The nucleation and growth of Si nanoparticle produced by pulsed laser ablation in helium gas ambient is investigated via direct simulation Monte Carlo method with a real physical scale of target-substrate configuration. The nucleation area is important for the formation of Si nanoparticles, and the average size and size distribution of Si nanoparticles formed in this region depend on its range. The narrower the nucleation area and, therefore, the less the maximum times of collisions between Si atoms in the region, the smaller and the more uniform the Si nanoparticles. A nucleation and growth process is clearly observed. It is shown that the nucleation region and the nucleation growth internal is changing with time. The ambient gas pressure is important to nucleation region. The suitable pressure range under certain conditions is given and our simulated results are approximately in agreement with the previous experimental data.
The single crystalline Si target with high resistivity was ablated by a XeCl excimer laser (wavelength 308nm) in pure Ar
gas under the ambient pressure of 10 Pa. The mask with a 1-10 mm diameter hole in the center was placed at a distance
of 1.5 cm to the Si target. The Si nanocrystalline films were systemically deposited on a glass or single crystalline Si
substrate placed behind the mask parallelly with a distance of 1.0 cm. The Raman and X-ray diffraction spectra indicate
that the films were nanocrystalline. Scanning electron microscope images of the films showed that the diameter of the
hole affected on the quantity and distributed range of Si nanoparticles on the substrate. It was obtained that the average
size of Si nanoparticles decreasing with the diameter of the hole increasing, the quantity of Si nanoparticles was
proportional to the power of 1.5 of the hole diameter. It is the nonlinear dynamic process to lead to the experimental
result.
The determination of the nucleation region for Si nanoparticles synthesized by pulsed laser ablation was helpful to find
proper parameter to obtain the high quality nanocrystalline silicon (nc-Si) thin films. A XeCl excimer laser was used to
ablate high-resistively single crystalline Si target under a deposition pressure of 10 Pa. Glass or single crystalline (111)
Si substrates, in parallel with the axial direction of silicon target, were located at a distance of 2.0 cm under the plasma to
collect a series of nc-Si thin films. The Raman spectra, X-ray diffraction spectra (XRD) and Scanning electron
microscopy (SEM) images show that Si nanoparticles deposited in substrates were only formed in the range 0.3~3.0cm
away from the target. In this area, the size of the nanoparticles increased firstly and then reduced, meanwhile, the
distributions for the size of the nanoparticles were also changed. According to the character of the beginning and the
terminus of "nucleation area", combining with the "Horizontal Projectile Motion", the range and position of "nucleation
area" were determination.
To investigate nucleation area and transport dynamics of Si nanoparticles, nanocrystalline silicon films were prepared by pulsed laser ablation. Subsequently, the additional laser beam as energy source was introduced, which crossed vertically the plasma plume from the top down in front of the target at a distance of 0.5 cm under same experiment condition. In this region, due to collision between the photon and the plasma plume, the transport of Si nanoparticles was impacted by the cross-laser beam. The Raman and x-ray diffraction spectra (XRD), scanning electron microscopy (SEM) images of the films showed that Si nanoparticles were formed in a certain range, and the average size of Si nanoparticles monotonically decreases with the increase of distance. Obviously, the range of Si nanoparticles deposited in substrates became narrower due to the influence of additional laser beam. Experimental results were analyzed in terms of the nucleation area model.
Amorphous Silicon (a-Si) films were prepared by pulsed laser ablation in vacuum chamber with base pressure of 2×10-4 Pa, then the a-Si deposited films were annealed by laser in vacuum chamber under different laser fluence and by heat in the electric furnace under ambient of nitrogen gas with different temperature. The crystallization of a-Si films presented similar process and results. According to similar characteristics, we analyzed the results via energy in order to control the relationship between two ways of crystallization, which may be able to help us to comprehend the mechanism of crystallization and prepare uniform-sized and symmetrical Si nanoparticles by controlling the energy accurately. This method can be readily adapted for mass production of optoelectronic devices ..
Er-doped nanocrystalline Si thin films were fabricated by pulsed laser ablation in high-purity Ar gas with different gas pressures at room temperature and post-annealing technology under different temperature in nitrogen. Scanning electron microscopy(SEM), x-ray diffraction(XRD) and Raman were employed to picture the microstructure of films. The SEM photographs showed that the morphology of film was transformed from the uniform nanoparticles in size to the web-like structure with the increase of gas pressure, which was attributed to the different collision cooling process of ablated particles. Raman and XRD spectra showed that the introduction of Ar gas could effectively improve the crystallinity degree of the samples and Si nanoparticle size could be controlled by adjusting the post-annealing temperature which was critical for improving the luminescent intensity of Er3+ ion. More uniform and higher crystallinity degree Er-doped Si thin films could be obtained at lower annealing temperature.
In He, Ne or Ar gas under a deposition pressure of 10Pa, nanocrystalline silicon films were prepared by pulsed laser ablation, which the deposition time was 5, 7, 13, 15, 69 and 350min, respectively. A Lambda Pyhsik XeCl excimer laser (wavelength 308nm, pulse duration 15ns, laser fluence 4J/cm2, repetition rate 1Hz) was used, and the distance between Si target and the substrate was 3cm. The Raman spectra indicate that the films are nanocrystalline. Scanning electron microscopy images show that the discrete nanoparticles are first formed, more and more nanoparticles are obtained with increasing of deposition time, and then some nanoparticles start to aggregate and form continuous film, and finally the film ruptures due to the stress. It is the complicated interaction between nanoparticles as-formed in the film and those produced subsequently to lead to the phenomena mentioned above. The morphology of the films deposited in different ambient gases is compared. The result shows that aggregation between nanoparticles, film-formation and rupture take place in a lighter gas earlier than those in a heavier gas. This is related to the different growing rate of the films deposited in different gases.
In this paper, vertical-cavity surface-emitting laser (VCSEL) with pillar structure which has potential applications in optical communication and optical interconnect is theoretically analyzed, the calculation model that used to discuss the modal performance of cylinder VCSEL with oxidized aperture is established by using vector field model. The numerical simulations show oscillating wavelength and threshold gain against inner and outer radius of laser, the layer refractive index and pair number of Bragg mirror, thickness, position and oxidized material’s refractive index of oxidized aperture, in detail. According to the standard that oscillating wavelength should approach to the designed one and threshold gain should be as low as possible, we give the appropriate range of parameters discussed in the paper.
The deposited dynamic process of Si nanoparticles prepared by pulsed laser ablation is numerically simulated by using the direct simulation Monte Carlo method. It is found that there is a mixed region where the high-density Si vapor peak and the gas peak are overlapped. The Si nanoparticles are formed by Si vapor condensation in the region and their sizes depend on the properties of the region such as density and range. The properties of the mixed region are changed constantly when its position are oscillated with time is increased, and get to stable state at certain a time, i.e., oscillating time. The oscillating time determines the distribution of nanoparticles in size. The influence of the experimental parameters on oscillating time of the mixed region is analyzed theoretically. The results show that the Si-based nanostructure materials with more uniform nanoparticles in size can be obtained by using appropriate proportion of He and Ar as ambient gas.
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