• Korean Journal of Materials Research
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• v.26 no.10
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• pp.577-582
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• 2016

$Dy^{3+}$ and $Eu^{3+}$-codoped $SrWO_4$ phosphor thin films were deposited on sapphire substrates by radio frequency magnetron sputtering by changing the growth and thermal annealing temperatures. The results show that the structural and optical properties of the phosphor thin films depended on the growth and thermal annealing temperatures. All the phosphor thin films, irrespective of the growth or the thermal annealing temperatures, exhibited tetragonal structures with a dominant (112) diffraction peak. The thin films deposited at a growth temperature of $100^{\circ}C$ and a thermal annealing temperature of $650^{\circ}C$ showed average transmittances of 87.5% and 88.4% in the wavelength range of 500-1100 nm and band gap energy values of 4.00 and 4.20 eV, respectively. The excitation spectra of the phosphor thin films showed a broad charge transfer band that peaked at 234 nm, which is in the range of 200-270 nm. The emission spectra under ultraviolet excitation at 234 nm showed an intense emission peak at 572 nm and several weaker bands at 479, 612, 660, and 758 nm. These results suggest that the $SrWO_4$: $Dy^{3+}$, $Eu^{3+}$ thin films can be used as white light emitting materials suitable for applications in display and solid-state lighting.

• Journal of Surface Science and Engineering
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• v.54 no.3
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• pp.112-118
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• 2021

Dy³⁺- and Eu³⁺-codoped BaWO₄ phosphors for white light-emitting diode were synthesized with different activator ions via a solid-state reaction process. The structural, morphological, and optical properties of the BaWO₄:Dy³⁺,Eu³⁺ phosphors were investigated as a function of Eu³⁺ concentration at a fixed concentration of Dy³⁺ ions. XRD patterns exhibited that all the synthesized phosphors had a tetragonal system, irrespective of the concentrations of Dy³⁺ and Eu³⁺ ions. The excitation spectra of the synthesized phosphors were composed of three intense bands centered at 251, 355, and 393 nm and several weak peaks. For the BaWO₄:Dy³⁺,Eu³⁺ phosphors synthesized with 1 mol% of Eu³⁺, the emission spectra under ultraviolet excitation at 393 nm showed two strong blue and yellow bands at 485 and 577 nm corresponding to the ⁴F_9/2⁶H_15/2 and ⁴F_9/2⁶H_13/2 transitions of Dy³⁺ ions, respectively and several weak bands in the range of 600-700 nm resulting from the 4f transitions of Eu³⁺ ions. As the concentration of Eu³⁺ ions increased, intensities of the blue and yellow emission bands gradually decreased while those of the red emissions increased rapidly and the energy transfer efficiency from Dy³⁺ to Eu³⁺ ions was 95.3% at 20 mol% of Eu³⁺. The optimum white light emission with x=0.363, y=0.357 CIE 1931 chromaticity coordinates was obtained for the sample doped with 5 mol% Dy³⁺ and 1 mol% of Eu³⁺.

• Korean Journal of Materials Research
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• v.31 no.6
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• pp.320-324
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• 2021

Effects of growth variables and post-growth annealing on the optical, structural and electrical properties of magnetron-sputtered Ga_0.04Mg_0.10Zn_0.86O films are characterized in detail. It is observed that films grown from pure oxygen plasma showed high resistivity, ~10² Ω·cm, whereas films grown in Ar plasma showed much lower resistivity, 2.0 × 10^-2 ~ 1.0 × 10^-1 Ω·cm. Post-growth annealing significantly improved the electrical resistivity, to 4.3 ~ 9.0 × 10^-3 Ω·cm for the vacuum annealed samples and to 1.3 ~ 3.0 × 10^-3 Ω·cm for the films annealed in Zn vapor. It is proposed that these phenomena may be attributed to the improved crystalline quality and to changes in the defect chemistry. It is suggested that growth within oxygen environments leads to suppression of oxygen vacancy (Vo) donors and formation of Zn vacancy (V_Zn) acceptors, resulting in highly resistive films. After annealing treatment, the activation of Ga donors is enhanced, Vo donors are annihilated, and crystalline quality is improved, increasing the electron mobility and the concentration. After annealing in Zn vapor, Zn interstitial donors are introduced, further increasing the electron concentration.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.293-293
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• 2014

Over last decade InGaN alloy structures have become the one of the most promising materials among the numerous compound semiconductors for high efficiency light sources because of their direct band-gap and a wide spectral region (ultraviolet to infrared). The primary cause for the high quantum efficiency of the InGaN alloy in spite of high threading dislocation density caused by lattice misfit between GaN and sapphire substrate and severe built-in electric field of a few MV/cm due to the spontaneous and piezoelectric polarizations is generally known as the strong exciton localization trapped by lattice-parameter-scale In-N clusters in the random InGaN alloy. Nonetheless, violet-emitting (390 nm) conventional low-In-content InGaN/GaN multi-quantum wells (MQWs) show the degradation in internal quantum efficiency compared to blue-emitting (450 nm) MQWs owing higher In-content due to the less localization of carrier and the smaller band offset. We expected that an improvement of internal quantum efficiency in the violet region can be achieved by replacing the conventional low-In-content InGaN/GaN MQWs with ultra-thin, high-In-content (UTHI) InGaN/GaN MQWs because of better localization of carriers and smaller quantum-confined Stark effect (QCSE). We successfully obtain the UTHI InGaN/GaN MQWs grown via employing the GI technique by using the metal-organic chemical vapor deposition. In this work, 1 the optical and structural properties of the violet-light-emitting UTHI InGaN/GaN MQWs grown by employing the GI technique in comparison with conventional low-In-content InGaN/GaN MQWs were investigated. Stronger localization of carriers and smaller QCSE were observed in UTHI MQWs as a result of enlarged potential fluctuation and thinner QW thickness compared to those in conventional low-In-content MQWs. We hope that these strong carrier localization and reduced QCSE can turn the UTHI InGaN/GaN MQWs into an attractive candidate for high efficient violet emitter. Detailed structural and optical characteristics of UTHI InGaN/GaN MQWs compared to the conventional InGaN/GaN MQWs will be given.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.259-259
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• 2010

Periodically polarity inverted (PPI) ZnO structures on (0001) Al2O3 substrates are demonstrated by plasmas assisted molecular beam epitaxy. The patterning and re-growth methods are used to realize the PPI ZnO by employing the polarity controlling method. For the in-situ polarity controlling of ZnO films, Cr-compound buffer layers are used.[1, 2] The region with the CrN intermediate layer and the region with the Cr2O3 and Al2O3 substrate were used to grow the Zn- and O-polar ZnO films, respectively. The growth behaviors with anisotropic properties of PPI ZnO heterostructures are investigated. The periodical polarity inversion is evaluated by contrast images of piezo-response microscopy. Structural and optical interface properties of PPI ZnO are investigated by the transmission electron microcopy (TEM) and micro photoluminescence ($\mu$-PL). The inversion domain boundaries (IDBs) between the Zn and the O-polar ZnO regions were clearly observed by TEM. Moreover, the investigation of spatially resolved local photoluminescence characteristics of PPI ZnO revealed stronger excitonic emission at the interfacial region with the IDBs compared to the Zn-polar or the O-polar ZnO region. The possible mechanisms will be discussed with the consideration of the atomic configuration, carrier life time, and geometrical effects. The successful realization of PPI structures with nanometer scale period indicates the possibility for the application to the photonic band-gap structures or waveguide fabrication. The details of application and results will be discussed.

• Korean Journal of Metals and Materials
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• v.47 no.1
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• pp.44-49
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• 2009

Indium tin oxide(ITO) thin films were deposited on the glass substrates by radio-frequency (RF) magnetron sputtering method. The influence of rapid thermal annealing (RTA) treatment on the optical and electrical properties of the films were investigated for the purpose of fabricating plasma display signboard. Structural properties, surface roughness, sheet resistance and transmittance of the ITO film were analysed by using x-ray diffraction method, atomic force microscopy (AFM), four point prove, and ultraviolet-visible spectrometer, respectively. It was found that the RTA treatment increased the transmittance and decreased the resistivity of the ITO film, respectively. Furthermore, we successfully demonstrated the direct-current plasma signboard by using ITO electrode and phosphors.

• Journal of Electrochemical Science and Technology
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• v.15 no.1
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• pp.67-95
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• 2024

Direct alcohol fuel cells (DAFCs) have gained much attention as promising energy conversion devices due to their ability to utilize alcohol as a fuel source. In this regard, Molybdenum-based electrocatalysts (Mo-ECs) have emerged as a substitution for expensive Pt and Ru-based co-catalyst electrode materials in DAFCs, owing to their unique electrochemical properties useful for alcohol oxidation. The catalytic activity of Mo-ECs displays an increase in alcohol oxidation current density by several folds to 1000-2000 mA mg_Pt^-1, compared to commercial Pt and PtRu catalysts of 10-100 mA mg_Pt^-1. In addition, the methanol oxidation peak and onset potential have been significantly reduced by 100-200 mV and 0.5-0.6 V, respectively. The performance of Mo-ECs in both acidic and alkaline media has shown the potential to significantly reduce the Pt loading. This review aims to provide a comprehensive overview of the bifunctional mechanism involved in the oxidation of alcohols and factors affecting the electrocatalytic oxidation of alcohol, such as synthesis method, structural properties, and catalytic support materials. Furthermore, the challenges and prospects of Mo-ECs for DAFCs anode materials are discussed. This in-depth review serves as valuable insight toward enhancing the performance and efficiency of DAFC by employing Mo-ECs.

• Korean Journal of Materials Research
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• v.24 no.9
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• pp.469-473
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• 2014

$SrSnO_3$ phosphor powders were synthesized with two different contents of activator ions $Eu^{3+}$ and $Tb^{3+}$ using the solid-state reaction method. The structural, morphological, and optical properties of the phosphors were investigated using X-ray diffractometry, field-emission scanning electron microscopy, and fluorescence spectrophotometry, respectively. All the phosphors showed a cubic structure, irrespective of the type and the content ratio of activator ions. For $Eu^{3+}$-doped $SrSnO_3$ phosphors, the intensity of the 620 nm red emission spectrum resulting from the $^5D_0{\rightarrow}^7F_2$ transition of $Eu^{3+}$ was stronger than that of the 595 nm orange emission signal due to the $^5D_0{\rightarrow}^7F_1$ transition in the range 0.01-0.05 mol of $Eu^{3+}$, but the ratio of the intensity was reversed in the range 0.10-0.20 mol of $Eu^{3+}$. The variation in the emission intensity indicates that the site symmetry of the $Eu^{3+}$ ions around the host crystal was changed from non-inversion symmetry to inversion. For the $Tb^{3+}$-doped $SrSnO_3$ phosphors under excitation at 281 nm, one strong green emission band at 550 nm and several weak bands were observed. These results suggest that the optimum red and green emission signals can be realized when the activator ion content for $Eu^{3+}$- or $Tb^{3+}$-doped $SrSnO_3$ phosphors is 0.20 mol and 0.15 mol, respectively.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.141-141
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• 2010

The effect of thermal anneal on the characteristics of structural properties and the enhancement of luminescence and photovoltaic (PV) characteristics of silicon-rich silicon-nitride films were investigated. By using an ultra high vacuum ion beam sputtering deposition, B-doped silicon-rich silicon-nitride (SRSN) thin films, with excess silicon content of 15 at. %, on P-doped (n-type) Si substrate was fabricated, sputtering a highly B doped Si wafer with a BN chip by N plasma. In order to examine the influence of thermal anneal, films were then annealed at different temperature up to $1100^{\circ}C$ under $N_2$ environment. Raman, X-ray diffraction, and X-ray photoemission spectroscopy did not show any reliable evidence of amorphous or crystalline Si clusters allowing us concluding that nearly no Si nano-cluster could be formed through the precipitation of excess Si from SRSN matrix during thermal anneal. Instead, results of Fourier transform infrared and X-ray photoemission spectroscopy clearly indicated that defective, amorphous Si-N matrix of films was changed to be well-ordered thanks to high temperature anneal. The measurement of spectral ellipsometry in UV-visible range was carried out and we found that the optical absorption edge of film was shifted to higher energy as the anneal temperature increased as the results of thermal anneal induced formation of $Si_3N_4$-like matrix. These are consistent with the observation that higher visible photoluminescence, which is likely due to the presence of Si-N bonds, from anneals at higher temperature. Based on these films, PV cells were fabricated by the formation of front/back metal electrodes. For all cells, typical I-V characteristic of p-n diode junction was observed. We also tried to measure PV properties using a solar-simulator and confirmed successful operation of PV devices. Carrier transport mechanism depending on anneal temperature and the implication of PV cells based on SRSN films were also discussed.

• JSTS:Journal of Semiconductor Technology and Science
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• v.16 no.5
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• pp.664-674
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• 2016

The electrical properties of Ti/p-type InP Schottky diodes with and without polyaniline (PANI) interlayer was investigated using current-voltage (I-V) and capacitance-voltage (C-V) measurements. The barrier height of Ti/p-type InP Schottky diode with PANI interlayer was higher than that of the conventional Ti/p-type InP Schottky diode, implying that the organic interlayer influenced the space-charge region of the Ti/p-type InP Schottky junction. At higher voltages, the current transport was dominated by the trap free space-charge-limited current and trap-filled space-charge-limited current in Ti/p-type InP Schottky diode without and with PANI interlayer, respectively. The domination of trap filled space-charge-limited current in Ti/p-type InP Schottky diode with PANI interlayer could be associated with the traps originated from structural defects prevailing in organic PANI interlayer.