Frenkel’-Poole effect for boron impurity in silicon in strong warming electric fields

A method based on measurement of the thermally stimulated conductivity of a weakly compensated semiconductor, which is doped with a deep impurity and which contains an impurity component that is shallower than the main component, has been developed for investigating the Frenkel’-Poole effect. The results of an investigation of the thermally stimulated conductivity of Si:Ga samples with gallium density N _A =(2–3)×10¹⁸ cm⁻³ and low accompanying impurity content (⩽10¹³ cm⁻³) are reported. The conductivity was measured after extrinsic photoexcitation of samples heated at a rate β=0.6 K/s in the temperature range T=4.2–24 K in electric fields E=20–1000 V/cm. It is shown that the maximum on the curves of the thermally stimulated conductivity is due to the thermally stimulated emptying of the boron impurity and shifts to lower values of T as E increases. The decrease of the ionization energy of impurity B in an electric field, which turns out to be somewhat weaker than the field according to the Frenkel’-Poole model for singly charged Coulomb centers, is found from the shift of the maximum. Fiz. Tekh. Poluprovodn. 31, 777–780 (July 1997)     

Absolute negative conductivity of graphene with impurities in magnetic field

Current-voltage and current-field characteristics for graphene with Anderson interaction of conduction and impurity electrons are calculated by the “average electron” method in the case of low temperatures. These characteristics are analyzed depending on the frequency of an external ac electric field and a magnetic field. A portion with absolute negative conductivity is detected.     

Optical and electrical effects of charges on defects in non-metals

Electrically charged defects, which are unavoidable in solids, create very strong electric fields that affect many properties of the host. In an insulator with ∿ 10 ppm charged impurities, for example, the average field strength is F ∿ 50 kV/cm. The polar lattice vibrations, which are defects of a different sort, create fields in ionic solids that are typically lO⁶V/cm at room temperature. Perhaps the most fundamental effect of these charges is the alteration of the density of electronic states. Near energy band edges this produces “band tails” that have important consequences in most electron transport properties quite apart from ordinary impurity scattering. Furthermore, the matrix elements for optical transitions among states near band edges are also affected with several significant consequences in the optical properties. Aside from these fundamental physical consequences of charged defects, a number of important practical effects occur, including the domination of semiconductor laser properties, limitation of light transmission in quartz fibers and behavior of amorphous semi-conductors.     

Polarization echoes in piezoelectric semiconductors

In many piezoelectric crystals, application of a se-quence of two or three microwave electric field pulses creates a polarization which radiates subsequent “echo” signals (analogous to spin echoes). Two distinct types of echo phenomena may be delineated. Both involve a nonlinear interaction of a forward propagating acoustic wave (ω,k) (launched by the first microwave pulse (ω,0), with the uniform microwave electric field of the second pulse (ω,0) . The nonlinearities are derivable on the basis of field induced ionization of electrons from shallow impurity states (traps). The first echo phenomenon, which we term “parametric echo”, is a backward wave parametric process. The pump being provided by the 2ω component of the conduction band electron density resulting from ionization by an ω electric field. In the second phenomenon, “holographic echo”, electrons are transferred among the traps to create a static charge distribution which is a stored hologram representing the interference pattern (with wave vectork) of the acoustic and electric fields. At low temperatures (4.2°K) in CdS, CdSe, and CdTe the pattern is stable in the dark for months. A third (or any subsequent) pulse, (ω,0) or (ω,k), produces an output echo (ω,k) or (ω,0), respectively, through electrostriction. Under illumination by low level white light the variation of echo amplitude with microwave power involves the energy distribution of the trap levels, and the latter may be obtained from the data. Results for a fully compensated CdS:S sample are presented.     

Ionization of impurity centers in a semiconductor quantum superlattice by nonlinear electromagnetic waves

The ionization of impurity centers in a semiconductor superlattice by nonlinear electromagnetic waves, which are the most general solution of the sine-Gordon equation and can be expressed in terms of the Jacobi elliptic functions, is investigated. The problem is solved in a quasiclassical approximation for arbitrary ratio of V (depth of the impurity energy level) and Δ (half-width of the conduction miniband). Results in agreement with those for solitary waves and sinusoidal (linear) electromagnetic waves are obtained in limiting cases. The effect of a uniform high-frequency electric field on the processes leading to the ionization of impurities by solitary waves is also investigated. Fiz. Tekh. Poluprovodn. 32, 334–337 (March 1998)     

Effects of hydrostatic pressure and external electric field on the impurity binding energy in strained GaN/AlxGa1?xN spherical quantum dots

The binding energy and Stark effect energy shifts of a shallow donor impurity state in a strained GaN/AlxGa1-xN spherical finite-potential quantum dot (QD) are calculated using a variational method based on the effective mass approximation. The binding energy is computed as a function of dot size and hydrostatic pressure. The numerical results show that the binding energy of the impurity state increases, attains a maximum value, and then decreases as the QD radius increases for any electric field. Moreover, the binding energy increases with the pressure for any size of dot. The Stark shift of the impurity energy for large dot size is much larger than that for the small dot size, and it is enhanced by the increase of electric field. We compare the binding energy of impurity state with and without strain effects, and the results show that the strain effects enhance the impurity binding energy considerably, especially for the small QD size. We also take the dielectric mismatch into account in our work.     

Steady-State Electron Transport and Low-Field Mobility of Wurtzite Bulk ZnO and Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O

Steady-state electron transport and low-field electron mobility characteristics of wurtzite ZnO and Zn_1−x Mg _x O are examined using the ensemble Monte Carlo model. The Monte Carlo calculations are carried out using a three-valley model for the systems under consideration. Acoustic and optical phonon scattering, intervalley (equivalent and nonequivalent) scattering, ionized impurity scattering, and alloy disorder scattering are used in the Monte Carlo simulations. Steady-state electron transport is analyzed, and the population of valleys is also obtained as a function of applied electric field and ionized impurity concentrations. The negative differential mobility phenomena is clearly observed and seems compatible with the occupancy and effective nonparabolicity factors of the valleys in bulk ZnO and in Zn_1−x Mg _x O with low Mg content. The low-field mobilities are obtained as a function of temperature and ionized impurity concentrations from the slope of the linear part of each velocity–field curve. It is seen that mobilities begin to be significantly affected for ionized impurity concentrations above 5 × 10¹⁵/cm³. The calculated Monte Carlo simulation results for low-field electron mobilities are found to be consistent with published data.     

Distribution of the electric field in high-resistivity MSM structures illuminated by nonmonochromatic light

The sensitivity of the electric field distribution to the spectrum of the incident radiation in highly biased, high-resistivity, semiconductor structures of the MSM (metal-semiconductor-metal) type illuminated by nonmonochromatic light is investigated theoretically. It is shown that in the presence of deep impurity levels the field distribution depends strongly on the spectral composition of the incident light. The frequency interval corresponding to optical thicknesses of the order of unity is found to significantly influence the space charge in the bulk of the structure and the electric field distribution E(x), even when the fraction of energy in this region of the spectrum relative to the total flux is extremely minimal. The trapping of holes by a deep impurity level in the bulk of the structure forms a positive space charge and produces qualitatively new field distributions, which increase near the dark electrode with a positive curvature of the function E(x). The impurity trapping of electrons near the illuminated anode imparts a negative space charge to the impurity levels. This phenomenon induces a substantial increase of the field in the electrode sheath and forms in the vicinity of the anode a region wherein the field varies only slightly. All the prominent features disclosed by the calculations in the electric field distributions are observed in experiment. Fiz. Tekh. Poluprovodn. 33, 815–823 (July 1999)     

Analysis of mechanisms for electron scattering in GaAs/AlxGa1−x As superlattices with doped quantum wells for longitudinal resonant current flow in high electric fields and at low temperatures

Formulas are derived for, and a numerical analysis made of, the dependence of the transverse phase relaxation time on electron energy for resonant current flow through GaAs/Al_xGa_1−x As superlattices with doped quantum wells. The parameters are chosen to be close to those of superlattices used for creating photodiodes for operation at λ⋍10 μm. The analysis is limited to the interactions of electrons with neutral atoms and impurity ions at low temperatures. Resonant current flow is ensured by an electric field that brings the ground state and the first excited state of the “Stark ladder” into resonance with neighboring, weakly interacting quantum wells. Fiz. Tekh. Poluprovodn. 33, 438–444 (April 1999)     

On the nature of charge carrier scattering in Ag<Subscript>2</Subscript>Se at low temperatures

The electric and thermoelectric properties of silver selenide in the temperature range of 4.2–300 K have been studied. The data obtained are interpreted within the theory of one-type carriers and Kane dispersion relation, with allowance for the character of electron-electron interaction. It is established that, for the concentrations n ≤ 7.8 × 10¹⁸ cm⁻³, charge carriers are scattered by impurity ions at T ≤ 30 K and by acoustic and optical phonons and point defects at T ≥ 30 K. Electron-electron interactions are found to be elastic at T < 30 K.     

Influence of strain on hydrogenic impurity states in a GaN/AlxGa1?xN quantum dot

Within the effective-mass approximation, we calculated the influence of strain on the binding energy of a hydrogenic donor impurity by a variational approach in a cylindrical wurtzite GaN/Al _x Ga_1−x N strained quantum dot, including the strong builtin electric field effect due to the spontaneous and piezoelectric polarization. The results show that the binding energy of impurity decreases when the strain is considered. Then the built-in electric field becomes bigger with the Al content increasing and the binding energy of hydrogenic donor impurity decreases when the Al content is increasing. For dot height L < 2 nm, the change of the binding energy is very small with the Al content variety. This work has been supported by the National Natural Science Foundation of China (No. 10564003) and the Key Project of the Science and Technology Research of the Educational Ministry of China (No. 208025)     

Current-voltage characteristics of Si:B blocked impurity-band structures under conditions of hopping-transport-limited photoresponse

The photoconductivity of Si:B blocked-impurity-band (BIB) structures with boron concentration in the active layer ∼10¹⁸ cm⁻³ has been studied. Measurements were performed in the temperature range 4.2–10 K at different intensities of the exciting radiation 10¹⁰–10¹⁵ photons/cm²·s. Photoexcitation at 5.5 μm was realized using a semiconductor laser. At temperatures below 6 K and low bias voltages (<0.5 V) the current-voltage characteristics were found to have a threshold-like character. The threshold voltage rises as the temperature is lowered and the radiation intensity is increased. A model based on the Frenkel’-Poole effect in the impurity band has been developed. This model can be used to numerically describe the current-voltage characteristics with accuracy better than 5%. As a result, it is found that the photoconductivity rises and then reaches a plateau as the radiation intensity increases. Under these conditions, as under equilibrium conditions (in darkness), the hopping conductivity also depends exponentially on the electric field. This fact is explained in terms of the destruction by the electric field of (A ⁺-A ⁻) impurity complexes which appear under nonequilibrium conditions. Fiz. Tekh. Poluprovodn. 32, 192–199 (February 1998)     

Nonradiative recombination at shallow bound states in quantum-confined systems in an electric field

The one-phonon recombination of carriers at shallow impurity states in parabolic quantum wells in a longitudinal electric field is investigated. It is shown that one-phonon recombination processes are more active in quantum-confined systems than in the bulk material. The possibility of electrically induced one-phonon transfer in a confined system is discussed. Fiz. Tekh. Poluprovodn. 33, 97–100 (January 1999)     

The problem of intermediate temperatures or electric fields in the scattering of hot electrons by acoustic phonons

An approximate expression is derived for the momentum relaxation time in the quasielastic scattering of hot electrons by acoustic phonons as a function of the electron energy and the lattice temperature. The mobility and the dependence of the impurity breakdown electric field on the degree of compensation are calculated in the electron-temperature approximation. The results of the calculations are in good agreement with the experimental for n-type Ge. Fiz. Tekh. Poluprovodn. 31, 1071–1073 (September 1997)     

掺氮4H-SiC电子输运特性的多粒子蒙特卡罗研究

在最新能带结构计算的基础之上,采用非抛物性能带模型对掺氮4H-SiC电子输运特性进行了多粒子蒙特卡罗(Ensemble Monte Carlo)研究.研究表明,低场下,掺杂浓度较低时,氮杂质不完全电离导致的中性杂质散射对4H-SiC横向电子迁移率影响较小.随着掺杂浓度的增加,中性杂质散射作用增强.掺杂浓度较高时,随着温度的增加,中性杂质散射的影响逐步减弱.4H-SiC电子迁移率较高且各向异性较小,温度为296K时得到的横向电子饱和漂移速度为2.18×10⁷cm/s;阶跃电场强度为1000KV/cm时,横向瞬态速度峰值接近3.3×10⁷cm/s,反应时间仅为百分之几皮秒量级.模拟结果同已有的测试结果较为一致.     

Characteristics of diffused P-N junctions in epitaxial layers

A one-dimensional analysis has been made to determine properties of diffused p-n junctions in epitaxial layers with nonuniform impurity concentration. Impurity diffusion from the surface and from the substrate is assumed to have complementary error function distribution. The transcendental equations obtained by analytical integration of Poisson's equation were evaluated numerically with the IBM 7090/94. Junction depth, impurity gradient and impurity level at the junction are given for a variety of diffusion parameters and impurity concentrations. In addition, graphs are presented, showing the relationship between reverse voltage and depletion layer thickness, capacitance per unit area, and peak electric field for the case of silicon. A comparison between the actual impurity profile and the usual linear approximation using the impurity gradient at the junction gives the range of depletion layer thickness or reverse voltage in which such an approximation is justified. Further, examples are presented of the electric field distribution in the depletion layer for several impurity concentration profiles. Calculated and experimentally determined values of some readily accessible junction characteristics show reasonably good agreement.     

Specific features of photoluminescence properties of copper-doped cadmium selenide quantum dots

The effect of doping with copper on the photoluminescence properties of cadmium selenide quantum dots 4 nm in dimension is studied. The quenching of the excitonic photoluminescence band related to the quantum dots and the appearance of an impurity photoluminescence band in the near-infrared region are observed after doping of the quantum dots with copper. It is established that, on doping of the quantum dots, the photoluminescence kinetics undergoes substantial changes. The photoluminescence kinetics of the undoped quantum dots is adequately described by a sum of exponential relaxation relations, whereas the photoluminescence kinetics experimentally observed in the region of the impurity band of the copper-doped samples follows stretched exponential decay, with the average lifetimes 0.3–0.6 μs at the photon energies in the range of 1.47–1.82 eV. The experimentally observed changes in the photoluminescence properties are attributed to transformation of radiative centers in the quantum dots when doped with copper atoms.     

Magnetic field and temperature dependence of the groundstate energy of weak-coupling magnetopolaron in quantum rods with hydrogenic impurity

The dependence of the ground-state properties of weak-coupling bound magnetopolarons in quantum rods (QRs) with hydrogenic impurity on magnetic field and temperature is studied by means of the Lee-Low-Pines (LLP) transformation method and Huybrechts linear combination operator method. The expression for the ground-state energy of the magnetopolaron is derived. Results of the numerical calculations show that the ground-state energy of weak-coupling bound magnetopolarons in QRs with hydrogenic impurity increases with increasing the cyclotron frequency of the magnetic field, the confinement strength of QRs and the temperature, but decreases with increasing the electron-phonon coupling strength and the dielectric constant ratio. The stability of the ground state of magnetopolarons is closely related to the aspect ratio e′of the QR. The ground state of magnetopolarons is the most stable at e′=1. The stability of the ground state of magnetopolarons can remarkably decrease when the value of the aspect ratio increases or decreases from 1.     

Solubility,diffusion and electrical activity of Na in bulk Ge crystals

By studying the drift of Na⁺ ions in the firstly grown Na-doped bulk Ge crystals as well as by analyzing optical and some other characteristics of this material, the following conclusions are made, many of which are different from the commonly accepted statements: (1) Ge can be uniformly doped with Na during the bulk Ge crystals growth from the melt; (2) maximum solubility at room temperature and distribution coefficient of Na in Ge are (0.3–1)×10¹⁵ cm⁻³ and (0.7–2.3)×10⁻⁷, respectively; (3) Na is a donor impurity in bulk Ge, and Na atoms introduced during the crystal growth are predominantly electrically active; (4) the evaluated values of diffusion parameters of Na in Ge are as follows: the diffusion coefficient D=3.6×10⁻⁷ cm²/s, pre-exponential factor D₀=0.13 cm²/s, the activation energy for diffusion Q=0.33 eV; (5) Na is an interstitial impurity in Ge and rather rapidly drifts in an electric field, most likely, via interstitial sites; (6) the resistance distribution along the crystal length may be changed by DC electric field application and remain stable at the long-term crystal storage. The stability in the Ge:Na properties opens the possibility for using Ge:Na crystals not only for creating passive optical elements of infrared imaging technique, as we are doing now, but also for the electrical appliances, in particular for the substitution of the thermally unstable Li for Na in germanium detectors of γ-radiation.     

EFFECT OF A WEAK LONGITUDINAL ELECTRIC FIELD ON WHISTLER WAVES

The effect of a longitudinal electric field on whistler waves is studied based onkinetic theory.A local Maxwellian distribution is taken as stationary distribution function ofelectrons which departs from thermodynamic equilibrium due to the applied electric field.Thedielectric tensor is derived by integrating along orbit of the particle in the unperturbed field.Dispersion relation and growth rate are analysed from Hermitian and anti-Hermitian parts ofthis tensor respectively.It is found that the waves are growing when the angle between the wavevector and the electric field is in range of θ<2θ_c, otherwise the whistler waves are damping.Thegrowth rate increases with wave frequency and decreases with the angle between the wave vectorand the applied field.In the case of ω_e(?)Ω the maximum of growth rate,which is at θ=O_l isproportional to the plasma density and anti-proportional to the magnetic field.Some computedresults for parameters at top of the F layer are given.