Effect Of Image Force Research Articles

Charge-transfer interatomic potential to reproduce 30° partial dislocation movements for 4H-SiC in the surface vicinity and its application to BPD-TED conversion

4H-SiC has recently attracted attention as a new power semiconductor material to replace silicon. One of the challenges impeding its practical use is the elimination of killer defects in its epitaxial layer, such as dislocations and stacking faults. The conversion of basal plane dislocations (BPDs) into threading edge dislocations (TEDs) is crucial for reducing harmful mobile dislocations. However, the atomistic mechanism underlying the process remains elusive. To clarify the BPD–TED conversion mechanism, we developed a charge-transfer interatomic potential to reproduce 30° partial dislocation dynamics in the surface vicinity of 4H-SiC. The proposed potential significantly improved the accuracy of estimating the stacking fault energy and dislocation mobility in the regions where the dislocation core and surface overlap. Using the developed potential, we investigated the mechanism underlying the contraction of a pair of 30° partial dislocations in the vicinity of the surface. From the energy landscape, the equilibrium distance between the two partials decreased as the depth from the surface decreased because of the strong image force near the surface. The equilibrium distance decreased to zero when the depth reached 0.25 nm. This result is qualitatively consistent with the dislocation theory and the known effect of image force. From the viewpoint of the activation energies of kink nucleation and migrations, the activation energy for C-core and Si-core partial contractions decreased below a depth of 1 nm from the C-face and S-face surfaces, respectively, and became almost equal to that of C-core and Si-core expansions, respectively, when the depth reached 0.25 nm. Therefore, the partial BPDs can contract easily in the corresponding structures, particularly when the depth from the surface is less than 1 nm.

Absence of any effect of the electric charging state of particles below 10 nm on their penetration through a metal grid

The effect of image force on the penetration of nanometer particles through metal grids remains a controversial issue. Experimental evidence of the existence and of the absence of such effect have both been reported in the past. A careful experimental work to measure penetration of particles in the mobility equivalent diameter range between 3.4 and 10 nm has been carried out. The possible particle size change between the aerosol generator and the filter has been considered, as well as the possible effect of particle number concentration on the filtration efficiency. The geometric dimensions of the filter allowed attainment of the fully developed parabolic flow velocity profile upstream the grid. Measurements were done at two values of the fiber Reynolds number, 0.09 and 0.12, much smaller than 1, as demanded by the currently accepted filtration theory. Penetration of charged particles, measured in three alternative ways, has been compared with penetration of uncharged and neutral particles (the latter consisting of a mixture of positive, negative, and uncharged particles). Two main conclusions have been reached: (1) the charging state of the particles does not affect their penetration through the metal grid and (2) the experimentally measured penetrations are fairly well predicted by the fan filter model of Cheng and Yeh.Copyright © 2018 American Association for Aerosol Research

Effect of Image Force on Tunneling Current for Ultra Thin Oxide Layer Based Metal Oxide Semiconductor Devices

Effect of Image Force on Tunneling Current for Ultra Thin Oxide Layer Based Metal Oxide Semiconductor Devices

Field emission of electrons from spherical conducting carbon nanotube tip including the effect of image force

An expression for the potential energy and field emission current density function of spherical conducting carbon nanotubes (CNTs), including the effect of image force, corresponding to the expression for the electron transmission coefficient obtained from the solution of the time-independent Schrödinger equation, has been derived. Numerical calculations of the potential energy, transmission coefficient, and the current density function have been carried out for the typical set of CNT parameters. It is found that the potential energy decreases with radial distance when image force is ignored. When image force is incorporated, it increases with radial distance in the beginning and then decreases. The transmission probability and the current density function increase, both with the normalized radial energy and normalized Fermi energy, but both the transmission probability and the current density function are larger in the presence of the image force for given values of the normalized radial energy and the normalized Fermi energy. In addition, the image force decreases the applied value of the field. The field emission current density function of emitted electrons decreases with the spherical CNT tip radius. Some of our theoretical results are in accordance with existing experimental observations.

Dislocation – interphase boundary interaction in HCP bicrystals Zn-X, X = Be, Co, Hf

A dislocation near an interface between elastically anisotropic materials is subjected to an image force, attractive or repulsive, depending on the elastic constants of crystals on both sides of the interface and the geometry of the configuration studied. We study the effect of image force on dislocations for bicrystals composed of two hexagonal materials (Zn-X where X is Be, Co, or Hf). The elastic interaction energy is calculated using the theory of Barnett and Lothe. The configuration is: a dislocation parallel to the interface with the Burgers vector b=a/3 [-2110] and an interface which is defined by a rotation axis R = [0001] and rotation angles in the interval [0°, 90°]. The interaction energies obtained are summarized in isoenergy maps. For a dislocation located in the crystal 1, if the crystal 2 is harder than the crystal 1 (μ2 > μ1), the elastic interaction energy is positive, therefore the dislocation is submitted to a repulsive image force.

The effect of image force and temperature on electrical characteristics of layer-type MTJs

Tunnel resistivity R (= V/J) as a function of voltage has been calculated for magnetic tunnel junction systems, such as ferromagnet-insulator-ferromagnet three-layer structures. Our early study [1] has been carried out for determining the barrier characteristics in the MTJs based on Ni80Fe20/Al2O3/Co systems, by fitting the experimental I-V characteristics to Simmons' and Brinkmann's models without image force. In this paper, the image force is taken into account for approaching closely to a real barrier potential. The effect of dielectric constant, and temperature on the tunnel characteristics has also been investigated.

Study of Dielectric Media with Relatively High Permittivity

The electrophysical characteristics of composite media consisting of finely dispersed T-10 000 ferroelectric ceramics and dielectric fluids are experimentally investigated. Transformer oil, dibutyl phthalate, cyclohexanol, and sulfolane are used as dielectric fluids. The permittivity of compositions e is directly proportional to that of the fluids. Its dependence on the concentration of the ferroelectric ceramics is nonlinear and is well consistent with the known Nielsen formula up to the concentration of 55 vol %. The permittivity of a 50% composition exceeds by a factor of five that of the fluid. The maximal coefficient of e increasing approximately equals 20 upon adding two ceramics fractions of different particle sizes to the composition. The specific resistance of compositions decreases as the permittivity of the fluid increases. The concentration dependence of the specific resistance of the T-10 000 ferroelectric ceramics–dibutyl phthalate mixture has a minimum at a concentration of 30–40 vol %. A new mechanism of the surface electric conductance is suggested, which is associated with a decrease in the dissociation energy in the surface layer under the effect of image force. The electric strength of compositions decreases as the permittivity of the fluid increases. The obtained experimental data agree with the bubble model of breakdown ignition.

Effect of image force on ion current density in plasma discharges

Ion current density at the bottom of narrow trenches has been calculated with a Monte Carlo method. It is found that in etching subquarter-micron trenches, the image force between the incident ions and the walls of the etched trench causes appreciable ion fluxes to the sidewalls and contributes to deviations from ideal etching anisotropy. The effect of the image force is particularly important in plasma etching discharges, which are characterized by low ion energies.

The effect of image force on the efficiency of electron tunneling through macromolecular structures

In electron tunneling (ET) along the boundary of two media with different high-frequency dielectric permittivities, the energy gap for the tunneling electron is affected by image forces. Macromolecules and macromolecular structures contain segments possessing different permittivities. Therefore, in any description of ET one must take image forces into account. Here, the spatial form of donor-acceptor ET through bridged molecular groups situated near macromolecular segments is investigated. It is shown that image forces can significantly reduce the energy levels of an electron on the bridge groups. As a result, the ET rate in the donor-bridge-acceptor system can increase by many orders of magnitude. the effect is more favorable for extended bridged chains. This fact may be used in explaining donor-acceptor conductivity of electron pathways in real macromolecular biological structures. Bridged groups in electron pathways may be provided by molecular groups whose affinity for the tunneling electron is higher than that of other groups. Furthermore, the electron overlap integral between the bridged groups should be large enough. That is, the role of bridge can be played by the polypeptide chain backbone, by side-chains of some amino acids (particularly those with aromatic rings), and by conjugated systems of the carotinoid type situated near macromolecular segments or near the surfaces of membranes or channels.

The effect of the dislocation image force on the brittle behaviour of materials

An exact solution for dislocations interacting with a surface crack in a thin plate material is derived. This solution is compared with the approximate solution proposed by Lung (1987). However, his result is only valid for a material with a sufficiently large thickness. It is found that the thinner materials have high toughness. Finally, the ratio of volume to area related to fracture is discussed.

The effect of the dislocation image force on the brittle behaviour of materials

The dislocation image force due to the free surface of a finite width specimen makes the plastic zone at a crack tip larger. The effect of the dislocation image force on the fracture behaviour of materials with different geometrical shapes is discussed. It is found that the ratio V/A as an indication of the brittle behaviour of structural components is reasonable for elastic-plastic fracture.

Bonding functions characterizing the effects of non-reducible cations upon catalytic properties over iron, palladium and platinum supported catalysts

The significant effects of cations upon ammonia synthesis over Fe/Al 2O 3, CO hydrogenation to form methanol over Pd/SiO 2 and propene hydrogenation over Pt/SiO 2 have been reported by us and others. In this article, the nature of the effects is disclosed using bonding functions correlated to the mechanisms of the reactions. It is demonstrated that the cation effect might be the direct interaction between cation and metal atom, through the overlap of the wave function, the effect of electron image force, or dipole interaction. The cation might also stabilize the negative valence of the carrier's oxygen atom, which indirectly affects the oxidation states of the metal atom. In CO hydrogenation to methanol, the cation might be also an active center for stabilizing the intermediate state of the formate ion. For non-reducible transition metal ions, the d-d interaction is shown to be more important than that of s-s or p-d interactions. A fairly good agreement between theory and experimental data is given. The theory has been used to find new effective catalysts.

Effect of minority-carrier injection on Schottky-barrier heights that approach the semiconductor band gap

A guideline for interpretation of capacitive and photothreshold Schottky-barrier energies has been set forth for metal–semiconductor systems which have barrier energies approaching or exceeding the band gap energy. For such systems the measured apparent barrier energies are always less than the true barrier energy if minority-carrier injection controls the incremental field at the interface. For very strong inversion the apparent barrier energy reaches a maximum value independent of the true degree of inversion. However, when the electric field penetration into the metal dominates over the effect of minority-carrier injection, the capacitively measured apparent barrier energies are greater than the true barrier energy and can be greater than the band gap energy without true surface inversion. An approximate universal relationship between the apparent and the true barrier energies has been found for Schottky barriers with a minority-carrier effective density of states Nv greater than 100× the dopant concentration Ns. Then we know the true barrier energy only when the observed capacitive barrier energy is more than one kT from the maximum possible apparent barrier energy predicted by the relationship for a given Nv/Ns. When Nv?100Ns, characterization of C–V data for Schottky barriers requires individual calculations of the detailed relationship between the apparent and the true barrier energies. Existing C–V data for barrier energies close to the band gap energies were interpreted: (1) true barrier energies of the Au–n-GaSb barrier measured by Mead and Spitzer are less than 26 meV greater than 0.61 eV at 298 K and there is no correction for the reported barrier energy, 0.75 eV, at 77 K, (2) bearing in mind all the existing data for Au–p-InAs Schottky barriers we suggest the true barrier energy should be specified as 470±20 meV in the temperature range 4.2 K to 77 K, (3) there appears to be no surface inversion under operating conditions for the Pb–p-PbTe and Sn–p-PbTe Schottky barriers measured at 77 K by Walpole and Nill, despite the fact that the apparent C–V barrier energy for the Sn–p-PBTe barrier exceeds the band gap energy. Finally we have shown that in the presence of strong minority-carrier injection the observed photothreshold barrier energy is greater the greater the applied bias or the higher the dopant concentration, contrary to the tendency expected by the effect of image force alone.

Effect of image force on emission of electrons from negatively charged solid particles

The effect of image force on field emission of electrons from negatively charged solid particles has been analytically investigated and some numerical results have been presented in the form of a graph between ln (nem/F2) and 1/F, where nem is the emission rate per unit area and F is the surface field strength. The results have been compared with those obtained for a corresponding plane surface using the WKB approximation. It is seen that the variation for a spherical surface has a significant departure from the corresponding approximate straight line for a plane surface.

Phenomenological Theory of Turnover in Point-Contact Rectifiers

Turnover relations associated with the general class of the isothermal current-voltage characteristics of the form I = A ( T ) V n exp [- f ( V )/ k T ] have been derived generally by using contact temperature T c as T c = T a ·exp ( P /2πγξ) and the turnover condition (∂ V /∂ T c ) T c =0, where T a is the ambient temperature, P the rate of power generation in a barrier, ξ/ T the thermal conductivity of a semiconductor, r the radius of a whisker tip. According to the results of the numerical calculations of the turnover relations, using the modified isothermal diode theory taking into account minority carriers (generated by intrinsic condition) as one kind of thermal equilibrium approximation and assuming the effect of image force and a Mott barrier, it was found that the above type of isothermal characteristics could not only lead to the essential features of the observed turnover behaviour that both turnover power and voltage decrease with increasing ambient temperature, but also agrees fairly well with the experimental results on germanium point-contact rectifiers obtained by Benzer. The simple isothermal diode theory taking into account majority carriers only can also lead to the essential features of the turnover behaviour although agreement between the theory and the experiment is very poor. From considerations of the rectification mechanism of the above mentioned modified isothermal diode theory, it was found that minority carriers, as well as majority carriers, played an important role in the turnover behaviour.

The theory of direct-current characteristics of rectifiers

It is shown how the image force can be taken into account very simply by making a minor improvement in the current theory of rectification. The equations obtained have been compared with experimental d. c. reverse characteristics of the copper oxide, selenium and germanium rectifiers, assuming in each case, first, that a Mott barrier is present, and, secondly, that a Schottky barrier is present. Agreement is satisfactory, the hypothesis of a Schottky barrier leading to a slightly better fit in all cases. As regards the numerical constants calculated from a fit of the experimental results, satisfactory values can be obtained if a Mott barrier is assumed. If a Schottky barrier is presupposed, the results for the copper oxide and selenium rectifier lead to very low values for the mobilities and high values for the impurity centre densities in the barrier region. This is discussed. The effect of the image force on the current carrier distribution is illustrated, and a general relation between the diffusion and diode theory established.