Influence of parameters of the potential barrier at the metal/polymer interface on the electronic switching in the metal/polymer/metal structure

The influence of the first-order phase transition on the parameters of the potential barrier at the indium/polymer interface has been investigated. It has been established that the phase transition occurring in the metal initiates switching of the polymer insulator into a high-conductivity state. Performed investigations have shown that the main charge transfer mechanism in the metal-polymer-metal structure at high temperatures is the current caused by the electron thermionic emission. The analysis of current-voltage characteristics has demonstrated that the first-order phase transition in indium leads to the variation in the potential barrier height at the metal/polymer interface by Δφ ≈ 0.18 eV. It is this phenomenon that is responsible for the electronic switching.     

Methods of calculating the band structure and low-energy secondary electron spectroscopy of iridium

A theoretical interpretation is put forward for the fine structure of the secondary electron emission spectra of Ir normal to the (111) surface and the total current spectrum of an Ir polycrystal. The calculations took into account the energy dependence of the broadening of the energy band levels, the electron-electron and electron-plasmon contributions to the nonequilibrium electron distribution function, and the isotropic component of the current from the electrons scattered at the surface. It is shown that the fine structure of the secondary electron emission spectrum and the total current spectrum is mainly attributable to the electron structure of the final states into which the electrons enter or from which they are emitted so that the characteristics of the band configuration in the energy band structure can be reconstructed directly from the experimental data. This method can be used to separate bulk effects from surface effects in the secondary electron emission and total current spectra. It is confirmed that the fine structure of the secondary electron emission and total current spectra depends on the geometric structure and the degree of ordering of the crystals. A reduction in the intensity of the fine structure serves as a measure of the defect structure in the surface region of the sample which can be successfully used to monitor the surface state during treatment. Zh. Tekh. Fiz. 69, 94–96 (June 1999)     

Critical (burst) electron emission from dielectrics, induced by injection of a dense electron beam

A dense pulsed electron beam and nanosecond pulse length has been used to inject negative electric charge into various dielectric materials (single crystals, glasses, composites, plastics) for initiation of electron field emission from the dielectric into a vacuum. It has been shown that upon reaching a critical electric field in the bulk and at the dielectric surface there is intense critical electron emission. The local current density from the emission centers reaches a record value (for dielectrics) of the order of 10⁶ A/cm². The emission occurs in the form of a single gigantic pulse. The measured amplitude of the emission current averaged over the emitting surface is the same order of magnitude as the injected electron current: 10–1000 A. the emission current pulse lages behind the current pulse of the primary electron beam injected into the sample. The delay time is in the range 1–20 nsec and decreases with increasing current density of the injected beam. Direct experimental evidence is found for intense generation of carriers (band or quasifree electrons) in the near-surface layer of the dielectric in a strong electric field due to the Frenkel-Poole effect and collisional ionization of traps, usually various donor levels. This process greatly strengthens the field emission from the dielectric. It has been shown experimentally that the emission is nonuniform and is accompanied by “point bursts” at the surface of the dielectric and ionized plasma spikes in the vacuum interval. These spikes are the main reason that the transition of the field emission into “bursts” is critical, similar to the current which has been previously observed in metals and semiconductors. However there are a number of substantial differences. For example the critical field emission current density needed for the transition into “bursts” is three orders of magnitude less than for metals. If we provide sufficient electron current at the surface or from the bulk of the dielectric to the emission centers, then the critical emission is always accompanied by a vacuum discharge between the surface of the dielectric and a metallic collector. A detailed computer model of the processes in the dielectric during injection of a high-density electron beam has been developed which allows one to understand the complex physical pattern of the phenomenon. Tomsk Polytechnic University. Institute of High-Current Electronics, Siberian Section, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–67, November, 1997.     

Mechanism of field emission from carbon materials

Field emission in diamond and graphite-like polycrystalline films is investigated experimentally. It is shown that the emission efficiency increases as the nondiamond carbon phase increases; for graphite-like films the threshold electric field is less than 1.5 V/μm, and at 4 V/μm the emission current reaches 1 mA/cm², while the density of emission centers exceeds 10⁶ cm⁻². A general mechanism explaining the phenomenon of electron field emission from materials containing graphite-like carbon is proposed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 56–60 (10 July 1998)     

Thermal and optical properties of electron beam irradiated cellulose triacetate

Samples from Cellulose triacetate (CTA) sheets were irradiated with electron beam in the dose range 10–200 kGy. Non-isothermal studies were carried out using thermogravimetric analysis (TGA) to obtain the activation energy of thermal decomposition for CTA polymer. The CTA samples decompose in one main break down stage. The results indicate that the irradiation by electron beam in the dose range 80–200 kGy increases the thermal stability of the polymer samples. Also, the variation of melting temperatures with the electron dose has been determined using differential thermal analysis (DTA). The CTA polymer is characterized by the appearance of one endothermic peak due to melting. It is found that the irradiation in the dose range 10–80 kGy causes defects generation that splits the crystals depressing the melting temperature, while at higher doses (80–200 kGy), the thickness of crystalline structure (lamellae) is increased, thus the melting temperature increases. In addition, the transmission of these samples in the wavelength range 200–2500 nm, as well as any color changes, were studied. The color intensity ΔE* was greatly increased on increasing the electron beam dose, and accompanied by a significant increase in the blue color component.      

Energy distribution of post-accelerated electrons field-emitted from carbon fibres

An electron gun system with post-acceleration is described suitable to operation with a carbon fibre field-emission tip. The system was tested in an electron optical bench with a vacuum pressure of about 10⁻⁶ torr. The electron current was most stable if the current to the extraction electrode was minimized. The half width of the energy distribution of the electrons accelerated up to 30 keV was 215 meV for small currents (≈10⁻⁹A). The half width of the distribution increases with growing emission current. This increase may be attributed to instabilities in the surface structure due to ion bombardment and to the circumstance that more than one emission centre contributes to the electron current.     

High-energy passive-ionization electrons and holes in dielectrics with pulsed irradiation by high-density electron beams

This article is a survey of works by the author and colleagues on the investigation of charging and discharging dynamics in solid dielectrics exposed to dense electron beams with subnanosecond resolution. Small high-current electron accelerators of theDzhin type, which were developed and fabricated at the Nonlinear Physics Laboratory, were used as the source of the primary electron beam. The primary electron beam parameters were: 0.25–0.45 MeV, 1–30 nsec, 0.1–10,000 A/cm³. The dielectric is investigated experimentally with its surface covered by a metallic electrode and with critical electron emission into the vacuum eliminated. In this case, the total current in the dielectric consists of three components: the primary beam current, the displacement current, and the conduction current. The first and last are responsible for charging and discharging of the dielectric volume. It is shown that the bulk nonequilibrium radiation conduction mechanism depends greatly on the dose intensity. For small dose intensities, the principal current carriers are the low-energy electrons of the conduction band and holes of the valence band, which are in quasi-equilibrium with the lattice phonon field before capture by defects or merging into excitons preceding recombination. This type of conduction has been well studied in the physics of semiconductors and dielectrics. However, over a broad interval of intermediate and high dose intensities, another type of nonequilibrium conduction dominates — the high-energy, which was discovered and studied by the author and colleagues. The principal carriers are then passive-ionization electrons and holes with energies of 0.1–10 eV in the process of phonon relaxation in which phonon emission dominates absorption. The high-energy conduction differs considerably from the low-energy in many properties, which determines the unusual dynamics of dielectric charging and discharging with irradiation by the dense electron beam of a high-current accelerator. Institute of High-Current Electronics, Siberian Section, Russian Academy of Sciences. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 109–119, November, 1996.     

Mirnov coil data analysis for tokamak ADITYA

The spatial and temporal structures of magnetic signal in the tokamak ADITYA is analysed using recently developed singular value decomposition (SVD) technique. The analysis technique is first tested with simulated data and then applied to the ADITYA Mirnov coil data to determine the structure of current peturbation as the discharge progresses. It is observed that during the current rise phase, current perturbation undergoes transition from m=5 poloidal structure to m=4 and then to m=3. At the time of current termination, m=2 perturbation is observed. It is observed that the mode frequency remains nearly constant (≈10 kHz) when poloidal mode structure changes from m=4 to m=2. This may be either an indication of mode coupling or a consequences of changes in the plasma electron temperature and density scale length.     

Barrier lowering effect and dark current characteristics in asymmetric GaAs/AlGaAs multi quantum well structure

In this study, we investigate dark current voltage characteristics of GaAs/AlGaAs staircase-like asymmetric multiquantum well structure at various temperatures experimentally. The activation energy is calculated by using Arrhenius plots at different voltages. It is found that the activation energy decreased with increasing electric field. This result is evaluated using a barrier lowering effect which is a combination of geometrical and Poole–Frenkel effects. Measured dark current density–voltage (J–V) characteristics compared with the Levine model, 3D carrier drift model and the emission capture model. The best agreement with the experimental results of dark current densities is obtained by the Levine model.     

Results of Determining the Electron Number Density in the Ionospheric E Region from Relaxation Times of Artificial Periodic Irregularities of Different Scales

We present the first results of determining the electron number density in the ionospheric E region by a novel technique based on the creation of artificial periodic irregularities when the ionosphere is affected by powerful radio emission at two frequencies. Using the results of the measurements performed in October 2006 during heating of the ionosphere by the “Sura” facility radiation at frequencies 4.7 and 5.6 MHz, we obtained the electron number density profiles in an altitude range of 100 to 110 km. Features of the procedure of measurement and calculation of the electron number density are described in detail. It is shown that the method can be used for a study of the irregular structure of the lower ionosphere. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 6, pp. 477–484, June 2008.     

Synthesis and characterization of LaB6 thin films on tungsten, rhenium, silicon and other substrates and their investigations as?field emitters

The paper deals with the comparative study of nanocrystalline Lanthanum hexaboride (LaB₆) thin films grown on various substrates by Pulsed laser deposition and Arc plasma method. Field emission studies were carried out on LaB₆ films deposited on various substrates show metallic behavior of the emitters. The high value of field enhancement factors, indicating that the electron emission from LaB₆ nanoscale protrusions deposited on emitter surface. The post field emission surface morphology of the emitters showed no significant erosion of the films during continuous operation. The observed behavior indicates that it is linked with the growth of LaB₆ films on substrate crystal structure. The LaB₆ nanocrystallites/nanowires films were synthesized using arc plasma method shows good emission current stability. The LaB₆ micro/nanocrystallites were also obtained by picosecond laser irradiation which gives high enhancement β factor, and good emission current stability along with high current density. The results reveal that nanocrystalline LaB₆ films, exhibit high resistance to ion bombardment and excellent structural stability and are more promising emitters for practical applications in field emission based new generation devices.     

Fast electron energy deposition in aluminium foils: Resistive vs. drag heating

The high current electron beam losses have been studied experimentally with 0.7 J, 40 fs, 6 10¹⁹ Wcm^-2 laser pulses interacting with Al foils of thicknesses 10-200 μm. The fast electron beam characteristics and the foil temperature were measured by recording the intensity of the electromagnetic emission from the foils rear side at two different wavelengths in the optical domain, ≈407 nm (the second harmonic of the laser light) and ≈500 nm. The experimentally observed fast electron distribution contains two components: one relativistic tail made of very energetic (T _h ^tail ≈ 10 MeV) and highly collimated (7° ± 3°) electrons, carrying a small amount of energy (less than 1% of the laser energy), and another, the bulk of the accelerated electrons, containing lower-energy (T _h ^bulk=500 ± 100 keV) more divergent electrons (35 ± 5°), which transports about 35% of the laser energy. The relativistic component manifests itself by the coherent 2ω₀ emission due to the modulation of the electron density in the interaction zone. The bulk component induces a strong target heating producing measurable yields of thermal emission from the foils rear side. Our data and modeling demonstrate two mechanisms of fast electron energy deposition: resistive heating due to the neutralizing return current and collisions of fast electrons with plasma electrons. The resistive mechanism is more important at shallow target depths, representing an heating rate of 100 eV per Joule of laser energy at 15 μm. Beyond that depth, because of the beam divergence, the incident current goes under 10¹² Acm^-2 and the collisional heating becomes more important than the resistive heating. The heating rate is of only 1.5 eV per Joule at 50 μm depth.     

Local electrical overstressing in polymer dielectrics

Using a spherical concentric capacitor as a microspike model, the electric field distribution in a polymer dielectric material with shallow and deep electron traps is investigated. It is established that in the course of field emission from microspikes on the electrode and space charge accumulation on the traps, the values of coefficient of electric overstresses depend on temperature, injection barrier height, and trap concentration and depth in the polymer material. The behavior of this coefficient is characterized as a function of the mean field strength. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–9, December, 2008.     

Circular ion diode with self-magnetic insulation

The results of a study of the generation of a gigawatt-level pulsed ion beam formed by a diode with an explosive-emission potential electrode in self-magnetic insulation mode are presented. The experiments have been performed on the TEMP-4M ion accelerator operating in double-pulse formation mode: the first pulse is negative polarity (300–500 ns, 100–150 kV) and the second is positive (150 ns, 250–300 kV). The ion current density is 20–40 A/cm²; the beam consists of protons and carbon ions. To increase the efficiency of the ion current generation, a circular geometry diode is proposed. It is shown that with the new design, the plasma is effectively formed over the entire working surface of the graphite potential electrode. During ion beam generation, magnetic insulation of the electrons is achieved over the entire length of the diode (B/B _cr ≥ 3). Because of the high drift velocity, the transit time of electrons in the anode-cathode gap is 3–5 ns, whilst the transit time of C⁺ carbon ions exceeds 8 ns. This indicates low efficiency self-magnetic insulation for this geometry of diode. At the same time, it has been observed experimentally that during ion current generation (the second pulse), the electron component of the total current is suppressed by a factor of 4–5. A new mechanism of limiting the electron emission, which explains the decrease in the electron component of the total current in the circular diode with self-magnetic insulation, is proposed.     

Structural and composition changes in superconducting ceramics locally irradiated by electrons

The structural changes and the cation and anion compositions of the surface of superconducting YBa₂Cu₃O_7−x and Tl₂Ba₂CuO_6+x ceramics after local irradiation for several seconds by an electron probe with an accelerating voltage of 25 kV and a current exceeding 10⁻⁷ A are investigated by secondary-electron emission and by cathodoluminescence and x-ray microanalysis in a scanning electron microscope. Morphologically altered regions are detected in the irradiation epicenter, where the structure, chemical composition, and phase composition of the original compound are completely lost. In the intermediate zone between the epicenter and the periphery a distribution of the secondary emission yield is observed with a complex character that differs for the yttrium and thallium ceramics, and anomalies appear in the cathodoluminescence spectra. The experimental data are interpreted on the basis of ideas of oxygen losses under the direct influence of the electron probe and related electronic processes in superconductors. Fiz. Tverd. Tela (St. Petersburg) 39, 452–456 (March 1997)     

Field electron emission form single-walled carbon nanotubes with deposited cesium atoms

It has been found that deposition g of cesium atoms on single-walled carbon nanotubes covered with potassium atoms not only drastically increases emission current but also considerably changes the shape of current-voltage characteristics of field electron emission, namely, the characteristics become nonlinear in Fowler-Nordheim coordinates. It has been assumed that this effect is associated with the fact that field electron emission in these layers comes from single-walled carbon nanotubes, which have p-type conductivity after potassium treatment, while deposition of cesium leads to the formation of p-n junctions near nanotube tips. Part of the applied voltage drops in p-n junction, thus causing a nonlinearity of current-voltage characteristics.     

FT-EPR study of the pH dependence of the photochemistry of sesamol in aqueous solution

A Fourier transform electron paramagnetic resonance (FT-EPR) study was made of the photochemistry of 3,4-methylenedioxyphenol (sesamol, SEOH)) in aqueous solution. FT-EPR measurements show that in alkaline (pH 11) solution, pulsed-laser excitation of SECT leads to photoionization giving the hydrated electron and SEO^⋅ free radical. Resonance signals from these paramagnetic species develop with instrument-controlled rise time. They exhibit a low-field emission/ high-field absorption (E/A) CIDEP pattern with the transition from emission to absorption occurring at the resonance of the hydrated electron. It is shown that the spin polarization stems from contributions from the ST₀ radical pair mechanism (E/A) and triplet mechanism (A). From this it is concluded that photoionization of sesamol occurs via the triplet excited state. In neutral and acidic (pH 4–7) aqueous solution, photoexcitation generates SEO^⋅ and cyclohexadienyl-type radicals. In this case, radicals grow in over a period of 1–2 μs and FT-EPR spectra display an E/A pattern with the inversion point in the center. The lowering of the pH of the solution apparently is accompanied by a strong reduction in the relative importance of photoionization. From the FT-EPR data it can be deduced that in neutral and acidic solutions the dominant reaction channel is H-atom transfer. In this respect, the photochemistry of sesamol differs from that of phenol andp-cresol. For these phenols the change in pH does not affect the appearance of the FT-EPR spectra. Apparently, the change in electronic structure caused by the methylenedioxy substituent strongly affects the excited state reactivity of sesamol.     

Transformation of auger electron spectra of ytterbium nanofilms under the action of adsorbed carbon monoxide and oxygen molecules

The transformation of the Auger electron spectra of ytterbium nanofilms as a result of chemisorption of CO and O₂ molecules on their surface has been studied. It has been shown that the adsorption of these molecules is accompanied by a radical transformation of the electronic structure of nanofilms, during which the 5d level of ytterbium drops below the Fermi level. As a consequence, one electron can transfer from the 4f level to the 5d level. In turn, this provides the conditions for a giant resonance 4d → 4f and a subsequent Coster-Kronig supertransition 4d ⁹4f ¹⁴ → 4d ¹⁰4f ¹² + Auger electron, which is accompanied by emission of one 4f electron to vacuum. The results obtained have demonstrated that molecules chemisorbed on the surface of nanofilms can cause qualitative changes in the properties of the surface and in the bulk of these films. It is obvious that this offers a means for designing nanoobjects with controlled properties.     

A “capacitor” model of the hysteresis of tunneling current in w -GaN/AlGaN(0001) structures

A “capacitor” model of the hysteresis is developed using the self-consistent calculation of the tunneling current in a w-GaN/AlGaN(0001) double-barrier structure. In the framework of this model, the current jumps and changes in the potential and the electric field in the structure upon transition from one branch of the current loop to the other branch are considered a result of the recharging of two joined capacitors with the plates located at the positions of the extrema of variations in the electron density in the regions of the emitter, the quantum well, and the collector. It is demonstrated that, when the external and internal fields in the quantum well compensate for each other, the tunneling current is sharply and irreducibly switched to the characteristics of the other resonance and forms a wide hysteresis loop so that, in the branches of this loop, the charge is redistributed between the quantum well and the collector. If the fields coincide with each other, there arises a narrow “singleresonance” hysteresis loop, which is accompanied by the transfer of the electron charge from the emitter to the collector. The developed model leads to agreement with the results of the self-consistent calculations and provides an illustrative interpretation of the complex electron tunneling processes. Original Russian Text ? A.N. Razzhuvalov, S.N. Grinyaev, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 1, pp. 168–177.     

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W

Nanocrystalline ZnO thin films have been deposited on rhenium and tungsten pointed and flat substrates by pulsed laser deposition method. An emission current of 1 nA with an onset voltage of 120 V was observed repeatedly and maximum current density ∼1.3 A/cm² and 9.3 mA/cm² has been drawn from ZnO/Re and ZnO/W pointed emitters at an applied voltage of 12.8 and 14 kV, respectively. In case of planar emitters (ZnO deposited on flat substrates), the onset field required to draw 1 nA emission current is observed to be 0.87 and 1.2 V/μm for ZnO/Re and ZnO/W planar emitters, respectively. The Fowler–Nordheim plots of both the emitters show nonlinear behaviour, typical for a semiconducting field emitter. The field enhancement factor β is estimated to be ∼2.15×10⁵ cm⁻¹ and 2.16×10⁵ cm⁻¹ for pointed and 3.2×10⁴ and 1.74×10⁴ for planar ZnO/Re and ZnO/W emitters, respectively. The high value of β factor suggests that the emission is from the nanometric features of the emitter surface. The emission current–time plots exhibit good stability of emission current over a period of more than three hours. The post field emission surface morphology studies show no significant deterioration of the emitter surface indicating that the ZnO thin film has a very strong adherence to both the substrates and exhibits a remarkable structural stability against high-field-induced mechanical stresses and ion bombardment. The results reveal that PLD offers unprecedented advantages in fabricating the ZnO field emitters for practical applications in field-emission-based electron sources.