MeV微集团束与物质的相互作用

简述了有关MeV微集团离子束与物质表面相互作用研究的概况．介绍了在北京大学技术物理系和重离子物理所１．７ＭＶ串列加速器上开展的有关实验研究及取得的初步结果． The recent development on investigation of MeV microcluster beam interaction with matter is outlined. And based on 5SDH 2 Pelletron of Peking University, some relative experimental results, such as identification and Rutherford backscattering measurement of MeV carbon cluster ions, stopping power of MeV silicon microcluster ions in Al film, damage producted in silicon by MeV silicon microcluster irradiation, etc. are briefly introduecd.     

Acetylene adsorption on silicon (100)-(4 × 2) revisited

The aim of this work is to revisit the problem of acetylene adsorption on silicon (100). Extending previous theoretical work and including van der Waals forces explicitly in the simulations we remove existing ambiguities about the adsorption sites. The simulated adsorption energies and scanning tunneling microscopy contours are in good agreement with experimental data, they support the interpretation of a two-dimer feature at the surface as resulting from the adsorption of two individual molecules. It is also found that the simulated apparent heights agree with experimental values, if the actual bandgap of silicon is taken into account.     

Grand canonical Monte Carlo simulations of phase equilibria of pure silicon tetrachloride and its binary mixture with carbon dioxide

Grand canonical histogram-reweighting Monte Carlo simulations were used to obtain the phase behaviour of pure silicon tetrachloride and its binary mixture with carbon dioxide. Two new potential models for pure silicon tetrachloride were developed and parametrized to the vapour-liquid coexistence properties. The first model, with one exponential-6 site and fixed electrostatic charges on atoms, does not adequately reproduce the experimental phase behaviour due to its inability to represent orientational anisotropy in the liquid phase. The second potential model, with five exponential-6 sites for the repulsive and dispersive interactions plus partial charges, accurately reproduces experimental saturated liquid and vapour densities as well as vapour pressures and the second virial coefficient for pure silicon tetrachloride. This model was used in simulations of the phase behaviour of the binary mixture carbon dioxide-silicon tetrachloride. Two sets of combining rules (Lorentz-Berthelot and Kong [1973, J. chem. Phys., 59, 2464]) were used to obtain unlike-pair potential parameters. For the binary system, the predicted phase diagram is in good agreement with experiment when the Kong combining rules are used. The Lorentz-Berthelot rules significantly overestimate the solubility of carbon dioxide in silicon tetrachloride.     

Structure and stability of a silicon cluster on sequential doping with carbon atoms

SiC is a highly stable material in bulk. On the other hand, alloys of silicon and carbon at nanoscale length are interesting from both technological as well fundamental view point and are being currently synthesized by various experimental groups (Truong et. al., 2015 [26]). In the present work, we identify a well-known silicon cluster viz., Si₁₀ and dope it sequentially with carbon atoms. The evolution of electronic structure (spin state and the structural properties) on doping, the charge redistribution and structural properties are analyzed. It is interesting to note that the ground state SiC clusters prefer to be in the lowest spin state. Further, it is seen that carbon atoms are the electron rich centres while silicon atoms are electron deficient in every SiC alloy cluster. The carbon–carbon bond lengths in alloy clusters are equivalent to those seen in fullerene molecules. Interestingly, the carbon atoms tend to aggregate together with silicon atoms surrounding them by donating the charge. As a consequence, very few Si–Si bonds are noted with increasing concentrations of C atoms in a SiC alloy. Physical and chemical stability of doped clusters is studied by carrying out finite temperature behaviour and adsorbing O₂ molecule on Si₉C and Si₈C₂ clusters, respectively.     

Multiscale simulation of loading and electrical resistance in silicon nanoindentation

Nanoindentation experiments are an excellent probe of micromechanical properties, but their interpretation is complicated by their multiscale nature. We report simulations of silicon nanoindentation, based on an extended version of the local quasicontinuum model, capable of handling complex Bravais crystals. Our simulations reproduce the experimental load vs displacement curves and provide microscopic information such as the distribution of transformed metallic phases of silicon underneath the indenter. This information is linked to the macroscopic electrical resistance, giving a satisfactory explanation of experimental results.     

Formation of oriented rodlike nickel silicide precipitates during magnetron deposition of carbon and nickel on silicon

Formation of rodlike structures elongated in the 〈100〉 and 〈010〉 directions of silicon crystal was observed during magnetron codeposition of carbon and nickel on (100) silicon substrate with a natural oxide layer. Rodlike structures did not form during deposition in similar conditions on (111) silicon substrate. It has been revealed that the rodlike structures represent epitaxial nickel silicide precipitates. The results of experimental study of the composition, structure, and shape of forming silicide clusters as a function of the silicon substrate orientation and the codeposited layer thickness are presented.     

Electronic excitation energy transfer between CdS quantum dots and carbon nanotubes

Stationary and transient photoluminescence of CdS quantum dots deposited on silicon substrates and carbon nanotubes is investigated. The photoluminescence spectrum of quantum dots on a silicon substrate is dominated by a band originating from electron transitions between the quantum-confinement levels in the dots. When the quantum dots are deposited on carbon nanotubes, the intensity of this band decreases significantly. Furthermore, the kinetics of the photoluminescence decay becomes faster, which brings evidence of an additional channel for the quantum-dot deexcitation. The analysis of the experimental data demonstrates that the Förster energy transfer from CdS quantum dots to carbon nanotubes is most probably responsible for this channel. The efficiency of this process exceeds 60%.     

Current-voltage characteristics of structures with a porous silicon layer at adsorption of carbon monoxide

The current-voltage characteristics of structures with a layer of porous silicon of 73% porosity were measured at adsorption of gas (carbon monoxide) at room temperature. Estimations are performed of the height of potential heterobarrier at the interface between porous silicon and p ⁺-type single-crystal silicon, of the perfectness factor and the resistance of a layer of porous silicon in air, in air with 0.4% CO, and in air with 2% CO. Physical causes explaining the experimental data are discussed.     

Bond-counting rule for carbon and its application to the roughness of diamond (001)

Despite that carbon is tetravalent identical to silicon, first-principles calculations reveal that stable step structures on diamond (001) are entirely different from those on silicon. Moreover, pristine Si(001) is flat; pristine diamond (001) could be rough due to negative step formation energies. A generic bond-counting rule is established, which should apply to most carbon structures where sp2 and sp3 hybrids coexist: e.g., it provides a qualitative account of the step energy order without detailed calculation. Our findings agree with experimental observations.     

Numerical modeling of ultrashort-pulse laser ablation of silicon

Silicon ablation by a single ultrashort laser pulse is simulated through a computer model. The agreement between results obtained through the model and experimental data found in the literature supports the hypothesis made by the authors in considering thermal evaporation as the dominant ablation mechanism in silicon. Two distinctive thresholds are defined for the ablation procedure leading to a better interpretation of experimental data. The dependence of ablation fluence thresholds on both wavelength and pulse width is discussed. An approximate analytical model describing the crater formation process is proposed and indicative results are presented.     

A theoretical examination of the chemisorption of benzene on Si(100)−(2 × 1)

A theoretical examination of the structure of the adsorbed state of benzene on the Si(100)-(2 × 1) surface is discussed. A number of potential candidate structures are examined. The local minimum in the total energy for each structure has been calculated with respect to variations in the atomic coordinates for the adsorbed benzene structure and the first four layers of the substrate. The preferred structure is the one with the lowest value for the total energy. This structure, in agreement with the experimental observations, describes a topology for the adsorbed benzene with no Si-H bonds and all the C-H bonds remaining intact. The benzene ring is oriented at an angle of 24° from the surface plane. Four of the carbon atoms are bonded to the silicon surface. Two silicon atoms are each bonded to two carbon atoms.     

非均匀光斑导致相变镜镜面偏转的研究

 针对高功率激光器中非均匀光斑热致变形导致光轴偏转的问题，提出用相变致冷镜作为腔镜来减小镜面变形峰谷值和角向偏转，在2kW连续二氧化碳激光器上进行了热变形和光束漂移实验。实验表明，与普通实心硅镜相比，相变致冷镜镜面温度分布更均匀，可有效地减小镜面偏转角和光斑中心漂移，提高远场光束质量。     

碳纳米管包裹的硅纳米线复合结构的热稳定性研究

采用经典分子动力学方法模拟一定直径[111]晶向的硅纳米线填充不同扶手椅型单壁碳纳米管复合结构的加热过程, 通过可视化和能量分析的方法判断复合结构中硅纳米线和碳纳米管的热稳定性. 通过讨论碳纳米管的空间限制作用和分子间相互作用力的关系, 对碳纳米管和硅纳米线的热稳定性变化进行初步解释. 研究发现碳纳米管中硅纳米线的热稳定性和碳纳米管的直径关系密切: 当管径较小时, 硅纳米线的热稳定性有所提高, 当管径增大到一定大小时, 硅纳米线的热稳定性会突然显著地下降, 直到硅纳米线与管壁不存在分子间相互作用力, 硅纳米线的热稳定性才会恢复. 而硅纳米线填充到碳纳米管中对碳纳米管的热稳定性有着明显的降低作用.     

Study of excimer laser induced melting and solidification of Si by time-resolved reflectivity measurements

In this paper we present the interpretation and analysis of time-resolved reflectivity measurements for excimer laser irradiated silicon. A nonequilibrium melting model is used to calculate the temperature distribution in the sample and the position of the solid-liquid interface. Calculations of the reflectivity of the probe beam are performed to obtain time-resolved reflectivity signals from basic principles, independent of the experiments, which can be compared with experimental data to reveal information about dynamics of the heating and cooling process. We propose more accurate methods for the determination of the melting threshold, the melt front position and the reflectivity of the sample for excimer laser light. From model calculations and experimental data we determine the reflectivity of the solid and liquid silicon for ArF excimer laser light.     

Intrinsic mobility of a dissociated dislocation in silicon

Dislocation velocities in silicon in the experimental range of temperature and stress are studied a priori by combining a mechanistic treatment of elementary kink processes with activation energies obtained by atomistic calculations. Pronounced effects of intrinsic coupling of the dissociated partial dislocations are captured in kinetic Monte Carlo simulations, which are consistent with observed velocity variations with applied stress. As a result, the nature of "weak obstacles" to kink propagation, a long-standing postulate in previous data interpretation, is clarified. A striking new effect is predicted and offered for experimental verification when dislocation velocity shows nonmonotonic oscillatory behavior with increasing stress.     

Superconductivity in doped sp3 semiconductors: the case of the clathrates

We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba(8)@Si-46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant lambda calculated from perturbative density functional theory. Further, the study of I(8)@Si-46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp(3) silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C60.     

Iron and its complexes in silicon

This article is the first in a series of two reviews on the properties of iron in silicon. It offers a comprehensive summary of the current state of understanding of fundamental physical properties of iron and its complexes in silicon. The first section of this review discusses the position of iron in the silicon lattice and the electrical properties of interstitial iron. Updated expressions for the solubility and the diffusivity of iron in silicon are presented, and possible explanations for conflicting experimental data obtained by different groups are discussed. The second section of the article considers the electrical and the structural properties of complexes of interstitial iron with shallow acceptors (boron, aluminum, indium, gallium, and thallium), shallow donors (phosphorus and arsenic) and other impurities (gold, silver, platinum, palladium, zinc, sulfur, oxygen, carbon, and hydrogen). Special attention is paid to the kinetics of iron pairing with shallow acceptors, the dissociation of these pairs, and the metastability of iron–acceptor pairs. The parameters of iron-related defects in silicon are summarized in tables that include more than 30 complexes of iron as detected by electron paramagnetic resonance (EPR) and almost 20 energy levels in the band gap associated with iron. The data presented in this review illustrate the enormous complexing activity of iron, which is attributed to the partial or complete (depending on the temperature and the conductivity type) ionization of iron as well as the high diffusivity of iron in silicon. It is shown that studies of iron in silicon require exceptional cleanliness of experimental facilities and highly reproducible diffusion and temperature ramping (quenching) procedures. Properties of iron that are not yet completely understood and need further research are outlined. Received: 14 December 1998 / Accepted: 22 February 1999 / Published online: 26 May 1999     

Bismuth onion thin film in situ grown on silicon wafer synthesized through a hydrothermal approach

Bismuth onion structured nanospheres with the same structure as carbon onions have been synthesized and observed. The nanospheres were synthesized through a hydrothermal method using bismuth hydroxide and silicon wafer as reactants. By controlling the heating temperature, heating time, and the pressure, nanoscale bismuth spheres can be in situ synthesized on silicon wafer, and forms a bismuth onion film on the substrate. The electronic property of the films was investigated. A formation mechanism of the formation of bismuth onions and the onion film has been proposed on the basis of experimental observations.     

Influence of irradiation on the thermal decomposition of lithium perchlorate

Abstract

Infrared absorption measurements were made before and after 90 °K electron irradiations of silicon samples which contained either dispersed oxygen, carbon, or carbon plus oxygen. Irradiation-produced absorption bands associated with two distinctly different defects are observed depending on the oxygen and carbon content of the silicon. One center is the well-known vacancy-oxygen A-center defect (836-cm^?1 band) and is formed on irradiation in oxygen-containing silicon with a magnitude which is independent of the carbon content. Measurements have correlated the formation of one A-center with the loss of one interstitial oxygen atom, thereby indicating that A-center formation occurs by vacancy trapping at interstitial oxygen atoms. A second center (922-and 932-cm^?1 bands) is formed only in silicon crystals which contain both oxygen and carbon. The results indicate that this center is formed by the trapping of a silicon interstitial at a carbon-oxygen complex.     

Characteristics of thermal distortions of the laser mirror substrates filled with phase-change materials

Under irradiating of the laser power of 2 kW, the thermal deformations of the silicon mirror substrates with phase change materials are experimentally measured and numerically analyzed by using finite element methods, respectively. The experimental results show that when the absorbed laser power is 120 W and the laser irradiating time gets to three seconds, the thermal distortion of the silicon mirror substrates with paraffin/carbon powder is 0.25 μm, that of the paraffin/aluminum powder 0.33 μm, and that of the paraffin/copper powder 0.37 μm. The numerical calculation coincides with the experimental results.