Ding Ze-Jun

A comparison of Monte Carlo simulations of electron scattering and X-ray production in solids

Monte Carlo simulations of electron scattering in silver have been performed at 6 and 20 keV to investigate the relation between electron transport, backscattering and depth distribution of characteristic X-ray production of tracer elements. The simulation models used combine the use of elastic cross sections, either of Rutherford or of Mott type, with approaches to inelastic scattering given by either the Bethe stopping power or by the energy loss- and momentum transfer-dependent dielectric functions derived from optical constants. A comparison of the simulation results shows that the model with Mott cross section and dielectric function results in better agreement between calculated and experimental energy spectra of backscattered electrons. However, the dominant term in models of Phi ( rho z) curves is elastic scattering and the Mott cross section yields better agreement with experimental results.

Theoretical Study on Structural and Elastic Properties of ZnO Nanotubes

A theoretical investigation on the structural and elastic properties of ZnO nanotubes is carried out by using atomistic calculations based on an inter-atomic pair potential within the shell-model approach. The calculation results are presented for the bond length, bond angle, radius dilation, strain energy, Young modulus and Poisson ratio as a function of tube radius. For small tube radius these properties depend on the helicity of the tube, while for the tube radius larger than 6.0 A, they are independent of the tube radius and helicity except for the strain energy which decreases with increasing tube radius.

Monte Carlo calculation of X-ray depth profiles in Si substrate coated with films

Using Monte Carlo methods, we have calculated depth profiles of X-ray production, Phi ( rho z) curves, for Si K alpha in a Si substrate coated with Cu, Ag and Au films of varying film thickness and electron beam voltages from 10 to 30 kV, as well as the characteristic radiation from films for comparison with August and Wernischs (1991) analytical approach. Calculation results shows that, for radiation from the film, general agreement is found with August and Wernischs formula, while it is not found for radiation from the substrate. We demonstrate that the Monte Carlo simulated Phi ( rho z) curves from Si substrate can be well described by the modified Gaussian profile of Packwood and Brown (1981), and the calculated profiles may be parameter-fitted into the analytical formula Phi ( rho z)= Theta ( rho z- rho Delta z)( gamma -(( gamma - Phi (0))exp(- beta rho z)))exp(

Effect of oxygen-deficiency of CuO2 plane on the structure, magnetism, and transport properties

- alpha 2( rho z)2).

Theoretical Investigation of Exchange Bias in Compensated Cases

Polycrystalline samples La2 − xSrxCuO4 − δ (0.06 ≤ x ≤ 1.0) and the corresponding vacuum annealed samples were investigated systematically by means of XRD, infrared (IR) spectra, ESR, and resistivity. It is found that the doping of Sr and the oxygen vacancies in CuO2 planes show different effects on the lattice parameters a and c, and the changes of a and c of La2 − xSrxCuO4 − δ samples (x ≥ 0.2) rest with the cooperation of the two factors. The two high frequency IR vibration modes at 500 and 689 cm−1, which are referred to the vibrations of apical oxygen and planar oxygen, are analyzed in detail. The disappearance of the mode at 689 cm−1 is interpreted in terms of the screened effect of charge carriers in CuO2 planes. ESR results indicate that oxygen deficiency of CuO2 layers not only decreases carrier concentration, but also destroys the antiferromagnetic spin correlation or spin fluctuation and results in localized Cu2+ spins. The effect of localized Cu2+ spins (or spin scattering) on the superconductivity and transport properties is discussed in the text.

Different effects of Fe and Ga doping on Tc between the Bi2201 system and the La214 system

The exchange bias (EB) of the ferromagnetic (FM)/antiferromagnetic (AFM) bilayers in a compensated case is studied by use of the many-body Greens function method of quantum statistical theory. The so-called compensated case is that there is no net magnetization on the AFM side of the interface. Our conclusion is that the EB in this case is primarily from the asymmetry of the interfacial exchange coupling strengths between the FM and the two sublattices of the AFM. The effects of the layer thickness, temperature and the interfacial coupling strength on the exchange bias HE are investigated. The dependence of HE on the FM layer thickness and temperature is qualitatively in agreement with experimental results. HE is nearly inversely proportional to FM thickness. When temperature varies, both HE and HC decrease with temperature increasing. The anisotropy of the FM layer only slightly influence HC, but does not influence HE.

Thickness dependence of the optical constants of oxidized copper thin films based on ellipsometry and transmittance

Single-phase samples Bi1.8Pb0.2Sr1.6 + xLa0.4-xCu1-xMxOy and La1.85-xSr0.15 + x Cu1-xMxOy (M = Fe3+,Ga3+) were synthesized by the solid state reaction method. The crystal structure, transport properties and spin dynamics were investigated by means of x-ray diffraction (XRD), resistivity, thermoelectric power (TEP) measurement and the electron spin resonance (ESR) spectrum. It is found that Fe doping suppresses Tc more effectively in the La214 system than in the Bi2201 system, while Ga doping shows a similar effect on Tc in both the Bi2201 system and the La214 system. The different effects of Fe doping on the broad peaks in S(T)-T curves between the La214 system and the Bi2201 system are clearly observed, while Ga doping shows a similar influence on the broad peaks in S(T)-T curves. ESR results show that a strong ESR signal from Fe3+ spins was detected in the La214 system, but no ESR signals were observed in the Bi2201 system. It is suggested that there exist different spin dynamic behaviours between the two systems. The superconductivity and transport properties are discussed based on these experimental results.

Mask characteristics for projection electron-beam lithography with demagnification imaging

Thin oxidized copper films in various thickness values are deposited onto quartz glass substrates by electron beam evaporation. The ellipsometry parameters and transmittance in a wavelength range of 300 nm-1000 nm are collected by a spectroscopic ellipsometer and a spectrophotometer respectively. The effective thickness and optical constants, i.e., refractive index n and extinction coefficient k, are accurately determined by using newly developed ellipsometry combined with transmittance iteration method. It is found that the effective thickness determined by this method is close to the physical thickness and has obvious difference from the mass thickness for very thin film due to variable density of film. Furthermore, the thickness dependence of optical constants of thin oxidized Cu films is analyzed.

Charging Effect in Plasma Etching Mask of Hole Array

We have designed and constructed an experimental tool for projection electron-beam lithography with demagnification imaging (PELDI).The tool was built to analyze the efficacy of this approach as an alternative to photolithography for future integrated circuit manufacturing. In this paper, we first introduce how to modify a transmission electron microscope into a PELDI tool. Then, we study the transmission and contrast of the mask through experiments and Monte Carlo simulation. The simulation is intended to help select the optimum materials and the thickness for use in a scattering mask. In addition, we will discuss some phenomena, such as the charging effect in the mask during exposure.

Temperature Dependence of the Fluorescence Emission Intensity of Eu/TiO2 Nanocrystals

It has already been found that the round shape of holes can be changed into hexagonal shape during plasma etching processes. This work aims to understand the mechanism behind such a shape change using particle simulation method. The distribution of electric field produced by electrons was calculated for different heights from the mask surface. It is found that the field strength reaches its maximum around a hole edge and becomes the weakest between two holes. The field strength is weakened as moving away from the surface. The spatial distribution of this electric field shows obvious hexagonal shape around a hole edge at some distances from the surface. This charging distribution then affects the trajectories of ions that fall on a mask surface so that the round hole edge is etched to become a hexagonal hole edge. The changing of this hole shape will again alter the spatial distribution of electric field to enhance the charging effect dynamically.