磷离子注入纳米金刚石薄膜的n型导电性能与微结构研究

系统研究了磷离子注入并在不同温度退火后的纳米金刚石薄膜的微结构和电学性能.研究表明,当退火温度达到800 ℃以上时,薄膜呈良好的n型电导.Raman光谱和电子顺磁共振谱的结果表明,薄膜中金刚石相含量越高和完整性越好,薄膜电阻率越低. 这说明纳米金刚石晶粒为薄膜提供了电导.1000 ℃退火后,薄膜晶界中的非晶石墨相有序度提高,碳悬键数量降低,薄膜电阻率升高.薄膜导电机理为磷离子注入的纳米金刚石晶粒提供了n型电导,非晶碳晶界为其电导提供了传输路径. 关键词： 纳米金刚石薄膜 n型 磷离子注入     

Study of electron-beam propagation through preionized dense foam plasmas

The transport of an intense electron-beam produced by the Vulcan petawatt laser through dense plasmas has been studied by imaging with high resolution the optical emission due to electron transit through the rear side of coated foam targets. It is observed that the MeV-electron beam undergoes strong filamentation and the filaments organize themselves in a ringlike structure. This behavior has been modeled using particle-in-cell simulations of the laser-plasma interaction as well as of the transport of the electron beam through the preionized plasma. In the simulations the filamentary structures are reproduced and attributed to the Weibel instability.     

Transverse modulation of an electron beam generated in self-modulated laser wakefield accelerator experiments

Low energy electron beams (E approximately 300 keV) generated in a self-modulated laser wakefield accelerator experiment were observed to filament and be deflected away from the laser axis forming radial jets in the electron beam profile. At higher energies (E>900 keV), the filamentation and jets were suppressed and smooth electron beams copropagating with the laser were observed. The observed electron beam filamentation likely results from laser beam filamentation in the plasma due to relativistic self-focusing effects. The radial jets of low energy electrons are likely caused by transverse ejection of the electrons due to the radial structure of the wakefield and space charge deflection of electrons as they exit the laser focus.     

Interplay of collisions and temperature on the filamentary structures of a relativistic electron beam in plasmas

A systematic study on the Weibel instability induced filamentation of a relativistic electron beam is made by including the effects of collisions and temperature. The stabilization and de-stabilization of the beam filamentation due to beam temperature and plasma collisions, respectively, is explored through an analytical model and PIC simulations. The de-stabilization of the beam filamentation is attributed to the collision driven negative energy wave generation in the beam plasma system.     

Early out-of-equilibrium beam-plasma evolution

We solve analytically the out-of-equilibrium initial stage that follows the injection of a radially finite electron beam into a plasma at rest and test it against particle-in-cell simulations. For initial large beam edge gradients and not too large beam radius, compared to the electron skin depth, the electron beam is shown to evolve into a ring structure. For low enough transverse temperatures, the filamentation instability eventually proceeds and saturates when transverse isotropy is reached. The analysis accounts for the variety of very recent experimental beam transverse observations.     

Filamentation of a rotating electron beam in a magnetized plasma

Filamentation of a nonrelativistic rotating electron beam in a magnetized plasma is investigated by solving the kinetic equation and finding its dielectric permitivity. The period and the establishment time of the filamentation structure and threshold for instability development are obtained. It will be shown that only when the external magnetic field strength becomes smaller than a characteristic value, filamentation appears.     

Characterization of the initial filamentation of a relativistic electron beam passing through a plasma

The linear instability that induces a relativistic electron beam passing through a plasma with return current to filament transversely is often related to some filamentation mode with the wave vector normal to the beam or confused with Weibel modes. We show that these modes may not be relevant in this matter and identify the most unstable mode on the two-stream or filamentation branch as the main trigger for filamentation. This sets both the characteristic transverse and longitudinal filamentation scales in the nonresistive initial stage.     

Highly concentrated Si1-xCx alloy with an ordered superstructure

An ordered Si-C alloy was observed in the products resulting from thermal reduction of molybdenum disilicide heating rods. High-resolution transmission electron microscopy measurements indicate that the Si1-xCx alloy with x approximately 25% is pure, single crystalline, and possesses a superlattice structure. The superlattice periodicity occurs along the diamond [001] direction and corresponds to the quintupling of the primary (002) periodicity. The possible growth mechanism is discussed. The Mo element is likely to play a crucial role in the growth process of the Si1-xCx alloy, since it can both reduce the energy required for breaking up C clusters and contribute to surface modification, which are of great benefit to enhance the carbon concentration and induce an ordered structure.     

黑腔中空间束匀滑光束成丝不稳定性的数值模拟北大核心CSCD

为深入研究黑腔内激光成丝不稳定性产生和发展,建立了空间束匀滑光束的传播模型,利用三维激光等离子体相互作用程序LAP3D模拟了黑腔物理条件下的大尺度成丝现象。模拟结果可正确描述成丝后光强的束发散现象及谱空间光强分布特征。从激光能量参数角度分析了导致成丝不稳定性发展的物理因素,并讨论了应用空间束匀滑技术抑制成丝不稳定性的局限性。研究表明:空间束匀滑光束发展成丝的能力取决于光斑内小焦斑的成丝表现,当具备成丝能力的小焦斑份额达到一定比例后,就会导致整个空间束匀滑光束成丝,因此对于光斑平均功率密度较高的光束,须考虑结合其他束匀滑手段来抑制成丝发展。     

Current filamentation instability in laser wakefield accelerators

Experiments using an electron beam produced by laser-wakefield acceleration have shown that varying the overall beam-plasma interaction length results in current filamentation at lengths that exceed the laser depletion length in the plasma. Three-dimensional simulations show this to be a combination of hosing, beam erosion, and filamentation of the decelerated beam. This work suggests the ability to perform scaled experiments of astrophysical instabilities. Additionally, understanding the processes involved with electron beam propagation is essential to the development of wakefield accelerator applications.     

Self-consistent-field CNDO/BW cluster model of covalent solids

We have suggested a parametrization scheme for the self -consistent CNDO/BW method suitable to model the electronic structure of perfect and locally perturbed covalent solids. By working on small clusters we have carried out calculations for diamond perfect crystal and for nitrogen replacing a carbon atom in the lattice. The results show that the methos is accurate to calculate one electron energy-eigenvalues and total energies.     

Dynamic control over mega-ampere electron currents in metals using ionization-driven resistive magnetic fields

The possibility of dynamically shaping mega-ampere electron currents generated in solids by ultraintense laser pulses in various conductor materials has been investigated. By tuning the target ionization dynamics, which depends both on the target material properties and on the input electron beam characteristics, we can control the growth of resistive magnetic fields that feedback on the current transport. As a result, collimation, hollowing, or filamentation of the electron beam can all be obtained. These results are beneficial for applications such as the production of secondary particles and radiation sources and fast ignition of inertial confinement fusion.     

Ground-state properties of boron-doped diamond

Boron-doped diamond undergoes an insulator-metal or even a superconducting transition at some critical value of the dopant concentration. We study the equilibrium lattice parameter and bulk modulus of boron-doped diamond experimentally and in the framework of the density functional method for different levels of boron doping. We theoretically consider the possibility for the boron atoms to occupy both substitutional and interstitial positions and investigate their influence on the electronic structure of the material. The data suggest that boron softens the lattice, but softening due to substitutions of carbon with boron is much weaker than due to incorporation of boron into interstitial positions. Theoretical results obtained for substitution of carbon are in very good agreement with our experiment. We present a concentration dependence of the lattice parameter in boron-doped diamond, which can be used for to identify the levels of boron doping in future experiments. The text was submitted by the authors in English.     

Structural transformations in carbon nanoparticles induced by electron irradiation

This paper reviews the electron-irradiation effects in graphitic nanoparticles. Irradiation-induced atomic displacements cause structural defects in graphite lattice forming the basis of carbon nanoparticles such as nanotubes or carbon onions. Defects of the type of non-six-membered rings induce topological alterations of graphene layers. The generation of curvature under electron irradiation leads to the formation of new structures, such as spherical carbon onions or coalescent nanotubes. At high temperatures, the self-compression of carbon onions can promote the nucleation of diamond cores or phase transformations of foreign materials that are encapsulated by onionlike graphitic shells. Under the nonequilibrium conditions of intense irradiation, the phase equilibrium between graphite and diamond can be reversed. It is shown that graphite can be transformed into diamond even if no external pressure is applied. All electron-irradiation and electron-microscopic studies described here were carried out using in situ transmission electron microscopy.     

质量分离低能离子束沉积碳膜及离子轰击效应

用质量分离的低能离子束沉积技术得到了非晶碳薄膜,X射线衍射、Raman谱以及俄歇深度谱的线形表明,此种非晶碳膜中镶嵌着金刚石颗粒.碳离子的浅注入是该碳膜SP³形成的主要机理.从一个侧面说明了化学气相沉积法中偏压预处理增加金刚石成核的主要原因是因为离子轰击效应. 关键词： 非晶碳 离子轰击 质量分离低能离子束     

Electrical and piezoresistive properties of ion beam deposited DLC films

In present study diamond like carbon (DLC) films were deposited by closed drift ion source from the acetylene gas. The electrical and piezoresistive properties of ion beam synthesized DLC films were investigated. Diode-like current–voltage characteristics were observed both for DLC/nSi and DLC/pSi heterostructures. This fact was explained by high density of the irradiation-induced defects at the DLC/Si interface. Ohmic conductivity was observed for DLC/nSi heterostructure and metal/DLC/metal structure at low electric fields. At higher electric fields forward current transport was explained by Schottky emission and Poole–Frenkel emission for the DLC/nSi heterostructures and by Schottky emission and/or space charge limited currents for the DLC/pSi heterostructures. Strong dependence of the diamond like carbon film resistivity on temperature has been observed. Variable range hopping current transport mechanism at low electric field was revealed. Diamond like carbon piezoresistive elements with a gauge factor in 12–19 range were fabricated.     

Analysis of polished polycrystalline diamond using dual beam focused ion beam microscopy

This paper presents a study on polycrystalline diamond (PCD) polished by dynamic friction polishing (DFP) with the aid of advanced dual beam FIB (focused ion beam) microscopy. After disclosing a variety of wear tracks by DFP using electron imaging in combination with the ion channelling effect, a dual beam FIB was successfully employed at wear track sites to specifically create both the large cross-sectional specimen for microanalysis and thin foil for nanoanalysis. The study concluded that the polished PCD subsurface was free from microscale cracking. However, the attached debris layer on the top surface contained metal oxides and non-diamond carbon phase with inhomogeneous distributions of C, Fe, Cr, Ni, Si and O across the layer. An attached layer directly above a diamond grain was composed of essentially amorphous carbon, suggesting that a direct phase transformation from diamond crystalline to amorphous occurred during DFP.     

Study of ultraintense laser-produced fast-electron propagation and filamentation in insulator and metal foil targets by optical emission diagnostics

The transport of an intense electron beam produced by ultrahigh intensity laser pulses through metals and insulators has been studied by high resolution imaging of the optical emission from the targets. In metals, the emission is mainly due to coherent transition radiation, while in plastic, it is due to the Cerenkov effect and it is orders of magnitude larger. It is also observed that in the case of insulators the fast-electron beam undergoes strong filamentation and the number of filaments increases with the target thickness. This filamented behavior in insulators is due to the instability of the ionization front related to the electric field ionization process. The filamentary structures characteristic growth rate and characteristic transversal scale are in agreement with analytical predictions.     

Fe-Si-Al合金结构的研究

本文叙述了利用电子显微镜对含10％Si和5％Al的Fe-Si-Al高导磁合金薄膜进行的结构研究。发现了基体为有序面心立方Fe₃(Al,Si)结构,其晶格参数为5.70±0.03?,该值在Fe₃Si和Fe₃Al晶格参数之间,说明部分Si原子为Al原子所置换。并获得三套基体和析出相合成的电子衍射图,相应确定了基体和析出相的取向关系:(001)_基‖(110)_相;[100]_基‖[00I]_相;[010]_基‖[I10]_相。对析出相的萃取碳复型,在电子显微镜中进行选区衍射,获得了立方Fe₃(Al,Si)C_x相的结构。提出:Fe原子占据面心;Al或Si原子占据顶角;C原子占据体心的看法。 关键词：     

New Form for the Coulomb Potential in Complex Crystals. Part I

The Coulomb potential is constructed for a crystal with a basis by the method of the Green's functions approximated by the lattice sum of spherically symmetric electron densities of free atoms. The application of the new method with improved convergence of the lattice series in the Green's function has allowed us to obtain an additional term in the electron part of the potential, which describes attraction and can be considered as a potential of the Coulomb hole. The method was applied to calculate the potentials for carbon in the diamond lattice and for TiC solid solution in the direction of four nearest neighbors. Some typical results are presented to illustrate the capabilities of the method.