Wen-Ching Shih

Growth of ZnO films on GaAs substrates with a SiO2 buffer layer by RF planar magnetron sputtering for surface acoustic wave applications

Abstract ZnO films were grown on GaAs substrates with or without SiO 2 buffer layers by RF planar magnetron sputtering. The as-deposited ZnO films were characterized, using X-ray diffraction (XRD) and X-ray rocking curve, scanning electron microscopy (SEM) and reflection high-energy electron diffraction (RHEED). The SiO 2 buffer layer is beneficial to the promotion of the ZnO film quality. The refractive indices of ZnO films obtained from m-line spectroscopy were 1.990 ± 0.002 and 2.004 ± 0.002, and are close to those of the ZnO single crystal. Surface acoustic wave (SAW) properties were also measured for SAW propagating along the [011] direction on (100) GaAs substrate.

Growth of ZnO thin films on interdigital transducer/Corning 7059 glass substrates by two-step fabrication methods for surface acoustic wave applications

ZnO films were grown on interdigital transducer (IDT)/Corning 7059 glass substrates by RF planar magnetron sputtering using two-step fabrication methods for surface acoustic wave (SAW) applications. The crystalline structure of the as-deposited ZnO films was characterized by x-ray diffraction (XRD) and x-ray rocking curve analysis. The SAW properties, including coupling coefficient and insertion loss, were evaluated and compared with the theoretical results. ZnO films deposited by the two-step fabrication method exhibited a lower insertion loss and a closer agreement between the experimental coupling coefficients and the corresponding theoretical values in comparison with films deposited by the one-step fabrication method. The results could be useful in the design of high-coupling low-loss SAW devices.

Theoretical investigation of the SAW properties of ferroelectric film composite structures

The characteristics of surface acoustic waves (SAW) propagating on a three-layered structure consisting of a perovskite-type ferroelectric film, a buffer layer and a semiconductor substrate have been studied theoretically. Large coupling coefficients (K(2)) can be obtained when the interdigital transducer (IDT) is on top of the perovskite-type ferroelectric film, with (type 4) and without (type 3) the floating-plane electrode at the perovskite-type ferroelectric film-buffer layer interface. In the above cases, the peak values of K (2) Of the Pb(Zr,Ti)O(3) (PZT) films (3.2%-3.8%) are higher than those of the BaTiO(3) (BT) and PbTiO(3) (PT) films. In the IDT configuration of type 4, there exists a minor peak of the coupling coefficients for the PZT and BT films, but not for the PT films when the normalized thickness (hK) of the perovskite-type ferroelectric film is about 0.3. The minor peak values of the coupling coefficients (0.62%-0.93%) for different layered structures (PZT/STO/Si, PZT/MgO/Si, and PZT/MgO/GaAs) all decrease when we increase hK value from 0 to 0.25. The results could be useful in the integration of ferroelectric devices, semiconductor devices, and SAW devices on the same substrate.

The effect of an SiO/sub 2/ buffer layer on the SAW properties of ZnO/SiO/sub 2//GaAs structure

The effect of an SiO/sub 2/ buffer layer on the surface acoustic wave (SAW) properties of ZnO/SiO/sub 2//GaAs structure is examined. Both theoretical and experimental results show that the coupling coefficient is increased appreciably by providing an SiO/sub 2/ film between the ZnO film and the GaAs substrate. Adding an SiO/sub 2/ film is also beneficial to the promotion of quality of ZnO thin film. The results could be useful for the further development of monolithic SAW devices.<<ETX>>

The induction of a graphite-like phase on diamond films by a Fe-coating/post-annealing process to improve their electron field emission properties

The electron field emission (EFE) process for diamond films was tremendously enhanced by Fe-coating and post-annealing processes. Microstructural analysis indicates that the mechanism for the improvement in the EFE process is the formation of nanographites with good crystallinity that surround the Fe (or Fe3C) nanoclusters. Presumably the nanographites were formed via the reaction of Fe clusters with diamond films, viz. by the dissolution of carbons into Fe (or Fe3C) clusters and the reprecipitation of carbon species to the surface of the clusters, a process similar to the growth of carbon nanotubes via Fe clusters as catalyst. Not only is a sufficiently high post-annealing temperature (900°C) required but also a highly active reducing atmosphere (NH3) is needed to give a proper microstructure for enhancing the EFE process. The best EFE properties are obtained by post-annealing the Fe-coated diamond films at 900°C in an NH3 environment for 5 min. The EFE behavior of the films can be turned on at E0 = 1.9 ...

Growth of c-Axis-Oriented LiNbO3 Films on ZnO/SiO2/Si Substrate by Pulsed Laser Deposition for Surface Acoustic Wave Applications

Highly c-axis-oriented LiNbO3(006) thin films were successfully grown on SiO2/Si substrates with a ZnO buffer layer by XeCl excimer pulsed laser deposition (PLD). The as-deposited films were characterized by X-ray diffraction, scanning electron microscopy, and atomic force microscopy to analyze their crystalline structure and surface morphology. The results show that the highly c-axis-oriented LiNbO3 thin films of 2.5 µm thickness were successfully grown on SiO2/Si substrates with a ZnO buffer layer by PLD. The full width at half maximum intensity of the LiNbO3(006) peak of the sample fabricated under the optimum deposition conditions is only 0.18°. The center frequency of the LiNbO3/ZnO/SiO2/Si substrate with line width of 4 µm is 188 MHz, and the phase velocity is 3010 m/s, which is slightly lower than that of the 36° Y–X LiNbO3 substrate.

The induction of nanographitic phase on Fe coated diamond films for the enhancement in electron field emission properties

A thin layer of iron coating and subsequent post-annealing (Fe-coating/post-annealing) is seen to significantly enhance the electron field emission (EFE) properties of ultrananocrystalline diamond (UNCD) films. The best EFE properties, with a turn on field (E0) of 1.98 V/μm and current density (Je) of 705 μA/cm2 at 7.5 V/μm, are obtained for the films, which were Fe-coated/post-annealed at 900 °C in H2 atmosphere. The mechanism behind the enhanced EFE properties of Fe coated/post-annealed UNCD films are explained by the microstructural analysis which shows formation of nanographitic phase surrounding the Fe (or Fe3C) nanoparticles. The role of the nanographitic phase in improving the emission sites of Fe coated/post-annealed UNCD films is clearly revealed by the current imaging tunneling spectroscopy (CITS) images. The CITS images clearly show significant increase in emission sites in Fe-coated/post-annealed UNCD films than the as-deposited one. Enhanced emission sites are mostly seen around the boundarie...

Effect of a Buffer Layer on the Surface Acoustic Wave Characteristics of Pb(Zr, Ti)O3 Film/Buffer Layer/Semiconductor Substrate Structures.

The effect of a buffer layer on the surface acoustic wave properties of Pb(Zr, Ti)O3 (PZT) film/buffer layer/ semiconductor substrate structures was examined theoretically. Large coupling coefficients (2.8–3.4%) can be obtained when the interdigital transducer (IDT) is on top of the PZT film, with (type 4) and without (type 3) the floating-plane electrode at the PZT film-buffer layer interface. In the type 4 IDT configuration, there exists a minor peak of the coupling coefficient when the hK value of the PZT film is about 0.3. The minor peak values of the coupling coefficient (0.62–0.93%) decrease when we increase the thickness of the buffer layer from 0 to 0.25. These results could be useful for the further development of ferroelectric devices, semiconductor devices and SAW devices on the same substrate.

Direct observation and mechanism of increased emission sites in Fe-coated microcrystalline diamond films

The electron field emission (EFE) properties of microcrystalline diamond (MCD) films are significantly enhanced due to the Fe coating and post-annealing processes. The 900 °C post-annealed Fe coated diamond films exhibit the best EFE properties, with a turn on field (E0) of 3.42 V/μm and attain EFE current density (Je) of 170 μA/cm2 at 7.5 V/μm. Scanning tunnelling spectroscopy (STS) in current imaging tunnelling spectroscopy mode clearly shows the increased number density of emission sites in Fe-coated and post-annealed MCD films than the as-prepared ones. Emission is seen from the boundaries of the Fe (or Fe3C) nanoparticles formed during the annealing process. In STS measurement, the normalized conductance dI/dVI/V versus V curves indicate nearly metallic band gap, at the boundaries of Fe (or Fe3C) nanoparticles. Microstructural analysis indicates that the mechanism for improved EFE properties is due to the formation of nanographite that surrounds the Fe (or Fe3C) nanoparticles.

Microplasma enhancement via the formation of a graphite-like phase on diamond cathodes

Enhanced electron field emission (EFE) properties in microcrystalline diamond (MCD) films that have been Fe-coated and postannealed are observed. Additionally, improved microplasma characteristics are also observed when these materials are used as cathodes. The turn-on field for inducing the EFE process decreases from 4.7 V/μm for pristine MCD films to 2.2 V/μm for the Fe-coated/postannealed ones, whereas the EFE current density at an applied field of 8.8 V/μm increases from 36.5 to 5327.1 μA/cm2. Transmission electron microscopy, in conjunction with high-angle annular dark field and 3D-tomography studies, reveals that enhanced EFE in the Fe-coated/postannealed MCD films is due to the graphite-like phase on the surface of diamond films. The authors infer that the Fe-coating interacts with the diamond in the postannealing process to dissolve carbons and reprecipitate them in nanographite networks. This process is similar to the formation of carbon nanotubes by the dissolution and reprecipitation of carbon ...