K. K. Tiong

Design of a Multiband Antenna for Mobile Handset Operations

A compact internal antenna is proposed for GSM/GPS/DCS/PCS/UMTS/Bluetooth/WLAN/WiFi/WiMAX applications. The proposed antenna is comprised of a loop antenna and a monopole antenna. The antenna system shows wide multi-operation band as well as good radiation patterns and small volume. The antenna can be printed on the system circuit board of the mobile phone and short-circuited to the system ground plane to form a loop-type structure. A comparison with that of the existing antennas showed that wide impedance bandwidth of the proposed structure can be achieved without increasing the implementation complexity.

Characterization of RuO2 thin films by Raman spectroscopy

Abstract Raman scattering has been used as a technique for the characterization of RuO 2 thin films deposited on different substrates by the metal-organic chemical vapor deposition method and reactive sputtering under various conditions. Red shift and broadening of the linewidth of the Raman peaks are analyzed by the spatial correlation model. The intrinsic linewidth for the RuO 2 films deposited at high and low temperatures has to be adjusted to different values to achieve a good fit for the features. The lineshape and position of the Raman features vary for films deposited on different substrates under the same condition. These differences indicate the existence of stress induced by lattice mismatch and the differential thermal expansion coefficients between RuO 2 and the substrates. After annealing at 650°C in an oxygen atmosphere for 3 h, the linewidth decreases significantly for the RuO 2 thin films deposited at lower temperatures. The results show the improvement of the crystallinity of the films during the annealing process.

Growth and characterization of well-aligned densely-packed rutile TiO2 nanocrystals on sapphire substrates via metal?organic chemical vapor deposition

Well-aligned densely-packed rutile TiO(2) nanocrystals (NCs) have been grown on sapphire (SA) (100) and (012) substrates via metal-organic chemical vapor deposition (MOCVD), using titanium-tetraisopropoxide (TTIP, Ti(OC(3)H(7))(4)) as a source reagent. The surface morphology as well as structural and spectroscopic properties of the as-deposited NCs were characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAED), x-ray diffraction (XRD) and micro-Raman spectroscopy. FESEM micrographs reveal that vertically aligned NCs were grown on SA(100), whereas the NCs on the SA(012) were grown with a tilt angle of ∼33° from the normal to substrates. TEM and SAED measurements showed that the TiO(2) NCs on SA(100) with square cross section have their long axis directed along the [001] direction. The XRD results reveal TiO(2) NCs with either (002) orientation on SA(100) substrate or (101) orientation on SA(012) substrate. A strong substrate effect on the alignment of the growth of TiO(2) NCs has been demonstrated and the probable mechanism for the formation of these NCs has been discussed.

Field emission from vertically aligned conductive IrO2 nanorods

We report on the preparation and field-emission properties of vertically aligned conductive IrO2 nanorods. The unique geometrical features of IrO2 nanorods, including nanosized structure and self-assembled sharp tip, exhibit a strong effect on field enhancement (β∼40 000), which result in a low threshold field (Eth∼0.7 V/μm) defined at the beginning of emission. A low turn-on field for driving a current of 10 μA/cm2 is about 5.6 V/μm, which is comparable with the carbon nanotube, diamond, and amorphous carbon. The potential of using IrO2 nanorods as an emitter material has been demonstrated.

Characterization of RuO2 thin films deposited on Si by metal-organic chemical vapor deposition

We report characterization of RuO2 thin films, deposited on Si substrates by metal-organic chemical vapor deposition (MOCVD), by scanning electron microscopy, X-ray diffraction, electrical conductivity, spectrophotometry, ellipsometry and Raman scattering measurements. As-deposited RuO2 films are specular, crack free, and well adherent on the substrate. The Auger electron spectroscopy depth profile shows good compositional uniformity across the thickness of the films. As confirmed by X-ray investigations, the films crystallize with the correct rutile structure. The results of the electrical and optical studies of the MOCVD RuO2 films show a metallic character of these films. The results of Raman investigation indicate that a nearly strain free and high quality RuO2 thin film could be deposited on a Si substrate by MOCVD.

Crystal structure and band-edge transitions of ReS2-xSex layered compounds

Abstract Single crystals of the ReS 2− x Se x solid solution series have been grown by the chemical vapor transport method using Br 2 as a transport agent. From analyzing the X-ray patterns, the series crystals are determined to be single-phase and crystallized in the triclinic layered structure. The lattice parameters of the compound series are evaluated and discussed. The optical absorption edge was measured on basal-plane at room temperature. The results reveal that ReS 2− x Se x are indirect semiconductors and their energy gaps are determined. The band gap energy varies smoothly with the Se composition x , indicating that the nature of band edges are similar for the end members ReS 2 and ReSe 2 , and the compounds of intermediate compositions.

In-plane anisotropy of the optical and electrical properties of ReS2 and ReSe2 layered crystals

ReS 2 and ReSe 2 belong to the family of transition-metal dichalcogenides crystallized in the distorted octahedral layer structure of triclinic symmetry. Crystals with triclinic symmetry are optically biaxial. The interband transitions of ReS 2 and ReSe 2 are expected to show anisotropic character for linearly polarized light, incident normal to the basal plane. The optical anisotropic effects for E∥b and E⊥b polarizations were studied through polarization-dependent piezoreflectance (PzR) and optical absorption measurements. The polarization dependence of the PzR spectra provides conclusive evidence that the features associated with the interband excitonic transitions are originated from different origins. The results of absorption measurements indicate that ReS 2 and ReSe 2 are indirect semiconductors, in which E parallel to b-axis polarization exhibits a smaller band gap and a single phonon makes an important contribution in assisting the indirect transitions. The electrical conductivity along the b-axis was shown to be several times higher than that perpendicular to b-axis for both crystals.

Characterization of IrO2 thin films by Raman spectroscopy

Abstract Raman scattering has been used as a technique for characterization of the sputtered IrO 2 thin films (SIROF) deposited on different substrates under various conditions. Compared with the spectrum of single crystal IrO 2 , red shift and broadening of the linewidth of the Raman peaks of SIROF are observed. X-ray diffraction measurements of SIROF were also carried out to assist the identification of the factor that influences the linewidth broadening of the Raman features. The results indicate that amorphous–crystalline transition for IrO 2 phase can be achieved at substrate temperature of 200–300 °C. The line-shape and position of the Raman features vary for films deposited on different substrates under the same conditions. These differences can be due to the existence of stress between IrO 2 and the substrates.

Temperature dependence of energies and broadening parameters of the band-edge excitons of single crystals

We have measured the temperature dependence of the spectral features in the vicinity of direct band-edge excitonic transitions of single crystals from 25 to 300 K using piezoreflectance (PzR). From a detailed lineshape fit of the PzR spectra, the energies and broadening parameters of the A and B excitons have been determined accurately. The origin of these excitonic transitions is discussed. The transition energies and their splittings vary smoothly with the tungsten composition x, indicating that the natures of the direct band edges are similar for the compounds. In addition, the parameters that describe the temperature variation of the energies and broadening function of the excitonic transitions are evaluated and discussed.

Optical absorption of ReS2 and ReSe2 single crystals

The optical absorption of synthetic ReS2 and ReSe2 single crystals is reported over a temperature range from 25 to 300 K. Analysis reveals that the absorption edges of ReS2 and ReSe2 are indirect allowed transitions. The indirect band gaps at various temperatures are determined and their temperature dependence is analyzed by the Varshni equation [Physica 34, 149 (1967)] and an empirical expression proposed by O’Donnel and Chen [Appl. Phys. Lett. 58, 2924 (1991)]. The parameters that describe the temperature dependence of energy gaps of these two materials are evaluated and discussed.