Comparative study on transport properties and scattering mechanism of group III doped SiC nanotube

The third group of elements is the acceptor of P-type silicon carbide (SiC). Therefore, studying the transport properties of group III doped SiC nanotube (SiCNTs) and revealing the carrier scattering mechanism have important scientific significance for improving the photoelectric properties and promoting the development of SiC nano-devices. In this article, the lattice structure and transport properties of IIIA-doped SiCNTs are investigated systematically using density functional theory. According to the first principle data, we calculate the temperature characteristics of the conductivity, carrier concentration, and mobility, then, analyze the contributions of optical phonon, ionized impurity, neutral impurity and inter-carrier scattering to the mobility. The calculation results show that the conductivity of IIIA-doped SiCNTs decreased with increasing temperature in the temperature range below 200 K, above 200 K, the conductivity increases with increasing temperature. The main scattering mechanisms are optical phonon scattering and neutral impurity scattering. In application, this results will help the selection of SiCNTs acceptor.     

Low-field carrier transport properties in biased bilayer graphene

Based on a semiclassical Boltzmann transport equation in random phase approximation, we develop a theoretical model to understand low-field carrier transport in biased bilayer graphene, which takes into account the charged impurity scattering, acoustic phonon scattering, and surface polar phonon scattering as three main scattering mechanisms. The surface polar optical phonon scattering of carriers in supported bilayer graphene is thoroughly studied using the Rode iteration method. By considering the metal–BLG contact resistance as the only one free fitting parameter, we find that the carrier density dependence of the calculated total conductivity agrees well with that observed in experiment under different temperatures. The conductivity results also suggest that in high carrier density range, the metal–BLG contact resistance can be a significant factor in determining the BLG conductivity at low temperature, and both acoustic phonon scattering and surface polar phonon scattering play important roles at higher temperature, especially for BLG samples with a low doping concentration, which can compete with charged impurity scattering.     

Electrical conductivity and colour centres in Tl-doped KCl crystal

The electrical conductivity and optical absorption of potassium chloride crystals doped with different concentrations of thallium have been measured before and after X-irradiation. The optical absorption spectrum exhibits the characteristic Tl⁺ ion band at 247 nm. The extrinsic conductivity and the absorption coefficient at 247 nm increase with impurity addition upto a certain concentration. Further increase in impurities decreases them. Room temperature X-irradiation decreases the Tl⁺ ion band and produces the F band. F centre concentration is higher in lightly doped crystal compared to pure or heavily doped KCl. These results have been interpreted in terms of formation of interstitial potassium ions and positive ion vacancies in the Tl-doped KCl lattice due to large ionic radius of Tl⁺ ions. The impurity ions precipitate into TlCl phase when the doping is heavy.     

电子-杂质和电子-声子相互作用对石墨烯磁光导的影响

在外加垂直磁场的石墨烯系统中,基于格林函数方法以自能的形式理论研究了电荷杂质散射和光学声子散射中心对朗道能谱的影响,采用久保(Kubo)公式研究了单层石墨烯的磁光电导谱以及跃迁选择定则。具体计算中电子-杂质库仑相互作用考虑了介电环境的屏蔽效应,对由散射引起的自能以及单粒子格林函数做自洽计算,另外在强磁场下单杂质散射是一个很好的近似模型。理论计算结果表明电荷杂质散射引起朗道能级对称展宽;同时考虑电荷杂质和光学声子两类散射后态密度表现为非对称的展宽。研究结果表明磁光电导谱的峰值和强度强烈依赖于填充因子和态密度。     

Different substitutions lead to differences in the transport and recombination properties of group V doped SiCNTs

Silicon carbide nanotubes (SiCNTs) has attractive application prospects in the field of micro-nanodevices. Based on first-principle, we find that a shallow and a deep impurity levels appearing when a group-V element replaces a C, while only one deep impurity level appears when a group-V replaces a Si. This indicates that different electronic properties will be generated when group-V replace different sites of SiCNTs. Further numerical simulation results show that when dopant replaces C, the conductivity is about an order of magnitude higher than dopant replaces Si, and the conductivity increase with increasing temperature; the non-equilibrium minority carrier lifetime decrease with increasing temperature, when group-V replace the C, they are strong n-type, when replace the Si, they are in weak n-type and strong p-type. These results will help reveal the doping mechanism of SiC nanomaterials and the selection of dopants, and provide a theoretical basis for the preparation of micro-nanodevices.     

Dynamic conductivity modified by impurity resonant states in doping three-dimensional Dirac semimetals

The impurity effect is studied in three-dimensional Dirac semimetals in the framework of a T-matrix method to consider the multiple scattering events of Dirac electrons off impurities. It has been found that a strong impurity potential can significantly restructure the energy dispersion and the density of states of Dirac electrons. An impurity-induced resonant state emerges and significantly modifies the pristine optical response. It is shown that the impurity state disturbs the common longitudinal optical conductivity by creating either an optical conductivity peak or double absorption jumps, depending on the relative position of the impurity band and the Fermi level. More importantly, these conductivity features appear in the forbidden region between the Drude and interband transition, completely or partially filling the Pauli block region of optical response. The underlying physics is that the appearance of resonance states as well as the broadening of the bands leads to a more complicated selection rule for the optical transitions, making it possible to excite new electron-hole pairs in the forbidden region. These features in optical conductivity provide valuable information to understand the impurity behaviors in 3D Dirac materials.     

Terahertz spectroscopy of AuFe spin glasses

The electrodynamic response of spin glasses (in the form of thin AuFe films) in the terahertz frequency range has been studied using backward-wave oscillator (BWO) spectroscopy (10–40 cm^?1) and optical ellipsometry (5000–33000 cm^?1) techniques at temperatures from 5 to 295 K. The room-temperature dynamic conductivity spectra of AuFe films are typical of metals and can be described within the framework of the Drude theory of conduction by free charge carriers. Changes in the microscopic parameters of charge carriers in AuFe films with increasing iron content, which are related to additional scattering of carriers on the impurity magnetic moments, have been studied on the quantitative level, including the carrier relaxation frequency and characteristic time, plasma frequency, and conductivity. It is established that the spin-glass phase at a temperature of ～5 K exhibits dispersion of the conductivity in the frequency range 10–40 cm^?1, which can be related to the appearance of a mobility gap in the subsystem of free electrons involved in the RKKY interaction between magnetic centers (Fe atoms).     

Scattering mechanisms and anomalous conductivity of heavily N-doped 3C-SiC in ultraviolet region

Using the first-principles density functional method, we investigate the band structures and conductivity spectra for N-doped 3C-SiC. It is found that conductivity peaks of heavily N-doped 3C-SiC are observed in the ultraviolet (UV), visible and infrared (IR) regions while the peaks can be only seen in the UV region for 3C-SiC. In the UV region, the conductivity peaks of 3C-SiC are obviously higher than those of N-doped 3C-SiC. According to the data of band structures, we calculate the ionized impurity scattering, inter-carrier scattering and neutral impurity scattering. The calculation results show that the scattering by incomplete ionization N to electrons and inter-carrier scattering have large effect on the conductive behavior of heavily N-doped 3C-SiC at room temperature. In the UV region, the conductivity of 3C-SiC depends on long-wavelength optical wave scattering, which has a longer relaxation time than that inter-carrier scattering and neutral scattering. This is the reason of anomalous conductivity of N-doped 3C-SiC in the UV region.     

Phonon drag conductivity in the Fröhlich-Peierls phase in quasi-one-dimensional electron phonon systems with impurities

The effect of impurities on the static conductivity in the Fröhlich-Peierls phase is discussed by using the technique of the temperature Green function. The diagrams of the phonon drag type especially are shown to give the most important contribution to the static conductivity at low temperatures. The temperature dependence of this contribution is of the activation type with an activation energy equal to the pinning frequency of the phase mode of the charge density wave. The dependence of the preexponential factor on the impurity concentrations is also discussed.On leave of absence     

Halleffekt und Leitfähigkeitsmessungen an Zinkoxid-Einkristallen mit Sauerstofflücken als Donatoren

The electrical conductivity and Hall effect of pure zinc oxide single crystals have been measured from 5 to 300 K. The temperature dependence of the conductivity and the charge carrier concentration is similar to silicon and germanium. The low temperature conduction mechanism depending on the impurity concentration is explained by means of hopping conductivity and impurity band conduction. The impurity band is supposed to be built up of overlapping wave functions of the excited donor states. The results have been discussed supposing that the donors are oxygen vacancies occupied by one or by two electrons.     

Optical and electrical properties of C60Tex films

The structure, composition, and electrical and optical properties of thin tellurium-intercalated fullerene films C₆₀Te_x are investigated. The samples of compositions from C₆₀Te_0.1 to C₆₀Te₆ are prepared by thermal evaporation. The sample composition and the impurity distribution are controlled by the Rutherford backscattering technique. The Raman vibrational spectra indicate changes in the symmetry of a C₆₀ molecule: the strain of the molecule increases with a decrease in the tellurium concentration and decreases as the tellurium impurity concentration increases. The evolution of the optical absorption spectra and the electrical conductivity suggests that intercalation of a tellurium impurity leads to modification of the electronic structure of the material. This process is accompanied by a shift and change in shape of the optical absorption edge and a change in the electrical conductivity of films by several orders of magnitude depending on the composition. The electrical conductivity is minimum at a low tellurium impurity content.     

Si含量对高锰硅化合物相组成及热电性能的影响研究

采用高频感应熔融、退火结合放电等离子烧结方法制备高锰硅(HMS)化合物MnSi1.70+x(x=0,0.05,0.1,0.15),系统研究了Si含量变化对材料相组成、微结构和热电性能的影响规律.结果表明,当x0.1时,样品由HMS和贫Si的MnSi金属相两相组成,随着Si含量x的增加,MnSi相相对含量减小;当x=0.1时,所得样品为单相HMS化合物;当x0.1时,样品由HMS和过量Si两相组成.随着x的增加,由于样品中高电导的金属相MnSi含量逐渐减少,样品的电导率逐渐下降,而Seebeck系数随之增加.室温下样品载流子浓度和有效质量随x增大逐渐减小,而迁移率逐渐增加.MnSi和Si杂相与HMS相比均为高热导相,因此当x=0.1时,由于样品为单相HMS,从而表现出最低热导率和最高ZT值.MnSi1.80样品在800K时热导率最小值达到2.25W·m-1K-1,并在850K处获得最大ZT值(0.45).     

Plateau in the temperature dependence of the thermal conductivity of solid hydrogen containing heavy isotopic neon impurities at ultimately low concentrations

The thermal conductivity of solid hydrogen with 0.0001–0.0002 at. % Ne in the form of equilibrium samples grown at a low rate after remelting is investigated in the temperature range 1.5–10.0 K. It is demonstrated that the temperature dependence of the thermal conductivity for the samples containing neon at concentrations considerably lower than the limiting solubility of the heavy impurity exhibits a symmetric plateau. This behavior of the temperature dependence of the thermal conductivity differs qualitatively from the previously observed resonance minimum in the temperature dependence of the thermal conductivity for desublimated samples. A relaxation model is proposed for explaining the observed effect. According to this model, the plateau is explained by the formation of linear impurity structures that are located along dislocation lines and considerably enhance phonon scattering by dislocation cores. The density of linear impurity structures is estimated. The influence of these structures on the thermal conductivity is compared with the corresponding effect of uniformly distributed individual neon atoms in solid hydrogen.     

Collective oscillations and interband absorption in granular deposits of lithium,prepared and studied at 6 K

Experimental results dealing with the interband optical conductivity of granular deposits of lithium at room temperature are in good agreement with most recent theories. In this paper, results obtained at 6 K are different from the theoretical predictions and may be explained on the basis of a phase change in lithium at low temperature     

Electron transport near the Mott transition in n-GaAs and n-GaN

In this paper, we study the temperature dependence of the conductivity and the Hall coefficient near the metal–insulator phase transition. A theoretical investigation is performed within the effective mass approximation. The variational method is used to calculate the eigenvalues and eigenfunctions of the impurity states. Unlike previous studies, we have included nonlinear corrections to the screened impurity potential, because the Thomas–Fermi approximation is incorrect for the insulator phase. It is also shown that near the phase transition the exchange interaction is essential. The obtained temperature dependencies explain several experimental measurements in gallium arsenide (GaAs) and gallium nitride (GaN).     

Electron-trapping centers in additively colored alkali halide crystals doped with divalent impurities

A study has been made of the temperature dependences of the dielectric loss and structure-sensitive conductivity of additively colored KCl single crystals having a Sr²⁺ impurity. The optical absorption spectra were recorded before and after heat treatment of the colored crystals. A model for Z-centers is proposed on the basis of the results.Translated from Izvestiya VUZ. Fizika, No. 10, pp. 26–29, October, 1969.In conclusion the author thank Professor E. K. Zavadovskaya for suggesting thistopic and for discussion of the results.     

Normal phase and superconducting instability in the attractive Hubbard model: a DMFT(NRG) study

We study the normal (nonsuperconducting) phase of the attractive Hubbard model within the dynamical mean field theory (DMFT) using the numerical renormalization group (NRG) as an impurity solver. A wide range of attractive potentials U is considered, from the weak-coupling limit, where superconducting instability is well described by the BCS approximation, to the strong-coupling region, where the superconducting transition is described by Bose condensation of compact Cooper pairs, which are formed at temperatures much exceeding the superconducting transition temperature. We calculate the density of states, the spectral density, and the optical conductivity in the normal phase for this wide range of U, including the disorder effects. We also present the results on superconducting instability of the normal state dependence on the attraction strength U and the degree of disorder. The disorder influence on the critical temperature T _c is rather weak, suggesting in fact the validity of Anderson’s theorem, with the account of the general widening of the conduction band due to disorder.     

氮掺入对N型纳米硅薄膜微观结构及光电特性影响

采用等离子体增强化学气相沉积技术(PECVD)通过改变NH₃流量制备出不同含氮量N型富硅氮化硅硅薄膜。利用Raman散射、红外吸收、紫外-可见光分光光度计及暗态I-V测量等技术分析了氮掺入对薄膜微观结构以及光电特性的影响。结果显示,随着NH₃的增加,薄膜由微晶硅向纳米硅结构转变,薄膜中晶粒尺寸减少,晶化度降低,微观结构有序性降低,所对应薄膜光学带隙增大,而带尾分布变窄。同时,红外吸收谱分析表明,Si—N键合密度增加,P掺杂受阻。暗态I-V测量显示,薄膜电导率随着NH₃掺入整体较微晶硅降低,但随NH₃增加,电导率受到迁移率和载流子浓度等特征共同作用先降低后变大,揭示了影响薄膜电导率的机制存在一定的竞争,然而过高的非晶网络结构将增大载流子的复合导致薄膜电导率显著降低。     

Defect centers in antimony doped NaCl crystals

The existence of both Sb²⁺ and Sb³⁺ in NaCl has been established by a series of correlated experiments like optical absorption, EPR, Electrical conductivity and dielectric loss. Sb²⁺ gives an optical band at 212 nm and in an arbitraryorientation gives an EPR signal whose direction dependence studies indicate three magnetically inequivalent sites. A vacancy occupying a second neighbour site with respect to the impurity ion satisfactorily, explains the observed spectra. The association energy of the impurity ion has been found to be 0·56 eV from ionic conductivity studies. Dielectric loss measurements give two prominent loss peaks which have been assigned to Sb²⁺ and Sb³⁺ on the basis of their temperature variation and other correlated experiments. Radiation damage reduces the Sb³⁺ content while Sb²⁺ is not affected.