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Hassadee A.,King Mongkut's University of Technology Thonburi | Jutarosaga T.,King Mongkut's University of Technology Thonburi | Jutarosaga T.,Research Center in Thin Film Physics | Onreabroy W.,King Mongkut's 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.


Chaya P.,King Mongkut's University of Technology Thonburi | Jutarosaga T.,Research Center in Thin Film Physics | Onreabroy W.,Research Center in Thin Film Physics
Advanced Materials Research | Year: 2014

The strontium hexaferrite (SrFe12O19) and Co-substituted strontium hexaferrite (SrCoFe11O19) were prepared by ceramic method. The milled mixture of Fe2O3, SrCO3 and CoO powders were calcined at 1100°C and pellets sintered at 1300°C in air. The crystal structure, morphology and magnetic properties of samples have been investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and vibrating sample magnetometer (VSM), respectively. The crystal structure of SrFe12O19 was hexaferrite with the crystallite size and the lattice constants a and c of 59. 6 nm, 5. 8 Å, and 23. 0 Å, respectively. Also, the crystal structure of SrCoFe11O19 was hexaferrite with the crystallite size and the lattice constants a and c of 63. 7 nm, 5. 9 Å and 23. 0 Å, respectively. The morphology of obtained samples changed from hexagonal rods to discs shape and grain sizes increased with the increase of doped Co in SrFe12O19. SrFe12O19 with the coercive force (Hc) of 2, 133 Oe was classified as hard ferrite magnetic. While, Co-substituted strontium hexaferrite (SrCoFe11O19) was soft ferrite magnetic with coercive force of 64 Oe. Results indicated that magnetic properties of samples such as hard ferrite magnetic and soft ferrite magnetic showed great dependence on the cobalt additive in strontium. © (2014) Trans Tech Publications, Switzerland.


Kanjanaput W.,Chulalongkorn University | Kanjanaput W.,Research Center in Thin Film Physics | Limkumnerd S.,Chulalongkorn University | Limkumnerd S.,Research Center in Thin Film Physics | And 2 more authors.
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2010

The energetically driven Ehrlich-Schwoebel barrier had been generally accepted as the primary cause of the growth instability in the form of quasiregular moundlike structures observed on the surface of thin film grown via molecular-beam epitaxy (MBE) technique. Recently the second mechanism of mound formation was proposed in terms of a topologically induced flux of particles originating from the line tension of the step edges which form the contour lines around a mound. Through large-scale simulations of MBE growth on a variety of crystalline lattice planes using limited-mobility, solid-on-solid models introduced by Wolf-Villain and Das Sarma-Tamborenea in 2+1 dimensions, we show that there exists a topological uphill particle current with strong dependence on specific lattice crystalline structure. Without any energetically induced barriers, our simulations produce spectacular mounds very similar, in some cases, to what have been observed in many recent MBE experiments. On a lattice where these currents cease to exist, the surface appears to be scale invariant, statistically rough as predicted by the conventional continuum growth equation. © 2010 The American Physical Society.


Sakdanuphab R.,Chulalongkorn University | Sakdanuphab R.,Research Center in Thin Film Physics | Chityuttakan C.,Chulalongkorn University | Chityuttakan C.,Research Center in Thin Film Physics | And 6 more authors.
Journal of Crystal Growth | Year: 2011

Cu(In,Ga)Se2 or CIGS thin films with NaF precursor are grown on Mo coated soda-lime glass (SLG) substrates using a molecular beam deposition (MBD) technique. The growth characteristics of the CIGS films deposited using the 3-stage process are examined by interrupting the deposition process at the end of each stage and the transition temperature at the beginning of the second stage. The evolution of the CIGS films derived from the compounds, e.g. γ-(In,Ga)2Se3, Cu(In,Ga)Se2Cu xSe, is investigated by comparing the properties of the films to those without an NaF precursor. The XRD spectra show the dominant peak of (1 0 5) preferred orientation of the γ-(In,Ga)2Se3 enhanced by the NaF precursor which is seen at the end of the first stage. The decrease of (2 2 0)(2 0 4) intensity of the CIGS film is found at the end of the second stage followed by the increase of (1 1 2) intensity at the end point. The AFM images at the end of the first stage show a smooth surface with similar grain shape in the films with the NaF precursor. During the second stage, the grain size of the Cu-rich CIGS film increases with slightly sharper grain boundaries. However, at the end point, the CIGS film is fully obtained and shows small sharp grains corresponding to the increase of (1 1 2) orientation. The cross-section SEM images show small columnar grains with deep grain boundaries at the end point. The AES depth profiles show that most Na atoms are located near the bottom layer in the first stage and diffuse to the surface of the film after increasing the temperature. Then the Na atoms uniformly distribute into the Cu-rich CIGS film during the second stage. Significantly high Na content is found at the surface of the CIGS film at the end point. In addition, a gradient of Ga composition in the CIGS film is also observed in the AES measurement. A simple model of Na-enhanced CIGS thin film growth based on the experimental results is proposed to describe the growth and doping mechanisms. © 2011 Elsevier B.V.


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.


Klawtanong M.,Chulalongkorn University | Klawtanong M.,Research Center in Thin Film Physics | Srinitiwarawong C.,Chulalongkorn University | Srinitiwarawong C.,Research Center in Thin Film Physics | And 2 more authors.
Journal of Crystal Growth | Year: 2015

We use a two-dimensional ball and spring model to model a heteroepitaxial system using fast kinetic Monte Carlo simulations. Effects of deposition flux, lattice mismatch, and growth temperature on the morphology of film surfaces are studied in detail. The two strain-relieve mechanisms, island formation and pit formation are investigated. We find the formation of islands at large lattice mismatch and high growth temperature, in agreement with the island nucleation theory. In limited mobility growth at high deposition flux, the formation of pits is found to be more preferable. The increase in the lattice mismatch leads to the decrease of critical thickness and island size. These values, however, are also significantly affected by growth conditions. Increasing the deposition flux results in larger critical thickness and smaller island size while higher growth temperature causes the critical thickness to be smaller but island size becomes larger. © 2014 Elsevier B.V. All rights reserved.


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.


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.


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.


Noikaew B.,Chulalongkorn University | Noikaew B.,Research Center in Thin Film Physics | Chatraphorn S.,Chulalongkorn University | Chatraphorn S.,Research Center in Thin Film Physics
Surface and Coatings Technology | Year: 2016

CuIn1 − xGaxSe2 (CIGS) polycrystalline thin film solar cells were deposited on 10 cm × 10 cm Mo–coated soda–lime glass (SLG) substrates using the three–stage co–evaporation process. The formation of (In,Ga)2Se3 precursor layer was strongly affected by the Se flux supplied during the first stage of the deposition. In our deposition system, the Se source temperature of 300 °C was sufficient for the growth of precursor and throughout the entire deposition. Then, the influence of the substrate temperatures during the first stage (T1), the second and third stages (T2) of the CIGS thin film depositions were investigated by varying T1 from 300 °C–410 °C and T2 from 500 °C–580 °C. The radiation from the surface of the growing films detected by a pyrometer and the output power of the substrate temperature controller were used to monitor the deposition process. For our system, the optimum values of T1 and T2 were 370 °C and 520 °C, respectively. The XRD results showed the highest (112):(220)/(204) intensity ratio of the chalcopyrite phase for T1 = 370 °C and T2 = 520 °C resulting in the device's maximum efficiency of 13.7%. © 2016 Elsevier B.V.

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