Seung Muk Lee

Effect of annealing temperature on microstructural evolution and electrical properties of sol-gel processed ZrO2/Si films

High-permittivity (k) ZrO2/Si(100) films were fabricated by a sol-gel technique and the microstructural evolution with the annealing temperature (Ta) was correlated with the variation of their electrical performance. With increasing Ta, the ZrO2 films crystallized into a tetragonal (t) phase which was maintained until 700 °C at nanoscale thicknesses. Although the formation of the t-ZrO2 phase obviously enhanced the k value of the ZrO2 dielectric layer, the maximum capacitance in accumulation was decreased by the growth of a low-k interfacial layer (IL) between ZrO2 and Si with increasing Ta. On the other hand, the gate leakage current was remarkably depressed with increasing Ta probably due to the combined effects of the increased IL thickness, optical band gap of ZrO2, and density of ZrO2 and decreased remnant organic components.

Hydrothermally Grown In-doped ZnO Nanorods on p-GaN Films for Color-tunable Heterojunction Light-emitting-diodes

The incorporation of doping elements in ZnO nanostructures plays an important role in adjusting the optical and electrical properties in optoelectronic devices. In the present study, we fabricated 1-D ZnO nanorods (NRs) doped with different In contents (0% ~ 5%) on p-GaN films using a facile hydrothermal method, and investigated the effect of the In doping on the morphology and electronic structure of the NRs and the electrical and optical performances of the n-ZnO NRs/p-GaN heterojunction light emitting diodes (LEDs). As the In content increased, the size (diameter and length) of the NRs increased, and the electrical performance of the LEDs improved. From the electroluminescence (EL) spectra, it was found that the broad green-yellow-orange emission band significantly increased with increasing In content due to the increased defect states (oxygen vacancies) in the ZnO NRs, and consequently, the superposition of the emission bands centered at 415 nm and 570 nm led to the generation of white-light. These results suggest that In doping is an effective way to tailor the morphology and the optical, electronic, and electrical properties of ZnO NRs, as well as the EL emission property of heterojunction LEDs.

Synergistic effect of Indium and Gallium co-doping on growth behavior and physical properties of hydrothermally grown ZnO nanorods

We synthesized ZnO nanorods (NRs) using simple hydrothermal method, with the simultaneous incorporation of gallium (Ga) and indium (In), in addition, investigated the co-doping effect on the morphology, microstructure, electronic structure, and electrical/optical properties. The growth behavior of the doped NRs was affected by the nuclei density and polarity of the (001) plane. The c-axis parameter of the co-doped NRs was similar to that of undoped NRs due to the compensated lattice distortion caused by the presence of dopants that are both larger (In3+) and smaller (Ga3+) than the host Zn2+ cations. Red shifts in the ultraviolet emission peaks were observed in all doped NRs, owing to the combined effects of NR size, band gap renormalization, and the presence of stacking faults created by the dopant-induced lattice distortions. In addition, the NR/p-GaN diodes using co-doped NRs exhibited superior electrical conductivity compared to the other specimens due to the increase in the charge carrier density of NRs and the relatively large effective contact area of (001) planes. The simultaneous doping of In and Ga is therefore anticipated to provide a broader range of optical, physical, and electrical properties of ZnO NRs for a variety of opto-electronic applications.

Optical study of sol-gel processed ZrO2/Si films by spectroscopic ellipsometry

We report optical properties of amorphous and tetragonal ZrO2 films grown on Si substrates by sol-gel deposition and formed by annealing at different temperatures. Pseudodielectric-function spectra ⟨e⟩ from 1.12 to 6.52 eV were acquired by spectroscopic ellipsometry at angles of incidence of 50, 55, 60, 65, and 70° with the samples at room temperature, then analyzed with the Tauc–Lorentz (TL) model for the refractive index and extinction coefficient of the films. These depend significantly on annealing temperature, and consistent with X-ray diffraction data, showing that amorphous ZrO2 crystallizes into the tetragonal phase between 300 and 500 °C, and that it coexists with the monoclinic phase after annealing at 700 °C. The dielectric functions of these materials can be calculated analytically within this spectral range from the TL parameters given.We report optical properties of amorphous and tetragonal ZrO2 films grown on Si substrates by sol-gel deposition and formed by annealing at different temperatures. Pseudodielectric-function spectra ⟨e⟩ from 1.12 to 6.52 eV were acquired by spectroscopic ellipsometry at angles of incidence of 50, 55, 60, 65, and 70° with the samples at room temperature, then analyzed with the Tauc–Lorentz (TL) model for the refractive index and extinction coefficient of the films. These depend significantly on annealing temperature, and consistent with X-ray diffraction data, showing that amorphous ZrO2 crystallizes into the tetragonal phase between 300 and 500 °C, and that it coexists with the monoclinic phase after annealing at 700 °C. The dielectric functions of these materials can be calculated analytically within this spectral range from the TL parameters given.

Surface-tunable bioluminescence resonance energy transfer via geometry-controlled ZnO nanorod coordination

The use of ZnO nanorods (NRs) as an effective coordinator and biosensing platform to create bioluminescence resonance energy transfer (BRET) is reported. Herein, a hydrothermal approach is applied to obtain morphologically controlled ZnO NRs, which are directly bound to luciferase (Luc) and carboxy-modified quantum dot (QD) acting as a donor-acceptor pair for BRET. BRET efficiency varies significantly with the geometry of ZnO NRs, which modulates the coordination between hexahistidine-tagged Luc (Luc-His6 ) and QD, owing to the combined effect of the total surface area consisting of (001) and (100) planes and their surface polarities. Unlike typical QD-BRET reactions with metal ions (e.g., zinc ions), a geometry-controlled ZnO NR platform can facilitate the design of surface-initiated BRET sensors without being supplemented by copious metal ions: the geometry-controlled ZnO NR platform can therefore pave the way for nanostructure-based biosensors with enhanced analytical performance.

Effects of Heating Time and Intermediate Heating on Sol–Gel-Processed ZrO2 Thin Films

Nanoscale-thick high-k ZrO2 films were fabricated via sol–gel method and the effects of heating time and an intermediate heat treatment on the microstructure and dielectric properties were studied. One set of films was spin-coated with a thickness of ~13 nm on Si substrates by two consecutive coating and drying sequences, followed by heating at 500 °C for 1, 5, or 10 h, while another sample was prepared by heating at 500 °C for 1 h after each drying step (intermediate heated sample). As the heating time was extended, the permittivity increased, but the leakage current property degraded. In contrast, both the permittivity and leakage current property were enhanced in the case of the intermediate heated sample. These improvements were likely due to the effective elimination of remnant organics and the development of a two-layer morphology consisting of lower amorphous and upper crystalline layers, the formation of which occurred via the minimization of surface and strain energies.

Enhanced Magnetic Property of MgB

Abstract-We fabricated carbon (C)-doped MgB₂ wires using phenol-formaldehyde resin (PF) as a C source to improve their critical current density (J_c) in a magnetic field. In order to induce homogenous C doping, boron (B) powders were mixed with PF via liquid mixing technique and then the PF-treated B and Mg powders were mixed. The wires were fabricated by conventional powder-in-tube (PIT) process, followed by heat treatment through an in-situ reaction route. We observed that C was substituted for the B sites in the PF-doped MgB₂ and that the actual C content increased with increasing PF content. The doped MgB₂ wires exhibited lower critical transition temperature (T_c ) and self-field J_c, but superior H_c2 value and better J_c(B) performance than the undoped sample. The 3 wt% PF sample had the highest J_c value of 7.77 × 10³ A/cm , which is significantly higher than that of the undoped sample (0.29 × 10³ A/cm²) at 5 K and 6.4 T.

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We report a novel GeInSnOx (GeITO) thin-film transistor (TFT) synthesized by a solution process, utilizing Ge as a charge carrier suppressor and amorphization-promoter, and the dependence of its microstructure, electronic structure and electrical properties on sintering temperature. The amorphous structure was maintained regardless of the sintering temperature. As the sintering temperature increased, the amount of oxygen vacancies increased and GeO2 bonds transformed into GeO bonds near the film surface above 400 °C. In addition, the In 5sp/Sn 5sp states appeared to act as the dominant electron source in the GeITO channel layers with increasing sintering temperature. These behaviours influenced TFT performances: the saturation mobility was increased from 0.004 to 6.4 cm2 V−1 s−1, while the threshold voltage was shifted in the negative direction by increasing the sintering temperature, which demonstrates the high sensitivity of the solution-deposited GeITO to the processing temperature.

Wire Using Phenol-Formaldehyde Resin

The dielectric functions, e, of amorphous LaAlOx (LAO) films grown by the sol–gel process are investigated using spectroscopic ellipsometry. The LAO precursor sols are prepared at a molar ratio of La:Al = 1:1 with different mole concentrations to control the film thickness. The films are deposited on p-type Si substrates treated with dilute HF, and the sols are sintered at 400 °C for 2 h in an ambient atmosphere. Room-temperature pseudodielectric function spectra, ⟨e⟩, are obtained from 0.7 to 8.6 eV, and best fits of the data are obtained with the Tauc–Lorentz (TL) model. The authors observe an increase in both the real and imaginary parts of e, a decrease in the TL threshold energy Eg, and an increase in film thickness with increasing mole concentration of the precursor solution.