Wei He-Lin

Monte Carlo simulation of thin-film growth on a surface with a triangular lattice

The thin-film growth process on a triangular lattice surface is studied by a Monte Carlo simulation (MCS). Four kinetic processes are considered: atom deposition, adatom diffusion, adatom nucleation and adatom reevaporation. We pay close attention to the substrate temperature and the interaction between the adatom and neighboring particles. The interaction energy between the adatom and the neighboring atoms around it is calculated by the Morse potential. The results show that the higher the temperature, the more compact the island. As the temperature is low, the film growth becomes fractal growth. At higher temperature, the island becomes more regular in shape that resembles the surface morphology.

Fabrication and photoluminescence characteristics of In2O3 nanohillocks

Uniformly distributed polycrystalline indium nanohillocks are synthesized on silicon substrates with Au catalyst by using the radio frequency magnetic sputtering technique. The results show that the Au catalyst plays a key role in the formation of indium nanohillocks. After thermally oxidizing the indium nanohillocks at 500 °C in air for 5 h, the indium nanohillocks totally transform into In2O3 nanohillocks. The energy-dispersive X-ray spectroscopy result indicates that many oxygen vacancies and oxygen—indium vacancy pairs exist in the In2O3 nanohillocks. Photoluminescence spectra under an Ne laser excitation at 280 nm show broad emissions at 420 nm and 470 nm with a shoulder at 450 nm related to oxygen vacancies and oxygen—indium vacancies at room temperature.

Spontaneous Hillock Growth on Indium Film Surface

Uniformly distributed indium hillocks are grown on silicon substrates by dc magnetron sputtering. The morphologies and the microstructures have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). From the TEM and SEM images, we find that, at the earlier stage, the grain coalescent process dominates. This coalescent process induces a larger compressive stress. We believe that the drive force for hillock growth comes from this compressive stress. Under this compressive stress, the grain locating in the middle of several grains are extruded from these grains, and then a hillock forms with the increasing deposition time. For low melting point and high diffusion coefficient metal, such as bismuth and indium, this spontaneous-hillock growth mechanism can be used to fabricate well aligned nanostructures.

Gold Nanobelt Reorientation by Molecular Dynamics Simulation

The embedded atom method is used to study the structure stability of gold nanobelt. The Au nanobelts have a rectangular cross-section with 100 orientation along the x-, y- and z-axes. Free surfaces are used along the x- and y-directions, and periodic boundary condition is used along z-direction. The simulation is performed at different temperatures and cross-section sizes. Our results show that the structure stability of the Au nanobelts depends on the nanobelt size, initial orientation, boundary conditions and temperature. A critical temperature exists for Au nanobelts to transform from initial 100 nanobelt to final 110 nanobelt. The mechanism of the reorientation is the slip and spread of dislocation through the nanobelt under compressive stress caused by tensile surface-stress components.

Monte-Carlo simulation for electron-neutral collision processes in normal and abnormal discharge cathode sheath region

Electron-neutral collision processes in helium DC normal and abnormal discharges have been studied using Monte-Carlo simulation (MCS). Four types of collisions (elastic, metastable excitation, excitation and ionization collision) are considered. Comparing normal discharge with abnormal discharge, we find that the number of collisions decreases and the collision rates increase through the cathode region in abnormal discharge. The collision rates calculated from electron instantaneous energy is different than that calculated from electron mean energy. The results also show that the elastic collision is the greatest proportion and plays an important role in electron-neutral collision processes in the cathode fall region. The effects of collision on the electron mean energy is also studied.

Growth of Indium Nanorods by Magnetron Sputtering

Indium nanorods are grown on silicon substrates by using magnetron-sputtering technique. Film morphologies and nanorod microstructure are investigated by using scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), and x-ray diffraction. It is found that the mean diameter of the nanorods ranges from 30 nm to 100 nm and the height ranges from 30 nm to 200 nm. The HRTEM investigations show that the indium nanorods are single crystals and grow along the [100] axis. The nanorods grow from the facets near the surface undulation that is caused by compressive stress in the indium grains generated during grain coalescence process. For low melting point and high diffusivity metal, such as bismuth and indium, this spontaneous nanorod growth mechanism can be used to fabricate nanostructures.

Electron diffusion in a non-uniform electric field and a uniform magnetic field

A Monte Carlo simulation technique has been used to model the electron transport behavior, especially the electron diffusion motion, in the cathode fall region of a glow discharge under the influence of a non-uniform electric field and a transverse magnetic field perpendicular to the cathode sheath electric field. Three types of collisions (elastic, excitation and ionization) are taken into account in our model. The electron free flying time is determined by the electron-neutral atom collision frequency. We focus attention on the electron diffusion distance and velocity. The electron-neutral atom collision processes and the electron drift velocity are also studied. The results indicate that with the increase of the magnetic field the electron diffusion distance increases and the electron diffusion velocity decreases. The results also show that the collision processes are enhanced by the magnetic field, this is in agreement with the experimental result. However, the axial magnetic field does not affect the electron transport behavior.

Electron Transport Behavior in a Mirror Magnetic Field and a Non-uniform Electric Field*

The behaviors of electrons as they move under the influence of mirror magnetic field and non-uniform electric field in a positive column of helium radio frequency gas discharge are studied by Monte Carlo simulation. Some types of collisions (elastic, excitation and ionization collisions) are considered. Graphs showing how the electron density, electron energy, electron current density, collision rate and the electron-scattering angular distribution are affected by the mirror magnetic field are presented. The results indicate that the mirror magnetic field can control the electron transport behavior in the positive column. In the presence of the mirror magnetic field, the electrons are restricted in the middle part of the positive column, and the electron density is greatly increased. The electron collision rate and the electron current density are enhanced in the middle region, and the electron-scattering angles are extended by the mirror magnetic fields. These results are in good agreement with experimental results.

Electron transport behavior in normal and abnormal direct current discharge cathode sheath

This paper studies the electron transport behavior in normal and abnormal helium direct current (dc) glow discharges using a Monte Carlo simulation, in which the influences of the electron initial energy and cathode surface reflection on the electron swarm behavior are considered. The electron-neutral atom collision processes and the electron swarm parameters (such as the number of collisions, collision rates, ionization coefficient, mean drift velocity, electron density, etc.) are analyzed. The results show that the variations of the initial energy of the electron emitted from the cathode and the cathode reflection cause a weak effect on the electron transport parameters in the two discharges. The results also indicate that the electron transport parameters are quite different in normal and abnormal discharges.

Theoretical investigation of electron-neutral atom collision process and electron transport parameters in DC glow discharge of plasma with a transverse magnetic field

A Monte Carlo simulation (MCS) technique is used to simulate the cathode sheath region of a helium dc glow discharge. In such a simulation, a nonuniform electric field and a transverse uniform magnetic field are considered. When the magnetic field intensity increases from 0 to 800 G, all types of collision considered in this paper are enhanced. This result is in agreement with the experimental result. The results also show that with the increase of magnetic field intensity, the electron transport time, the electron density increase, and the electron mean energy decreases.