Analysis of electrical conduction in an <Emphasis Type="BoldItalic">n</Emphasis> -type GaAs epilayer

The magnetic field dependence of conductivity tensor components, magnetoresistance, and the Hall coefficient have been analyzed in an n-type Si-doped GaAs epilayer at temperatures from 11 to 295 K. Carriers from the conduction band and the impurity band take part in the electrical conduction. The conduction band is located in the epilayer and the impurity band is located in a narrow layer, less than 0.1 m thick, between the GaAs buffer and GaAs semi-insulating substrate. At temperatures below 20 K the localization and magnetic freeze-out of the conduction band electrons have been taken into account as quantum corrections to the electrical conduction. The dependence of the mobility on energy has been considered in the analysis of the experimental data. A wide peak of partial conductions versus mobility appears in the mobility spectrum. From the analysis of the mobility spectrum of conduction band electrons it follows that at low temperatures the mobility of non-degenerated conduction band electrons is limited by scattering on screened charge centers. The mobility spectrum technique has been used as a tool for interpolation and extrapolation of the experimental data beyond the experimentally investigated magnetic field range. PACS 72.20.-i; 72.60.+g     

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.     

The theory of the carriers mobility in layer-inhomogeneous semiconductors

An effective mobility of the charge carriers is calculated in a diffusional approximation for a semiconductor with the uniaxial quasiperiodical distribution of the doping impurity, in a wide temperature interval including the regions of the weak and of the strong impurity ionisation. At the conditions when the compensation is significant in parts with minimal concentration of doping impurity, the influence of the non-uniformity of the distribution sharply increases. The effective mobility in the direction normal to the layers μ⊥ becomes essentially different in value and in the temperature dependence from the mobility along the layers. The extrema can appear in the dependence μ⊥(T).     

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).     

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 experimental investigation of the photovoltaic effect in some crystals without a center of symmetry

The photovoltaic effect in some crystals without a center of symmetry (ZnO, α-HgS, and α-quartz) is investigated. It is shown that the impurity centers of different nature give a different contribution to the photovoltaic effect. The influence of the optical recharging of the impurity centers on the spectral distribution and on the kinetics of the photovoltaic effect is noted. The optical activity strongly influences the angular dependence of the photovoltaic current, observed in the linearly polarized light.     

Excitations in doped quantum dot induced by randomly fluctuating magnetic field: influence of impurity

We explore the excitation profile of a repulsive impurity doped quantum dot under randomly fluctuating magnetic field. We have considered Gaussian impurity centers. The investigation reveals the roles subtly played by the dopant coordinate, dopant strength, and the region of influence of the dopant to modulate the excitation pattern. The rate of transition to the excited states has been invoked to analyze the roles played by the above impurity parameters in influencing the excitation process. Quantum phase space plots are often exploited to support the findings.     

Effect of crossed electric and magnetic fields on donor impurity binding energy

By using an appropriate coordinate transformation, we have calculated variationally the ground state binding energy of a hydrogenic donor impurity in a quantum well in the presence of crossed electric and magnetic fields which are applied tilted at an angle to the layers. The dependence of the donor impurity binding energy on the well width, on the strength of the electric and magnetic fields, on the impurity position and on the directions of the external fields is discussed. PACS 71.55.Eq; 71.55.-i     

Microscopic theory of dipole–dipole interaction in ensembles of impurity atoms in a Fabry–Perot cavity

We develop a consistent quantum theory of the collective effects that take place when electromagnetic radiation interacts with a dense ensemble of impurity centers embedded in a transparent dielectric and placed in a Fabry–Perot cavity. We have calculated the spontaneous decay dynamics of an excited impurity atom as a specific example of applying the developed general theory. We analyze the dependence of the decay rate on the density of impurity centers and the sample sizes as well as on the characteristic level shifts of impurity atoms caused by the internal fields of the dielectric. We show that a cavity can affect significantly the pattern of collective processes, in particular, the lifetimes of collective states.     

Grain Boundary Migration in Metals: Recent Developments

Current research on grain boundary migration in metals is reviewed. For individual grain boundaries the dependence of grain boundary migration on misorientation and impurity content are addressed. Impurity drag theory, extended to include the interaction of adsorbed impurities in the boundary, reasonably accounts quantitatively for the observed concentration dependence of grain boundary mobility. For the first time an experimental study of triple junction motion is presented. The kinetics are quantitatively discussed in terms of a triple junction mobility. Their impact on the kinetics of microstructure evolution during grain growth is outlined.     

Spectroscopic characteristics of single dibenzanthanthrene molecules isolated in a low-temperature naphthalene matrix

With the use of the methods of single-molecule spectroscopy the authors determined homogeneous widths of the band of a pure electron transition of individual impurity centers of dibenzanthanthrene in a naphthalene matrix and the character of their temperature dependence. The high level of the fluorescence signal and the photostability of the impurity enabled the authors to determine using an autocorrelation procedure the rate constants of intercombination conversion and decay of the triplet state and to observe the process of spectral diffusion for a number of impurity centers. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 323–330, May–June, 1999.     

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.     

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.

     

Electron mobility in a modulation doped AlGaN/GaN quantum well

We consider a two dimensional electron gas confined to a modulation doped AlGaN/GaN quantum well and study the dependence of low field mobility on various parameters such as composition, well width, remote impurity and interface roughness as a function of temperature. GaN is assumed to be in the zincblende structure. Acoustic and optical phonon, ionized remote impurity and interface roughness scatterings are taken into account in mobility calculations. The scattering rates are calculated using the self-consistently calculated wave functions obtained from the numerical solution of Poisson and Schr?dinger equations. Also found from the self-consistent solutions are the potential profile at the junction, the energy levels in the well and electron concentrations in each level. Ensemble Monte Carlo method is used to find the drift velocities of the two dimensional electrons along the interface under an applied field. The mobility of two dimensional electrons is obtained from the drift velocity of electrons. It is found that while remote impurity scattering is very effective for small values of spacer layer and doping concentrations, increasing Al concentration reduces the mobility of electrons. The effect of surface roughness, on the other hand, on mobility is almost independent of well width. The results of our simulations are compatible with the existing experimental data.     

Hall mobility of degenerate semiconductors

The dependence ofHall mobility of carriers on impurity content has been theoretically investigated for degenerate semiconductors, taking scattering by lattice vibrations as well as ionized impurities into account. The scattering due to optical modes of lattice vibrations has been neglected, for the sake of simplicity of calculations. This omission is not serious at temperatures, when the impurity scattering is important.     

Calculation of the anisotropic mobility in (110) AlAs quantum wells at zero temperature

We calculate the mobility of the two-dimensional electron gas as realized in (110) AlAs quantum wells at zero temperature. In this structure the mass is strongly anisotropic which gives rise to an anisotropic mobility. By using a theoretical approach developed by Tokura [Phys. Rev. B 58, 7151 (1998)] we numerically calculate the anisotropic mobility. We study impurity scattering in quantum wells having an ellipsoidal Fermi surface. We find that increasing the electron density and/or the well width results in reduction of the anisotropy of the mobility while the anisotropy in the scattering time is increased. A strong dependence of the mobility anisotropy on the impurity position is predicted. Excellent agreement with a recently published experimental result is found under the assumption that impurities are located at the edge of the quantum well.     

Some empirical relations for electron mobility in II–VI compound semiconductors

Electron mobility has been calculated in a number of binary II–VI compound semiconductors using a displaced Maxwellian distribution function and taking the various scattering mechanisms into consideration at different lattice temperatures and for various amounts of ionized impurity concentrations. It is observed that the low field mobility values can be expressed by a cubic power relationship with lattice temperature and with ionized impurity concentration using a least mean square fit technique with an accuracy better than 5 per cent. Similarly, the field dependence of mobility can also be expressed as a power series of the applied electric field. It is suggested that these equations can be profitably used for a quick estimation of mobility values as a check on experiments and also as sufficiently accurate formulae for simulation and modelling purposes.     

Anomalous mobility enhancement in Si doping superlattices

Enhanced Hall mobility has been measured in Si doping superlattices with pipi or nini doping profiles grown by molecular beam epitaxy. Although the carrier concentration is large, in the range of 10¹⁸cm⁻³, the mobility of holes in the pipi doping superlattice has the temperature dependence such as that expressed in the T^−2.2 law which is characteristic of high purity bulk crystals. The hole mobility is about twice that of high purity crystals, rising up to 40,000 cm²/V.s at about 30 K. In the case that p-type impurity concentration is comparable to n-type impurity concentration, no mobility enhancement is observed and the mobility is anomalously depressed. The mechanisms for drastical change in the mobility of doping superlattices have not yet been clarified.     

Determination of the LO phonon energy by using electronic and optical methods in AlGaN/GaN

The longitudinal optical (LO) phonon energy in AlGaN/GaN heterostructures is determined from temperature-dependent Hall effect measurements and also from Infrared (IR) spectroscopy and Raman spectroscopy. The Hall effect measurements on AlGaN/GaN heterostructures grown by MOCVD have been carried out as a function of temperature in the range 1.8-275 K at a fixed magnetic field. The IR and Raman spectroscopy measurements have been carried out at room temperature. The experimental data for the temperature dependence of the Hall mobility were compared with the calculated electron mobility. In the calculations of electron mobility, polar optical phonon scattering, ionized impurity scattering, background impurity scattering, interface roughness, piezoelectric scattering, acoustic phonon scattering and dislocation scattering were taken into account at all temperatures. The result is that at low temperatures interface roughness scattering is the dominant scattering mechanism and at high temperatures polar optical phonon scattering is dominant.     

Temperature and impurity concentration dependences of the efficiency of Frenkel defect accumulation in alkali halide crystals

A simple phenomenological model of the efficiency of accumulation of F centers in alkali halide crystals as affected by temperature and homologous cation impurity is considered. The conclusion is drawn that the temperature dependence is probably due to elastic interaction of primary Frenkel defects — F , H centres affecting their diffusion-controlled separation (similar to metals) whereas the impurity concentration dependence is due to the tunneling recombination of F , H_A pairs.