• Advances in materials Research
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• v.12 no.3
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• pp.179-192
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• 2023

We have studied the structural and optical properties of the non-doped and Co 0.08 at.%, Co 0.02 at.%, and Co 0.11 at.% doped ZnO nanorods (NRs) synthesized using the simple low-temperature chemical bath deposition (CBD) method at 95℃ for 2 hours. The scanning electron microscope (SEM) images confirmed the morphology of the ZnO NRs are affected by Co incorporation. As observed, the Co 0.08 at.% doped ZnO NRs have a larger dimension with an average diameter of 153.4 nm. According to the International Centre for Diffraction Data (ICDD) number #00-036-1451, the x-ray diffraction (XRD) pattern of non-doped and Co-doped ZnO NRs with the preferred orientation of ZnO NRs in the (002) plane possess polycrystalline hexagonal wurtzite structure with the space group P63mc. Optical absorbance indicates the Co 0.08 at.% doped ZnO NRs have stronger and blueshift bandgap energy (3.104 ev). The room temperature photoluminescence (PL) spectra of ZnO NRs exhibited excitonicrelates ultraviolet (UV) and defect-related green band (GB) emissions. By calculating the UV/GB intensity, the Co 0.08 at.% is the proper atomic percentage to have fewer intrinsic defects. We predict that Co-doped ZnO NRs induce a blueshift of near band edge (NBE) emission due to the Burstein-Moss effect. Meanwhile, the redshift of NBE emission is attributed to the modification of the lattice dimensions and exchange energy.

• Nuclear Engineering and Technology
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• v.55 no.7
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• pp.2556-2566
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• 2023

A first-principle approach within the framework of density functional theory was employed to study the effect of vacancy defects and fission products (FPs) doping on the mechanical, electronic, and thermodynamic properties of uranium monocarbide (UC). Firstly, the calculated vacancy formation energies confirm that the C vacancy is more stable than the U vacancy. The solution energies indicate that FPs prefer to occupying in U site rather than in C site. Zr, Mo, Th, and Pu atoms tend to directly replace U atom and dissolve into the UC lattice. Besides, the results of the mechanical properties show that U vacancy reduces the compressive and deformation resistance of UC while C vacancy has little effect. The doping of all FPs except He has a repairing effect on the mechanical properties of U_1-xC. In addition, significant modifications are observed in the phonon dispersion curves and partial phonon density of states (PhDOS) of UC_1-x, Zr_xU_1-xC, Mo_xU_1-xC, and Rh_xU_1-xC, including narrow frequency gaps and overlapping phonon modes, which increase the phonon scattering and lead to deterioration of thermal expansion coefficient (α_V) and heat capacity (C_p) of UC predicted by the quasi harmonic approximation (QHA) method.

• Journal of Powder Materials
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• v.31 no.2
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• pp.132-136
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• 2024

This study investigated the effects of revolution speed and ball size in planetary milling on the microstructure and dehydrogenation behavior of TiH₂ powder. The particle size analysis showed that the large particles present in the raw powder were effectively refined as the revolution speed increased, and when milled at 500 rpm, the median particle size was 1.47 ㎛. Milling with a mixture of balls of two or three sizes was more effective in refining the raw powder than milling with balls of a single size. A mixture of 3 mm and 5 mm diameter balls was the optimal condition for particle refinement, and the measured median particle size was 0.71 ㎛. The dependence of particle size on revolution speed and ball size was explained by changes in input energy and the number of contact points of the balls. In the milled powder, the endothermic peak measured using differential thermal analysis was observed at a relatively low temperature. This finding was interpreted as the activation of a dehydrogenation reaction, mainly due to the increase in the specific surface area and the concentration of lattice defects.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.284-284
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• 2016

Topological insulator (TI) is a bulk-insulating material with topologically protected Dirac surface states in the band gap. In particular, $Bi_2Se_3$ attracted great attention as a model three-dimensional TI due to its simple electronic structure of the surface states in a relatively large band gap (~0.3 eV). However, experimental efforts using $Bi_2Se_3$ have been difficult due to the abundance of structural defects, which frequently results in the bulk conduction being dominant over the surface conduction in transport due to the bulk doping effects of the defect sites. One promising approach in avoiding this problem is to reduce the structural defects by heteroepitaxially grow $Bi_2Se_3$ on a substrate with a compatible lattice structure, while also preventing surface degradation by encapsulating the pristine interface between $Bi_2Se_3$ and the substrate in a clean growth environment. A particularly promising choice of substrate for the heteroepitaxial growth is hexagonal boron nitride (h-BN), which has the same two-dimensional (2D) van der Waals (vdW) layered structure and hexagonal lattice symmetry as $Bi_2Se_3$. Moreover, since h-BN is a dielectric insulator with a large bandgap energy of 5.97 eV and chemically inert surfaces, it is well suited as a substrate for high mobility electronic transport studies of vdW material systems. Here we report the heteroepitaxial growth and characterization of high quality topological insulator $Bi_2Se_3$ thin films prepared on h-BN layers. Especially, we used molecular beam epitaxy to achieve high quality TI thin films with extremely low defect concentrations and an ideal interface between the films and substrates. To optimize the morphology and microstructural quality of the films, a two-step growth was performed on h-BN layers transferred on transmission electron microscopy (TEM) compatible substrates. The resulting $Bi_2Se_3$ thin films were highly crystalline with atomically smooth terraces over a large area, and the $Bi_2Se_3$ and h-BN exhibited a clear heteroepitaxial relationship with an atomically abrupt and clean interface, as examined by high-resolution TEM. Magnetotransport characterizations revealed that this interface supports a high quality topological surface state devoid of bulk contribution, as evidenced by Hall, Shubnikov-de Haas, and weak anti-localization measurements. We believe that the experimental scheme demonstrated in this talk can serve as a promising method for the preparation of high quality TI thin films as well as many other heterostructures based on 2D vdW layered materials.

• Journal of the Korean Vacuum Society
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• v.22 no.6
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• pp.313-320
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• 2013

The $AlAs_xSb_{1-x}$ step-graded buffer (SGB) layer was grown on the Silicon (Si) substrate to overcome lattice mismatch between Si substrate and $Al_{0.3}Ga_{0.7}As$/GaAs multiple quantum wells (MQWs). The value of root-mean-square (RMS) surface roughness for 5 nm-thick GaAs grown on $AlAs_xSb_{1-x}$ step-graded buffer layer was ~1.7 nm. $Al_{0.3}Ga_{0.7}As$/GaAs MQWs with AlAs/GaAs short period superlattice (SPS) were formed on the $AlAs_xSb_{1-x}$/Si substrate. Photoluminescence (PL) peak at 10 K for the $Al_{0.3}Ga_{0.7}As$/GaAs MQW structure showed relatively low intensity at ~813 nm. The RMS surface roughness of the $Al_{0.3}Ga_{0.7}As$/GaAs MQW structure was ~42.9 nm. The crystal defects were observed on the cross-sectional transmission electron microscope (TEM) images of the $Al_{0.3}Ga_{0.7}As$/GaAs MQW structure. The decrease of PL intensity and increase of RMS surface roughness would be due to the formation of the crystal defects.

• Korean Journal of Crystallography
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• v.5 no.2
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• pp.51-66
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• 1994

Y3Al5O2 and Nd: Y3Al5012 single crystals were grown by Czochralskl technique. The effectt of pulling rate rotation rate, and doping level of Nd3+ ion on the crystal quality were studied Various types of defects were analysed by photo-elastic effect and chemical etching method Finally, spectroscopic and laser poputies of grown crystal were measured. Optirmum pulling rate for good quality was dependant on the doping level of Nd3+ ion. It was found that the suitable pulling rates for pure Y3Al5O12 for 3.0∼3.5 a/o Nd3+ ion doped Y3Al5012 and for more than 40 a/o Nd3+ ion doped Y3Al5012 were 2∼4mm/hr, 0.6∼0.5mm/hr, and less than 0.4mm/hr respectively. Solid-liquid interface was convex at the rotation rate of 27∼60rpm, and concave at the rotation rate of 80∼100rpm. Growth axis was confired to <111> direction and lattice parameter was measured to 12.017A. Core (211) facets,striations, inclusions of metal particles, dislocations and optical inhonngeneities were detected. Four level laser transition of Nd3+ion in YIAls012 single crystal were identified by the spectroscopic measurements. Laser rod with tam diameter and 63mm length was fabricated from grown Nd3+ Y3Al5012 sin91e crystals. 1.8lJ of lasing threshould and 0.49% of soope efficiency were measured by the Pulsed laser action.

• Journal of the Mineralogical Society of Korea
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• v.28 no.1
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• pp.9-16
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• 2015

A mineralogical analysis on the factors affecting the luster of pearls was carried out using gravity measurement, optical microscope observation, X-ray diffraction analysis, and scanning electron microscopy. We divided the seawater cultured pearls from Tongyeong into the following four types based on luster and shape; good luster and round (LR), lackluster and round (LLR), lackluster and baroque (LLB), and lackluster and two nucleus (LTN) pearls. Pearls with high-quality luster had slightly lower specific gravity as compared to pearls with low-quality luster, but both these types of pearls are within the specific gravity range of commercial pearls. Regarding the cross-sectional thickness of the mother-of-pearl layer, LR pearls showed a uniform thickness of about 0.3 mm in average. On the other hand, LLR pearls were characterized by relatively thinner, but uniform thickness. LTN and LLB pearls showed a tendency of significantly large variation in thickness even within a single pearl. For the surface of pearls, pearls with high-quality luster showed narrower and clearer growth lines of aragonite crystals as compared to pearls with low-quality luster. Pearls with high-quality luster were characterized by fewer aragonite crystal lattice defects as compared to pearls with low-quality luster, and the former showed parallel arrangement, thinner thickness, and less difference in thickness on the surface and inside. If a pearl has a prismatic layer, it is composed of aragonite with calcite in the prismatic and nacreous layer, and calcite content is very high in the lackluster pearl. Pearls without a prismatic layer were devoid of calcite irrespective of their quality of luster, and were composed of aragonite.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.28 no.4
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• pp.152-158
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• 2018

PVT (Physical Vapor Transport) method has advantages in producing high quality, large scale wafers where many researches are being carried out to commercialize nitride semiconductors. However, complex process variables cause various defects when it had non-equilibrium growth conditions. Annealing process after crystal growth has been widely used to enhance the crystallinity. It is important to set appropriate temperature, pressure, and annealing time to improve crystallinity effectively. In this study, the effect of the annealing conditions on the crystalline structure variation of the AlN single crystal grown by PVT method was investigated with synchrotron whitebeam X-ray topography, electron backscattered diffraction (EBSD), and Rietveld refinement. X-ray topography analysis showed secondary phases, sub-grains, impurities including carbon inclusion in the single crystal before annealing. EBSD analyses identified that sub-grains with slightly tilted basal plane appeared and the overall number of grains increased after the annealing process. Rietveld refinement showed that the stress caused by the temperature gradient during the annealing process between top and bottom in the hot zone not only causes distortion of grains but also changes the lattice constant.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.06a
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• pp.327-327
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• 2009

ZnO with a large band gap (~3.37 eV) and exciton binding energy (~60 meV), is suitable for optoelectronic applications such as ultraviolet (UV) light emitting diodes (LEDs) and detectors. However, the ZnO-based p-n homojunction is not readily available because it is difficult to fabricate reproducible p-type ZnO with high hall concentration and mobility. In order to solve this problem, there have been numerous attempts to develop p-n heterojunction LEDs with ZnO as the n-type layer. The n-ZnO/p-GaN heterostructure is a good candidate for ZnO-based heterojunction LEDs because of their similar physical properties and the reproducible availability of p-type GaN. Especially, the reduced lattice mismatch (~1.8 %) and similar crystal structure result in the advantage of acquiring high performance LED devices. In particular, a number of ZnO films show UV band-edge emission with visible deep-level emission, which is originated from point defects such as oxygen vacancy, oxygen interstitial, zinc interstitial[1]. Thus, defect-related peak positions can be controlled by variation of growth or annealing conditions. In this work, the undoped ZnO film was grown on the p-GaN:Mg film using RF magnetron sputtering method. The undoped ZnO/p-GaN:Mg heterojunctions were annealed in a horizontal tube furnace. The annealing process was performed at $800^{\circ}C$ during 30 to 90 min in air ambient to observe the variation of the defect states in the ZnO film. Photoluminescence measurements were performed in order to confirm the deep-level position of the ZnO film. As a result, the deep-level emission showed orange-red color in the as-deposited film, while the defect-related peak positions of annealed films were shifted to greenish side as increasing annealing time. Furthermore, the electrical resistivity of the ZnO film was decreased after annealing process. The I-V characteristic of the LEDs showed nonlinear and rectifying behavior. The room-temperature electroluminescence (EL) was observed under forward bias. The EL showed a weak white and strong yellowish emission colors (~575 nm) in the undoped ZnO/p-GaN:Mg heterojunctions before and after annealing process, respectively.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.24 no.5
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• pp.202-206
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• 2014

In order to investigate the photoluminescence (PL) properties of $V_2O_5$ films, amorphous and crystalline films were prepared by using RF sputtering system, and the PL spectra of the films were measured at the temperatures ranging from 300 K to 10 K. In the amorphous $V_2O_5$ film grown at room temperature, a PL peak centered at ~505 nm was only observed, and in the crystalline $V_2O_5$ film, two peaks centered at ~505 nm and ~695 nm, which is known to correspond to oxygen defects, were revealed. The position of PL peak centered at 505 nm for both the amorphous and crystalline $V_2O_5$ films showed a strong dependence on temperature, and the positions were 2.45 eV at 300 K and 2.35 eV at 10 K, respectively. The PL at 505 nm was due to the band energy transition in $V_2O_5$, and also, the reduction of the peak position energy with decreasing temperature was caused by a decrement of the lattice dilatation effect with reducing electron-phonon interaction.