分子动力学模拟C+离子与聚变材料钨的相互作用

采用分子动力学方法模拟不同能量的C+离子与聚变材料钨的相互作用.模拟结果表明:当C+离子入射剂量为3.11×1016 cm-2,入射能量为50eV时,样品表面形成一层碳膜;而入射离子能量为150和250eV时,C+离子入射到样品内与钨原子共同形成碳钨混合层,样品表面没有形成碳膜;碳的沉积率随能量的增大先减小后增加,溅射率随能量的增大先增大后减小;轰击后的样品中,碳原子密度、C-W键密度及C-C键密度分布都随能量的增加逐渐向样品内移动,且C-W键分布厚度随能量的增加而逐渐增加,C-C键分布厚度几乎不随能量变化;在作用过程中极少量的钨原子发生溅射,但引起钨品格损伤严重;碳在轰击后的样品中主要以Csp3杂化形式存在.     

金刚石状薄膜

本文以化学键的观点,解释了金刚石的(C-C,SP~3键)的许多性质。主要由C-C、SP~3键构成的碳膜定义为金刚石状碳薄膜(DLC)。DLC膜的特性接近于金刚石晶体。因为SP~2键在常温常压下是稳定的,所以DLC膜形成于诸如等离子体过程等非平衡状态。在一些等离于体工艺中,我们着重于反应性溅涂,它和一般的物理溅涂不同。在反应性溅涂的沉积层中,氢游离基被认为是对石墨标板呈化学溅射,并有选择地在基片上沉积出主要由SP~3键构成的薄膜。我们还讨论了用来区刀DLC薄膜中化学键和微结构的表征技术。     

氟化类金刚石(F-DLC)薄膜的红外分析

以CF4和CH4为源气体,利用射频等离子体增强化学气相沉积法,在不同条件下制备了氟化类金刚石(F-DLC)薄膜,并进行了退火处理。红外分析表明,F-DLC薄膜中主要有C-Fx(x=1,2,3)、C-C、C-H2、C-H3和C=C化学键等。低功率下制备的薄膜主要由C-C、CF和CF2键构成,功率增加时,薄膜内C-C键含量相对增加;气体流量比R(R=CF4/[CF4 CH4])增大,薄膜内F的含量增加,C-C键相对减少;高温退火后,薄膜内部分F和几乎全部的H从膜内逸出,薄膜的稳定温度在300℃以上。低功率、高流量比下制备的薄膜,F含量相对较大,介电常数较低。     

分子动力学模拟不同入射能量的SiF2与SiC的相互作用

采用分子动力学模拟方法研究了入射能量对SiF2与SiC样品表面相互作用的影响。本次模拟选择的入射初始能量分别为0.3,1,5,10和25 eV。模拟结果显示SiF2分解率与Si和F原子的沉积率有密切的关系。沉积的Si和F原子在SiC表面形成一层SixFy薄膜。随入射能量的增加,薄膜厚度先增加后减小,薄膜中Si-Si键密度增大。构成薄膜的主要成分SiFx(x=1~4)中主要是SiF和SiF2,随入射能量的增加,薄膜成分由SiF2向SiF转变。     

蓝宝石衬底上6H-SiC单晶薄膜的化学气相淀积生长

采用低压化学气相淀积方法以相对较低的生长温度(900～1050℃)在蓝宝石(0001)衬底上外延生长了6H-SiC单晶薄膜。其晶体质量和结构由X射线衍射谱和喇曼散射谱的测量结果得到确定。俄歇电子能谱和X射线光电子能谱的观察表明所制备的6H-SiC薄膜中的Si-C键的结合能为181．4eV,Si与C的原子比符合SiC的化学配比。扫描电子显微镜的分析显示6H-SiC外延层和蓝宝石衬底间的界面相当平整。由紫外及可见光波段吸收谱的结果得到所生长的6H-SiC的禁带宽度为2．83eV、折射系数在2．5～2．7之间。均与6H-SiC体材料的相应数据一致。     

低能He+、Ar+、Xe+轰击SiC的蒙特卡诺模拟

应用蒙特卡诺程序SRIM对He 、Ar 、Xe 轰击SiC的微观过程进行了模拟.对不同能量(100～500eV)以及不同角度(0～85°)下He 、Ar 、Xe 轰击SiC引起的溅射率、溅射原子分布、溅射原子能量以及入射离子在SiC中的分布情况进行了分析比较.结果表明对于原子量较小的He 入射SiC所引起的溅射主要是由进入表面之下的背散射离子产生的碰撞级联造成的,溅射原子具有较高的能量;对于原子量较大的Ar 、Xe 入射所引起的溅射主要是由进入SiC内部的离子直接产生的碰撞级联产生,溅射原子的能量相对较低.随着离子入射角度的逐渐增加,SiC的溅射率逐渐增加,在70°左右达到溅射峰值,随着入射角度的继续增加,入射离子的背散射不能使碰撞级联充分扩大,反冲原子的生成效率急剧降低,导致溅射率开始急剧下降.     

非晶壳层SixN/SiyC包覆硅量子点的微结构表征

通过磁控溅射技术和1100℃的高温后退火处理,在富硅碳化硅薄膜中形成高密度小尺寸的硅量子点,硅量子点的结构由X射线光电子能谱和高分辨透射电镜进行表征,结果表明,在高温退火过程中,碳化硅薄膜发生了相分离,硅和碳的化学结合态在热力学的驱动下形成稳定的Si-Si键和Si-C键,同时,氮原子钝化了分解过程中形成的Si悬挂键,在硅量子点的表面形成SixN/SiyC非晶壳层。这种非晶壳层包覆量子点的结构配置非常有利于形成稳定的超小硅量子点 （1-3 nm）,此结构的量子效应所产生的光吸收了从绿光到紫外光的光谱范围,大幅度提高光伏太阳能电池的光电转换效率。     

磁控溅射制备SiC薄膜的高温热稳定性

采用磁控溅射方法在Si基底上制备SiC薄膜,研究了SiC薄膜经不同温度和气氛条件高温退火前后结构、成份的变化.结果表明,薄膜主要以非晶为主,由Si-C键,C-C键和少量Si的氧化物杂质组成;在真空条件下经高温退火后,薄膜C-C键的含量减少,而Si-C键的含量增加,真空退火有利于SiC的形成;在800℃空气中退火后,薄膜表面生成一层致密的SiO2薄层,阻止了氧气与薄膜内部深层的接触,有效保护了内部的SiC.在空气条件下,SiC薄膜在800℃具有较好的热稳定性.     

化学键性质对机械合金化过程中非晶形成能力的影响

研究了Ｂ２型金属间化合物ＦｅＡｌ和ＦｅＴｉ的机械合金化过程发现在相同的试验条件下，ＦｅＡｌ形成化学无序的固溶体，ＦｅＴｉ形成结构无序的非晶态。此现象可用ＦｅＡｌ的化学键为金属—离子混合键，而ＦｅＴｉ的化学键为金属—共价—离子混合键来解释。     

氢化非晶硅(a-Si:H)薄膜稳定性的研究进展

氢化非晶硅(a-Si:H)是一种重要的光敏感薄膜材料,其稳定性的好坏是决定能否应用于器件的重要因素之一.介绍了a-Si:H薄膜稳定性的研究进展,论述了a-Si:H薄膜的稳定性与Si-Si弱键的关系,分析了先致衰退效应(S-W效应)产生的几种机理,提出了在薄膜制备和后处理过程中消除或减少Si-Si弱键以提高a-Si:H薄膜稳定性的方法.     

Tandem mass spectrometry characteristics of silver-cationized polystyrenes: internal energy, size, and chain end versus backbone substituent effects

The Ag(+) adducts of polystyrene (PS) oligomers with different sizes (6-19 repeat units) and initiating (alpha) or terminating (omega) end groups mainly decompose via free radical chemistry pathways upon collisionally activated dissociation. This reactivity is observed for ions formed by matrix-assisted laser desorption/ionization as well as electrospray ionization. With end groups lacking weak bonds (robust end groups), dissociation starts with random homolytic C-C bond cleavages along the PS chain, which lead to primary and benzylic radical ions containing either of the chain ends. The primary radical ions mainly depolymerize by successive beta C-C bond scissions. For the benzylic radical ions, two major pathways are in competition, namely, depolymerization by successive beta C-C bond scissions and backbiting via 1,5-H rearrangement followed by beta C-C bond scissions. The extent of backbiting decreases with internal energy. With short PS chains, the primary radical ions also undergo backbiting involving 1,4- and 1,6-H rearrangements; however, this process becomes negligible with longer chains. If the polystyrene contains a labile substituent at a chain end, this substituent is eliminated easily and, thus, not contained in the majority of observed fragments. Changes in the PS backbone structure can have a dramatic effect on the resulting dissociation chemistry. This is demonstrated for poly(alpha-methylstyrene), in which backbiting is obstructed due to the lack of benzylic H atoms; instead, this backbone connectivity promotes 1,2-phenyl shifts in the primary radical ions formed after initial C-C bond homolyses as well as H atom transfers between the incipient primary and benzylic radicals emerging from these homolyses.     

4H-SiC晶体中的层错研究

采用物理气相传输法(PVT法)在4英寸(1英寸=25.4 mm)偏<11¯20>方向4°的4H-SiC籽晶的C面生长4H-SiC晶体。用熔融氢氧化钾腐蚀4H-SiC晶体, 并利用光学显微镜研究了晶体中的堆垛层错缺陷的形貌特征和生长过程中氮掺杂对4H-SiC晶体中堆垛层错缺陷的影响。结果显示, 4H-SiC晶片表面的基平面位错缺陷的连线对应于晶体中的堆垛层错, 并且该连线的方向平行于<1¯100>方向。相对于非故意氮掺杂生长的4H-SiC晶体, 氮掺杂生长的4H-SiC晶体中堆垛层错显著偏多。然而, 在氮掺杂生长的4H-SiC晶体的小面区域, 虽然氮浓度高于其他非小面区域, 但是该小面区域并没有堆垛层错缺陷存在, 推测这主要是由于4H-SiC晶体小面区域特有的晶体生长习性导致的。     

Ab Initio HF and Density Functional Theory Studies of C60@Si60

Ab initio HF and DFT calculations on C₆₀@Si₆₀ confirmed our previous semiempirical results that the interaction of two fullerene layers brings about extensive modification of the bond parameters, the C-C, Si-Si and Si-C bond distances being close to the respective single bond lengths. However, the present calculations revised the previous HOMO-LUMO picuture and the stability relations among C₆₀@Si₆₀, C₆₀ and Si₆₀ species.     

Anisotropic diffusion of point defects in a two-dimensional crystal of streptavidin observed by high-speed atomic force microscopy

The diffusion of individual point defects in a two-dimensional streptavidin crystal formed on biotin-containing supported lipid bilayers was observed by high-speed atomic force microscopy. The two-dimensional diffusion of monovacancy defects exhibited anisotropy correlated with the two crystallographic axes in the orthorhombic C 222 crystal; in the 2D plane, one axis (the a-axis) is comprised of contiguous biotin-bound subunit pairs whereas the other axis (the b-axis) is comprised of contiguous biotin-unbound subunit pairs. The diffusivity along the b-axis is approximately 2.4?times larger than that along the a-axis. This anisotropy is ascribed to the difference in the association free energy between the biotin-bound subunit-subunit interaction and the biotin-unbound subunit-subunit interaction. The preferred intermolecular contact occurs between the biotin-unbound subunits. The difference in the intermolecular binding energy between the two types of subunit pair is estimated to be approximately 0.52?kcal?mol(-1). Another observed dynamic behavior of point defects was fusion of two point defects into a larger defect, which occurred much more frequently than the fission of a point defect into smaller defects. The diffusivity of point defects increased with increasing defect size. The fusion and the higher diffusivity of larger defects are suggested to be involved in the mechanism for the formation of defect-free crystals.     

Environmental influence on the mechanical strength of chemical bonds in solids-ab initio quantum calculations

To elucidate the mechanism of the adsorption-induced strength decrease in solids with chemical bonds, the quantum-mechanics calculations have been performed of force and energy parameters of single and double carbon-carbon bonds during mechanical deformation, and the influence of hydrogen cations on this process has been studied. The Hartry-Fock-Roothaan method (HONDO program) and 5-31G basis set have been used. In both systems the proton decreases mechanical strength of the C-C bond (i.e. the force required to break the bond) and this influence is very sensitive to the geometry of the carbocation formed. The results obtained show the possibility of strength reduction of polymers by strong acids and confirm the relation between the Rehbinder effect and catalysis.     

Electron paramagnetic resonance characterization of aluminum ion implantation-induced defects in 4H-SiC

Deep-level defects in silicon carbide(SiC) are critical to the control of the performance of SiC electron devices. In this paper, deep-level defects in aluminum ion-implanted 4 H-SiC after high-temperature annealing were studied using electron paramagnetic resonance(EPR) spectroscopy at temperatures of 77 K and 123 K under different illumination conditions. Results showed that the main defect in aluminum ion-implanted 4 H-SiC was the positively charged carbon vacancy( VC~+), and the higher the doping concentration was, the higher was the concentration of VC~+. It was found that the type of material defect was independent of the doping concentration,although more VC~+ defects were detected during photoexcitation and at lower temperatures. These results should be helpful in the fundamental research of p-type 4 H-SiC fabrication in accordance with functional device development.     

Microscopic structure and energy transfer of vacancy-related defect pairs with Erbium in wide-gap semiconductors

Electron Paramagnetic Resonance (EPR) measurements of Erbium-doped 6H-SiC and wurtzite GaN samples are compared to total energy calculations based on density functional theory (DFT) in order to investigate the well-known luminescence of the intra 4f-shell transition at 1540 nm, useful in light-emitting diodes or lasers. The highly correlated f-electrons of Erbium (Er) have been treated within an LDA+U approach. We discus how pairs of an Er-ion with intrinsic defects can be responsible in GaN and SiC for relaxing the selection rules for intra 4f-shell transitions: In GaN our EPR investigation indicates the presence of a nitrogen vacancy next to the Er-ion. Through controlled generation of intrinsic defects in 6H-SiC single crystals and EPR measurements we support the corresponding model in SiC, that predicts defect pairs of an Er-ion and a neighboring carbon vacancy. In other words, low-energy irradiation seems to be a promising way to enhance the Er-luminescence desired for device applications.     

Material quality improvements for high voltage 4H-SiC diodes

The influence of thin 4H-SiC buffer layers grown by liquid phase epitaxy (LPE) on structural quality of 4H-SiC low-doped epitaxial layers, grown by chemical vapor deposition (CVD) was investigated in detail. A dramatic defect density reduction in CVD epitaxial layers grown on commercial wafers with buffer LPE layer was detected. P⁺n junctions were formed on these CVD layers by high dose Al ion implantation followed by rapid thermal anneal. It was shown that both the increase of diffusion lengths of minority carriers (Lp) in CVD layers and the forming of p⁺-layers after Al ion implantation and high temperature anneal lead to superior device characteristics.     

Ion trap versus low-energy beam-type collision-induced dissociation of protonated ubiquitin ions

The beam-type and ion trap collision-induced dissociation (CID) behaviors of protonated bovine ubiquitin ions were studied for charge states ranging from +6 to +12 on a modified triple quadrupole/linear ion trap tandem mass spectrometer. Both beam-type CID and ion trap CID were conducted in a high-pressure linear ion trap, followed by proton-transfer ion/ion reactions to reduce the charge states of product ions mostly to +1. The product ions observed under each activation condition were predominantly b- and y-type ions. Fragmentation patterns showed a much stronger dependence on parent ion charge state with ion trap CID than with beam-type CID using nitrogen as the collision gas, with preferential cleavages C-terminal to aspartic acid at relatively low charge states, nonspecific fragmentation at moderate charge states, and favored cleavages N-terminal to proline residues at high charge states. In the beam-type CID case, extensive cleavage along the protein backbone was noted, which yielded richer sequence information (77% of backbone amide bond cleavages) than did ion trap CID (52% of backbone amide bond cleavages). Collision gas identity and collision energy were also evaluated in terms of their effects on the beam-type CID spectrum. The use of helium as collision gas, as opposed to nitrogen, resulted in CID behavior that was sensitive to changes in collision energy. At low collision energies, the beam-type CID data resembled the ion trap CID data with preferential cleavages predominant, while at high collision energies, nonspecific fragmentation was observed with increased contributions from sequential fragmentation.     

Kinematics, electromechanics, and kinetics of dielectric and piezoelectric crystals with lattice defects

A mathematical framework is formulated to address the electromechanical behavior of dielectric and piezoelectric solids containing lattice imperfections. The macroscopic displacement gradient encompasses recoverable elasticity, deviatoric plasticity arising from dislocation glide, and volumetric deformation attributed to point vacancies in the crystal. A linear connection on the spatial manifold of deformed lattice vectors describes gradients of stretch and rotation at the microscale caused by continuous distributions of various classes of crystal defects. It is shown that parallel transport of a lattice director vector with respect to this connection about a closed loop yields a discontinuity with contributions from the torsion of the connection (physically, from dislocations) and its curvature (physically, from rotational defects such as domain walls, and from gradients in vacancy concentration). Classical balance laws of electrostatics and mass and momentum conservation are invoked. A free energy function dependent upon lattice distortion, polarization, temperature, and defect densities is suggested. Thermodynamically consistent kinetic relations for dislocation glide and vacancy diffusion are then derived, with the chemical potential for the latter depending upon defect density, electric potential, hydrostatic pressure, and vacancy energy. The theory also explicitly considers mass rearrangement at the free surface of the substance. Two forms of the contribution of vacancies to the free energy are investigated in detail: a logarithmic function common in chemical mixing theory, and a quadratic function analogous to the convex strain energy used in continuum elasticity theory. For the latter case, the analytical solution of the diffusion equation in one spatial dimension, at steady state, illustrates the effects of defect charge, defect energy, and mechanical stress on the distribution of vacancies in a dielectric thin film. A specific example is given of how compressive residual stresses observed in experiments may be correlated with the equilibrium concentration of vacancies within grains of a polycrystalline dielectric thin film, influencing its electrical performance.