Role of the electron forward-focusing effect in the formation of Kikuchi patterns of single-crystal silicon

Kikuchi patterns produced by quasi-elastic backscattering of electrons with energies of 0.6–2 keV from a thin Si(111)7×7 near-surface layer are studied. It is shown that experimental data obtained for silicon, just like those for metals, can be described satisfactorily by calculations made in single-scattering cluster approximation, as well as in terms of a model taking phenomenologically into account the forward-focusing of backscattered electrons as they escape from the crystal. It has thus been demonstrated that the forward-focusing effect at an energy E=2 keV plays a dominant role in the formation of Kikuchi patterns, which permits their use for visualization of the atomic structure of a surface. The dependences of the focusing efficiency on the parameters of the atomic chains along which electrons propagate have been established for the closest-packed crystal directions. Fiz. Tverd. Tela (St. Petersburg) 39, 752–757 (April 1997)     

Theoretical study of sorption and diffusion of lithium atoms on the surface of crystalline silicon and inside it

The energy of the sorption and diffusion of lithium atoms on the reconstructed (4 × 2) (100) silicon surface in the process of their transport into near-surface layers, as well as inside crystalline silicon, at various lithium concentrations have been investigated within the density functional theory. It has been shown that single lithium atoms easily migrate on the (100) surface and gradually fill the surface states (T3 and L) located in channels between silicon dimers. The diffusion of lithium into near-surface silicon layers is hampered because of high potential barriers of the transition (1.22 eV). The dependences of the binding energy, potential barriers, and diffusion coefficient inside silicon on distances to the nearest lithium atoms have also been examined. It has been shown that an increase in the concentration of lithium to the Li_0.5Si composition significantly reduces the transition energy (from 0.90 to 0.36 eV) and strongly increases (by one to three orders of magnitude) the lithium diffusion rate.     

Dependence of the surface energy of metals on pressure

A theoretical model has been developed within the method of electron-density functional to describe the effect of pressure on the surface energy of pure metals.     

Effect of the thermophysical properties of the material of a local energy source on conditions and characteristics of ignition of metallized composite propellants

Solid-phase ignition of metallized composite propellants by a single particle heated to a high temperature under conditions of an ideal thermal contact has been numerically studied. The effect of the thermophysical properties of the material of a local energy source on the conditions and characteristics of ignition of composite propellants has been analyzed. It has been found that sources with a high heat storage capacity exhibit shorter ignition delay times for metallized propellants (by 10–60%) and lower initial temperatures required to initiate the combustion process (by 170 K). In addition, it has been found that the presence of particles of metals (boron, aluminum, magnesium, lithium) in the propellant composition leads to an increase in the effective thermal conductivity of the propellant. The cumulative effect of the thermophysical properties of the materials of the “particle heated to a high temperature–metallized composite propellant” system leads to an increase in the ignition delay times (by 25–65%) and the heat penetration depth of the near-surface layer of the propellant (by 25–40%) at the time of combustion initiation compared with metal-free compounds.     

Effect of adsorption on the surface mobility of charge carriers in a semiconductor substrate

The effect of adsorption on the carrier mobility in the near-surface region of a semiconductor substrate has been investigated within the framework of the Schrieffer model. The dependence of the carrier surface mobility on the concentration of adatoms has been determined. The systems chosen for the study are the gases adsorbed on the surface of semiconductor oxides. Empirical estimates of the surface mobility, which are based on modification of conventional volume scattering mechanisms, have been proposed.     

Two level metal vapor lasers with thermal creation of population inversion

We present a new technique for direct conversion of thermal energy into coherent radiation. The near-surface layer of evaporated excited atoms of rare earth metals is proposed to be used as a converter. There is an inverted population in this layer. Operation principles of two level lasers on the transitions to the ground state using Eu are considered. The fact of detachment of Sm atoms in the excited 4f(N-1)5d6s2 state during the thermal evaporation process has been proved experimentally.     

Field injection of low-energy electrons into the ZnSe/CdSe/ZnSe heterostructure with the use of an ultra-high-vacuum tunneling microscope

The field emission injection of low-energy electrons (E _e ?? 10 eV) into the ZnSe/CdSe/ZnSe heterostructure has been considered. The probe of the ultra-high-vacuum tunneling microscope has been used as a field emitter. It has been shown that the energy of injected electrons is sufficient for impact ionization in ZnSe. The impact ionization creates a high concentration of nonequilibrium carriers in the near-surface ZnSe layer. The transport of nonequilibrium carriers in the heterostructure under study has been simulated. The electric field of the near-surface space charge and surface recombination have been taken into account. The calculation has demonstrated that filling the active region of CdSe with nonequilibrium carriers is highly efficient.     

Laser-ultrasonic surface wave dispersion measurements on surface-treated metals

Surface acoustic wave (SAW) velocity spectroscopy has been long considered to be one of the leading candidates for nondestructive characterization of surface-treated metals because of its ability to probe the material properties at different penetration depths depending on the inspection frequency. We developed a high-precision laser-ultrasonic technique to study the feasibility of SAW dispersion spectroscopy for residual stress assessment on shot-peened metals. This technique is capable of measuring SAW dispersion with a relative error of 0.1% over a frequency range from 2 to 15 MHz. Our experimental results obtained from shot-peened aluminum 2024-T351 samples indicate that the dispersion of the surface wave is a superposition of different effects of surface treatment in the material, including surface roughness, compressive residual stress, and cold work. Although the surface roughness induced component is often the dominating part of the overall dispersion, the experimental results also indicate that it is feasible to observe a perceivable change in the dispersion of the SAW when the specimen is heat-treated at different temperatures, which has no perceivable effect on the surface roughness. The part of the dispersion, which changes during annealing via thermal relaxation, is due to near-surface residual stresses and the decay of texture, although at high frequencies nonuniform grain coarsening could also play a significant role.     

草地下垫面地表温度与近地面气温的对比研究

地表温度是气候系统中的关键参数,它与近地面气温的差异极大地影响着地-气界面上能量和水汽通量的交换。根据大气辐射国际联合观测网合肥站2002年9月至2005年8月三年期间无云晴天的地面观测资料,研究了该地区草地下垫面的地表温度(辐射测温法)和近地面气温的年变化、季节变化和日分布特征。着重讨论了两者差值的变化情况以及两者的相关性,提出了基于近地面气温的地表温度的经验计算方法。研究发现,当地表比辐射率ε的取值范围为0.94～1.00时,ε每改变0.01,草地地表温度的平均变化小于0.2 K。该研究结果可为卫星资料的地面验证提供参考。     

激光在近表面弹性性质梯度变化的材料中激发超声波的数值分析

激光激发超声波为评价材料近表面弹性性质提供了有效的手段. 考虑到由于冲击硬化、表面热处理、表面氧化等引起的金属材料近表面层弹性性质的变化，建立了一种激光在基底上的梯度材料中激发超声波的理论模型. 用有限元方法模拟了热弹条件下脉冲激光作用于材料上表面激发出的超声波及其传播过程，研究了近表面层离散的层数对超声波的产生和传播的影响，并分别讨论了表面层“变硬”和“变软”两种情况下声表面波的模式变化及用二维傅里叶变换得到各模式的色散曲线. 为进一步研究近表面层的弹性性质建立合理的计算模型及材料性质的反演提供了理论依 关键词： 超声波 有限元法 近表面弹性性质 色散     

Spectroscopic studies of electron surface states on liquid helium

Field-ion microscopy has been previously used [1–4] to determine the surface energy anisotropy of tungsten and iridium. Drechsler and Nicholas [5] have used field emission patterns of equilibrium shapes of emitters to determine the surface energy anisotropies for a number of fcc and bcc metals including α-iron. The purpose of the present paper is to report some observations by field-ion microscopy on the vacuum faceting of α-iron and the surface energy anisotropy at about half the absolute melting point. Qualitative results on the effect of hydrogen on the surface energy anisotropy are also presented.     

Reversible superstructural transitions on the GaAs(001) surface under the selective effect of iodine and cesium

The selective interaction of the iodine and cesium atoms with the GaAs(001) surface, which leads to a decrease in the bond energy of the Ga and As surface atoms, respectively, owing to the redistribution of the electron density in the near-surface region under the effect of electronegative and electropositive adsorbates, has been experimentally investigated. This selective interaction makes it possible to remove alternately the Ga and As monolayers in the iodine and cesium adsorption followed by heating at T ≤ 450°C and, thus, to implement reversible low-temperature transitions between the Ga-and As-stabilized superstructures, as well as the atomic layer etching of the semiconductor with the physically ultimate monolayer accuracy.     

Formation of Kerr waveguide and microgratings on a germanium surface under the action of femtosecond radiation of the mid-IR range

A new nonlinear physical model of the formation of ordered structures of a surface topography of a semiconductor by ultrashort pulses of linearly polarized laser radiation has been proposed. The model is based on the formation of the dynamic near-surface waveguide due to the optical Kerr effect, interference with the part of waveguide modes, and the formation of microgratings on the surface, which are oriented by polarization of radiation. The model is confirmed by experimental data.     

Simulation of transport and relaxation of hot electrons in the near-surface layers of nanostructured and crystalline silicon dioxides

Computer simulation has been performed to investigate the transport and energy relaxation of photoelectrons in the near-surface layers of nanostructured and crystalline silicon dioxides in the presence and absence of an electric field. Calculations have shown that nanostructured samples have a shorter hot-electron thermalization time and exhibit weaker influence of an electric field on the electron energy relaxation process than the bulk crystal. The size effect calculated in terms of electron thermalization time is most pronounced at particle sizes less than 5 nm.     

Surface properties of 3d transition metals

Using the projector augmented wave method within density functional theory, we present a systematic study of the layer relaxation, surface energy and surface stress of 3d transition metals. Comparing the calculated trends for the surface energy and stress with those obtained for 4d and 5d metals we find that magnetism has a significant effect on the surface properties. Enhanced surface magnetic moments decrease the size of the surface relaxation, lower the surface energy and surface stress, leading to compressive stress in Cr and Mn.     

Generalized model of metal adsorption with inclusion of the effects of substrate lattice relaxation: Adsorption of gold atoms onto aluminum

The geometric and energy characteristics of Au atom adsorption onto an Al(100) surface have been calculated self-consistently in the framework of the electron density functional method. These calculations take into account the lattice relaxation of two near-surface substrate layers and the first Au adlayer. It is shown that the interstitial regions on the Al(100) surface are the most advantageous adsorption regions.     

Decisive role of polydispersity in the relaxation spectrum of saturated hydrocarbons from plasma-induced thermoluminescence data

The method of plasma-induced thermoluminescence for the first time has been used to investigate the molecular mobility in near-surface nanolayers of molecular crystals (paraffins) with different chain lengths. The investigations have been performed using a NanoLuminograph device (PlasmaChem, GmbH, Germany) under conditions excluding the modifying effect of gas discharge plasma emission on the surface structure under study. The origin of charge stabilization sites on the surface of molecular crystals as well as the influence of the chain length of paraffins and the purity of their chemical composition on the thermoluminescence intensity and the shape of the glow curves have been discussed.     

Calculation of the surface energy of hcp-metals with the empirical electron theory

A brief introduction of the surface model based on the empirical electron theory (EET) and the dangling bond analysis method (DBAM) is presented in this paper. The anisotropy of spatial distribution of covalent bonds of hexagonal close-packed (hcp) metals such as Be, Mg, Sc, Ti, Co, Zn, Y, Zr, Tc, Cd, Hf, and Re, has been analyzed. And under the first-order approximation, the calculated surface energy values for low index surfaces of these hcp-metals are in agreement with experimental and other theoretical values. Correlated analysis showed that the anisotropy of surface energy of hcp-metals was related with the ratio of lattice constants (c/a). The calculation method for the research of surface energy provides a good basis for models of surface science phenomena, and the model may be extended to the surface energy estimation of more metals, alloys, ceramics, and so on, since abundant information about the valence electronic structure (VES) is generated from EET.     

Effect of deuteron density distribution on the deduction of screening potential from the D(d,p)T reaction in Be metals

The D(d,p)T reaction in Be metal environments has been measured to investigate the electron screening effect in metals in an energy region of from 5.5 keV to 10 keV in a center of mass system(CMS)at a temperature of 121 K.The depth distribution of deuteron density in Be metals has an impact on the observed reaction yields.A model of deuteron density distribution in metal has been proposed to obtain the original yields.A screening energy of(116±46)eV has been obtained with the assumed deuteron density distribution model.     

金属纳米球-纳米圆盘结构中表面等离激元共振模式的电磁场增强研究

表面等离激元自诞生以来已有一百多年的历史,并逐渐形成了一门新的学科——表面等离激元光子学.位于金属纳米结构中的局域表面等离激元可产生非常显著的近表面电场增强,并成功应用于诸多研究领域当中,而对局域表面等离激元与外界入射光中磁场的相互作用的研究则相对较少.该研究在前期已有的研究基础之上模拟计算了金属纳米球-纳米圆盘结构间...