if the bias voltage Vbias = –200 V. The increase of the bias voltage (Vbias = –600 V) led to a decrease in the conductivity (µn0 = 1011…1012 сm–1·s–1·V–1). It was found that there was the double injection regime with bimolecular recombination in this structure with the dependence I = V3/2 for CVCs of SiC. The luminescence spectrum of SiC coating on non-dielectric ceramics (if Vbias = – 200 V during deposition) was significantly different from the luminescence spectrum of SiC coating on dielectric ceramics. Increasing the applied voltage to the substrate Vbias during deposition led to increasing the fraction of hexagonal polytypes. Directions in the crystal lattice according to the photoluminescence spectra were identified from the comparing the values of the width of the non-phonon parts of stacking faults and deep level spectra in the low-temperature photoluminescence with arrangements of atoms in the SiC lattice structure. The displacement of each atom participating in photoluminescence allowed to find the correlation with technology of SiC deposition and to develop technology of SiC coating on the dental materials.

Significant photoresponsivity enhancement of polycrystalline BaSi2 films formed on heated Si(111) substrates by sputtering

We fabricated approximately 200-nm-thick BaSi2 films on Si(111) substrates at 600 °C. The formation of BaSi2 was demonstrated by X-ray diffraction and Raman spectroscopy. A reduction in the electron concentration (n = 2 × 1016 cm−3) by 3 orders of magnitude compared to that previously reported (n = 7 × 1019 cm−3) and resultant photoresponsivity enhancement by more than two orders of magnitude were achieved. The photoresponsivity increased with the bias voltage Vbias applied between the top and bottom electrodes, and reached approximately 0.19 A/W at 2.0 eV, room temperature, and |Vbias| = 0.5 V.

Effect of interleaved Si layer on the magnetotransport and semiconducting properties of n-Si/Fe Schottky junctions

Heterostructure films of the form n-Si/Si(tSi)/Fe(800 Å) were prepared by DC magnetron sputtering. In these films, the Si and Fe (800 Å) films were deposited onto n-Si(100) substrates. Substrates with different doping concentration ND were used. The thickness tSi of the interleaved Si layer is varied. For tSi = 0, the heterostructures form n-Si/Fe Schottky junctions. Structural studies on the samples as performed through XRD indicate the polycrystalline nature of the films. The magnetization data showed that the samples have in-plane easy axis of magnetization. The coercivity of the samples is of the order of 90 Oe. The I-V measurements on the samples showed nonlinear behavior. The diode ideality factor η = 2.6 is observed for the junction with ND = 1018 cm-3. The leakage current I0 increases with the increase of ND. Magnetic field has less effect on the electrical properties of the junctions. A positive magnetoresistance in the range 1 – 10 % was observed for the Si/Fe Schottky junctions in the presence of magnetic field of strength 2 T. The origin of the MR is analyzed using a model where the ratio of the currents across the junctions with and without the applied magnetic field, IH=2T/IH=0 is studied as a function of the bias voltage Vbias. The ratio IH=2T/IH=0 shows a decreasing trend with the Vbias, suggesting that the contribution to the MR in our n-Si/Fe Schottky junctions due to the spin dependent scattering is very less as compared to that due to the suppression of the impact ionization process.

Advances in Soft Functional Materials Research

Advances in Soft Functional Materials Research

Thin Film Polycrystalline Silicon Solar Cells Studied by Transient Terahertz Probe Spectroscopy

Abstract Thin film polycrystalline silicon (poly-Si) solar cells were annealed in hydrogen plasma or in water vapour to suppress electrical activity of defects and impurities (passivation). The hydrogen plasma passivation procedure was optimized and a modification of several hydrogenation parameters was suggested, e.g. higher hydrogen pressure of 300 Pa and keeping bias voltage Vbias constant during the whole passivation process until the plasma was switched off. Since the plasma hydrogenation process was run as a closed system (without hydrogen flow) and the achieved passivation results (up to 497 mV) were close to the limit for poly-Si (500 mV), the generally accepted necessity to operate the process with a hydrogen flow was called into question. Samples passivated in a hydrogen plasma at different processing conditions were analyzed by transient terahertz probe spectroscopy and Suns-VOC method to measure a solar cell's charge carrier lifetime and its open-circuit voltage VOC, respectively. A correlation of these measured parameters was observed even though both these techniques characterize solar cells in completely different states. The transient terahertz probe spectroscopy appears to be a useful tool for a contactless investigation of materials where ultrafast processes of ns-long lifetimes play a crucial role, e.g. nanostructured objects and materials.

Flow driven transmission of charged particles against an axial field in antistatic tubes at the sample outlet of a Differential Mobility Analyzer

Electrophoretic losses at the outlet of Differential Mobility Analyzers result from the need to transport the aerosol from the high voltage VBias typically applied to the inner electrode, to the ground potential at which a particle detector is usually connected. Here we examine the sample transmission efficiency η for mobility-selected ions brought to ground through a commercial Static Dissipative Polyurethane tube. η(VBias) is defined as the ratio of the ion current transmitted through the tube with bias voltage VBias over that transmitted when VBias=0. We find η values close to unity under common DMA operational conditions. η is measured also under adverse fields strong enough for all charged particles to be lost, with a sample transmission efficiency comparable to that expected for a uniform velocity profile within the tube. Outlet tube Reynolds numbers are examined in the range 446<Re<4460, with an increased transmission associated to decreasing Re. The sample transmission efficiency is close to ideal, but geometrical non-idealities are apparent from small decreases in transmission under favorable bias voltages.

New class of photocatalytic materials and a novel principle for efficient water splitting under infrared and visible light: MgB2 as unexpected example.

Water splitting is unanimously recognized as environment friendly, potentially low cost and renewable energy solution based on the future hydrogen economy. Especially appealing is photocatalytic water splitting whereby a suitably chosen catalyst dramatically improves efficiency of the hydrogen production driven by direct sunlight and allows it to happen even at zero driving potential. Here, we suggest a new class of stable photocatalysts and the corresponding principle for catalytic water splitting in which infrared and visible light play the main role in producing the photocurrent and hydrogen. The new class of catalysts - ionic or covalent binary metals with layered graphite-like structures - effectively absorb visible and infrared light facilitating the reaction of water splitting, suppress the inverse reaction of ion recombination by separating ions due to internal electric fields existing near alternating layers, provide the sites for ion trapping of both polarities, and finally deliver the electrons and holes required to generate hydrogen and oxygen gases. As an example, we demonstrate conversion efficiency of ~27% at bias voltage Vbias = 0.5V for magnesium diboride working as a catalyst for photoinduced water splitting. We discuss its advantages over some existing materials and propose the underlying mechanism of photocatalytic water splitting by binary layered metals.

Flux Control of Carbon Nanoparticles Generated due to Interactions between Hydrogen Plasmas and Graphite Using DC-Biased Substrates

Flux control of dust particles in a nanometer size range using dc bias voltage is discussed based on dust collection in a divertor simulator employed helicon hydrogen discharges. To discuss mechanisms of flux control, we have estimated etching rate of deposited dust particles due to hydrogen plasma irradiation and have measured current density toward the dc biased substrates. We have found the contribution of the etching can be negligible in a dc bias voltage Vbias range between -50 and 70 V. Clear correlation between Vbias dependence of current density and that of dust flux shows electrostatic force is one of important forces for controlling flux of dust particles.

Formation of polycrystalline BaSi2 films by radio-frequency magnetron sputtering for thin-film solar cell applications

Polycrystalline BaSi2 films were prepared by means of radio-frequency magnetron sputtering using a BaSi2 target and by post-annealing at 500°C and above in ultrahigh vacuum. Striped Al electrodes spaced at 1.5mm were evaporated on the surface. Photocurrents were clearly observed for photon energies greater than approximately 1.25eV under bias voltages applied between the electrodes. The photoresponsivity increased with bias voltage Vbias and reached approximately 5mA/W at 1.5eV when Vbias=2.0V.

Large photoresponsivity in semiconducting BaSi2 epitaxial films grown on Si(0 0 1) substrates by molecular beam epitaxy

Approximately 900- and 400-nm-thick BaSi2 epitaxial films were grown on Si(111) and Si(001) substrates, respectively, by molecular beam epitaxy, and their photoresponse properties were compared at room temperature. When the bias voltage Vbias applied between the 1.5-mm-spacing stripe-shaped electrodes on the BaSi2 surfaces increased, photocurrents were clearly observed for photon energies greater than the band gap for both samples. However, the photoresponsivity for BaSi2 on Si(001) was more than 8 times larger than that for BaSi2 on Si(111); reaching approximately 50 and 5mA/W at 1.6eV, respectively, when Vbias was 1.0V. This is attributed to the difference in the grain size of BaSi2 films confirmed by plan-view transmission electron microscopy.