On the analysis of the vibrational Boson peak and low-energy excitations in glasses

Implications of reduction procedures applied to the low energy part of the vibrational density of states in glasses and supercooled liquids are considered by advancing a detailed comparison between the excess – over the Debye limit – vibrational density of states g(ω) and the frequency-reduced representation g(ω)/ω² usually referred to as the Boson peak. Analyzing representative experimental data from inelastic neutron and Raman scattering, we show that reduction procedures distort to a great extent the otherwise symmetric excess density of states. The frequency of the maximum and the intensity of the excess experience dramatic changes; the former is reduced while the latter increases. The frequency and the intensity of the Boson peak are also sensitive to the distribution of the excess. In the light of the critical appraisal between the two forms of the density of states (i.e. the excess and the frequency-reduced one) we discuss changes of the Boson peak spectral features that are induced under the presence of external stimuli such as temperature (quenching rate, annealing), pressure, and irradiation. The majority of the Boson peak changes induced by the presence of those stimuli can be reasonably traced back to simple and expected modifications of the excess density of states and can be quite satisfactorily accounted for by the Euclidean random matrix theory. Parallels to the heat capacity ‘Boson peak’ are also briefly discussed.     

Bandwidth narrowing in n-type many-valley semiconductors

A simple method that takes into account the many-valley effects of the host conduction band is outlined for the calculation of the impurity density of states. N random impurity sites were simulated with a computer, within a diamond lattice as a host. A Kohn-Luttinger donor wave function was associated with each impurity. It was found that the low energy side (or the high energy side when the effect of non-orthogonality is taken into account) of the density of states is strongly reduced to a small tail when compared to the case of neglecting many-valley effects. At the Fermi energy, the density of states is considerably enhanced which leads to an enhancement of the extrinsic specific heat. The inverse participation ratio varies with the impurity concentration, presenting different states which are responsible for different physical properties. An analytical calculation taking into account these effects was carried out for the sake of comparison.     

Electronic structure and properties of liquid caesium up to critical point by LMTO calculations

It is well-known that liquid caesium shows some unusual properties at low densities. We used supercell technique within linear muffin-tin orbital method to investigate this phenomenon. Electronic structure of liquid caesium for different temperatures from the melting point up to critical point was obtained. The atomic structure was simulated for a cluster of 2000 atoms by the Reatto method on the base of experimental structure factors of Cs obtained by Winter and co-workers for different temperatures (from 323 K up to 1923 K). The Kubo–Greenwood formula was applied for the calculations of melts conductivity. The received results indicate that metal–nonmetal transition in liquid cesium is connected not to the gap at Fermi energy in density of electronic states, but more likely with electrons localization on some kind of atomic clusters.     

Capture of adatoms by clusters in thin film deposition

The adatom diffusion and capture by clusters are considered as a random-walk problem. The method of solution is developed. Green's function for a square substrate lattice is obtained. This permits one to give a general solution for a steady-state adatom distribution around clusters and for Halpern's arrival probability. This solution is applicable for calculating capture numbers of some small capture centres. An interpolation formula for the capture number, as dependent on the perimeter length of the capture centre, is proposed for λ a. The adatoms fluxes to the 10 and 11 steps on the surface are calculated. The fluxes are anistropic but the anisotropy disappears with λ/a → ∞. The applicability of the steady-state approach is discussed. It is shown that this approximation is valid for low adatom density. The adatom fluxes to monatomic and diatomic immobile clusters are calculated under conditions when these clusters cannot be treated as capture centres. The conservation equations for these clusters are formulated whereby two different configurations of three-atomic clusters are taken into account.     

Investigation of charge carrier lifetime in high-resistivity semiconductor layers by the method of small charge photocurrent

A method of determination of charge carrier lifetime from experimental data of small charge current transient under constant electric field strength is discussed. On the basis of experimental results on transient of small charge photocurrent of holes and electrons which were obtained at different electric field strengths, an analysis of charge carrier lifetimes in a-Se layer has been performed. For interpretation of experimental results the approximation, according to which photogenerated electrons or holes can be trapped in levels of three types, has been used. We found that the process of localized charge carrier release is influenced by the electric field strength and that in a-Se layer, in the vicinity of the substrate, there is a narrow region where the density of localized states exceeds its mean in a layer.     

Numerical modeling of thermally-stimulated currents for the density-of-states determination in thin-film semiconductors

The paper reports on thermally stimulated conductivity studies used for characterization of the density of states profile in thin film semiconductors, by numerically solving the non-linear time-dependent rate equations for free and trapped charge. We explore the derivation of energy and density scales from temperature and conductivity data. We examine the distinction between ‘strong’ and ‘weak’ re-trapping and the use of low ‘effective’ values of attempt-to-escape frequencies in establishing an energy scale, and the ad hoc inclusion of a temperature-dependent lifetime. It is confirmed for several illustrative model systems that the technique can afford surprisingly good fidelity in recovery of the density of states under a range of conditions.     

Electron density distribution in crystals with potassium perchlorate structure

Within the theory of local electron density functional, the difference densities in crystals of perchlorates, perbromates, and sulfates of metals with potassium perchlorate structure are calculated by the sublattice method. It is shown that the difference-density distribution qualitatively coincides with the experimental deformation-density distribution. Oxygen atoms in nonequivalent crystallographic positions exhibit a different character of electron charge redistribution, which results in their different charge states and different forces of chemical bonding in molecular anions. The mechanisms of charge redistribution in sulfates differ qualitatively from those for perchlorates.     

Mg掺杂对CaMnO3基热电氧化物电子结构及电性能的影响研究

采用超软赝势密度泛函理论计算的方法研究了Mg掺杂Ca位CaMnO3基复合氧化物的能带结构、电子态密度和电荷分布状况,在此基础上分析了Mg掺杂氧化物的电性能.结果表明,Mg掺杂CaMnO3氧化物仍然呈间接带隙型能带结构,带隙宽度由0.756eV减小到0.734eV.CaMnO3氧化物和Mg掺杂CaMnO3氧化物的自旋态密度曲线极值点均位于为-0.8eV附近.Mg掺杂CaMnO3氧化物中Mn原子对体系费米面态密度的贡献有所减小,O原子和Ca原子对体系费米面态密度的贡献有所增大.Mg原子比Ca原子具有更强的释放电子的能力,Mg掺杂对于CaMnO3氧化物属于电子型掺杂.Mg掺杂CaMnO3氧化物导电性能增强,电性能提高.     

Numerical simulation of a low- and a high-electric-field photocurrent decay in a-Si:H

We report a numerical simulation of the photocurrent decay (PCD), from the steady state, in two different structure configurations based on the a-Si:H. The standard DOS of the a-Si:H is used. The high-electric-field PCD is considered in a structure configuration based on a metal/a-Si:H junction. Poisson’s and the two continuity equations are numerically solved in a one-dimensional space to calculate the current density. Two different boundary conditions of the a-Si:H film are considered. The low electric field PCD, which may occurs in a coplanar configuration, is calculated from the solution of a system of two non linear coupled rate equations which govern the free carriers concentrations and the different charges on the localized states in the gap. The calculated PCD versus time curves, for the two configurations, show a shoulder around 1 μs which separate two main regions. We can see that the initial current decay is dominated by the electron emission from the conduction-band tail and the recombination via the dangling bonds states. The second current decay is mainly due to the electrons emission from the dangling bonds. We show also that the PCD curve tends towards the PCD of the coplanar configuration when the electric field decreases.     

立方晶系Ca2Si声子色散和热力学性质的第一性原理计算

采用模守恒贋势和超软贋势两种方法分别计算立方晶系Ca2Si的声子色散关系、声子态密度和热力学特性.运用线性响应方法和有限位移方法确定立方晶系Ca2Si的声子色散关系和声子态密度,得到立方晶系Ca2Si的声子振动模式是各向异性,且立方晶系的Ca2Si是不稳定结构.在声子色散关系确定的基础上分别计算定容热容、焓、熵和自由能随温度的变化关系,得到Cv-T曲线在低温条件下符合德拜第三定律;当T=1000K时,立方晶系Ca2Si的热熔Cr=21J·mol-1 ·K-1;在高温条件下,随着温度的增加Cv→3R.通过分析声子色散、振动关系和热力学特性得到立方晶系Ca2Si的振动系统由声子和电子组成.     

Local microstructure of silica glass

We present a microstructure study of different amorphous states of SiO₂, which are constructed by compress–decompress procedure. The local microstructure is analyzed through the coordination number, bond-angle statistic and the characteristic of void. The simulation reveals that the number of SiO₄, SiO₅ and SiO₆ units varies in different silica states, but their topology is identical. The densest model (highest density) is obtained upon compress pressure around 20 GPa. The change in void space for different silica states is also calculated and discussed here.     

Electron Transport in Configurational Model of Ferromagnetic Fulleride with Triple Orbital Degeneracy

Abstract

Phenomenon of ferromagnetic ordering was for a long time associated exclusively with transition metal and rare-earth compounds. Nowadays this view is challenged by growing evidence that in molecular carbon-based systems the ferromagnetic alignment of spins can be observed as well. We have developed a microscopical model of a fulleride electronic subsystem taking into account triple orbital degeneracy of energy states within the configurational-operator approach. Using the Green function method the energy spectrum of the model has been calculated. Conditions for the ferromagnetic state stabilization have been determined. Static electrical conductivity and effective masses of current carriers in the system with orbitally degenerated energy band have been obtained. In the ground state and for low temperatures at different forms of unperturbed density of electronic states the concentration dependences of transport characteristics for less-then-half-filled lower quasiparticle subband have been calculated.     

Determination of the defect density in thin film amorphous and microcrystalline silicon from ESR measurements: The influence of the sample preparation procedure

Accurate evaluation of the defect density (N_D) is of high relevance for the optimization of thin film silicon. The spin density (N_S) measured in ESR experiments is often used as a measure for the density of deep defects in the material, assuming that all defects are in a paramagnetic charge state. However, exposure to air, water, or acid during ESR sample preparation can potentially change the N_S in a sample and lead to misinterpretation of N_D. We have investigated how the preparation procedures of a Si thin film ESR sample may affect the properties of its ESR spectrum. Samples of different structural composition from highly crystalline μc-Si:H to a-Si:H deposited by PECVD on Mo-foil, Al-foil and ZnO:Al were studied for different states of exposure to ambient conditions and annealing. N_S measured directly after sample preparation and after air exposure was found to be higher than N_S measured in the annealed state. Particularly in highly crystalline material this discrepancy may reach one order of magnitude. On the other hand in a-Si:H and medium crystalline μc-Si:H relevant for applications, the difference in N_S between air-exposed and annealed conditions is smaller. ESR measurements performed at 40 K suggest that atmospheric exposure leads to charging of the defect states, which in turn influences the evaluated spin density.     

The nature of the defect states above midgap and their role in the light-induced metastability of a-Si:H

Defect states of a-Si:H above midgap, with which the electrons interact through trapping and thermal release and make an important contribution to the imaginary (Y) term of the modulated photocurrent, are determined. Analysis of the ‘transition’ region between the low- and high-frequency regimes enables the densities and capture coefficients of defect states to be probed. A reasonable fit to the data is obtained by assuming three defect states, and it is suggested that the defects with the higher capture probability are hydrogen-related centers that are removed during the initial stage of light degradation.     

Deep hole trap level of nitrogen-doped ZnSe grown by metalorganic vapor phase epitaxy

We investigated the hole trap level of nitrogen-doped ZnSe grown by a metalorganic vapor phase epitaxy method. The deep level transient spectroscopy (DLTS) signal and the C-V profile were measured to obtain the trap level, the capture cross section and the trap concentration. A deep hole trap level of about 1 eV from a valence band (labeled TLIS) was extracted from the tail of the DLTS peak. The distributions of the capture cross section and the trap level of the samples for different growth conditions were drawn in a figure to investigate the validity of the resolution of the DLTS signal. The origin of TLIS is thought to be in relation to the ionized acceptor or the charged acceptor-like localized defects.     

Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series

Experimental results for the temperature dependence of the density of propyl – cyanobiphenyl (3CB), butyl – cyanobiphenyl (4CB) and hexyl – cyanobiphenyl (6CB) are presented. The results are compared with previous results for temperature dependence of other members of the alkyl cyanobiphenyl series (nCB). The deviation of the density from linear temperature dependence is discussed in terms of the recently discovered corresponding rule for nematic liquid crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Light-induced annealing of hole trap states: A new aspect of light-induced changes in hydrogenated amorphous silicon

Details of light-induced annealing of hole trap state in undoped hydrogenated amorphous silicon (a-Si:H) have been studied; it has been found that prolonged illumination significantly reduces the density of hole trap states in the energy range deeper than 0.5 eV, and subsequent thermal annealing increases the density of hole trap states and restored the sample to the initial state before the illumination. We can speculate, from the experimental results and discussion in this work, that defect conversion processes are taking place during the long exposure to light; Si dangling bonds are generated from the precursors or latent sites which manifested as hole trap states located between 0.5 and 0.7 eV from the top of the valence band.     

Au掺杂浓度对Si纳米线电子结构和磁性的影响

利用基于密度泛函理论的第一性原理,对不同直径和浓度Au掺杂Si纳米线的态密度和磁性进行了计算。结果表明:杂质Au的形成能随Si纳米线直径的增大和掺杂浓度的降低而下降,这说明直径越小的Si纳米线掺杂越困难,杂质浓度越高的Si纳米线越不稳定。其磁性是由于Au d轨道和Si p轨道的p-d耦合而产生的;改变Au的掺杂浓度可改变Si纳米线的带隙,也改变了其磁矩的大小。     

Structure simulation and study of electronic and dielectric properties of two derivatives of benzamide

First-principles calculations based on density functional theory have been done on the chlorinated phenyl benzamides; 4-chloro-phenyl-benzamide – C13H10ClNO, and 2-chloro-3-chloro-phenyl-benzamide – C13H9Cl2NO. The triclinic structure of 4-chloro-phenyl-benzamide and orthorhombic structure of 2-chloro-3-chloro-phenyl-benzamide have been simulated and the structural parameters have been found out. Electron density of states has been computed in the materials using the electronic structure calculation code of Quantum-Espresso which gives a band gap of 0.74 eV in case of 4-chloro-phenyl-benzamide. This value is close to the value exhibited by semiconducting materials and photonic band gap materials. Band gap in case of 2-chloro-3-chloro-phenyl-benzamide comes out to be 3.08 eV. This value is in the range exhibited by NLO materials. Dielectric constant of the materials has been computed. The value of dielectric constant in 4-chloro-phenyl-benzamide comes out to be 2.78, 3.14, and 3.92 along X, Y, and Z axes, respectively, and the average value comes out to be 3.28. The value of dielectric constant in case of 2-chloro-3-chloro-phenyl-benzamide comes out to be 1.39, 1.53, and 1.34 along X, Y, and Z axes, respectively, and the average value comes out to be 1.42. Thus, additional chlorination is increasing the band gap and decreasing the dielectric constant in phenyl benzamide.     

The validity of the double layer model on the amorphous semiconductor surface including the bulk localized state within the gap

Under the two assumptions that the origin of surface states may be different from that of bulk localized states within the gap and the density of surface states is sufficiently high, the validity of the double layer model on the amorphous semiconductor surface is investigated in comparison with the case of a crystal. It is suggested that the criteria concerning the double layer should be determined by the relative value of the surface states to that of bulk localized states. The existence of the double layer can be confirmed when the bulk localized state density n(E_f) is smaller than 10¹⁹ cm^?3 eV^?1. When n(E_f) is high at about 10²⁰ cm^?3 eV^?1, surface states cannot be distinguished from the localized states within the gap. This double layer model is strongly supported by the results of previous experiments by others who have measured the dependence of the Schottky barrier height on the work function of metal and the dependence of the surface potential on the preparation conditions of a-SiH.