Dong Heon Kang

PHOTOELECTROCHEMICAL PROPERTIES OF COPPER OXIDE THIN FILMS COATED ON AN N-SI SUBSTRATE

Abstract The photoelectrochemical properties of the copper oxide thin film coated on the n-type silicon electrode were investigated as a function of film deposition temperature. The variation in the deposition temperature affected the film morphology and the ratio of copper to oxygen. In case of the films deposited below 200°C, the main phase was found to be CuO while the amount of the Cu2O phase increased with further increases in deposition temperature. The n-silicon photoelectrode showed enhanced photocurrent–potential (I–V) properties by forming a copper oxide/n-silicon heterojunction. In particular, the electrode, which mainly consisted of a CuO phase, showed better photoelectrochemical conversion efficiencies compared to the Cu2O phase. This result was explained in terms of the electrical conductance and transmittance of the copper oxide film.

Characteristics of (Pb1-xSrx)TiO3 thin film prepared by a chemical solution processing

Abstract (Pb1-xSrx)TiO3 (PST) (0≤x ≤0.7) thin films on Pt/Ti/SiO2/Si(100) substrate were prepared by the modified sol-gel process and their characteristics were investigated as a function of strontium content (x). A stable PST sol was obtained by mixing lead and strontium acetates dissolved in deionized water and ethylene glycol and a titanium alkoxide modified by chelating acetylacetone in 2-methoxyethanol. The multi-layer annealing at 700 °C was adopted to prepare the 4000 °PST thin films with perovskite phase and dense microstructure. With increasing x, the tetragonality (c/a) was slightly decreased and approached to cubic phase above x ≅ 0.5 within XRD detection limit. The dielectric constant at room temperature abnormally increased and showed the maximum one (er = 1377, tan8 = 0.037 at 1 kHz) for the Pb0.5Sr0.5TiO3 thin film. Also strontium substitution suppressed the leakage current density of the films effectively. These properties can be explained in terms of variations of phase transition boundary (tetragonal to cubic), resulting from the substitution of nonvolatile strontium into lead site of PbTiO3 structure.

Physical and photoelectrochemical properties of the TiO2–ZnO system

Abstract The phases, microstructure, and electrical and photoelectrochemical properties of the TiO 2 –ZnO system were investigated as a function of processing variables such as ZnO content and heat treatment condition. In the specimens containing >1 wt% ZnO, the rutile TiO 2 and Zn 2 TiO 4 phases were observed. When the sintered specimens were heat-treated in reducing atmosphere, the content of Zn 2 TiO 4 phase decreased and the surface became apparently porous. These results were related to zinc evaporation. Electrical resistivity was influenced by the formation of defects and the Zn 2 TiO 4 phase, rather than microstructural characteristics such as grain size and density. A high photocurrent density of 2.2 mA/cm 2 at 0 V (vs. SCE) was observed for 30 wt% ZnO added to TiO 2 , resulting from an increase in electrical conductivity and surface reactivity.

Dielectric and field-induced strain behaviour of (Pb1-xBax) ZrO3 ceramics

The effects of Ba2+ substitution on the dielectric properties and induced strain behaviour of the (Pb1−xBax) ZrO3 ceramics (0.05≤x≤0.3) have been investigated as a function of x. A new phase diagram of the (Pb1−xBax) ZrO3 system, indicating the field effect on the phase transition, is also presented. As the Ba2+ content increases, the Curie temperature decreases linearly, whereas maximum dielectric constant increases for up to 20 mol% Ba2+ addition, and then decreases with further Ba2+ addition. Based on the hysteresis loops, the temperature range of the ferroelectric phase as an intermediate phase between the antiferroelectric and paraelectric phases, increases with increasing electric field and Ba2+ content. The ferroelectric loops are induced at room temperature for the specimens containing above 10 mol% Ba2+ by applying an electric field up to ∼25 kV cm−1. However, for the 5 mol% Ba2+-substituted specimen, no ferroelectric loop is induced, even with applied fields up to 55 kV cm−1. The phase transition due to electric field and Ba2+ addition is also confirmed by the measurement of the field-induced longitudinal strain.The effects of Ba2+ substitution on the dielectric properties and induced strain behaviour of the (Pb1−xBax) ZrO3 ceramics (0.05≤x≤0.3) have been investigated as a function of x. A new phase diagram of the (Pb1−xBax) ZrO3 system, indicating the field effect on the phase transition, is also presented. As the Ba2+ content increases, the Curie temperature decreases linearly, whereas maximum dielectric constant increases for up to 20 mol% Ba2+ addition, and then decreases with further Ba2+ addition. Based on the hysteresis loops, the temperature range of the ferroelectric phase as an intermediate phase between the antiferroelectric and paraelectric phases, increases with increasing electric field and Ba2+ content. The ferroelectric loops are induced at room temperature for the specimens containing above 10 mol% Ba2+ by applying an electric field up to ∼25 kV cm−1. However, for the 5 mol% Ba2+-substituted specimen, no ferroelectric loop is induced, even with applied fields up to 55 kV cm−1. The phase transition due to electric field and Ba2+ addition is also confirmed by the measurement of the field-induced longitudinal strain.

Dielectric and electrical properties of preferentially (111) oriented zr-rich 0.1Pb(Mg1/3Nb2/3)O3-0.9Pb(ZrxTi1-x)O3 thin films by chemical solution deposition

Abstract Ferroelectric thin films with rhombohedral composition of 0.1Pb(Mg 1/3 Nb 2/3 )O 3 –0.9Pb(Zr x Ti 1− x )O 3 (0.6≤ x ≤0.9) have been prepared by a chemical solution deposition on the (111) Pt/Ti/SiO 2 /Si substrates. The films were consisted of nearly pure perovskite phases with the (111) preferred orientation. Fine grains below approximately 200 nm in average size tended to be the clusters of grains when x was higher than 0.8. The dielectric constant of the films, which gradually decreased with x , showed a minimum (∼650) at the composition of x =0.9. Meanwhile the values of remanent polarization ( P r ) of the films increased with x up to 0.8 and then decreased for x =0.9. The increasing tendency is thought to be due to the preferred (111) orientation and distortion of perovskite unit cell. While the decreased polarization properties of x =0.9 implies the role of antiferroelectric properties of the lead zirconate phase, which can be possibly explained by DC bias-dependent capacitance behavior. The storage charge densities of the films decreased with x , but significantly increased upon the field-induced ferroelectric phase transition of the films.

Effect of n-WO3 thin film deposition on the photoelectrochemical properties of the textured p-Si electrode

The effect of WO3 thin film deposition on the photoelectrochemical properties of the textured p-Si electrodes, which were prepared by anisotropic etching of (100) p-Si in alkaline solution, have been investigated. The textured p-Si surface was covered with small pyramids having (111) planes, which led to decrease in surface reflectance and to increase in effective surface area. The photocurrents of both textured p-Si and WO3/textured p-Si electrodes tended to increase with increasing etching time up to 40 min and then slightly decreased with further etching. But the WO3/textured p-Si electrode showed relatively higher photocurrent at a less cathodic potential compared to the textured p-Si electrode without WO3 layer. These results were explained in terms of a degree of surface texturing, an intrinsic property of WO3 thin film on the surface reflectance and a charge transfer behavior due to the p-n heterojunction.

Effects of substrate pre-annealing on the preferred orientation of lead magnesium niobium titanate thin films by chemical solution deposition

Abstract The orientation of sol-gel derived lead magnesium niobium titanate (0.5PMN-0.5PT) films on the Pt/Ti/SiO2/Si(100) substrate was investigated as a function of substrate annealing temperature. The substrates were annealed in the temperature range from 300°C to 700°C for 30 min in the air atmosphere. Above 500°C, the substrate surface roughness increased drastically from 10 A to 278 A. The Rutherford back scattering revealed that outdiffusion of inner metal Ti was hindered by pre-annealing treatment at 400°C. When the films were prepared on the substrate annealed below 400°C, the (100) preferential orientation was observed. However, the (111) orientation appeared in case of the substrate annealed above 500°C. These results were explained in terms of surface roughness of substrate and Pt hillock formation.

Effect of ZnO on dielectric properties of Pb(Mg⅓Nb⅔)O3-PbTiO3

Abstract 0.9Pb(Mg⅓Nb⅔)O3−0.1PbTiO3 phase with the addition of ZnO was prepared by NaCl-KCl molten salt synthesis. Sinterability and dielectric properties of the specimens were investigated as a function of amount of ZnO. For the specimens sintered at 1100°C with 2 mol% ZnO, the pyrochlore phase of 1-2% present disappeared completely and the relative density reached a maximum value 97%. With further additions, the density value decreased. These results were mainly related to the microstructure changes including grain size and formation of a second phase. Dielectric properties had the same trend as the variation in density. Dielectric constant of about 23,500 was obtained for specimens sintered at a low temperature of 1100°C with 2 mol% ZnO.

Crystal structure, microstructure and ferroelectric properties of PZT(55/45) and PZT(80/20) thin films due to various buffer layers

Abstract Correlations between Pb(ZrxTi1−x)O3 thin films with different Zr contents (x=0.55, 0.8) and buffer layers of ferroelectric-PbTiO3 (PT), ferro/antiferroelectric-(PbBa)ZrO3 (PBZ) and antiferroelectric-PbZrO3 (PZ) thin films have been investigated in terms of the crystal structure, microstructure and electric phase of both layers. PT layer improved the crystalline character and surface morphology of both PZT films, while PZ and PBZ layers were only effective in the PZT80 film. Increasing tendency of dielectric properties was more remarkable for the PZT80 film rather than the PZT55 when PT and PZ layers were inserted, respectively. The effect of compositions of PZT and buffer layer on the fatigue character was also investigated.

Compositional modification of Pb-BASED ferroelectric thin films in chemical solution processing

Abstract Pb(Zr0.5Ti0.5)O30.5Pb(Zr0.5Ti0.5)O3-0.5Pb(Mg1/3Nb2/3/)O3 were prepared on the Pt-passivated substrates using a modified chemical solution. Modified methoxyethanol route including propyleneglycol and water based solvent and chelating agent was adopted to prepare a homogeneous multi-components solution precursor and stable solution for preparation of the thin films. Differential thermal analysis showed that the pyrolysis behavior of the films depended on the type of the chelating agent employed. The triethanolamine (TEA)-derived films were characterized at higher pyrolysis temperature than that of the acetylacetone (AcAc)-derived films. Several of these films with different compositions were studied to determine the effect of composition on crystallization, microstructure and dielectric properties.