M.A. Timofeyev

Nanocrystalline graphite: Promising material for high current field emission cathodes

Electron field emission properties of nanocrystalline graphite (NCG) films, grown by plasma enhanced chemical vapor deposition method on conductive Si substrates without using of any catalyst, were investigated. Current-voltage characteristics were measured in pulse-periodic regime. It was shown that grown NCG films can operate at field emission current density up to 10 A/cm2. It was found that NCG films contain, along with the normally oriented to the substrate nanoflakes, carbon whiskers consisted of graphene nanoribbons and nanowires with length considerably higher than of the nanoflakes.Electron field emission properties of nanocrystalline graphite (NCG) films, grown by plasma enhanced chemical vapor deposition method on conductive Si substrates without using of any catalyst, were investigated. Current-voltage characteristics were measured in pulse-periodic regime. It was shown that grown NCG films can operate at field emission current density up to 10 A/cm2. It was found that NCG films contain, along with the normally oriented to the substrate nanoflakes, carbon whiskers consisted of graphene nanoribbons and nanowires with length considerably higher than of the nanoflakes.

Performance of nanocrystalline graphite field emitters

Nanocrystalline graphite field emitters fabricated on silicon substrates have been characterized in terms of current–voltage, pressure dependency, long term stability, work function and lateral momentum. The work function is 4.0–4.2 eV and the cone angle of emission due to the lateral momentum of the emitting electrons in less than 1°. It is shown that with proper pre-testing treatment and copper anodes, these emitters can operate for over 5000 h without significant changes in emission current. They also operate at pressures of 5×10−5 Torr with current fluctuations ΔI/I of less than 1%. A quite complex emission pattern develops in conjunction with CRT phosphors. The transmission coefficient of electrons exiting a grid can vary from 1–50% depending on which phosphor is being used.

Effect of the tin content on the composition and optical and electrical properties of ITO films deposited onto silicon and glass by ultrasonic spray pyrolysis

With the aim of optimizing the properties of tin-doped indium oxide (ITO) films as applied to silicon solar cells, ∼100-nm-thick ITO films were deposited onto (nn+)-Cz-Si and glass substrates by ultrasonic spray pyrolysis in argon at a temperature of 380°C. The relative Sn and In content in the film-forming solution was varied in the range of [Sn]/[In] = 0–12 at %. Optimal parameters are exhibited by the films produced at [Sn]/[In] = 2–3 at % in the solution ([Sn]/([In] + [Sn]) = 5.2–5.3 at % in the film). For such films deposited onto glass substrates, the effective absorptance weighted over the solar spectrum in the wavelength range from 300 to 1100 nm is 1.6–2.1%. The sheet resistance Rs of the films deposited onto silicon and glass is, correspondingly, 45–55 and 165–175 Ω▭−1. After eight months of storage in air, the resistance Rs of the optimal films remained unchanged; for the other films, the resistance Rs increased: for the films on silicon and glass, the resistance Rs became up to 2 and 14 times higher, respectively.

Optimization of the deposition and annealing conditions of fluorine-doped indium oxide films for silicon solar cells

Fluorine-doped indium oxide (IFO) films are deposited onto (pp+)Si and (n+nn+)Si structures made of single-crystal silicon by ultrasonic spray pyrolysis. The effect of the IFO deposition time and annealing time in an argon atmosphere with methanol vapor on the IFO chemical composition, the photovoltage and fill factor of the Illumination-Uoc curves of IFO/(pp+)Si structures, and the sheet resistance of IFO/(n+nn+)Si structures, correlating with the IFO/(n+)Si contact resistance, is studied. The obtained features are explained by modification of the properties of the SiOx transition layer at the IFO/Si interface during deposition and annealing. Analysis of the results made it possible to optimize the fabrication conditions of solar cells based on IFO/(pp+)Si heterostructures and to increase their efficiency from 17% to a record 17.8%.

COMPARATIVE-STUDY OF MICROCRYSTALLINE DIAMOND

Abstract Raman spectra of diamond powders and polycrystalline CVD diamond films with diamond crystalline size from 4 μm to 5 nm have been measured as a function of the crystallite size. The Raman diamond line at 1332 cm −1 was found for the powder to become more asymmetric with decreasing particle size to 5 nm. A strong influence of substrate temperature and methane-hydrogen gas mixture pressure during diamond films growth by the d.c. glow discharge method on linewidths and peak position have been found. The observed results are explained by “phonon confinement” and by strain effects. CVD and laser-evaporated nanophase diamond films were produced and have shown a Raman line at 1140 cm −1 .

Diamond film deposition by downstream d.c. glow discharge plasma chemical vapour deposition

Abstract A new d.c. glow discharge chemical vapour deposition (CVD) method was developed for diamond film (DF) deposition onto insulator substrates. This method uses ordinary d.c. plasma CVD equipment, but the anode system was modified both to insulate the substrate holder from the anode and to set the substrate downstream of the plasma. This is achieved by two methods. First, the anode was made from the tungsten grid and the insulated substrate was placed under the grid and out of the discharge area. Second, the anode was made from the thick-wall molibden tube and the substrates were placed on the insulator inside the tube. The experiments were carried out under a hydrogen pressure of 50–150 Torr and methane concentration of 1%–2%. The substrates (Mo or Si) were set on the silica substrate holder. The temperature was approximately 1000 °C. Diamond films were grown by both methods. Scanning electron microscopy, cathodoluminescence microscopy and spectroscopy, Raman scattering and X-ray diffractometry were used to study and compare the diamond films.

Scanning tunneling microscope study of diamond films for electron field emission

An experimental method has been developed for measuring the emission parameters of diamond-like materials using a scanning tunneling microscope modified for both tunneling and emission regimes. With this technique, nanodiamond films with emission site densities as high as 106 cm−2 were investigated. The emission sites were found to be located on the grain slopes or in the areas between grains and consist of multiple smaller emission points.

OES study of plasma processes in d.c. discharge during diamond film deposition

A systematic study has been performed of optical emission spectra from d.c. glow discharge during diamond film deposition. The emission intensity within the region 400–700 nm was found to be a function of methane concentration and interelectrode position. The experiments were carried out under hydrogen pressure 160 Torr and methane concentration 0%–3%. The optical emission had a different character for different discharge regions (anode and cathode regions, plasma volume). The optical spectra observed were also essentially different from the flame and jet arc emission spectra. The most intensive lines in our experiments were emitted by hydrogen atoms, Hα and Hβ. Using a ratio of these lines the electron temperature in CH4 + H2 plasma was estimated. The R-branch of the H2 molecular (G1∑+g−B1∑+u (00) band in the emission spectra near λ = 463 nm and vibration structures in the emission spectra of the radical C2 (d3пg−a3пu) near λ = 515 nm were also examined. These data were used to determine a gas temperature.

The study of electron source with flat diamond field emission cathode

There is a particular interest to possible usage of various field emission cathodes in electron sources. It seems very attractive to apply diamond flat cold cathodes as they show good emission parameters, are reliable, stable and rather insensitive to vacuum. Some measurements of divergence of e-beams emitted from diamond films and diamond-coated FEAs have been published. In this work we studied the divergence of the electron beam emitted from flat diamond cold cathode, passed through the split in the extracting metal anode. The aim was to analyse the influence of split edges on electrons trajectories and to obtain information about initial angular and energy distribution of emitted electrons comparing experimental and calculated results. We used the split of rectangular form as it did not affect on electrons trajectories in direction along the split (axis y), by this means allowing to see initial electrons distribution, and its affects in the perpendicular direction (axis x).

Laminated grid solar cells (LGCells) on multicrystalline silicon. Application of atomic hydrogen treatment

For the first time, solar cells of laminated grid cell (LGCell) design are fabricated on multicrystalline nontextured silicon (mc-Si). An efficiency of 15.9% is achieved. The effect of (n+pp+)-mc-Si structure treatment by atomic hydrogen generated by a hot filament and microwave plasma is studied. Hydrogenation improves the parameters describing the dependence of the open-circuit voltage on the radiation intensity and the long-wavelength (λ = 1000 nm) sensitivity of the solar cell by 10–20%, which indicates that defects in mc-Si are passivated. Hydrogenation of the emitter side results in an increase in the series resistance of the solar cell, a decrease in the short-wavelength (λ = 400 nm) sensitivity by 30–35%, and the appearance of an oxygen peak in the energy-dispersive spectra (EDS). These effects are eliminated by fine etching of the emitter.