Yung-Kuan Tseng

Two-step oxygen injection process for growing ZnO nanorods

Uniform hexagonal prismatic zinc oxide rods were grown over the entire alumina substrate at 550 °C using a two-step oxygen injection process, whether the substrates were coated with a catalyst or not. X-ray diffraction showed that all of the depositions exhibited a preferred orientation in the (002) plane. The influence of oxygen concentration was investigated by changing the oxygen flow rate. Oxygen concentration affected the size of ZnO nanorods, especially the diameter. The ZnO nanorods were further checked using high-resolution transmission electron microscopy, photoluminescence, Raman spectroscopy, and room-temperature ultraviolet lasing. The results showed that the rods were single crystals and had excellent optical properties. By observing the growth process, we found that the diameter increased slowly, but the longitudinal growth rate was very high. The growth of ZnO nanorods revealed that the uniform hexagonal prismatic ZnO nanorods were synthesized through vapor deposition growth and a self-catalyzed vapor-liquid-solid (VLS) process.

PYROELECTRIC PROPERTIES OF (PB1-XLAX)TIO3 THIN FILMS DEPOSITED USING SRRUO3 AS A BUFFER LAYER

In this study, we observed that the ferroelectric properties of (Pb1−xLax)TiO3, (PLT) thin films deposited on Pt/Ti/Si substrates using SrRuO3 as a buffer layer change markedly with the substrate temperature and the composition. All the films are perovskite with no secondary phases when deposited at 500–600 °C. However, only the films deposited below 520 °C possess a satisfactory small leakage current density, for example, JL⩽10−7 A/cm2, under a 50 kV/cm applied field. Both PLT10 (x=0.10) and PLT5 (x=0.05) thin films thus obtained possess large pyroelectric coefficient (p=0.009–0.018 °C−1). However, the PLT10 thin films show pyroelectric properties markedly superior to the PLT5 thin films, although the PLT5 thin films own much better ferroelectric properties. This phenomenon is explained by the lower Curie temperature (Tc) of the PLT10 materials.

Low-temperature growth of ZnO nanowires

ZnO nanowires with diameters of 40-200 nm were grown with a gold catalyst in bulk quantities on alumina substrates and sapphire substrates. This synthesis procedure was achieved by heating a 1: I mixture of ZnO and Zn powder to 500 °C with trace water vapor as an oxidizer. X-ray diffraction and transmission electron microscopy revealed that the nanowires were in the pure wurtzite phase. Photoluminescence spectroscopy showed two peaks: one was a strong ultraviolet emission at around 380 nm, which corresponds to the near-band-edge emission; the other was a weak near-infrared emission around 750 nm, which indicates a low concentration of oxygen vacancy. Moreover, we observed that the Zn/Au alloy droplets appeared on the tips of ZnO nanowires. As a consequence, we can select areas to grow ZnO nanowires by patterning the thin metal film on the substrates. These findings prove that the low-temperature growth mechanism is via vapor-liquid-solid rather than vapor transport deposition or vapor supersaturation (vapor-solid) mechanism. On the basis of the site-specific growth and the low-temperature requirement developed from this work, the synthesis of ZnO is compatible to microelectric machining system processing.

Growth of Epitaxial Needlelike ZnO Nanowires on GaN Films

Epitaxial needlelike ZnO nanowires were grown vertically over an entire epi-GaN/sapphire substrate at 550°C by low-pressure vapor phase deposition without employing any metal catalysts. A two-step oxygen injection processis the key of successful synthesis. The length of ZnO wires was up to 3.0 μm. The diameters of the roots and tips of the ZnO nanowires were around 80-100 and 15-30 nm, respectively. X-ray diffraction showed the epitaxial orientation relationship between ZnO and GaN as [001] Z n O //[001] G a N along the normal to the plane, and [100] Z n O //[100] G a N along the in-plane direction, consistent with the selective area electron diffraction pattern taken at the ZnO/GaN heterointerface. High-resolution transmission electron microscopy confirmed that nanowire was a single crystal. A room-temperature photoluminescence spectrum of the wires revealed a low concentration of oxygen vacancy in the ZnO nanowires and showed high optical quality.

Improvement on ferroelectric properties of metal-organic decomposited PZT thin film prepared by using prenucleation layer

Abstract Perovskite SrRuO3 (SRO) layer was, for the first time, been successfully synthesized by using metal-organic decomposition (MOD) process. The presence of SRO buffer layer on Pt(Si) substrates has significantly enhanced the crystallization and densification behavior of the subsequently deposited Pb(Zr0.52Ti0.48)O3 films. The pyrochlore free perovskite phase can be obtained by post-annealing the PZT/SRO/Pt(Si) films at 500°C, which is 50°C lower than that needed in PZT/Pt(Si) films. The fine grain (∼0.3 μm) microstructure can be attained by post-annealing at 650°C for PZT/SRO/Pt(Si) films and 700°C for PZT/Pt(Si) films. The ferroelectric hysteresis properties of the two PZT films are comparable to each other. The leakage current properties of PZT/SRO/Pt(Si) films increased pronouncedly with post-annealing temperature, resulting in inferriar leakage behavior to PZT/Pt(Si) films.

Interdiffusion in (Pb1-xLax)(ZryTi1-y)1-x/4O3/SrRuO3 Multilayer Thin Films Examined by Secondary Ions Mass Spectroscopy

The interdiffusion between layers in multilayer thin films, including (Pb0.97La0.03)(Zr0.66Ti0.34)0.9875O3 (PLZT), SrRuO3, Ru, Pt and Si, was examined using secondary ions mass spectroscopy (SIMS). The PLZT thin films deposited on Pt-coated Si-substrate using SrRuO3 as buffer layer possessed ferroelectric properties substantially superior to those grown on Si (or Si3N4) surface. Procoating Ru-layer prior to the deposition of SrRuO3 layer further improved the electrical response of PLZT thin films, that was ascribed to the efficient suppression of Sr- and Ru-outward diffusion into PLZT materials. The optimum properties attained are remanent polarization Pr=25.6 µC/cm2, coercive force Ec=47.1 kV/cm, relative dielectric constant K=1,200 and leakage current <10-5 A/cm2 (under 50 kV/cm).

INFLUENCE OF BUFFER MATERIALS ON THE PYROELECTRIC PROPERTIES OF (PB0.9LA0.1)TIO3 THIN FILMS

The effects of the LaNiO3 (LNO) and SrRuO3 (SRO) buffer layers on the characteristics of the subsequently deposited (Pb0.9La0.1)TiO3 (PLT) thin films were examined. The Pt-layer precoated on Si-substrates results in tetragonal perovskite PLT/LNO/Pt/Si and PLT/SRO/Pt/Si films, which possess ferroelectric properties superior to the cubic films (PLT/LNO/Si3N4/Si). Using SrRuO3 layers in lieu of LaNiO3 layers as buffer materials significantly improves the ferroelectric behavior of PLT thin films. The optimized electrical properties are: K=460, Pr=15 µC/cm2, Ec=25 kV/cm, JL = 8 ×10-7 A/cm2 (at 50 kV/cm) and p=0.304 µC/cm2K for the PLT/SRO/Pt/Si thin films deposited at 500°C substrate temperature.