Chih Ta Chia

Infrared and Raman-scattering studies in single-crystalline GaN nanowires

Abstract Infrared and Raman-scattering studies of high-purity and -quality GaN nanowires are presented. The nanosize dependences of the peak shift and the broadening of the four first-order Raman modes agree with those calculated on the basis of the phonon confinement model. Additionally, the appearance of one Raman mode at ∼ 254 cm −1 is attributed to zone-boundary phonon activated by surface disorders and finite-size effects. Moreover, the Raman-scattering intensities of certain phonons show a different resonantly enhanced behavior, which can be used to verify the information on the electronic structures and the electron–phonon interaction in GaN nanowires.

Correlation of microwave dielectric properties and normal vibration modes of xBa(Mg1/3Ta2/3)O3–(1−x)Ba(Mg1/3Nb2/3)O3 ceramics: II. Infrared spectroscopy

The relationship between the microwave dielectric properties and the IR active phonons of xBa(Mg1/3Ta2/3)O3–(1−x)Ba(Mg1/3Nb2/3)O3 ceramics was investigated. The IR modes were assigned, and the origin of dielectric response was determined. Among the 15 prominent IR modes, we found that the normal vibrations of the O layers and that of the Ta/Nb layers are strongly correlated to the measured dispersion parameters, such as the resonant strength (4πρ) and the damping coefficient (γ). The frequency shifts of the normal modes of the O layers and that of the Ta/Nb layers explain the linear decrease of microwave dielectric constant (K) as x increases, while the width of these modes correlate with the Q×f value.

Self-assembled nanorings in Si-capped Ge quantum dots on (001)Si

Nanorings with an average height and diameter of 1.2 and 65 nm, respectively, were observed to form in Si-capped Ge quantum dots grown at 600 °C by ultrahigh-vacuum chemical vapor deposition. The nanorings were captured with the rapid cooling of the samples with appropriate amount of Si capping. Based on the results of transmission electron microscopy and Raman spectroscopy, the formation of nanorings is attributed to alloying and strain relief in the Si/Ge/(001)Si system. The self-assembly of nanorings provides a useful scheme to form ultrasmall ring-like structure and facilitates the characterization of the physical properties of unconventional quantum structures.

Microwave dielectric relaxation in cubic bismuth based pyrochlores containing titanium

Cubic pyrochlore (Bi1.5Zn0.5)(Zn0.5−x∕3TixNb1.5−2x∕3)O7 ceramics with 0⩽x⩽1.5 were synthesized and investigated between 100Hz and 100THz by means of broadband dielectric spectroscopy, time-domain terahertz transmission spectroscopy, Fourier transform infrared reflectivity spectroscopy and Raman scattering. Bi1.5ZnNb1.5O7 exhibits a microwave dielectric relaxation which slows down and broadens remarkably on cooling. Careful structural investigations confirmed that the relaxation originates in hopping of disordered Bi and part of Zn atoms in the A sites of the pyrochlore structure. Substitution of Ti atoms to the B sites, i.e., increasing x, results in an increase of the microwave permittivity from 150 to 200 of the relaxation frequency and also of the microwave quality Q. Low temperature Raman scattering experiments did not reveal any phase transition in the samples under study.

Micro-Raman spectroscopy of a single freestanding GaN nanorod grown by molecular beam epitaxy

Micro-Raman spectra were measured on a single freestanding GaN nanorod, which was grown by molecular beam epitaxy. A sharp linewidth of E2(high) mode of 2.1cm−1 measured in the x(y,y)x¯ configuration indicates the high crystalline quality of the nanorod. The angle-dependent Raman spectroscopy shows that the integrated intensities of these first-order Raman modes follow the theoretical sinusoidal functions. The forbidden E1(LO) mode that appeared in the x(z,z)x¯ scattering configurations is assigned to the quasi-LO phonon mode. Power-dependent Raman spectroscopy shows redshift with increasing laser power density due to sample heating which is confirmed by Stokes and anti-Stokes measurements. The broadband centered at 708.5cm−1 is ascribed to the surface mode of the nanostructure.

RHEED studies of nucleation of Ge islands on Si(001) and optical properties of ultra-small Ge quantum dots

Abstract The initial stages of Ge growth on the Si(001)-(2×1) surface have been studied by using a RHEED pattern zero-streak profile analysis technique. Thicknesses for {105} and {113} facets formation, corresponding to the nucleation of coherent ‘hut’-clusters and dislocated ‘dome’ three-dimensional (3D) islands respectively, were determined in a growth temperature range of about 200–600°C. Multilayer structures containing ultra-small Ge quantum dots (QDs) with a plane size of about 10 nm and a height of 1.5 nm have been studied by photoluminescence (PL). PL bands assigned to QDs show an intensity comparable to data in the literature, but a band width five times smaller.

Time-dependent in-situ Raman observation of atomic hydrogen etching on diamond-like carbon films

Abstract In-situ time-dependent Raman spectra were measured on the thermal annealing of diamond-like carbon films produced by Pulsed Laser Deposition (PLD-DLC), with or without the presence of atomic hydrogen etching. It was found that annealing transformed DLC films into a graphitic structure in both cases and this transformation was completed within 30 min of annealing. Atomic hydrogen etches away the G*=1500 cm −1 band, revealing the existence of the D*=1200 cm −1 band. Both G*- and D*- phases are as stable as graphite during thermal annealing. These observations separate the effects of thermal annealing and annealing with atomic hydrogen etching on diamond-like carbon films.

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.

Impedance spectroscopic studies on congruent LiNbO3 single crystal

Electrical impedance measurements on a congruent LiNbO3 single crystal were performed as a function of both temperature and frequency. The measurements were carried out in the directions along the c- and a-axes of the crystal. The temperature and frequency dependence of various dielectric properties have been studied. The result has revealed two remarkable dynamic relaxations: dielectric dipolar relaxation and ionic conductivity relaxation. The dipolar relaxation peaks were found at frequencies around 4 × 106 and 2 × 106 Hz for the c-axis and a-axis, respectively, and they were only slightly temperature dependent. The ionic conductivity relaxation was found at the lower-frequency end but it was temperature dependent. The temperature dependence of the dc electrical conductivity follows the Arrhenius law. It corresponds to the long-range ionic motion of Li+ ions which are thermally activated with activation energy of 0.90 and 0.87 eV along the c- and a-axis directions, respectively. The dc conductivities measured along the c- and a-axes are very close to each other, and the value increases from 1.7 × 10−6 to 1.9 × 10−3 Ω−1 cm−1 as the temperature is raised from 300 to 700 °C. The sample crystal becomes an ionic conductor as the temperature is raised.

Hydrogen effects on the post-production modification of diamond-like carbon produced by pulsed laser deposition

This work utilizes Raman spectroscopy to examine the thermal annealing of diamond-like carbon (DLC) films produced by pulsed laser deposition (PLD). Post-production modification of DLC films by atomic hydrogen etching reveals distinct results and is observed for the first time. Both ex-situ and in-situ thermal annealing and annealing with atomic hydrogen on the as-prepared DLC samples are compared herein. The Raman spectra reveal that the typical D- and G-bands evolve from unresolved to well separated above 400°C as the temperature increases from room temperature to 700°C. Atomic hydrogen enhances the annealing process at temperatures above 500°C. Thermal annealing transforms the DLC films into graphitic structures while atomic hydrogen etches away the disordered carbons according to both the ex-situ and in-situ Raman spectra.