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Limkumnerd S.,Chulalongkorn University | Limkumnerd S.,Research Center in Thin Film Physics
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2014

Interest in thin-film fabrication for industrial applications have driven both theoretical and computational aspects of modeling its growth. One of the earliest attempts toward understanding the morphological structure of a film's surface is through a class of solid-on-solid limited-mobility growth models such as the Family, Wolf-Villain, or Das Sarma-Tamborenea models, which have produced fascinating surface roughening behaviors. These models, however, restrict the motion of an incidence atom to be within the neighborhood of its landing site, which renders them inept for simulating long-distance surface diffusion such as that observed in thin-film growth using a molecular-beam epitaxy technique. Naive extension of these models by repeatedly applying the local diffusion rules for each hop to simulate large diffusion length can be computationally very costly when certain statistical aspects are demanded. We present a graph-theoretic approach to simulating a long-range diffusion-attachment growth model. Using the Markovian assumption and given a local diffusion bias, we derive the transition probabilities for a random walker to traverse from one lattice site to the others after a large, possibly infinite, number of steps. Only computation with linear-time complexity is required for the surface morphology calculation without other probabilistic measures. The formalism is applied, as illustrations, to simulate surface growth on a two-dimensional flat substrate and around a screw dislocation under the modified Wolf-Villain diffusion rule. A rectangular spiral ridge is observed in the latter case with a smooth front feature similar to that obtained from simulations using the well-known multiple registration technique. An algorithm for computing the inverse of a class of substochastic matrices is derived as a corollary. © 2014 American Physical Society. Source

Klawtanong M.,Research Center in Thin Film Physics | Srinitiwarawong C.,Chulalongkorn University | Chatraphorn P.,Research Center in Thin Film Physics
Physica A: Statistical Mechanics and its Applications | Year: 2013

Chiral symmetry breaking in the frustrated antiferromagnetic XY (FAXY) model on a two-dimensional triangular lattice is investigated. The roughness exponent method is used instead of the standard Metropolis method. Spin configurations are mapped to adatoms on a solid-on-solid (SOS) growth model. Statistical properties of the grown film surface are analyzed. Results show that the chiral transition can be indicated by the sharp increase in the roughness of the film morphologies. The critical temperature at the transition can be identified either by the peak of the noise-reduced interface width (W *) or the peak of the noise-reduced roughness exponent (α*). The critical temperature and exponent (ν) obtained here are consistent with those obtained from conventional methods. © 2013 Elsevier B.V. All rights reserved. Source

Thongkham W.,Chulalongkorn University | Thongkham W.,Research Center in Thin Film Physics | Pankiew A.,Thai Microelectronics Center | Yoodee K.,Chulalongkorn University | And 3 more authors.
Solar Energy | Year: 2013

The fabrication of Cu(In,Ga)Se2 (CIGS) thin film solar cells on flexible stainless steel (SS) foils or Na free substrates needs the impurity blocking barrier to prevent the diffusion of undesired elements from the substrate into the CIGS as well as the addition of alkali doping especially Na in the CIGS absorber layer. The amount Na in terms of the thicknesses of NaF was varied from 30Å to 200Å in order to study its contributions to the efficiency of the CIGS solar cells. The results show that the Na content in the CIGS films has a direct influence to the open-circuit voltage leading to the energy conversion efficiency and affects the distribution of Ga in the CIGS film. The influence of Na was studied and compared, based on the results of the performance of the solar cells, by using the NaF co-evaporation in various steps during the CIGS deposition process. The optimum thickness of NaF is approximately 50Å to achieve the maximum efficiency of 15.8% without antireflection coating. In addition, the quantum efficiency (QE) indicated different absorption in the long wavelength regions depending upon the methods of Na addition. © 2013 Elsevier Ltd. Source

Hassadee A.,King Mongkuts University of Technology Thonburi | Jutarosaga T.,King Mongkuts University of Technology Thonburi | Jutarosaga T.,Research Center in Thin Film Physics | Onreabroy W.,King Mongkuts University of Technology Thonburi | Onreabroy W.,Research Center in Thin Film Physics
Procedia Engineering | Year: 2012

Zinc-substituted cobalt ferrites, Co1-xZnxFe 2O4 (x = 0.0 - 0.5), were prepared by ceramic processing. The crystal structural, morphological and magnetic properties of the products were determined by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM) respectively. The results revealed that the spinel structure was also modified by the substitute ions. In Co1-xZnxFe2O4 samples, Zn 2+ commonly substitute for Co2+, resulting in an increase in the lattice parameter from 8.381 - 8.412 Å. Magnetization measurements indicated that Co1-xZnxFe2O4 samples with x = 0.0 - 0.5 showed ferrimagnetic behavior at room temperature. The decrease in the maximum magnetization of the Co1-xZn xFe2O4 samples from 134 to 100 emu/g and the decrease in the coercivity of the Co1-xZnxFe 2O4 samples from 140 to 4 Oe by increasing the zinc content from 0.0 to 0.5 can be attributed to the magnetic characteristic and the anisotropic nature of cobalt. © 2010 Published by Elsevier Ltd. Source

Chatraphorn P.,Chulalongkorn University | Chatraphorn P.,Research Center in Thin Film Physics | Chomngam C.,Chulalongkorn University | Chomngam C.,Research Center in Thin Film Physics
International Journal of Modern Physics B | Year: 2012

Most studies of thin film growth simulations are performed on flat substrates. However, in reality, a substrate is usually miscut leading to a vicinal surface with a small tilt. The goal of this work is to study effects of an initial configuration of a miscut substrate on the grown film. The Das SarmaTamborenea model with modified diffusion rules is used for the simulations. The modification is done to allow variation in the surface diffusion length and mobility of adatoms. The results show that the optimum conditions that lead to step-flow growth are long diffusion length and small step height. © 2012 World Scientific Publishing Company. Source

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