Electron Transport Parameters Research Articles

Backward propagated ion-acoustic instability in collision-dominated argon plasma

The possibility of backward propagation of the ion-acoustic instability (IAI) in collision dominated weakly ionized plasmas placed in external d.c. electric field, previously predicted within the frames of the kinetic theory is further studied. The developed theory indicates that there is a narrow band of wavelengths in which ion-acoustic waves (IAW) in the strongly collisional domain are excited. The instability of IAW is seen to set in for electron drifts above (or, alternatively, for i-n collision rates below) some critical value depending on Eo/nn. The electron transport parameters determining the instability criterion are evaluated for argon plasmas using two particular steady-state electron distribution functions (EDFs), which take into account the specific feature of the dominant elastic e-n momentum transfer cross section in argon in the range of electron energies up to 16 eV. It is shown that in a weakly ionized, low-temperature, non-equilibrium argon plasma the backward propagated IAI can be expected to set in for higher values of Eo/nn, provided the ratio of i-n to e-n collision rates is of the order 10−3.

Charge transport and device parameters from noise measurements

A select number of examples where measurements of number fluctuation noise, carrier multiplication noise and velocity fluctuation noise can be used to determine electronic transport or device parameters are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electronic transport properties and thickness dependence of the giant magnetoresistance in Co/Cu multilayers

The transport properties of electrons in Co/Cu multilayered thin films are of special interest for the giant magnetoresistance (GMR) of this system. The magnitude of this effect depends on the mean free paths and on the strength of the interface scattering which in turn are strongly related to film structure. In this article, we discuss the results of resistance and magnetoresistance measurements carried out during film growth. We characterize the electronic transport parameters of these films and the growth mechanism of the layers. The new technique of the in situ measurement of the magnetoresistance furthermore provides a tool to find correlations of the growth mechanism with the dependence of the GMR on the Co thickness.

Electron transport study of nanocrystallization in FeSiB based alloys

The influence of Cu, Nb and Ta additions on the crystallization of amorphous FeSiB based alloys was studied. For each alloy, coherent electrical resistivity and thermoelectric power behaviors were observed and related to the mean grain sizes. Anomalous electron transport parameters strongly influenced by the electron scattering at grain boundaries were explained using the Mayadas-Shatzkes model. From values of the local Avrami exponent, the nucleation rate and the growth kinetics were determined. The influence of relaxation processes on Avrami exponent was investigated.

High-temperature transport of electrons in diamond

The transport parameters of electrons in diamond has been investigated at high temperatures of 500 and 700 K for electric fields ranging from 0.4 to 100 kV/cm. It is found that the diffusion coefficient decreases with increasing temperature and electric fields. The drift velocity at 100 kV/cm decreased from 1.5 × 10 7 to 1.0 × 10 7 cm/s as the temperature increased from 300 to 700 K. At fields above 50 kV/cm, a nonparabolicity factor of 0.25 eV -1 softens excessive heating of the electrons and the increase in the diffusion coefficient.

Electron transport parameters in a weakly ionized gas with vibrationally excited molecules

The influence of vibrationally excited molecules on electron transport in a weakly ionized gas in an electric field is investigated. It is shown that, due to the dependence of the electron energy distribution function on the molecular temperature Tv, a new type of anisotropic electron transport, proportional to Del Tv and delta Tv/ delta t, appears. By approximately solving the electron energy balance equation the nature of this transport is described. The electron energy distribution function and transport coefficients (new transport coefficients, longitudinal diffusion coefficient, etc.) are calculated in vibrationally excited N2, CO, and CO2.

Theoretical Analysis of Electron Velocity Distribution Functions and Transport Coefficients in HCl in a DC Electric Field

The electron transport parameters in HCl in a DC electric field have been calculated over a wide range of reduced field strength E/N, 1≤E/N≤2000 Td using the direct numerical method of the Boltzmann equation. The velocity distributions related to each swarm parameter have also been obtained and the structure is discussed.

Numerical solution of coupled steady-state hot-phonon-hot-electron Boltzmann equations in InP.

A matrix method for the numerical determination of steady-state hot-phonon and nondegenerate hot-electron distribution functions is presented. The coupled Boltzmann equations for the longitudinal-optical phonons and for the electrons, in the \ensuremath{\Gamma} and L valleys of III-V semiconductor compounds, are iteratively solved with high accuracy under typical hot-electron conditions. The phonon distribution is shown to be significantly disturbed from thermal equilibrium. In turn, nonequilibrium phonons induce a substantial disturbance of the hot-electron distribution. Computations are performed for InP at 300 K, taking into account all relevant scattering mechanisms. It turns out that the electron transport parameters are strongly affected in the field range from 0 to 10 kV/cm: the drift velocity is increased by about 42% at 2 kV/cm and the Ohmic mobility by 37%. These effects are shown to be due to the drag and reduced cooling of the carriers by the nonequilibrium LO phonons.

A theoretical study on the fluorescence induction curve of spinach thylakoids in the absence of DCMU

Recent progress in studying Photosystem II (PS II) makes it possible to describe quantitatively each step of the electron transport in PS II centers. Based on the current reaction scheme, the fluorescence induction curve originating from PS II of isolated thylakoids is numerically simulated and compared with the experimental curve. The effects of changing the values of various electron transport parameters on the shape of the simulating curve are also studied. I find that it is impossible to fit the experimental curve without making additional assumption. I propose that the number of plastoquinone molecules functionally connected to each PS II center is likely to be heterogeneous. The hypothesis is also tried on the fluorescence induction curves measured at various light intensities and in the presence of various amounts of DCMU.

Effects of electron-electron collisions on the characteristics of DC and microwave discharges in argon at low pressures

The authors present a systematic investigation of the effects caused by electron-electron collisions on the electron kinetics in Ar under the action of DC and microwave fields. The analysis is based on solutions to the homogeneous electron Boltzmann equation using the effective field approximation, i.e. assuming that the electron energy distribution function is time independent. The calculated electron transport parameters and ionization coefficient are used together with a generalized charged particle balance equation to derive the characteristics for the maintenance field and the mean absorbed power per electron for steady-state DC and microwave discharges in Ar. Results are provided for the cases of (i) single-step ionization; (ii) multi-step ionization, via the metastable and the resonance levels of Ar.

The influence of surface roughness on electronic transport in thin films

In thin films, the structure of the surfaces considerably influences the transport of conduction electrons. For mesoscopic roughnesses in the range of a few nm, this is due to the varying film thickness, which gives rise to a spatially fluctuating conductance. Moreover, microscopic roughnesses can contribute to the scattering of the electrons and therefore additionally enhance the thin-film resistivity. For a quantitative understanding of the transport in these systems, a detailed investigation of the surface roughness combined with measurements of the electronic properties are necessary. Here, we discuss STM imaging of various metal films and the application of these results to the interpretation of electronic thin-film properties. Provided reasonable resolution of STM in the nm range, a good correspondence of STM results with the electrical behaviour of growing metal films can be established. Furthermore, a detailed two-dimensional analysis allows for a calculation of the potential on current-carrying thin films. On the other hand, this method supplies reliable values for the electronic transport parameters.

Hot-electron transport properties of rare-gas solids derived from low-energy electron transmission experiments

The results of the analysis of available experimental low-energy electron transmission (LEET) data on thin solid rare-gas films deposited on a metal substrate are presented. The analysis is carried out on the basis of a most recent semiclassical electron transport model. Transport parameters inferred from the experiments are: (i) the entrance probability of the incident electrons at the vacuum-film interface; (ii) the total (elastic and inelastic) electron scattering mean free path (MFP); and (iii) the probability of inelastic scattering in the bulk of the studied materials (namely solid argon, krypton, and xenon). Comparisons made with recent energy band-structure calculations reveal that both the entrance probability and the inverse of the total MFP (or the total scattering frequency per unit pathlength) are highly correlated with the conduction-band density of states of these materials. A comparison of two sets of experimental data obtained for solid argon under different conditions of preparation of the metallic surface illustrates the reliability of our analysis to infer electron transport parameters from LEET experiments.

Electron Transport in a-Si1-xGex:H Solar Cells

ABSTRACTWe report a study of the correlation between solar cell parameters and electron transport parameters in a series of a-Si1-xGex:H alloys with bandgaps spanning the range 1.42 – 1.72 eV. Fill factors and open circuit voltages were measured in p-i-n solar cells with 0.32 μm thick i-layers. The electron drift mobility and deep-trapping mobility-lifetime product were measured using transient photocurrent techniques on p-i-n cells with thicker i-layers. The open circuit voltage tracked the bandgap accurately. Both the electron drift mobility and the deep-trapping mobility lifetime product correlated reasonably well with the fill factor measured with illumination absorbed near the p-i interface. We present a discussion of the relationship of the fill factor to these transport parameters.

Charged Particle Transport in Gaseous Nitrogen at Intermediate E/N using the Voltage Transient Method

A pulsed swarm of charged particles crossing an inter-electrode gap under the influence of an applied electric field E will produce a pulsed current in the external circuit which, when integrated over time, will result in a transient voltage pulse, the shape and magnitude of which is characteristic of the number and type of charged particles. This voltage transient technique has been used to investigate a gas discharge in nitrogen gas at values of EIN (the ratio of applied electric field to gas number density), such that ionisation is non-negligible. The voltage transients have been subjected to a theoretical analysis, which has previously been reported, which includes not only cathode and anode image terms but also both electron and ion diffusion terms. Electron transport parameters are reported for EIN ::; 350 Td (1 Td = 10-17 V cm2). Data are also obtained for the drift velocities and diffusion coefficients of the ions operative within the nitrogen discharge. An estimate is obtained for the collisional decay rate of Nt.

Electronic transport in metallic films — a tool for scanning tunneling microscopy investigations

In this contribution, we show, that the evaluation of electronic transport parameters in confined systems can be considerably improved by additional Scanning Tunneling Microscopy (STM) imaging of the limiting surfaces. The thin films which are experimentally available usually show both a mesoscopic and a microscopic (i.e. atomic) surface roughness. These two roughnesses, however, are of well separated magnitudes and therefore can be treated either by classical averaging or by quantum mechanics. In order to ensure reliable STM results, the resolution especially of mesoscopic surface features will be discussed. Provided reasonable STM resolution, the experimental data for the resistivity can be interpreted for the first time with a realistic, two dimensional model for the current transport in thin films. Forthcoming applications concerning the distribution of the potential related to current transport in thin films will be discussed.

Transport parameters and electric field profile in amorphous silicon solar cells

We present a detailed study of the electronic transport parameters in amorphous silicon based single junction solar cells. Employing the time-of-flight technique we determine the drift mobility and mobility-lifetime product for electrons and holes, as well as the electric field profile in optimized devices having conversion efficiencies between 10% and 12%. We have also studied the trends in these parameters due to alterations in the device design and material. Here we discuss the importance of buffer layers, and the changes induced by light-soaking. Finally, we discuss our results with respect to future improvements in the conversion efficiency.

Anisotropic low energy electron collision cross sections for methane derived from transport coefficients

Based on a comprehensive set of transport parameters of electrons in methane derived from single electron spectra in the range of field 0.01-15 Td the authors determined a set of elastic and inelastic scattering cross sections up to 3 eV. The necessity to include anisotropic elastic scattering in a Boltzmann analysis of methane is demonstrated. In addition the drift velocity in a mixture of methane with helium was used to enhance the reliability of the derived cross sections. The low energy part of the cross sections for elastic scattering is subject to a MERT analysis and is compared with total cross section data from single scattering experiments.

Electron kinetics in weakly ionized helium under DC and HF applied electric fields

The electron kinetics in weakly ionized helium under the action of DC and HF fields of angular frequency omega is investigated by solving the homogeneous electron Boltzmann equation using the classical two-term expansion approximation. The analysis is based on a consistent set of electron cross sections which is derived by adjusting experimental cross section data in such a way that calculated and measured electron swarm parameters are in agreement. In the case of HF fields the analysis is based on the DC effective field approximation which is valid for omega > tau e-1, where tau e is the characteristic time for electron energy relaxation by collisions with the atoms. The influence of omega on the electron energy distribution function, transport parameters, rate coefficients and fractional power transfer is investigated and a detailed comparison of the DC and HF situations is made. It is shown that for reduced effective fields in the range 10-16-10-15 V cm2, as typically found in low-pressure discharges, the mean electron kinetic properties are nearly the same in the whole range of omega > tau e-1 as for the DC case.

Modeling of low-pressure microwave discharges in Ar, He, and O/sub 2/: similarity laws for the maintenance field and mean power transfer

A kinetic study of low-pressure microwave discharges in Ar, He, and O/sub 2/ is carried out using electron-transport parameters and rate coefficients derived from solutions to the Boltzmann equation, together with the continuity and transport equations for the charged particles, taking into account stepwise-ionization processes. The Boltzmann equation is solved over a wide range of the applied frequency, omega /2 pi , but assuming that the angular frequency omega > tau /sub e//sup -1/, with tau /sub e/, denoting the characteristic time for electron-energy relaxation by collisions. The formulation provides discharge characteristics for the maintenance field and for mean absorbed power per electron in the three gases, which are shown to agree satisfactorily with experimental data obtained from surface-wave discharges. It is shown that such an agreement would not always be obtained without consideration of the role played by stepwise-ionization processes in sustaining the discharge. >

Reverse dark I– V characteristics of the heterojunction based on oriented polycrystalline CdSe and amorphous As 2Se 3 films

The reverse dark current-voltage characteristics of amorphous-polycrystalline heterojunctions based on an oriented polycrystalline N-type CdSe and a high-resistance amorphous P-type As 2Se 3 have been examined. Electronic states at the interface which are introduced during processing cause the low dark reverse current. Experimental I– V characteristics are in agreement with a theoretical model of electronic transport parameters of a-c heterojunctions with a built-in charge at the interface.