CPA density of states and conductivity in a double-exchange system containing impurities

We study density of states and conductivity of the doped double-exchange system, treating interaction of charge carriers both with the localized spins and with the impurities in the coherent potential approximation. It is shown that under appropriate conditions there is a gap between the conduction band and the impurity band in paramagnetic phase, while the density of states is gapless in ferromagnetic phase. This can explain metal-insulator transition frequently observed in manganites and magnetic semiconductors. Activated conductivity in the insulator phase is numerically calculated. Received 13 June 2000 and Received in final form 5 January 2001     

The metal-insulator transition in extrinsic semiconductors

The main purpose of this article is to describe the electrical and magnetic properties of extrinsic semiconductors when the concentration of donors varies, particularly for concentrations near that for which a metal-insulator transition takes place. Since the donor centres are distributed at random in space, the combined effects of correlation and Anderson localization have to be considered. As an introduction, in § 1 we give an outline of our present understanding of the transition in some crystalline materials, particularly V₂O₃. For this the metallic phase, a highly correlated electron gas, is discussed, as is also the question of a discontinuous change in the number of current carriers. In § 2 we discuss Si : P and similar materials. For uncompensated samples, the material near the transition is to be described by overlapping Hubbard bands, and the metal-insulator transition occurs when the states at the Fermi energy show Anderson localization. Just on the insulator side of the transition conduction is by variable-range hopping for uncompensated as for compensated materials, and a variation of log (resistivity) as T ^?1/4 is observed. The existence or otherwise of moments in the metallic phase is discussed, as is also the magnetoresistance, Hall effect and Knight shift. In § 3 the transition is described for narrow-band semiconductors, particularly La_1?x Sr _x VO₃. In § 4 some compounds of rare earths are treated, including Sm_1?x Nd _x Se.     

Bound ferromagnetic and paramagnetic polarons as an origin of the resistivity peak in ferromagnetic semiconductors and manganites

A theory of resistivity is developed for ferromagnetic semiconductors, possibly, including manganites. The theory is based on analysis of the interaction of the free and bound charge carriers with the magnetization of the crystal. The temperature dependence of free energy for nonionized donors and free electrons is calculated for the spin-wave and paramagnetic regions. In addition to the trapping by the ferromagnetic fluctuations (the ferromagnetic polarons), the electron trapping by the random magnetization fluctuations as T → is taken into account (the paramagnetic polarons). For the nondegenerate semiconductors, the theory makes it possible to explain a nonmonotonic temperature dependence of the activation energy, with the value for T = 0 being lower than that for T → ∞. For degenerate semiconductors, the theory explains a metal-insulator transition that occurs with increasing temperature in samples with relatively low charge carrier density. If the density is larger, a reentrant metal-insulator transition should take place, so that the crystal is highly conductive as T → ∞.     

Electrostatic models of insulator-metal and metal-insulator concentration phase transitions in Ge and Si crystals doped by hydrogen-like impurities

Two electrostatic models have been developed that allow calculation of the critical concentration of hydrogen-like impurities in three-dimensional crystalline semiconductors corresponding to the insulator-metal and metal-insulator transition in the zero temperature limit. The insulator-metal transition manifests itself as a divergence of the static permittivity observed in lightly compensated semiconductors as the concentration of polarizable impurities increases to the critical level. The metal-insulator transition is signaled by the divergence of the dc electrical resistivity in heavily doped semiconductors as the compensation of the majority impurity increases (or its concentration decreases). The critical impurity concentration corresponds to the coincidence of the percolation level for the majority carriers with the Fermi level. The results of the calculations made with these models fit the experimental data obtained for n-and p-type silicon and germanium within a broad range of their doping levels and impurity compensation.     

A model of hopping and band DC photoconduction in doped crystals

Expressions for the screening length and the ambipolar diffusion length are derived, for the first time, for the case where hopping conduction and band conduction coexist in semiconductors with hydrogen-like impurities. A method is proposed for calculating the diffusion coefficient of electrons (holes) hopping between impurity atoms from data on the Hall effect, in the case where the hopping and band conductivities are equal. An interpretation is given of available experimental data on hopping photoconduction between acceptors (Ga) and donors (As) in p-Ge at T=4.2 K doped by a transmutation method. It is shown that the relative magnitude of the mobilities of electrons hopping between donors and holes hopping between acceptors can be found from the hopping photoconductivity measured as a function of the intensity of band-to-band optical carrier excitation.     

Recent test and new applications of the unified theory of electron emission

A review is presented of the recent experimental research confirming the unified theory of electron emission from metals in vacuum, insulators and semiconductors in the whole range between the extreme conditions of field and thermionic emission. Some new applications of the theory for calculation of the electron current in vacuum discharge, for determination of the metal-insulator (semiconductor) work function and for estimation of the effective (conduction band) electron mass in insulators and semiconductors, are also considered.     

A quasi-zero-gap semiconductor at high pressures as a model of amorphous semiconductor

A data summary is made for the energy spectrum and electron transport under uniform pressure in heavily doped compensated semiconductors with a deep impurity band in the tail of the intrinsic band state density. It is shown that these semiconductors subject to high pressure can be used for modeling amorphous semiconductors. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 20–24, July, 2008.     

Correlated defects,metal-insulator transition,and magnetic order in ferromagnetic semiconductors

The effect of disorder on transport and magnetization in ferromagnetic III-V semiconductors, in particular (Ga,Mn)As, is studied theoretically. We show that Coulomb-induced correlations of the defect positions are crucial for the transport and magnetic properties of these highly compensated materials. We employ Monte Carlo simulations to obtain the correlated defect distributions. Exact diagonalization gives reasonable results for the spectrum of valence-band holes and the metal-insulator transition only for correlated disorder. Finally, we show that the mean-field magnetization also depends crucially on defect correlations.     

External control of a metal-insulator transition in GaMnAs wires

Quantum transport in disordered ferromagnetic (III,Mn)V semiconductors is studied theoretically. Mesoscopic wires exhibit an Anderson disorder-induced metal-insulator transition that can be controlled by a weak external magnetic field. This metal-insulator transition should also occur in other materials with large anisotropic magnetoresistance effects. The transition can be useful for studies of zero-temperature quantum critical phase transitions and fundamental material properties.     

Electron Metal-Dielectric Phase Transitions in Semiconductors under Pressure

The effect of uniform pressure on the character of metal-dielectric transitions in compensated weakly- and highly doped semiconductors (WHDCS) is considered. The influence of hybridization of resonance quasilocalized impurity states with band continuum states on the transition is shown. Minimum metallic conductivities in p-CdSnAs₂ are determined for Mott and Anderson transitions. Special features of the metal-dielectric transformation in weakly-doped narrow- and high energy-gap semiconductors are discussed for the case of hydrogen-like impurities. Anderson localization in WHDCS is also considered. Phase diagrams are presented.     

Coulomb gap,its critical behavior,and the metal-insulator transition in doped semiconductors

To measure a density of localized states in the vicinity of the Fermi level variable range hopping spectroscopy procedure has been established. As a result the parabolic Coulomb gap has been discovered on a set of the series of doped and compensated semiconductors. The gap has a single- or multielectron origin for the case of strong localization when a compensation degree is close to unity or moderate correspondingly. All the gaps turned out to be collapsing in the critical point for the metal-insulator transition. One can consider the transition as Coulomb gap collapsing phenomenon in a frame of the scaling theory describing it as a second order phase transition. The critical indices for a localization radius and for a metallic conductivity are nearly equal to unity.     

Far-infrared spectroscopy of impurities in semiconductors

Far-infrared spectroscopy of the electronic transitions between bound states of impurities provides a very high resolution technique for studying chemical shifts and thereby identifying residual contaminants. The use of photoconductivity generated within the sample itself, usually by the photothermal mechanism (“photothermal ionisation spectroscopy”), enables very high sensitivity to be achieved even with very thin films or ultrahigh-purity material. The current knowledge about the identity of the residual shallow donors in GaAs, InP, InAs and InSb obtained with this technique is reviewed. With high-purity materials the magneto-optical spectrum of the shallow donors can be particularly rich and more than fifty lines can be observed with both GaAs and InP.

Hydrostatic pressure provides a valuable additional experimental parameter in studies of impurities. Not only does the pressure-induced increase in mass improve the resolution of the “fine structure” due to different chemical species but additional states can be introduced into the forbidden energy gap. Results with both InSb and GaAs have shown that generally donors in direct-gap III-V materials may be expected to have three types of state: the familiar gamma-associated donors, localised states with A₁ symmetry which are normally resonant within the conduction band and metastable DX states.

Negatively charged shallow donor states (D^- states) and “molecular” combinations where the electrons are shared between two or more donor sites have been studied by infrared spectroscopy of III-V materials. These states are important precursors of the metal-insulator transition.

Recently there have been a number of studies of impurities within quantum wells and heterostructures. The dependence of impurity energy on distance from the well edge has been established and it has been shown that high concentrations of D^- states can be formed by remote deping of the structures.     

Zener tunneling in one-dimensional organic semiconductors

Tunneling effect in one-dimensional organic semiconductors in the presence of an external electric field is studied within the framework of a tight-binding model and a nonadiabatic dynamical method. It is found that under a high electric field, electrons can transit from the valence band (VB) to the conduction band (CB), which is demonstrated to be Zener tunneling in organic semiconductors. The results also indicate a field-induced insulator-metal transition accompanied by the vanishing of the energy gap. It is found that, after the field is turned off, the Peierls phase cannot be recovered.     

硅中的过渡元素杂质能级

本文提出了半导体中过渡元素杂质的一个简单模型,用格林函数方法计算了硅中替代和间隙原子产生的杂质能级和波函数。发现两者的性质有很大的差别。替代原子只有当d原子能级V_d低于价带顶时才能产生杂质能级。它的波函数主要是悬键态,当能级靠近导带边时变成正键态。间隙原子只有当V_d高于价带顶时才能产生杂质能级。它的波函数主要是中心原子d态,当能级靠近导带边时变成弱反键态。最后定性地说明了过渡元素杂质能级的化学趋势和一些实验事实。 关键词：     

Possibility of investigation the metastable donor states in ii-vi semiconductors under hydrostatic pressure

Abstract

The model of bistable donor centers was proposed for describing the metastable electron traps in cubic semiconductors. The detailed analysis of the model suggests that the bistable donors should be very common in II-VI semiconductors doped with VII group impurities. Under normal conditions the additional metastable states are degenerated with the conduction band. However, the analysis of the energy bands pressure coefficients for II-VI materials shows that under hydrostatic pressure the donor traps can change the electrical properties of the sample.     

n型碲镉汞MIS器件动态存储时间研究

通过对动态存储时间的测量，分析了ｘ＝０．３１的ｎ型碲镉汞（Ｈ_1-xＣｄ_xＴｅ）ＭＩＳ器件的少子暗电流机理．理论计算和实验结果都表明，在其工作温度区域（～７７Ｋ），通过禁带中深能级中心辅助的间接隧道电流将限制器件的电学性能。 关键词：     

低补偿度n-Hg1-xCdxTe的磁致金属-绝缘体相变和相变后的温度激活输运行为

报道了ｘ＝０．２１４组份、低补偿度（Ｋ《１）ｎ－Ｈｇ_1-xＣｄ_xＴｅ晶体在０．３─３０Ｋ温度范围，０─７Ｔ强磁场下的横向磁阻、电子霍耳迁移率、霍耳系数测量结果，观测到了磁致金属－绝缘体相变和相变后的温度激活输运行为。分析实验数据，提出：低补偿度、组份：ｘ＝０．２附近的ｎ－Ｈｇ_1-xＣｄ_xＴｅ，磁致金属－绝缘体相变（ＭＩＴ）发生的机理是载流子在浅施主杂质态上的磁冻结；发生磁冻结的前提是热冻出（ｔｈｅｒｍａｌ ｆｒｅｅｚｅ 关键词：     

Thermal donors in silicon: Consistent interpretation of Hall-effect and capacitance transient spectroscopy

Thermal donors are generated in oxygen-rich silicon by an anneal at 450 °C. The energy-levels of these donors are investigated by capacitance transient spectroscopy (DLTS) and the Hall effect. A level at 125 meV below the conduction band observed with DLTS exhibits a strong dependence on the electric field (Poole-Frenkel effect). Evaluation with a novel model for the Poole-Frenkel effect leads to a corrected zero-field energy of 200 meV for this level. The Hall effect indicates two thermal donors at 57 and 120 meV below the conduction band; both centers cannot be observed with our capacitance-DLTS method due to the Poole-Frenkel energy reduction by 70 meV±10 meV. The 45 meV current-DLTS level [1,2] is shown to correlate with the 120 meV Hall level.     

Metal-insulator transitions in the periodic Anderson model

We solve the periodic Anderson model in the Mott-Hubbard regime, using dynamical mean field theory. Upon electron doping of the Mott insulator, a metal-insulator transition occurs which is qualitatively similar to that of the single band Hubbard model, namely, with a divergent effective mass and a first order character at finite temperatures. Surprisingly, upon hole doping, the metal-insulator transition is not first order and does not show a divergent mass. Thus, the transition scenario of the single band Hubbard model is not generic for the periodic Anderson model, even in the Mott-Hubbard regime.