Charge carriers and mobility of Cu-Ti ferrite

The concentration and drift mobility of charge carriers in Cu_1–x Ti _x Fe₂O₄ ferrite are calculated, over a wide range of temperatures (300–773 K), employing d.c. conductivity and thermoelectric power data. With increasing temperature the concentration of charge carriers decreases whilst the drift mobility exhibits an exponential increase. Over the above-mentioned temperature range, the obtained density of charge carriers varies between 10²¹ and 10²² cm^–3 whereas the drift mobility has values between 10^–8 and 10^–4 cm²/V s. The results are discussed on the basis of a small-polaron hopping conduction. The activation of the d.c. conductivity has been attributed to the thermal activation of the mobility.     

Switching processes and the electronic subsystem in ferroelectric ceramic of the barium-lead niobate type

This paper is devoted to the study of the relation between switching processes and the states of the electronic subsystem in a ferroelectric ceramic. Experimental observations of the dielectric hysteresis loops at 5·10^–3 Hz, thermally stimulated currents, the relaxation of the conductivity, and the temperature dependence of the stationary conductivity in the temperature range 280–423 K were performed. Formulas are proposed for calculating the hysteresis loop and the coefficient of dielectric viscosity from the parameters of the hysteresis loop. It is established that for a ferroelectric ceramic of the BLN type charge carriers localized at the levels 0.18, 0.25, 0.27, and 0.37 eV play the determining role in the formation of the equilibrium ferroelectric state far from the phase-transition point. It is shown that the coefficient of dielectric viscosity for polycrystalline samples is determined by the relaxation time of the electronic subsystem and the orientation of the internal field in the sample.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 9–14, February, 1989.In conclusion we thank V. P. Kamentsev and A. V. Nekrasov for a useful discussion of this work.     

环氧树脂高温分子链松弛与玻璃化转变特性

环氧树脂是电力设备中广泛应用的一种绝缘材料, 其介电性能受到分子链运动特性的影响. 本文制备了直径为50 mm、厚度为1 mm的环氧树脂试样, 采用差示扫描量热仪和宽频介电谱仪测试了环氧树脂的玻璃化转变温度和介电特性. 实验结果表明, 环氧树脂的玻璃化转变温度为105 ℃, 在玻璃化转变温度以上, 高频段出现了由分子链段运动造成的松弛过程, 低频段出现了由载流子在材料中迁移造成的直流电导过程. 发现环氧树脂不同尺寸分子链段的松弛时间不同, 其松弛时间分布较宽, 计算得到了分子链段在不同温度下的松弛时间分布特性. 分子链松弛峰频率和直流电导随温度的变化关系服从Vogel-Tammann-Fulcher公式. 拟合实验结果得到分子链松弛峰频率和直流电导的Vogel温度和强度系数. 由Vogel温度计算得到了与差示扫描量热测试结果一致的玻璃化转变温度, 约为102 ℃. 结果表明玻璃化转变温度以上环氧树脂的自由体积增大, 分子链段有足够的空间来响应外电场从而产生分子链松弛极化, 载流子有足够的能量在材料中迁移形成电导.     

Terahertz spectroscopy of AuFe spin glasses

The electrodynamic response of spin glasses (in the form of thin AuFe films) in the terahertz frequency range has been studied using backward-wave oscillator (BWO) spectroscopy (10–40 cm^?1) and optical ellipsometry (5000–33000 cm^?1) techniques at temperatures from 5 to 295 K. The room-temperature dynamic conductivity spectra of AuFe films are typical of metals and can be described within the framework of the Drude theory of conduction by free charge carriers. Changes in the microscopic parameters of charge carriers in AuFe films with increasing iron content, which are related to additional scattering of carriers on the impurity magnetic moments, have been studied on the quantitative level, including the carrier relaxation frequency and characteristic time, plasma frequency, and conductivity. It is established that the spin-glass phase at a temperature of ～5 K exhibits dispersion of the conductivity in the frequency range 10–40 cm^?1, which can be related to the appearance of a mobility gap in the subsystem of free electrons involved in the RKKY interaction between magnetic centers (Fe atoms).     

Time variation of the concentration of excess carriers in dielectrics under irradiation

In this paper, a theoretical analysis is performed of charge carriers in dielectrics which are irradiated by electrons. A phenomenological model of a uniform bulk charging is used in the approximation of unipolar conductivity. It is shown that the process consists of two or three stages, corresponding to a certain accumulation regime of excess charge. Using the results of the analysis, the time dependence of the equilibrium parameter is calculated for certain numerical values of the model parameters. It turns out that this dependence consists of a fast and a slow component; moreover, thermodynamic equilibrium between free and captured carriers exists only for the slow component. Results of experiments on the bulk charging of polymers, described in the paper, confirm the obtained theoretical conclusions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 47–52, March, 1990.     

Electronic Heat Conductivity of a Partially Ionized Hydrogen Plasma

The electronic contribution of the translational heat conductivity of a partially ionized hydrogen plasma is determined, starting from a quantum kinetic equation, which takes into account the two particle scattering between the free charge carriers as well as electron-atom collisions. Plasma effects are included by statically screened potentials. In connection with a mass action law, the influence of the different collision terms on the electron heat conductivity is shown. An estimation is given for the contribution of the atoms. The pressure ionization (Mott effect) is described by a minimum behaviour of the heat conductivity.     

Thermostimulated current of dielectric relaxation in metal-vanadium phosphate glass-metal systems

The nature of the maxima of the thermostimulated current of dielectric relaxation in the thinfilm system metal-vanadium phosphate glass-metal is discussed. The experimental data is analyzed, taking into account the presence of a blocking barrier layer at the metalglass interface and allowing for the fact that the charge carriers in the glass have an activated mobility. The mobility of the charge carriers and their activation energy as well as the energetic position of the impurity levels and their concentration are determined from the curves of the temperature dependence of relaxation current.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 92–100, February, 1977.     

Relaxation processes on A ZnO surface irradiated by electrons

The dependence of effective charge carrier lifetime and the surface conductivity relaxation constant of polycrystalline zinc oxide as functions of electron irradiation dosage at constant residual gas pressure, partial oxygen pressure at constant irradiation dosage, and partial water vapor pressure at constant irradiation dosage are studied. Surface state types and the charge carrier relaxation mechanism are determined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 81–85, June, 1985.     

Long-lived excited impurity states in diamond-like semiconductors

The lifetime of charge carriers in the lowest excited states of some impurities of groups III and V in diamond, silicon, and germanium can be several (four to six) orders of magnitude longer that the lifetime of free carriers. Accumulation of carriers in these long-lived states may give rise to several new effects, such as hopping photoconductivity via long-lived excited states of impurities in dc and microwave electric fields, slow relaxation of induced absorption, and infrared absorption at energies lower than the impurity ionization energy. Zh. éksp. Teor. Fiz. 112, 221–236 (July 1997)     

Relaxation time for a charge carrier due to its scattering from other charge carriers in superlattices

A calculation of relaxation time for (i) electron–electron scattering in a modulation-doped superlattice of type-I and (ii) electron–electron, hole–hole and electron–hole scattering processes in a compositional superlattice of type-II has been performed, using Fermi's golden rule. As compared to a two-dimensional electron gas system, both intralayer and interlayer interactions, between charge carriers in a superlattice, contribute to relaxation time. It is found that scattering processes at all possible value of momentum transfer contribute to relaxation time, for a given value of temperature and carrier density. We further find interlayer interactions in a superlattice make a significant contribution to relaxation time. Relaxation time is found to decrease on increasing temperature, carrier density and single particle energy, in a superlattice. The computed relaxation time for an electron (hole) in a superlattice enhances on increasing the width of layer consisting of electrons (holes). The electron–hole (hole–electron) scattering process in a type-II superlattice yields maximum contribution to the relaxation time when a hole layer lies exactly in between two consecutive electron layers.     

Poole-Frenkel effect in chalcogenide semiconductors with various crystalline structures

The results of investigation of electrical conductivity of a large group of chalcogenide semiconductors having the layered, cubic, and orthorhombic structures in strong electric fields up to 10⁵ V/cm are presented. The revealed increase in electrical conductivity σ in strong electric fields has been explained by the Frenkel thermionic ionization. This has made it possible, along with other parameters (for example, activation energy and trap concentration, charge-carrier mean free path, permittivity), to evaluate the concentration and mobility of charge carriers in semiconductors under study. It has been shown that in strong electric fields in semiconductors, when the thermionic ionization of the traps occurs, their permittivity ?, which is caused by the electron polarization, is determined by the simple formula ? = n ², where n is the refractive index of light.     

Effect of the Nickel Impurity on the Rate of Surface Generation of Charge Carriers at the Si–SiO2 Interface

The effect of the nickel impurity on the generation characteristics of the Si–SiO₂ interface is investigated by the method of isothermal capacitance relaxation. It is demonstrated that doping by the nickel impurity increases the rate of surface generation of charge carriers and the spread of this parameter from structure to structure. The effects observed are associated with the existence of defect-impurity complexes formed in doped structures as a result of interaction of impurity clusters (precipitates) with native defects in the transition layer at the Si–SiO₂ interface.     

AC conductivity and relaxation behavior in ion conducting polymer nanocomposite

We report the ac conductivity and relaxation behavior analysis for a heterogeneous polymer–clay nanocomposite (PNC) having composition (polyacrylonitrile)₈LiCF₃SO₃ + x wt.% dodecylamine modified montmorillonite. Charge transport behavior in an ionically conducting PNC has been analyzed systematically and correlated with the macroscopic parameters like polymer glass transition temperature and available free mobile charge carriers. Intercalation of cation coordinated polymer into the nanometric clay channels has been confirmed by high-resolution transmission electron microscopy. The electrical properties of the intercalated PNC films have been studied using complex impedance/admittance spectroscopy. Excellent correlation of relaxation behavior with polymer glass transition temperature (T _g) confirmed the objectives of the work. An analysis of dielectric relaxation indicates that PNC films are lossy when compared with polymer–salt film. This result is a direct outcome of faster ion dynamics leading to strong electrode polarization effect due to the accumulation of charge carriers at the interface.     

Electrophysical properties of heat-treated gallium arsenide

The electrical conductivity, Hall effect, ionization energy, and defect concentration of GaAs samples subjected to various forms of heat treatment were studied. The original material comprised single crystals grown by the Bridgman and Czochralski methods with electron concentrations of 2·10¹⁵–7·10¹⁷ cm^–3. The ionization energy and defect concentration were calculated with an electronic computer. The thermal conversion of GaAs was attributed to traces of copper, lattice defects, and residual impurities. The mobility varied in a complicated manner with the temperature of heat treatment in GaAs samples retaining their original n-type conductivity.Translated from Izvestiya VU Z, Fizika, No. 3, pp. 69–76, March, 1973.     

一种计算稠密等离子体电导率的半经验模型

为了准确计算稠密等离子体电离平衡,在理想Saha 方程的基础上,加入了压致电离理论和德拜理论相结合的修正。此电离模型可以描述稠密等离子体内电离机制随着密度的增加由热电离转变为压致电离的现象。压致电离理论结合德拜理论修正的Saha 方程的计算结果与其它电离模型结果作了对比和分析。电离气体区域采用完全电离等离子体模型计算电子弛豫时间,而在凝聚态区域采用电子平均自由程计算电子弛豫时间。提出了将两个区域平滑连接起来的公式,从而建立了一种可以计算从理想等离子体态到凝聚态物质的半经验电导率模型,半经验模型计算结果与实验数据作了对比。     

Generalization of the Hutson and White theory to the case of piezosemiconductors with mixed conductivity

The Hutson-White theory is extended to the case when there are mobile charge carriers of both signs in a piezeosemiconductor. Expressions are obtained for the damping and velocity of the longitudinal sound wave being propagated along the z axis in the piezosemiconductor as a function of the magnitude of the longitudinal electrical field, as well as for the constant current density originating here. It is shown that the acoustic wave changes the conductivity of a piezosemiconductor with mixed conductivity. A method is proposed for determining the mobility simultaneously of both the positive and negative carriers in an intrinsic piezosemiconductor.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 55–58, November, 1984.     

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为了准确计算稠密等离子体电离平衡,在理想Saha方程的基础上,加入了压致电离理论和德拜理论相结合的修正。此电离模型可以描述稠密等离子体内电离机制随着密度的增加由热电离转变为压致电离的现象。压致电离理论结合德拜理论修正的 Saha 方程的计算结果与其它电离模型结果作了对比和分析。电离气体区域采用完全电离等离子体模型计算电子弛豫时间,而在凝聚态区域采用电子平均自由程计算电子弛豫时间。提出了将两个区域平滑连接起来的公式,从而建立了一种可以计算从理想等离子体态到凝聚态物质的半经验电导率模型,半经验模型计算结果与实验数据作了对比。     

Charge relaxation in conducting dielectric films with shallow and deep traps

A nonsteady-state boundary-value problem of the relaxation of a charge injected into a dielectric film is solved analytically with allowance for the film conductivity and capture of charge carriers by traps having a finite emptying rate. The one-and two-zone modes of charge relaxation are considered. The obtained general expressions reduce to earlier published formulas derived in particular cases. Numerical calculations and an analysis of the experimental data available in the literature on the electret state of oxide films deposited on metal substrates confirm the applicability of the proposed model of dielectric relaxation.     

Drift mobility and kinetics of transport of excess charge carriers in glassy CdGexAs2

Conclusions We tried to measure transient conductivity response to pulse strongly absorbed excitation (light, accelerated electrons) in sandwich type samples of glassy CdGe _x As₂ compounds. We observed the signal due to transport of free excess carriers. From analysis of experimental results we conclude that in our materials strong trapping effects are present, so the range of excited carriers is very short (10^–4-10^–3 cm) even in the highest electrical fields used (to 10⁴ V. cm^–1). Estimates of upper limit of drift mobility give the values 10^–1- 1 cm² V^–1 sec^–1. We did not succeed in determining the type of carriers which are responsible for the observed effects.     

Electroluminescence and internal screening of domains upon polarization switching of ferroelectric ceramics in a pulsed self-consistent field

This paper reports on the results of investigations into the electroluminescence induced by polarization switching of a ferroelectric ceramic material in an electric field with a rapidly increasing strength. A correlation between the polarization switching and luminescence kinetics is revealed. It is demonstrated that tunneling of electrons to the conduction band and impact ionization in field concentrators are the main mechanisms of generation of free charge carriers responsible for the electroluminescence and internal screening of switching domains.