Relaxation of the electric field in high-resistivity, strongly biased MISIM structures with deep impurity levels

The relaxation of the field and current in a high-resistivity metal-insulator-semiconductor (MISIM) structure containing a considerable concentration of deep impurity levels after the removal of strongly absorbed light is investigated numerically. It is established that the time dependence of the field distribution is determined by the relation between the times for the thermal generation of electrons (τ _n ) and holes (τ _p ) by an impurity. In the case of the temporal variation of the field in the bulk of the semiconductor is nonmonotonic. The drift of the photogenerated carriers after removal of light leads to the formation of a negative space charge layer of increased density and a significant increase in the field near the anode. Its maximum value can be as high as 5–6 times the mean field E _e =V/d. Consideration of the additional injection of holes from the anode leads to an increase in the current, restriction of the maximum field at the anode, and appreciable acceleration of the relaxation of the field to the dark distribution. Fiz. Tekh. Poluprovodn. 32, 203–208 (February 1998)     

Modeling the gate bias dependence of contact resistance in staggered polycrystalline organic thin film transistors

We have modeled the dependence on the gate voltage of the bulk contact resistance and interface contact resistance in staggered polycrystalline organic thin film transistors. In the specific, we have investigated how traps, at the grain boundaries of an organic semiconductor thin film layer placed between the metal electrode and the active layer, can contribute to the bulk contact resistance. In order to the take into account this contribution, within the frame of the grain boundary trapping model (GBTM), a model of the energy barrier E_B, which emerges between the accumulation layer at the organic semiconductor/insulator interface and injecting contact, has been proposed. Moreover, the lowering of the energy barrier at the contacts interface region has been included by considering the influence of the electric field generated by the accumulation layer on the injection of carriers at the source and on the collection of charges from the accumulation layer to the drain contact. This work outlines both a Schottky barrier lowering, determined by the accumulation layer opposite the source electrode, as well as a Poole-Frenkel mechanism determined by the electric field of the accumulation layer active at the drain contact region. Finally it is provided and tested an analytical equation of our model for the contact resistance, summarizing the Poole-Frenkel and Schottky barrier lowering contribution with the grain boundary trapping model.     

The influence of the illumination direction on the field distribution in high-resistivity metal-semiconductor structures

The dependence of the field distribution on the illumination intensity in a heavily biased high-resistivity metal-semiconductor structure exposed to intrinsic-absorption monochromatic light was investigated. The distinctions in the field distribution under illumination from the anode and cathode sides are revealed. In the bulk, these distinctions are caused by a difference in the photocarrier mobility, whereas near the surface they are caused by the different directions of diffusion and drift flows for more mobile electrons. It is demonstrated that if the cathode is illuminated, the field distribution is nonuniform. In this case, the field decreases in a thin layer with a distance from the illuminated cathode, passes through a minimum, and increases toward the anode. With increasing illumination intensity, the quasi-neutral region in the vicinity of the field minimum expands toward the anode and the lowest field decreases. On the other hand, near the illuminated cathode, the field increases for low intensities and decreases for high intensities. For sufficiently high illumination intensities, the field dependence on the coordinate in the bulk of pure crystals as a function of the distance from the illuminated electrode is independent of the illumination direction.     

Modeling of photoinduced charge separation in germanosilicateoptical fibers during UV-excited poling

We report numerical solutions of a proposed model for charge separation and trapping during poling of germanosilicate fiber in the presence of ultraviolet (UV) light. The model was solved quantitatively in the steady state to determine the space charge field distribution after UV-excited poling of a germanosilicate optical fiber with internal electrodes. The resulting internal electric field was found to be up to an order of magnitude higher than the initial poling field, sufficient to produce an effective second-order nonlinearity consistent with experimental observations by the internal field acting on the inherent third-order nonlinearity. The effects of core-cathode spacing, nonuniform defect distributions, and photo-electron recombination rate on the induced χ_eff⁽²⁾ were also investigated. It is shown that a small core-cathode spacing is advantageous. Our UV-poled field solutions may also apply to thermal poling, provided we swap the anode and cathode designations. The results suggest that it is optimal to have the core located in the depletion region regardless of poling method     

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.     

Donor impurity-related optical absorption coefficients and refractive index changes in a rectangular GaAs quantum dot in the presence of electric field

Within the quasi-one-dimensional effective potential model and effective mass approximation, we obtain the wavefunctions and energy eigenvalues of the ground (j=1) and first 2 excited states (j=2 and 3) of a donor impurity in a rectangular GaAs quantum dot in the presence of electric field. The donor impurity-related linear and nonlinear optical absorption as well as refractive index changes for the transitions j=1-2 and j=2-3 are investigated. The results show that the impurity position, incident optical intensity and electric field play important roles in the optical absorption coefficients and refractive index changes. We find that the impurity effect induces the blueshift for j=1-2 and redshift for j=3-2 in the absence of the electric field, but it leads to redshift for j=1-2 and blueshift for j=3-2 in the existence of the field. Also, the optical coefficient for the higher energy transitions j=2-3 is insensitive to variation of impurity positions, while that for the low energy transition j=1-2 depends significantly on the positions of impurity. In addition, the saturation and splitting phenomenon of the optical absorption are observed as the incident optical intensity increases.     

Characterization of Cd1−xZnxTe crystals grown from a modified vertical bridgman technique

Cd_1−xZn_xTe (CZT) crystals grown from a modified vertical Bridgman technique were characterized by means of an optical polarized transmission technique using the Pockels effect, low-temperature direct current (DC) photo-conductivity technique, low-temperature photoluminescence (PL) spectroscopy, room-temperature PL mapping technique, and detector performance measurements. Electric field mapping indicates that an approximation of a uniform electric field distribution approximation is generally satisfied for CZT detectors operated at room temperature under typical working conditions. A nonuniform electric field distribution is observed under intense infrared (IR) light illumination, and a model is proposed based on charge generation of defects, trapping, and space-charge effects. The largest hole mobility-lifetime product (μτ)_h of the CZT detector measured by DC photoconductivity is 7.0 × 10⁻⁴ cm²/V. The detector treated with 2% bromine in methanol chemical etch has a relatively small surface recombination velocity at room temperature, which was obtained from DC photocurrent and detector performance tests, as measured by irradiation of 5.5-MeV α particles and 59.6-keV γ-rays, respectively. We have clearly shown the equivalence of charge collection efficiency results measured by both DC photocurrent and α particle response. Low-temperature DC photocurrent measurements show that surface recombination velocity increases significantly with decreasing temperature from 300 K to 250 K. The effective electron mobility-lifetime product—combination effects of bulk and surface of CZT crystal—increases with increment of temperature. Room-temperature PL mapping measurements indicate uniformity of zinc concentration within CZT crystals. Low-temperature PL spectroscopy shows that the dominant emission peaks are excitons, which are bound to either shallow neutral donors (D⁰, X) or neutral acceptors (A⁰, X), depending on the temperature, concentration of donors and acceptors, and the incident light intensity. It was found that the luminescence of (D⁰, X) depends linearly on the incident laser intensity, while (A⁰, X) has a nonlinear dependence.     

Migration of laser-induced point defects in IV–VI compounds

The laser-induced (hω<E _g) transformation of defects in IV–VI compounds is studied. The rate of defect production is found to increase with the free carrier concentration and the electric field strength in the laser wave. The directed migration of both the intrinsic and impurity defects under the combined action of laser radiation and an external electric field is analyzed in terms of a model where the defects are believed to arise mainly from the accumulation of inclusions whose sizes are significantly smaller than the laser wavelength in the crystal. It is proposed that the directed migration and other displacements of the induced defects over the crystal are due to the drag of the activated atoms by the free charge carriers in the laser field E _L and the external electric field E _ex. The diffusion coefficient and the effective charge of dragged laser-induced defects are estimated.     

Drift velocity of electrons in quantum wells of selectively doped In0.5Ga0.5As/AlxIn1 ? xAs and In0.2Ga0.8As/AlxGa1 ? xAs heterostructures in high electric fields

The field dependence of drift velocity of electrons in quantum wells of selectively doped In_0.5Ga_0.5As/Al _x In_{1 − x} As and In_0.2Ga_0.8As/Al _x Ga_{1 − x} As heterostructures is calculated by the Monte Carlo method. The influence of varying the molar fraction of Al in the composition of the Al _x Ga_{1 − x} As and Al _x In_{1 − x} As barriers of the quantum well on the mobility and drift velocity of electrons in high electric fields is studied. It is shown that the electron mobility rises as the fraction x of Al in the barrier composition is decreased. The maximum mobility in the In_0.5Ga_0.5As/In_0.8Al_0.2As quantum wells exceeds the mobility in a bulk material by a factor of 3. An increase in fraction x of Al in the barrier leads to an increase in the threshold field E _th of intervalley transfer (the Gunn effect). The threshold field is E _th = 16 kV/cm in the In_0.5Ga_0.5As/Al_0.5In_0.5As heterostructures and E _th = 10 kV/cm in the In_0.2Ga_0.8As/Al_0.3Ga_0.7As heterostructures. In the heterostructures with the lowest electron mobility, E _th = 2–3 kV/cm, which is lower than E _th = 4 kV/cm in bulk InGaAs.     

Simulation of hysteresis in a metal-ferroelectric-semiconductor structure

The hysteresis in the dependence of the polarization P on the electric field E was simulated for a metal-ferroelectric-semiconductor structure with a perovskite semiconductor. The simulation is based on the analysis of an experimental P(E) hysteresis loop observed in a metal-ferroelectric-metal structure and approximated by hyperbolic tangent. Poisson’s equation is numerically integrated with consideration for the dependence of the ferroelectric permittivity on electric field. The depolarizing action of the semiconductor reduces the remanent polarization several times, with the depolarization effect more pronounced for a semiconductor with lower impurity concentration.     

Hall effect in quasi-two-dimensional superlattices in nonquantizing magnetic and strong electric fields

The transverse electric field E _y arising in quasi-two-dimensional superlattices (SLs) in a strong pulling electric field E _x and a weak magnetic field oriented in a direction perpendicular to the plane of the SL (H ∥ Z) is calculated. In the case where the electronic energy spectrum is nonadditive, the field E _y includes the Hall factor and the spontaneous transverse electric field that exists without H. The field E _y is a multivalued and sign-variable function of E _x . The asymptotically stable branches of the function E _y are determined. The (kinetic) “potential,” whose minimum corresponds to a stationary state of the nonequilibrium electron gas, is used. Fiz. Tekh. Poluprovodn. 31, 916–919 (August 1997)     

Effective photoelectric converters of ultraviolet radiation with graded-gap ZnS-based layers

The use of ultrathin (～10 nm) stable p-Cu_1.8S films as a transparent component of the p-Cu_1.8S-n-ZnS heterojunction as well as of the graded-gap layers made it possible to obtain effective photoconverters of ultraviolet radiation. The results of examination of the properties of photoactive Cu_1.8S-ZnS junctions grown on the CdS or CdSe substrates with intermediate graded-gap layers CdS-Zn _x Cd_{1 ? x} S or CdSe-(ZnS) _x (CdSe)_{1 ? x} , respectively, are presented. With the correct selection of parameters of the substrates, the graded-gap layers allows one to attain the optimal characteristics of the p—n junction, to realize high electric fields at the Cu_1.8S-ZnS contact, and to solve the problem of fabrication of the back ohmic contact to ZnS without additional doping of all components of the heterostructure with a foreign impurity. Varying the thickness of a thin ZnS layer, it is possible to control the extension of the space charge in the graded-gap layer and thereby to control the long-wavelength edge of photoconverter sensitivity.

     

平面平行真空微电子二极管中二分之三次方关系式的应用

在一定的假定条件下，考虑空间电荷影响，平面平行真空微电子（P—VMD）二极管中电流一电压近似按二分之三次方关系式工作。本文在此关系式及Forler—Nordheim场发射方程的基础上，通过解简化立方方程，进一步推导出管内的电位、电场强度、电子速度和空间电荷密度的分布函数。P-VMD二极管在保持管内结构与阴极表面电场强度不变，并工作在典型工作状态（归一化电位系数P=2／3）情况下，考虑空间电荷影响时的阳极电压、阳极电场强度和阳极电子速度分别比无空间电荷影响时增加约50．00％，73．21％和22．47％。P—VMD二极管内的空间电荷密度分布函数为正割函数，在阳极表面附近为最小，在阴极表面处为无穷大，这是由于本文假设在阴极表面处的电子初速度为零的缘故。     

Multisubband electron mobility in a parabolic quantum well structure under the influence of an applied electric field

We study the multisubband electron mobility in a barrier delta doped AlχGal-χAs parabolic quantum well structure under the influence of an applied electric field perpendicular to the interface plane. We consider the alloy fraction χ = 0.3 for barriers and vary x from 0.0 to 0.1 for the parabolic well. Electrons diffuse into the well and confine within the triangular like potentials near the interfaces due to Coulomb interaction with ionized donors. The parabolic structure potential, being opposite in nature, partly compensates the Coulomb potential. The external electric field further amends the potential structure leading to an asymmetric potential profile. Accordingly the energy levels, wave functions and occupation of subbands change. We calculate low temperature electron mobility as a function of the electric field and show that when two subbands are occupied, the mobility is mostly dominated by ionised impurity scattering mediated by intersubband effects. As the field increases transition from double subband to single subband occupancy occurs. A sudden enhancement in mobility is obtained due to curtailment of intersubband effects. Thereafter the mobility is governed by both impurity and alloy disorder scatterings. Our analysis of mobility as a function of the electric field for different structural parameters shows interesting results.     

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.     

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)     

A detailed analysis of the metal-semiconductor contact

A general quantum and electronic theory able to explain the electric and photoelectric experimental properties of the metal-semiconductor contacts is proposed. The theory consists firstly in calculating the electric space charge due to the quantum mechanical tunneling of the electrons from the metal into the semiconductor, and vice-versa, and to the metal and semiconductor bands bending. Then the electric charge so obtained is utilised to solve in an appropriate and complete way the Poisson equation so as to determine the electric field and potential as functions of the abscissa x. The electric field F(x) is employed to obtain a new expression for the junction capacitance C, holding in the general case of a non-uniform charge, whereas the electric potential ν_i(x) is used to calculate general expressions for the thermionic and photoelectric currents i and i_ph, respectively, taking into account in this both the tunneling probability through the energy barrier and the many-valley structure of the semiconductor energy bands. Finally, from ν_i(x), C, i and i_ph four new expressions of the energy barrier height of the contact are deduced. The theoretical results relative to the barrier height so determined (which hold for both n-and p-type semiconductors) are compared with published experimental values obtained, by means of capacitance and photocurrent measurements: (a) on contacts between n-type CdS and Au, Cu, Ag and Pt; (b) on contacts between n-type GaAs and Au, Ag, Cu, Sn, Al and Pt and; (c) on contacts between p-type GaAs and Au and Al. The agreement between the theoretical and experimental values is very good.     

Overcoming the Unfavorable Kinetics of Na3V2(PO4)2F3//SnPx Full‐Cell Sodium‐Ion Batteries for High Specific Energy and Energy Efficiency

In this work, a full‐cell sodium‐ion battery (SIB) with a high specific energy approaching 300 Wh kg^?1 is realized using a sodium vanadium fluorophosphate (Na₃V₂(PO₄)₂F₃, NVPF) cathode and a tin phosphide (SnP_x) anode, despite both electrode materials having greatly unbalanced specific capacities. The use of a cathode employing an areal loading more than eight times larger than that of the anode can be achieved by designing a nanostructured nanosized NVPF (n‐NVPF) cathode with well‐defined particle size, porosity, and conductivity. Furthermore, the high rate capability and high potential window of the full‐cell can be obtained by tuning the Sn/P ratio (4/3, 1/1, and 1/2) and the nanostructure of an SnP_x/carbon composite anode. As a result, the full‐cell SIBs employing the nanostructured n‐NVPF cathode and the SnP_x/carbon composite anode (Sn/P = 1/1) exhibit outstanding specific energy (≈280 Wh kg^?1_{(cathode+anode)}) and energy efficiency (≈78%); furthermore, the results are comparable to those of state‐of‐the‐art lithium‐ion batteries.     

Effect of successive implantation of Ag+(Cu+) and Xe+ ions on the recombination properties of CdxHg1−x Te crystals

The effect of successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions on the recombination properties of Cd_xHg_1−x Te (0.2<x<0.3) crystals has been investigated. It is shown that after implantation of ions of one chemical element, followed by diffusion thermal annealing at temperatures below 150–200 K, recombination through local levels lying 30±5 meV below the conduction band bottom dominates. Successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions followed by diffusion thermal annealing changes the course of the temperature dependence of the lifetime of the nonequilibrium charge carriers. It was determined that for Cd_xHg_1−x Te crystals with x⋍0.20–0.25 in the temperature interval 700–200 K the lifetime of the nonequilibrium charge carriers is low (τ<0.15 μs) and does not depend on the temperature. For Cd_xHg_1−x Te crystals with x⋍0.3 recombination of nonequilibrium charge carriers occurs through two types of levels: in the temperature range 140–200 K — deep levels E _t1⋍E _c −51 meV and at lower temperatures (77–140 K) — through shallower levels E _t2⋍E _c −(16±2) meV. Fiz. Tekh. Poluprovodn. 31, 786–789 (July 1997)