Mobility of electrons upon scattering by a multicomponent correlated system of impurity centers

A system that contains two sorts of impurity centers spatially distributed in a random way is considered. Not all impurities of the first sort are ionized, and all the impurities of the second sort are ionized. Spatial correlations in the system of impurity ions of the first sort are investigated under conditions when the correlation radius of an impurity ion is limited from above due to a deficit of neutral impurities. The influence of randomly spatially arranged small-sized donors (impurities of the second sort) on correlations in the system of impurity ions is analyzed. The equations for describing the effect of small-sized donors on correlations in the system of impurity ions are obtained. The electron mobility at zero temperature is calculated by the example of HgSe: Fe (the correlated system of impurity centers consists of iron atoms and small-sized donors whose concentration is higher than the Mott concentration).     

Density of states in the impurity d band and inelastic electron scattering by a system of mixed-valence iron ions in HgSe: Fe crystals

The resistivity and the Hall coefficient of HgSe: Fe crystals with various iron content are experimentally studied in the temperature range 1.3≤T≤300 K and in magnetic fields up to 60 kOe. The temperature dependences of the density and mobility of conduction electrons in these crystals are determined. The influence of spatial charge ordering in the system of mixed-valence iron ions on impurity states in HgSe: Fe crystals is considered. The density of states in the impurity d band is theoretically analyzed, and inelastic electron scattering in which bi-and trivalent iron ions are recharged is discussed. It is shown that the experimentally detected features in the dependence of the density and mobility of electrons on temperature and iron impurity content can be explained by the influence of Coulomb correlations in the mixed-valence iron ion system on the structure of the impurity d band.

     

Effect of charged impurity correlations on transport in monolayer and bilayer graphene

We study both monolayer and bilayer graphene transport properties taking into account the presence of correlations in the spatial distribution of charged impurities. In particular we find that the experimentally observed sublinear scaling of the graphene conductivity can be naturally explained as arising from impurity correlation effects in the Coulomb disorder, with no need to assume the presence of short-range scattering centers in addition to charged impurities. We find that also in bilayer graphene, correlations among impurities induce a crossover of the scaling of the conductivity at higher carrier densities. We show that in the presence of correlation among charged impurities the conductivity depends nonlinearly on the impurity density n_i and can increase with n_i.     

Transverse Nernst-Ettingshausen effect in HgSe:Fe,Ga crystals containing mixed-valence iron impurities

The transverse Nernst-Ettingshausen effect is investigated experimentally and theoretically for HgSe:Fe,Ga samples with various iron and gallium impurity contents. It is shown that the unusual dependence of the magnitude and sign of the effect on the gallium impurity content are attributable to their influence on the degree of spatial ordering of the trivalent iron ions and, accordingly, on the nature of the scattering of conduction electrons. The results of the calculations agree qualitatively with the experimental data. Fiz. Tverd. Tela (St. Petersburg) 39, 1767–1774 (October 1997)     

Free-carrier absorption in a parabolic quantum well with consideration of scattering on ionized impurities

Intra-subband transitions caused by light absorption in a parabolic quantum well is considered taking into account the scattering by ionized impurity centers. To calculate the scattering matrix element, the Born approximation is used and the interaction with the impurity is described by the Coulomb potential. An analytical expression for the absorption coefficient of processes with the initial absorption of photon and further scattering by an ionized impurity center is obtained. For absorption coefficient the frequency characteristics and dependence on the width of quantum well are examined.     

HgSe:Fe—a mixed-valency system and the problem of the ground state

The dependence of the electron mobility on the iron impurity content N _Fe and temperature is studied for three variants of the ordering of Fe³⁺ ions in crystalline HgSe:Fe, a weakly correlated gas, states with near ordering like that in a strongly correlated Coulomb liquid, and long-range ordering. The electron mobilities owing to scattering on the correlated system of Fe³⁺ ions are determined. The temperature dependence of the mobility is analyzed for electron scattering on fluctuations in the charge density in a system of Fe²⁺-Fe³⁺ iron ions with mixed valency, and the correlation length is determined. It is shown that the ordering region for the Fe³⁺ ions encompasses only the first coordination sphere, i.e., near ordering in the position of the Fe³⁺ ions is established, as in a liquid. The coupling between the ordering of the Fe³⁺ ions and the formation of a correlation gap in the density of impurity d-states and its effect on the low-temperature behavior of the electron mobility in HgSe:Fe crystals are examined. Fiz. Tverd. Tela (St. Petersburg) 40, 425–432 (March 1998)     

Scattering theory and ground-state energy of Dirac fermions in graphene with two Coulomb impurities

We study the physics of Dirac fermions in a gapped graphene monolayer containing two Coulomb impurities. For the case of equal impurity charges, we discuss the ground-state energy using the linear combination of atomic orbitals (LCAO) approach. For opposite charges of the Coulomb centers, an electric dipole potential results at large distances. We provide a nonperturbative analysis of the corresponding low-energy scattering problem.     

Simulation of spatial correlations of impurity ions in solids using configurational entropy and the hard-sphere model

Based on the hard-sphere model, the spatial correlations are considered in a system of impurities with variable valency. In a zeroth approximation, the configurational entropy of the spatially correlated system of impurity ions is identified with the configurational entropy of a system of hard spheres. The electron mobility limited by scattering on the correlated system of impurity ions at finite temperatures is found. The theory developed explains experimentally observed anomalies of the carrier mobility in an iron-doped HgSe gapless semiconductor.     

Anomalous Hall effect in a two dimensional electron gas with magnetic impurities

Magnetic impurities play an important role in many spintronics-related materials. Motivated by this fact, we study the anomalous Hall effect in the presence of magnetic impurities, focusing on two-dimensional electron systems with Rashba spin-orbit coupling. We find a highly nonlinear dependence on the impurity polarization, including possible sign changes. At small impurity magnetizations, this is a consequence of the remarkable result that the linear term is independent of the spin-orbit coupling strength. Near saturation of the impurity spins, the anomalous Hall conductivity can be resonantly enhanced, due to interference between potential and magnetic scattering.     

Mobility of two-dimensional electrons upon scattering by a correlated distribution of impurity ions

Correlation effects in the mobility of two-dimensional electrons upon scattering by a correlated distribution of impurity ions are described in the framework of the hard-sphere model. The theory is developed for the case of partially ionized impurity centers when correlations in the distribution of impurity ions are weakened as the result of a deficit of free positions for impurity holes. The calculations are performed for heterostructures with a wide spacer when small-angle scattering of electrons dominates.     

The effect of electrostatic interactions on hot electron transport in n-GaAs

Hot electron transport in bulk n-GaAs is investigated in a three dimensional numerical simulation. An emphasis is placed on understanding the effect of the electrostatic interaction and, in particular, that of the ionized impurity scattering on drift velocity vs electric field characteristics. In order to account for the spatial correlations between electrons and scattering centers, the numerical model combines molecular dynamics and Monte Carlo techniques. A major result is that the impurities enhance the drift velocity of hot electrons. This emphasizes the inappropriateness of the concept of a phenomenological mean free flight time when the interaction potential is long range. An explanation of the effect is proposed which applies to a general multi-valley semiconductor; and the model is compared to the traditional Brooks-Herring treatment of electron-ionized impurity scattering.     

“The energy spectrum of quadrupole impurity centers of different types in antiferromagnets

We study the energy spectrum and some properties of various quadrupole centers (magnetic impurities or magnetic impurity complexes that are symmetric with respect to the magnetic sublattices of the antiferromagnet). We allow for the effect of spin-phonon coupling on the quadrupole splitting parameter and show that such coupling can lead to a considerable decrease in the value of this parameter and even change its sign. We investigate the behavior of quadrupole centers with an orbitally degenerate ground state and of quadrupole impurity complexes formed by mixed-valence ions. We demonstrate that such centers may greatly affect the resonant, magnetic, and thermodynamic properties of antiferromagnets. Finally, we analyze the existing experimental data and show that several new effects can be observed in systems with such centers (in particular, a magnetic analog of the Jahn-Teller effect and a strong magnetoelectric effect). Zh. éksp. Teor. Fiz. 111, 964–978 (March 1997)     

Diffusion of electrons scattered by short-range impurities in a quantizing magnetic field

Formulas for transverse diffusion and conductivity in a semiconductor are obtained for electrons scattered by neutral impurities in a quantizing magnetic field. The formulas are valid for an impurity potential of arbitrary depth. Based on Kubo’s theory [1], calculations are performed using electron wavefunctions of the problem of single-impurity scattering in a magnetic field [2]. The poles of the scattering amplitude correctly determine electron eigenstates and magnetic impurity states. As a result, an exact expression is found for the dependence of transverse diffusion coefficient D _⊥ on longitudinal electron energy ? due to scattering by short-range (neutral) impurities. The behavior of D _⊥(?) is examined over an interval of magnetic field strength for several values of impurity potential depth. The experimental observability of diffusion and conductivity using IR lasers is discussed.     

Artificial carrier heating due to the introduction of ab initio Coulomb scattering in Monte Carlo simulations

Accurately treating ionised impurity scattering in a way suitable to describe the influence of random dopant fluctuations on device characteristics is important in next generation MOSFETs. Statistical variations are unobservable using a continuous treatment of the doping, requiring a discrete representation of impurities. In particle-based simulations the P³M method, which resolves the Coulomb interaction into long- and short-range components, is in principle capable of describing Coulomb scattering through propagation in this accurately resolved potential. However, numerically the integration of the equations of motion is inaccurate and controlling the errors in practical simulations is vital. In this paper we investigate the effect of the choice of short-range correction strategy and integration time step on accuracy in a 3D self-consistent ensemble Monte Carlo simulations featuring random discrete dopants. We illustrate the importance of the ‘ab initio’ Coulomb scattering comparing the effect of a single trapped charge in drift-diffusion and Monte Carlo simulations.     

Scattering theory of electrons on impurities in compensated semiconductors

In this paper we study the temperature behavior of the electron mobility in n-type compensated semiconductors, caused by scattering on neutral, ionized, and dipole impurity centers. With increased temperature, neutral impurity atoms and paired oppositely charged donor-acceptor dipole scattering impurity centers transform into separate ionic scattering centers. These transformations of scattering centers take place at various temperatures. We find that the mobility caused by electron scattering on separate impurity ions does not change with temperature monotonically — there appear two minima under certain conditions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 106–109, October, 1987.     

Temperature dependent weak field Hall resistance in two-dimensional carrier systems

Using the Drude-Boltzmann semiclassical transport theory, we calculate the weak-field Hall resistance of a two-dimensional system at low densities and temperatures, assuming carrier scattering by screened random charged impurity centers. The temperature-dependent 2D Hall coefficient shows striking nonmonotonicity in strongly screened systems, and, in particular, we qualitatively explain the recent puzzling experimental observation of a decreasing Hall resistance with increasing temperature in a dilute 2D hole system. We predict that the impurity scattering limited Hall coefficient will eventually increase with temperature at higher temperatures.     

Application of the modified electrostatic model to the impurity diffusion in nickel

The modified electrostatic model (Neumann and Tölle 1995) is applied to the impurity diffusion in nickel.Z ₀ = 0.4 is used for the effective charge of the nickel ion.The comparison of calculated and experimental diffusion parameters reveals that the sign of J Q, the difference between impurity diffusion and self-diffusion energy, and the sign of the difference between impurity diffusion and self-diffusion coefficient is correctly predicted in all cases. On the other hand the comparison exhibits some systematic deviations for 5p impurities, which cannot be explained in terms of the current impurity diffusion models.     

Metastable states and physical characteristics of a subsystem of interstitial impurities

The behavior of mobile interstitial impurities in a crystal matrix has been simulated numerically. The contribution of the impurity subsystem to the heat capacity, and the effect of clusterization on the temperature dependence of the impurity diffusion coefficient have been studied. Fiz. Tverd. Tela (St. Petersburg) 40, 475–480 (March 1998)     

Impurity effects in the optical absorption of quantum rings

We study the interplay between impurity scattering and Coulomb interaction effects in the absorption spectrum of neutral bound magnetoexcitons confined in quantum-ring structures. Impurity scattering breaks the rotational symmetry of the ring system, introducing characteristic features in the optical emission. Signatures of the optical Aharonov–Bohm effect are still present for weak scattering and strong Coulomb screening. Furthermore, an impurity-induced modulation of the absorption strength is present even for a strong impurity potential and low screening. This behavior is likely responsible of recent experimental observations in quantum-ring structures.