Duck-Kyun Choi

Dye-sensitized solar cells using network structure of electrospun ZnO nanofiber mats

Nanostructured semiconducting metal oxides and particularly nanofiber-based photoelectrodes can provide enhanced energy conversion efficiencies in dye-sensitized solar cells (DSSCs). In this study ZnO/poly(vinyl acetate) composite nanofiber mats were directly electrospun onto a glass substrate coated with F:SnO2, then hot pressed at 120°C and calcined at 450°C. This resulted in multiple nanofiber networks composed of a twisted structure of 200–500nm diameter cores with ∼30nm single grains. The DSSCs using ZnO nanofiber mats exhibited a conversion efficiency of 1.34% under 100mW∕cm2 (AM-1.5G) illumination.

Formation of hillocks in Pt/Ti electrodes and their effects on short phenomena of PZT films deposited by reactive sputtering

The formation of hillocks was examined in various Pt/Ti electrodes with similar thermal histories, which were annealed prior to PZT film deposition by the sputtering method. The Pt electrode with no Ti underlayer did not exhibit hillocks. An increase of the Ti thickness or a decrease of the Pt thickness in the Pt/Ti electrodes were found to result in an increase in hillock density and the reduction of hillock size. A pre-annealing in Ar reduced the oxidation rate of Ti during the subsequent oxygen annealing, resulting in the reduction in density as well as the size of hillocks. The density and the size of hillocks were also reduced by a decrease of oxygen partial pressure during oxygen annealing. Hillocks in Pt/Ti electrodes are formed to relieve the compressive stress of the Pt films, which is mainly developed by the volume expansion of the Pt layer, due to the oxidation of diffused Ti along the Pt grain boundaries. A comparison of the short probabilities of the PZT films with the observed hillocks suggests that the electrical short of the PZT capacitors should not depend on the density, but on the size of hillocks.

Electrical characteristics of thin-film transistors using field-aided lateral crystallization

A polycrystalline silicon thin-film transistor (TFT) technology, field-aided lateral crystallization (FALC), has been explored. Polycrystalline silicon thin film, as an active layer, was prepared by applying an electric field to amorphous silicon film during Ni metal-induced lateral crystallization (MILC). Compared with the conventional metal-induced lateral crystallization thin-film transistors (MILC TFTs), these field-aided lateral crystallization thin-film transistors (FALC TFTs) show a low off-state leakage current of 1.79×10−11 A at Vg=−10 V and a high on/off current ratio of 8.82×105. Moreover, the threshold voltage is lower and field-effect mobility is higher than those of MILC TFTs. Therefore, the possibility of high-performance and low-temperature (<500 °C) polycrystalline silicon TFTs was demonstrated by using FALC technology.

Piezoelectric Properties and Densification Based on Control of Volatile Mass of Potassium and Sodium in (K0.5Na0.5)NbO3 Ceramics

Recently lead-free ceramic systems have been intensively studied in order to fabricate environmentally friendly piezoelectric ceramics and replace widely used lead-free piezoelectric ceramics. The (K0.5Na0.5)NbO3 (KNN) piezoelectric ceramics with non-lead piezoelectric ceramics were manufactured by conventional solid-state synthesis, and we obtained sintered bodies with relative densities over 97.4% by adding excess K (0.44 mol %)/Na (0.22 mol %) to improve sinterability. The composition of the sintered bodies was close to stoichiometric compounds. The electromechanical coupling factor (kp) and piezoelectric constant (d33) of KNN + excess K (0.44 mol %)/Na (0.22 mol %) composition ceramics showed optimal values of 0.341 and 126 pC/N, respectively.

Correlation of band edge native defect state evolution to bulk mobility changes in ZnO thin films

The evolution of native defect states near conduction band present in ZnO thin films is correlated with the bulk electron density and mobility changes driven by the thermal structure modification. The evolution of band edge electronic structures of ZnO thin films was studied via the spectroscopic detection of empty localized defect states in conduction band (CB) edge and occupied defect states in valence band using spectroscopic ellipsometry and x-ray photoemission spectroscopy. The energy depth of native defect states against CB edge revealed the direct correlation to Hall mobility values for ZnO thin films.

H2 gas-sensing characteristics of SnOx sensors fabricated by a reactive ion-assisted deposition with/without an activator layer

Abstract SnOx-based gas-sensor devices have been fabricated by a reactive ion-assisted deposition technique. SnOx films of 400 A thickness are deposited on sputtered amorphous SiO2 substrates and then Pt films (600 A) are sputtered on top of them as upper electrodes. From XRD and TEM studies, as-deposited SnOx films are seen to be amorphous and become polycrystalline after annealing at 500°C in air. In quantitative XPS analyses, the O/Sn atomic ratio in the annealed films shows the nearly stoichiometric value of two and the binding energy of Sn 3d5/2 is 486.43 eV, very close to 486.75 eV of the standard tin oxide powder. When sensitivity is defined as a relative decrement in resistance, these sensor devices show nearly 100% sensitivities to 1000–5000 ppm H2 in air at 200°C and above. By attaching an ultra-thin metal activator, the response time can be reduced to less than one tenth of its previous value.

Erratum: “Influences of Various Metal Elements on Field Aided Lateral Crystallization of Amorphous Silicon Films”

In this study, the effects of various metals on field aided lateral crystallization (FALC) behaviors of amorphous silicon (a-Si) were investigated. Under the influence of the electric field, some metals such as Cu, Ni and Co were found to induce the lateral crystallization toward the metal-free region while Au, Al and Cr were not able to induce the crystallization of a-Si. On the other hand, the effect of the electric field on the lateral crystallization was not obvious for Pd. These phenomenological differences could be interpreted in terms of the dominant diffusing species (DDS) in the reaction between the metal and Si. It is judged that the applied electric field can enhance the crystallization velocity by accelerating the diffusion of metal atoms because the occurrence of lateral crystallization is known to rely on the diffusion of metal atoms than that of Si atoms. Therefore, it is thought that the only metal-dominant diffusing species in the reaction between metal and Si can strongly result in the crystallization of a-Si in metal-free region.

Dielectric relaxation of atomic-layer-deposited HfO2 thin films from 1kHzto5GHz

The dielectric relaxation of HfO2 thin films grown by atomic-layer deposition (ALD) was studied as a function of frequency from 1kHzto5GHz. The dielectric relaxation of the ALD HfO2 films followed a power-law dependence known as the Curie–von Schweidler relaxation law both in the kHz and GHz ranges, and the relaxation exponents were consistent with the measured dielectric losses. The behavior of the dielectric response for the HfO2 thin films may be attributed to defect sites in the HfO2 layer and∕or interface.

Evaluation of tailored electrode (Ba,Sr)RuO3 for (Ba,Sr)TiO3

The conducting oxide electrode (Ba,Sr)RuO3 (BSR), which is expected to be advantageous for (Ba,Sr)TiO3 (BST) dielectric materials due to its similarity in structure and chemical composition, was evaluated. Ba/Sr and (Ba+Sr)/Ru ratios in BSR films were varied to investigate the effect of composition in BSR films on the BST properties. BSR films deposited at 1/9 ratio of O2/Ar resulted in the lowest resistivity. BST films on stoichiometric BSR electrodes demonstrated the best electrical properties. The dielectric constant was 450 and the leakage current density was 10−8 A/cm2,which is about two orders of magnitude lower than BST on RuO2 processed in the same laboratory conditions. The dielectric constant of BST films was affected more by the chemical composition, while the leakage current was influenced both by the chemical composition and the lattice constant of BSR. Minimization of reaction at the interface between BST and BSR through a suppression of diffusion due to the compositional similarity seems to...

Fabrication and Characterization of Pb(Zr,Ti)O3 Microcantilever for Resonance Sensors

A microcantilever beam that includes the Pb(Zr,Ti)O3 (PZT) layer was fabricated. In this cantilever structure, RuO2 conductive oxide thin film was adopted to simplify the structure. Deposition parameters for PZT film were optimized and the etching conditions of Si were studied. From the hysteresis loop of PZT thin film, we found that the remanent polarization (Pr) of PZT thin film increased as the deposition temperature decreased and the post-annealing temperature increased. When the PZT thin films were deposited at 375°C by the radio frequency (RF) magnetron sputtering method and were annealed at 725°C by a rapid thermal annealing (RTA) process, the Pr of PZT thin film reached the maximum value of 11 µC/cm2. The mode of Si etching changed from isotropic one to anisotropic one with the addition of O2 gas in SF6 gas. Using these optimized conditions, 50 µm wide microcantilevers with different lengths from 50 µm to 150 µm were fabricated. It turned out that the measured resonance frequency ranged from 41.6 kHz to 180 kHz and the resonance frequency decreased as the length of cantilever increased.