硅中钛杂质对激光电场分布影响的数值模拟

采用T-matrix数值方法模拟光学系统常用的光学材料硅中金属钛杂质对激光电场分布的影响。分析了不同球形杂质数目、不同杂质球半径、不同球形杂质间距、不同球形杂质排列以及不同相对介电常数杂质对激光电场分布的影响,通过数值模拟了解到:电场幅值的最大值总是出现在沿入射波极化方向上,电场幅值的增大与相邻两球的耦合有关,且两球间距越小电场幅值的最大值越大;相对介电常数实部越小、虚部越大的杂质会导致较大的电场幅值最大值。     

A lattice model of thermopower in hopping conduction: Application to neutron-doped crystalline germanium

A model for describing hopping thermopower is proposed. Within the model, the majority and compensating impurities form a simple cubic lattice in a crystal matrix. The thermopower associated with hole (electron) hopping over hydrogen-like impurities is calculated with inclusion of their excited states. The results of calculations are compared with available experimental data on the low-temperature thermopower of Ge: Ga and the specific heat of Si: P in the dielectric region of the insulator-metal transition.     

A threshold voltage model MOSFETs considering for high-k gate-dielectric fringing-field effect

In this paper, a threshold voltage model for high-k gate-dielectric metal-oxide-semiconductor field-effect transistors （MOSFETs） is developed, with more accurate boundary conditions of the gate dielectric derived through a conformal mapping transformation method to consider the fringing-field effects including the influences of high-k gate-dielectric and sidewall spacer. Comparing with similar models, the proposed model can be applied to general situations where the gate dielectric and sidewall spacer can have different dielectric constants. The influences of sidewall spacer and high-k gate dielectric on fringing field distribution of the gate dielectric and thus threshold voltage behaviours of a MOSFET are discussed in detail.     

The effects of core structure on radiative and non-radiative recombinations at metal ion substituents in semiconductors and phosphors

Carrier binding and recombination processes at transition metal (TM) and post-transition metal impurities in solids are reviewed. The presence of low-lying empty core orbitals in these impurities introduces novel binding and recombination phenomena and leads naturally to a classification of impurity centres as ‘simple’ or ‘structured’. ‘Simple’ impurities, generally the main group elements, introduce only effective-mass-like states into the forbidden gap of the host lattice, whereas ‘structured’ impurities show localized atomic-like transitions below the lattice absorption edge. The fact that donor or acceptor centres generated by TM ions in semiconductors are usually deep is discussed with reference to a simple oxidationstate correlation diagram based on the variable chemical valency characteristic of these ions. Many metal ion impurities in solids generate iso-electronic centres however, and evidence is assembled to show how the normally accepted range of iso-electronic centres can be extended to include such impurities. It is emphasized that the formation of bound states at these ‘structured’ cationic iso-electronic centres is fairly common, in contrast to the situation normally found for anionic substituents in semiconductors. A possible explanation for this general property of structured iso-electronic centres is given in terms of the low-lying core levels which they may introduce in the band gap and their enhanced polarizability. Simple arguments then suggest that those centres showing electron ionization thresholds or atomic inter-configurational transitions just below E _g should be hole-attractive, whereas those showing charge transfer transitions should be electron-attractive. It is shown that two kinds of defect Auger recombination (DAR) may be significant for excitons localized at structured impurities. The first involves hole ionization at a deep neutral acceptor, and the conditions which must be met to make this process significant at room temperature are reviewed. The second DAR process involves the transfer of carrier recombination energy to the core electrons of the structured impurity. A simple model developed for this energy transfer process predicts that the coupling between exciton and core states will be larger if the impurity shows strong, broad dipole-allowed transitions quasi-resonant with the lattice absorption edge. Throughout the paper we stress the importance of recombination processes at structured impurities to the problems of phosphor activation and of shunt recombination mechanisms in semiconductor LEDs. In the final sections this general framework of ideas is used to explain the relative cathodoluminescence efficiencies of different rare-earth activators in crystals of Y₃Al₅O₁₂, with good agreement between the theoretical predictions and the experimental observations. Possible evidence for the occurrence of bound exciton states at structured TM impurities is found in the appearance of anomalously weak zero-phonon lines in the near-edge luminescence of GaP crystals prepared under special conditions.     

Effect of organic dyes on the dielectric properties of KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystals

The effect of organic dyes on the dielectric properties of KH₂PO₄ (KDP) crystals is studied over a wide range of temperatures. The dielectric properties of KDP crystals doped with molecules of the Chicago Sky Blue and Amaranth organic dyes are investigated for the first time. The dye molecules can be incorporated into the crystal lattice of KDP and selectively paint the pyramidal growth sectors of the crystal. The influence of dye organic impurities on the domain contribution to the permittivity is analyzed with due regard for the sectoral crystal structure. It is demonstrated that, upon doping of KDP crystals with organic dyes, the blocking effect of background impurities on domain walls is substantially weakened in the prismatic growth sector of the crystal in the polar phase. This leads to a noticeable change in the dielectric properties, specifically to an increase in the domain contribution to the permittivity of the crystal.     

Quenching of self-diffusion by impurities in plastic crystals as a tentative explanation of experimental discrepancies

A model has been developed which can explain observed discrepancies in the determination by different techniques of the activation energy for self-diffusion in plastic crystals. In this model, impurities play an essential role. The basic hypothesis is that an impurity stabilizes the crystal lattice by reducing the self-diffusion around it, to a range of several molecular distances. This hypothesis allows a semi-quantitative fit of published experimental N.M.R. data on perfluorocyclohexane, and the parameters of the model are found to be consistent with previous work on cyclohexane. Moreover, as observed experimentally, it leads to the conclusion that activation energies deduced from N.M.R. linewidth measurements are in general smaller than those deduced from radiotracer data. It is concluded that for infinitely pure samples, all techniques should lead to convergent results for self-diffusion activation energies.     

The dielectric response of quantum paraelectrics containing dipole impurities

The presence of even a very small concentration of slowly reoriented dipole impurities in a quantum paraelectric leads to strong changes in the dielectric response of the sample. This is manifested by the appearance of a giant dielectric susceptibility, colossal frequency dispersion, and several peaks in the temperature dependence of the susceptibility; in addition, the temperature dependence of the relaxation time deviates from exponential. The experimental results are presented for KTaO₃ samples slightly or moderately doped with lithium. General relationships between the dielectric response and the concentration of impurities, the temperature, and the frequency are established. A theory is proposed which qualitatively explains the observed complex phenomena and sometimes quantitatively describes the results with high precision.     

Enhancement of carrier mobility in semiconductor nanostructures by dielectric engineering

We propose a technique for achieving large improvements in carrier mobilities in 2- and 1-dimensional semiconductor nanostructures by modifying their dielectric environments. We show that by coating the nanostructures with high-kappa dielectrics, scattering from Coulombic impurities can be strongly damped. Though screening is also weakened, the damping of Coulombic scattering is much larger, and the resulting improvement in mobilities of carriers can be as much as an order of magnitude for thin 2D semiconductor membranes, and more for semiconductor nanowires.     

Kinetics of mechanical impurities electroseparation from dielectric liquids

This paper deals with the problem of dielectric liquids purification. The separation process of mechanical impurities from dielectric liquids in an electric field is analyzed and an experimentally validated mathematical model is proposed for the electroseparation process. The main objective is to find a relationship to describe the kinetics of particles electroseparation, solving the problem of establishing a relationship of dependency between initial concentration and final concentration with time, and determining the electroseparation time depending on particles and liquid dielectric properties.     

Microwaves properties of MgO single crystals computed from the IR, and measured with a resonator technique

The dielectric properties of MgO single crystals are carefully measured in the microwaves and compared with the predictions given by a dynamical model made of 4 Lorentz oscillators with parameters fitting quite well the IR spectra. The microwave losses can be made very low (e.g. 4 × 10^–7 for T=80 K and f=8 GHz), with the same order of magnitude expected from the model. It is a hint that Debye relaxation of small amounts of impurities (e.g. Mn which is sometimes detected by ESR analysis), bring a negligible contribution, and MgO crystal plates are indeed a good support to grow High Tc superconducting films.     

Defects in semiconductor nanostructures

Impurities play a pivotal role in semiconductors. One part in a million of phosphorous in silicon alters the conductivity of the latter by several orders of magnitude. Indeed, the information age is possible only because of the unique role of shallow impurities in semiconductors. Although work in semiconductor nanostructures (SN) has been in progress for the past two decades, the role of impurities in them has been only sketchily studied. We outline theoretical approaches to the electronic structure of shallow impurities in SN and discuss their limitations. We find that shallow levels undergo a SHADES (SHAllow-DEep-Shallow) transition as the SN size is decreased. This occurs because of the combined effect of quantum confinement and reduced dielectric constant in SN. Level splitting is pronounced and this can perhaps be probed by ESR and ENDOR techniques. Finally, we suggest that a perusal of literature on (semiconductor) cluster calculations carried out 30 years ago would be useful.      

Self-consistent theory of screening in a two dimensional electron gas under strong magnetic field

The dielectric response of a two dimensional electron gas under a strong transverse magnetic field is obtained within a self-consistent procedure in which the broadening of Landau levels due to collisional damping from the impurities both determines and is determined by the static dielectric function of the system. The dielectric function is evaluated in random phase approximation with the impurity scattering treated in a self-consistent Born approximation. Explicit results are presented for the zero temperature, extreme quantum limit. It is found that this “no parameter” theory is in good qualitative agreement with experimental results for the broadening as extracted from magnetotransport data.     

杂质对K2ZnCl4晶体无公度结构相变的影响

对K₂ZnCl₄铁电晶体在无公度-公度转变的相变点附近的孤子行为进行了研究。发现在相变点附近杂质使介电函数明显偏离居里-外斯定律,并导致介电峰值的明显下降;同时还发现杂质对升温相变点T_c^h较之对降温相变点T_c^c有更大的影响,这与Rb₂ZnCl₄等晶体中的情况恰好相反。采用朗道自由能理论可较好地描述杂质在相变点附近对介电函数的影响。分 关键词：     

Influence of kondo effect on the specific heat jump of anisotropic superconductors

A calculation for the specific heat jump of an anisotropic superconductor with Kondo impurities is presented. The impurities are treated within the Matsuura - Ichinose - Nagaoka framework and the anisotropy effect is described by the factorizable model of Markowitz and Kadanoff. We give explicit expressions for the change in specific heat jump due to anisotropy and impurities which can be tested experimentally.     

Influence of Gap Spacing between Dielectric Barriers in Atmospheric Pressure Discharges

The breakdown activity in helium atmospheric pressure dielectric barrier discharge (DBD) plasma is strongly modified by introducing small impurities (nitrogen (N₂) and air in ppm), although its precise implications for the behavior of discharge plasma is not evident under several constraints. In this simulation study, we investigate the influence of gap spacing between the dielectric barriers to explore the dynamic modification in the structure of discharge plasma in distinct phases of the discharge current pulse using a two‐dimensional fluid model in He‐air gas mixture. Specifically, the impact of nitrogen and air impurities is contrasted by exploring the spatial distributions of electrons in the breakdown phase under similar operating conditions. The filamentary mode of DBD plasma in He‐N₂ is transformed into uniform glow discharge in He‐air gas mixture by the dominant effect of Penning ionization. Finally, the outcomes of two‐dimensional fluid model are validated by comparing with three‐dimensional fluid model to provide the reliability of numerical simulations. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spin Hall effect induced by resonant scattering on impurities in metals

The spin Hall effect is a promising way for transforming charge currents into spin currents in spintronic devices. Large values of the spin Hall angle, the characteristic parameter of the yield of this transformation, have been recently found in noble metals doped with nonmagnetic impurities. We show that this can be explained by resonant scattering off impurity states split by the spin-orbit interaction. By using as an example copper doped with 5d impurities we describe the general conditions and provide a guide for experimentalists for obtaining the largest effects.     

Effects of doping and free carriers on the refractive index of direct-gap semiconductors

A model for the doping and current dependence of the refractive index of direct-gap semiconductors has been developed and applied to GaAs in order to explain certain properties of single heterostructure injection lasers. The model involves the concept of an effective energy gap which takes account of the effects of doping and free carriers via the Burstein shift, the exchange interaction, and the average screened Coulomb potential of the impurities. This effective gap, together with empirical results for the energy dependence of the refractive index, facilitates the calculation of refractive index changes with doping and current. Numerical results are given for GaAs, and the model is applied particularly to the substrate and active regions of single heterostructure injection lasers. In these devices the refractive index values are particularly important in determining the dielectric waveguide properties. With the aid of our model, this feature can be discussed in detail, with special reference to the breakdown of the waveguide effect which occurs (a) for substrate doping levels below a critical value, and (b) for currents well above threshold ('saturation').     

Site-dependent donor and acceptor levels in 6 H-SiC

The photoluminescence of N- and Al- or Ga-doped 6 H-SiC has been studied. Radiative transitions between free electrons and holes bound to acceptors in cubic-like and hexagonal-like sites are assigned. Donor-acceptor pair transitions between donor and acceptor impurities in cubic-like and hexagonal-like sites are analyzed. From studies on these transitions, the ionization energies of impurities in cubic-like and hexagonal-like sites in 6 H-SiC are estimated to be 248.5 and 239 meV for Al acceptors, 333 and 317 meV for Ga acceptors, and 155 and 100 meV for N donors, respectively. The ratios of the ionization energies of acceptors in the cubic-like sites to the hexagonal-like sites are almost constant, ≈1.04-1.05. The origin of the difference of the ionization energies of impurities in the different sites is discussed on the basis of the quantum defect model. For acceptors this is mainly caused by differences in the local dielectric constants.     

A physical scattering model of repeat-pass InSAR correlation for vegetation

A physical scattering model of repeat-pass InSAR correlation over forested areas is derived by accounting for the changes in the dielectric properties and positions of the scatterers in the scene between overpasses. This derivation is based on the discrete representation of a sparse random medium (such as forest canopy) along with the solution to the Foldy–Lax multiple scattering equations. In addition to taking into account the random motion of scatterers, which has been investigated in previous work, the derived repeat-pass InSAR correlation model in this paper includes the effects of moisture-induced dielectric fluctuations. This is accomplished by incorporating a separate correlation profile that takes into account these fluctuations into the framework. Once constructed, the mathematical formulation of this scattering model is cast into a modified version of the Random Volume over Ground model such that it can separately take into account dielectric fluctuations in the ground and volume components. This model is then validated using a modified version of ESA’s PolSARproSim simulation to show that similar results can be obtained from both the simulation and the theoretical model.     

Structural and radiation color centers and the dielectric properties of doped yttrium aluminum garnet crystals

The results of experimental and theoretical studies of the spectral and dielectric properties of garnet Y₃Al₅O₁₂ crystals are analyzed. The crystals studied were grown under different conditions and were either pure or contained iron-group ion impurities (Ti, Cr, Fe). The crystals were studied before and after irradiation by γ rays and high-energy electrons. A model is proposed for the formation of color centers and their influence on luminescent and other optical and dielectric properties.