Nonequilibrium Conductivity Research Articles

USE OF POROUS LAYERS OF GALLIUM ARSENIDE AS ATOMIC HYDROGEN SENSORS

The modern development of nanotechnologies, gadgets, and robotics is closely related to the use of nanosized particles of metals, dielectrics, and semiconductors. Composite heterosystems based on semiconductors are used in the creation of advanced opto- and nanoelectronic devices. The operation of such devices is based on the phenomena of excitation of the electronic subsystem of the conductor. The paper investigates the occurrence of non-equilibrium chemiconductivity in gallium arsenide (GaAs) samples when the sample surface is in contact with atomic hydrogen. The effect occurs due to the non-thermal generation of electron-hole (e–h) pairs in the semiconductor wafer, thanks to the energy released during the chemical interaction of hydrogen atoms with the GaAs surface (adsorption and recombination of atoms with the formation of H2 molecules). The kinetics of chemiconductivity was studied, as well as the dependence of chemiconductivity on the density of the flow of atoms per sample and temperature. It was found that the process of generation of (e–h) pairs involves both acts of adsorption of atoms and acts of recombination of atoms into a molecule. The revealed effect of the appearance of non-equilibrium conductivity in samples of single-crystal gallium arsenide during recombination on the surface of atomic hydrogen clearly indicates the presence of effective channels for the accommodation of chemical energy by the conductor when a chemical reaction occurs on its surface. To increase the sensitivity of such a sensor by increasing the area of the "active" surface, it is proposed to use a porous layer of GaAs (por-GaAs). The phenomenon of non-equilibrium chemiconductivity in porous gallium arsenide, discovered and investigated for the first time in this paper, opens up wide prospects for the use of this semiconductor for the direct conversion of chemical energy into electrical energy. Semiconductors with a narrow bandgap of this type have prospects in the development of devices for the direct conversion of chemical energy into electrical energy, as well as in the creation of chemical sensors. The possibility of using this material as a detector of hydrogen atoms in the gas phase is substantiated.

Nonequilibrium optical response of a one-dimensional Mott insulator

We define, compute and analyze the nonequilibrium differential optical conductivity of the one-dimensional extended Hubbard model at half-filling after applying a pump pulse, using the time-dependent density matrix renormalization group method. The melting of the Mott insulator is accompanied by a suppression of the local magnetic moment and ensuing photogeneration of doublon-holon pairs. The differential optical conductivity reveals $(i)$ mid-gap states related to parity-forbidden optical states, and $(ii)$ strong renormalization and hybridization of the excitonic resonance and the absorption band, yielding a Fano resonance. We offer evidence and interpret such a resonance as a signature of nonequilibrium optical excitations resembling excitonic strings, (bi)excitons, and unbound doublon-holon pairs, depending on the magnitude of the intersite Coulomb repulsion. We discuss our results in the context of pump and probe spectroscopy experiments on organic Mott insulators.

Numerical simulation of gap length influence on energy deposition in spark discharge

The aim of the work is to study the influence of the length of the spark gap on energy input into the discharge channel during its gas-dynamic expansion. Methodology. The research is carried out by numerical modeling of the process of spark discharge development at variable values of the discharge gap length and at invariable other discharge conditions. The length of the gap was set in the range from 1 mm to 20 mm. The study was conducted using a numerical model of spark development, which takes into account the processes of nonstationary gas-dynamic expansion of the spark channel, the transient process in the electric circuit, nonequilibrium chemical processes, gas ionization, heat transfer and electrons thermal conductivity. The simulation was performed in atmospheric pressure nitrogen. The calculation was performed for various parameters of the RLC circuit, such as capacitance, inductance, resistance and voltage across the capacitor. Results. The study evaluates the influence of the spark length on the discharge current, the resistance of the spark channel, the energy deposited in the spark channel, and the distribution of thermodynamic parameters of the gas during the development of the spark discharge. It is confirmed that increasing the length of the gap increases the resistance of the spark. The deviation from the linear relationship between the deposited energy or the radiated energy and the length of the spark gap is estimated. Scientific novelty. A linear relationship between the gap length and the deposited energy is revealed when the total energy is above tens of Joules. Deviations from the linear dependence were detected in the discharge circuit when the total energy is below one of Joules. Practical value. The research results allow predicting the effect of the spark gap length on the energy input into the discharge channel under conditions of a slight change in the discharge current. In the conditions of essential change of amplitude of discharge current it is expedient to apply numerical researches for specification of changes in the energy deposited into a spark discharge.

Method of information entropy for convergence assessment of molecular dynamics simulations

The lack of a reliable method to evaluate the convergence of molecular dynamics simulations has contributed to discrepancies in different areas of molecular dynamics. In the present work, the method of information entropy is introduced to molecular dynamics for stationarity assessment. The Shannon information entropy formalism is used to monitor the convergence of the atom motion to a steady state in a continuous spatial domain and is also used to assess the stationarity of calculated multidimensional fields such as the temperature field in a discrete spatial domain. It is demonstrated in this work that monitoring the information entropy of the atom position matrix provides a clear indicator of reaching steady state in radiation damage simulations, non-equilibrium molecular dynamics thermal conductivity computations, and simulations of Poiseuille and Couette flow in nanochannels. A main advantage of the present technique is that it is non-local and relies on fundamental quantities available in all molecular dynamics simulations. Unlike monitoring average temperature, the technique is applicable to simulations that conserve total energy such as reverse non-equilibrium molecular dynamics thermal conductivity computations and to simulations where energy dissipates through a boundary as in radiation damage simulations. The method is applied to simulations of iron using the Tersoff/ZBL splined potential, silicon using the Stillinger–Weber potential, and to Lennard–Jones fluid. Its applicability to both solids and fluids shows that the technique has potential for generalization to other areas in molecular dynamics.

Nonequilibrium dynamics from BCS to the bosonic limit

We investigate the quench dynamics of strongly coupled superconductors within the time-dependent Gutzwiller approximation from the BCS to the BEC regime and evaluate the out-of-equilibrium transient spectral density and optical conductivity relevant for pump probe experiments. Fourier transformation of the order parameter dynamics reveals a frequency $\Omega_J$ which, as in the BCS case, is controlled by the spectral gap. However, we find a crossover from the BCS dynamics to a new strong coupling regime where a characteristic frequency $\Omega_U$, associated to double occupancy fluctuations controls the order parameter dynamics. The change of regime occurs close to a dynamical phase transition. Both, $\Omega_J$ and $\Omega_U$ give rise to a complex structure of self-driven slow Rabi oscillations which are visible in the non-equilibrium optical conductivity where also side bands appear due to the modulation of the double occupancy by superconducting amplitude oscillations. Analogous results apply to CDW and SDW systems.

Current-constrained one-electron reduced density-matrix theory for non-equilibrium steady-state molecular conductivity.

In the effort to create ever smaller electronic devices, the idea of single molecule circuit elements has sparked the imagination of scientists for nearly fifty years. While traditional theories for non-equilibrium steady-state molecular conductivity like the non-equilibrium Green's function density functional theory determine the current from an applied voltage, the recently proposed current-constrained density-matrix theory computes the voltage from a current constraint on the molecule. In the present paper we extend the current-constrained density-matrix theory from its two-electron reduced density-matrix (2-RDM) formulation to a one-electron reduced density matrix (1-RDM) formulation that is applicable to Hartree-Fock, density functional, and tight-binding theories. We demonstrate the current-constrained 1-RDM method through the computation of the theoretical, intrinsic resistance of acenes and phenacenes.

Optical absorption of semiconductor quantum dot solids

Several aspects concerning optical absorption in 3D assemblies of semiconductor quantum dots (QD solids) were studied. Considering the numerically simulated spectral dependences of the absorption coefficient in the case 3D QD assemblies with finite crystal size distribution, described by log-normal probability distribution functions (PDFs), several fundamental predictions were derived. First of all, it is predicted that the band gap energy of the QD solid should exhibit a certain ‘red-shift’ upon enlargement of the dispersion of the crystal size at a fixed average value thereof when the size-quantization regime in individual QDs has been entered. Furthermore, very large Urbach energy values are predicted, comparable to those characteristic for amorphous materials, for semiconductor QD solids with finite dispersion of the crystal size when the average QD size falls within the size-quantization interval. The Urbach energy in 3D assemblies composed by strongly quantized QDs with large dispersions of the PDFs characterizing the size distributions could be 100 times larger than the values in the non-quantized case (regardless of the size distribution in the latter case). Such large values are not due to the structural disorder inherent to individual QDs constituting the array, but a consequence of the disorder on the superlattice scale. Analogous arguments could be applied to predict large Urbach energies corresponding to the absorption tails in absorption spectra constructed from the spectral response of stationary nonequilibrium conductivity (photoconductivity). All these predictions are in excellent agreement with the available experimental data. Moreover, the presented approach could enable exact prediction of the optical absorption of a semiconductor QD solid if the PDF of the crystal size is known from the experiment. The smearing of excitonic absorption peaks in QD solids induced as a consequence of particle size distribution is considered and analyzed to some extent as well.

Nonequilibrium conductivity at quantum critical points

Quantum criticality provides an important route to revealing universal non-equilibrium behaviour. A canonical example of a quantum critical point is the Bose-Hubbard model, which we study under the application of an electric field. A Boltzmann transport formalism and $\epsilon$-expansion are used to obtain the non-equilibrium conductivity and current noise. This approach allows us to explicitly identify how a universal non-equilibrium steady state is maintained, by identifying the rate-limiting step in balancing Joule heating and dissipation to a heat bath. It also reveals that the non-equilibrium distribution function is very far from a thermal distribution.

Photoconductivity of pyrolytic CdS films alloyed with Cs

The investigation of the influence of Cs alloy on recombination processes has been carried out for the pyrolytic CdS films. The effect of photomemory has been observed for such structures, both pure and alloyed. Cs impurity introduction leads to a five- to tenfold photocurrent increase in comparison with pure samples. It also leads to increase of the nonequilibrium conductivity relaxation time from 300 to 104 s. The stored conductivity is due to the present of the internal potential barriers between the different conductivity areas, which are related with the heterogeneity of the investigated polycrystalline structures. Here, the non-equilibrium conductivity relaxation kinetic of alloyed CdS films corresponds to the case of square-law recombination. It is determined that the potential barrier increases from 0.33 to 0.44 eV along the nonequilibrium conductivity relaxation.

Investigation of magnetohydrodymamic interaction in a supersonic air flow at M = 8

The influence of magnetohydrodynamic interaction localized before a model on the position of a shock wave attached to a wedge is experimentally and numerically investigated. The investigation is carried out in an air flow with a Mach number of 8. It is shown that, for a hydromagnetic interaction parameter on the order of 0.1, the slope angle of the shock wave can be increased by 10°. Experiments are conducted for the case when the flow is ionized by an electron beam or by a pulsed electric discharge. Good agreement between experimental and numerical results is obtained for both ways of ionization if the Joule heating of the gas is insignificant. The conclusion is drawn that the way of providing a nonequilibrium conductivity of the flow has a minor effect on the position of the oblique shock wave near the wedge with the hydromagnetic interaction parameter being the same.

Symmetrized General Hopping Current Equation

We show that within the validity range of local thermal equilibrium--therein, however, irrespective of the magnitude of the driving force--a simple current equation can be formulated that expresses the current in terms of a product of a local nonequilibrium conductivity and a sinh function of half the electrochemical potential drop (normalized with respect to kBT) over the local hopping distance. This local current/driving force relation takes account of both electrical and compositional effects and can be generalized as to include interactions and structural variations.

Size-Dependent Properties of Sonochemically Synthesized Three-Dimensional Arrays of Close-Packed Semiconducting AgBiS2 Quantum Dots

3D arrays of close-packed AgBiS2 quantum dots (QDs) in thin film form were synthesized for the first time using novel, convenient sonochemical approach. Structural, optical, and photoelectrical properties of the synthesized material were investigated with an emphasis on their dependence on crystal size. The sonochemically synthesized AgBiS2 colloidal crystals have an average QD radius of 4.2 nm, twice as small compared to the QD solid obtained without ultrasonic irradiation. The optical band gap energy of sonochemically synthesized AgBiS2 QD thin films of 1.40 eV is strongly blue-shifted in comparison to that of the macrocrystal (0.90 eV) and that of nanostructured films synthesized by conventional chemical route (1.10 eV). Upon annealing, Eg exhibits a red shift to 1.00 eV. Spectral dependence of stationary nonequilibrium conductivity of the 3D QD assemblies suggests that the thin films’ photoconductivity is modulated by the intercrystalline barrier height decrease. Eg of the films calculated on the basi...

A Study of Photophysics, Photoelectrical Properties, and Photoconductivity Relaxation Dynamics in the Case of Nanocrystalline Tin(II) Selenide Thin Films

Charge-carrier transport properties in chemically deposited thin films composed of close-packed tin(II) selenide nanocrystals were studied both in the dark and under conditions when internal photoelectric effect is manifested in the nanostructured material. Stationary and time-resolved experiments were performed to characterize the photophysics of quantum-confined nanocrystals synthesized in thin film form. The charge-carrier transport through the films was found to occur predominantly by the thermionic emission mechanism in temperature range from 300 to 585 K. The corresponding crystal boundary barrier height in the case of as-deposited films is ≈160 meV, and the trap states which cause a partial charge-carrier depletion of the nanocrystals are situated below the Fermi level. Thermal band gap energy was found to be 0.90 eV, which is somewhat lower than the value calculated from optical spectroscopic data (1.10 eV). A physical basis for these differences is proposed. Both the internal photoelectric effect and the crystal boundary barrier height modulation upon illumination were found to contribute to the photoresponse of nanocrystalline films, the surface and bulk recombination velocities being comparable. Indirect and direct band gap energy values of 1.23 and 1.54 eV were calculated for the nanocrystalline films on the basis of photoresponse data. The nonequilibrium conductivity was found to relax exponentially with a time constant of 1.78 ms, which corresponds to the average lifetime of minority charge carriers (electrons). Lux-ampere characteristics of the films further support the linear photoconductivity relaxation mechanism, contrary to what is usually found in the case of amorphous materials.

Calculations of the parameters of the air plasma produced by an electron beam in the channel of an MHD generator with a nonequilibrium conductivity

Approximate analytic expressions for calculating the electron density in both steady and unsteady plasmas produced by pulsed electron beams are derived and proved to agree well with numerical calculations. It is shown that the algorithm for calculating the parameters of a nonequilibrium plasma in the channel of an MHD plasma generator depends on the type of generator. The effect of the magnetic field strength on the electron density and electric conductivity of the air plasma produced by an electron beam in the channel of a Faraday MHD generator is investigated. The influence of the parameters of the flow and ionizer on the efficiency of an MHD generator with a nonequilibrium conductivity is analyzed.

The shape of the signals of transient photoconductivity in silicon doped with gold or sulfur

Experimental results of studying the photoconductivity kinetics in silicon doped with either gold or sulfur are reported. The nonequilibrium charge-carrier concentration was generated by the effect of pulsed laser radiation. The time dependence of nonequilibrium conductivity was determined by contactless measurements from a variation in the reflected power of the microwave field. The long-term photoconductivity with the time constant of τ ≈ 1.6 ms at room temperature is observed in the silicon samples overcompensated with sulfur. Conventional short-term processes (τ ≈ 2–3 µs) characteristic of the centers with deep levels are observed in the silicon samples doped with gold. These differences are accounted for using the known data on the energy characteristics of the gold and sulfur levels in the silicon band gap and their charge states.

Universal critical exponents in percolation systems with tunneling

Some specific dependence of micro-crystalline solar cells efficiency on the crystallite size was reported recently [R. Beserman, A. Chak, R. Weil, T. Roschek, in: Proceedings of the 19th European Photovoltaic Solar Energy Conference, Paris, 2004, p. 1520]. The simple explanation of this dependence is suggested in the framework of percolation phenomena for non-equilibrium carriers. Percolation problem for non-equilibrium carriers was solved in continuous space with tunnel coupling between conducting grains. It was shown that local micro-geometry is not essential if the inter-grains transition time is less than the lifetime of the non-equilibrium carriers inside the conducting grains. In this case the main parameter of the problem is the lifetime of the non-equilibrium carriers in the conducting grains. A new bonding criterion was formulated. An amorphous hydrogenated silicon (a-Si:H) matrix with a big number of crystalline silicon (c-Si) inclusions was investigated as an example of the system. It was shown that universal dependence exists for non-equilibrium conductivity on the mean crystalline size. This prediction was verified by experiments with real solar cells produced from a-Si:H with c-Si inclusions.

Importance of nonequilibrium thermal conductivity during short-pulse laser-induced desorption from metals

Importance of nonequilibrium thermal conductivity during short-pulse laser-induced desorption from metals

The interaction of thermal nonequilibrium and heterogeneous conductivity effects in forced convection in layered porous channels

Forced convection in a parallel plate channel filled with a porous medium, consisting of two layers with the same porosity and permeability but with different solid conductivity, and saturated by a single fluid, is analyzed using a two-temperature model. It is found that the effect of local thermal nonequilibrium is particularly significant when the solid conductivity in each layer is greater than the fluid conductivity, and in these circumstances the effect is to reduce the Nusselt number defined in terms of the overall effective thermal conductivity.

GaAs peculiarities related with inhomogeneities and the methods for reveal of their properties

The inhomogeneities in as-grown and irradiated GaAs crystals and structures, their properties and role have been analysed. The influence of inhomogeneities on the properties of crystal and Schottky diode structure are presented. The possibilities of different methods for material and structure characterisation (Hall effect and magnetoresistance, noise spectra, thermally stimulated current and polarisation, photoconductivity kinetics and spectrum, light-induced diffraction) are discussed. Existence of the inhomogeneity of the crystal structure has been revealed by diffusion of copper through the GaAs wafer, and a percolation of it through doped crystal has been determined by means of the Hall effect. Modification of the recombination-induced defect system has been analysed to get deeper insight into the crystal and device degradation under illumination by ionising radiation. Role of free carriers in local electric field generation has been analysed via modification of the EL2 centre in GaAs. Complicated behaviour of non-equilibrium conductivity during quenching and activation of EL2 centres has been determined by precise measurement of the photoconductivity spectra in different crystals. Influence of the EL2 centre on the non-linear polarisation of sample and on the additional non-active light absorption has been analysed.

Probing the electronic density of states in semiconductor quantum wires using nonequilibrium acoustic phonons

We have used nonequilibrium phonon-induced conductivity (phonoconductivity) measurements to probe the electronic states in semiconductor quantum wire devices. The devices were based on high mobility two-dimensional electron systems (2DESs) in GaAs/Al 0.3Ga 0.7As heterostructures and quantum wires formed using the well-known split-gate technique. Short (20 ns-long) pulses of nonequilibrium acoustic phonons were generated by heating a metal film on the back surface of the substrate. These phonons propagated ballistically across the substrate and were incident on the quantum wire. The electron–phonon interaction was detected via the phonon-induced change in electrical conductance of the device. We observed giant oscillations of the phonoconductivity with increasing (negative) gate bias. Maxima occurred when the Fermi energy was coincident with the bottom of any one-dimensional electronic subband. In this paper we argue that the phonoconductance is due to phonon-induced backscattering of the electrons in the quantum wire and present evidence that the strength of the phonon signal is proportional to the density of electronic states in the quantum wire.