Controlled synthesis of monodispersed superparamagnetic nickel ferrite nanoparticles

Nickel ferrite nanoparticles have been prepared through a gentle chemistry route, starting from iron nitrate, nickel nitrate and stearic acid. The nickel ferrite crystalline phase, the particle size and shape, and the homogeneity of the resulting nanoparticles were studied by X-ray diffraction and transmission electron microscopy. Fourier transform infrared techniques were used to study the composition characteristics of the as-prepared sample. Magnetization studies at room temperature showed superparamagnetic behavior for the nanoparticles. Magneto-optic rotation studies at different wavelengths of He-Ne lasers reveal non-linear behavior.     

Effect of Pretreatment of TaN Substrates on Atomic Layer Deposition Growth of Ru Thin Films

The polycrystalline ruthenium films are grown on TaN substrates by atomic layer deposition （ALD） using bis（cyclopentadienyl） ruthenium [RuCp2] and oxygen as ruthenium precursor and reactant respectively at a deposition temperature of 330℃. The low-energy Ar ion bombardment and Ru pre-deposition are performed to the underlying TaN substrates before ALD process in order to improve the Ru nucleation. X-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy are carried out to characterize the properties of ALD Ru films. The results show that the nucleation density of Ru films with Ar^＋ bombardment to the underlying TaN substrates is much higher than that of the ones without any pretreatment. The possible reasons are discussed.     

Growth and Characterization of A1GaN/A1N/GaN HEMT Structures with a Compositionally Step-Graded A1GaN Barrier Layer

A new A1GaN/A1N/GaN high electron mobility transistor （HEMT） structure using a compositionally step-graded A1GaN barrier layer is grown on sapphire by metalorganic chemical vapour deposition （MOCVD）. The structure demonstrates significant enhancement of two-dimensional electron gas （2DEG） mobility and smooth surface morphology compared with the conventional HEMT structure with high A1 composition A1GaN barrier. The high 2DEG mobility of 1806 cm2/Vs at room temperature and low rms surface roughness of 0.220 nm for a scan area of 5μm×5 μm are attributed to the improvement of interracial and crystal quality by employing the stepgraded barrier to accommodate the large lattice mismatch stress. The 2DEG sheet density is independent of the measurement temperature, showing the excellent 2DEG confinement of the step-graded structure. A low average sheet resistance of 314.5Ω/square, with a good resistance uniformity of 0.68%, is also obtained across the 50 mm epilayer wafer. HEMT devices are successfully fabricated using this material structure, which exhibits a maximum extrinsic transconductance of 218 mS/ram and a maximum drain current density of 800 mA/mm.     

Long-time stabilization of porous silicon photoluminescence by surface modification

We present results on the photoluminescence (PL) properties of porous silicon (PS) as a function of time. Stabilization of PL from PS has been achieved by replacing silicon-hydrogen bonds terminating the surface with more stable silicon-carbon bonds. The composition of the PS surface was monitored by transmission Fourier transform infrared (FTIR) spectroscopy at intervals of 1 month in ageing time up to 1 year. The position of the maximum PL peak wavelength oscillates between a blue-shift and a red-shift in the 615-660 nm range with time.     

Growth of thin Si oxide in a cyclic oxygen plasma environment below 200 °C

The growth of Si oxide by means of a cyclic radio-frequency (rf) plasma oxidation process has been explored in a low temperature range of 100-200 °C. The growth mechanism exhibits Cabrera-Mott (CM) oxidation, that is, the transport of mobile ionic species is assisted by an electric field. The low activation energy of 0.3 eV is attributed to the small size of O⁻ and the assistance of the electric field. The oxide becomes off-stoichiometric as one approaches to the exterior surface of the oxide layer.     

Growth of undoped and Zn-doped GaN nanowires

Undoped and Zn-doped GaN nanowires were synthesized by chemical vapor deposition (CVD), and the effects of substrates, catalysts and precursors were studied. A high density of GaN nanowires was obtained. The diameter of GaN nanowires ranged from 20 nm to several hundreds of nm, and their length was about several tens of μm. The growth mechanism of GaN nanowires was discussed using a vapor–liquid–solid (VLS) model. Furthermore, room-temperature cathodoluminescence spectra of undoped and Zn-doped GaN nanowires showed emission peaks at 364 and 420 nm, respectively.     

Compositional studies of near-room-temperature thermal CVD poly(chloro-p-xylylene)/SiO2 nanocomposites

Chemical-vapor-deposited (CVD) nanostructured thin films have been recently developed to overcome the limitations of thin films from one material class. In particular polymer/SiO₂ nanocomposite thin films have been developed to reduce power consumption, cross-talk, and RC delay in the next generation of ultralarge-scale integrated devices. Since polymers mainly possess electronic polarization they inherently have a low dielectric constant. However, they often suffer from poor dielectric anisotropy, low elastic and shear moduli, and have poor resistance to metallic diffusion. As a proof of concept, poly(chloro-p-xylylene)/SiO₂ thermal CVD nanocomposites have been developed to overcome such material deficiencies. Additionally, the CVD process allows for high manufacturing throughput and compositional control in situ, both potentially advantageous for IC fabrication. The study here focuses on the polymeric phase of the nanocomposite, which as a homopolymer can possess ≈60% crystallinity and a positive optical birefringence of 0.034, both post-deposition-annealed just before the polymer’s melting point. With increasing volume percent of SiO₂, the percent crystallinity is reduced, the thin film becomes more isotropic and the index of refraction can be varied depending on the volume percent SiO₂. Received: 15 December 1999 / Accepted: 7 January 2000 / Published online: 5 April 2000     

Growth and properties of ZnO nanorod and nanonails by thermal evaporation

ZnO nanorods and nanonails have been synthesized on silicon wafers by a three-step catalyst-free thermal evaporation method in oxygen atmosphere. All the samples were hexagonal phase ZnO with highly c-axis preferential orientation. Different morphologies of ZnO nanostructures, i.e. ZnO nanorods and two kinds of nanonails, were observed at various temperature regions. Photoluminescence, transmission electron microscopy, and energy-dispersive X-ray spectroscope were employed to elucidate the reason for the formation of such different rod-like structures. The analysis results demonstrated that the caps of nanonails possess a large number of oxygen vacancies, which may play a key role in determining the formation of nanonails and the high intensity of green emission.     

Fabrication of a GaN lateral polarity junction by metalorganic chemical vapor deposition

A fabrication process for growth of GaN lateral polarity junctions consisting of Ga-polar and N-polar domains grown simultaneously side-by-side on c-plane sapphire was developed using the polarity control scheme. An ammonia-annealing step following deposition and patterning of a thin low-temperature AlN nucleation layer played a crucial role in avoiding mixed-polarity growth of the remaining AlN nucleation layer, as well as in nitriding the bare sapphire surface to facilitate growth of N-polar GaN. The achievement of both polar domains, free from inversion domains within a contiguous domain, led to Ga-polar domain exhibiting featureless morphology with highly resistive characteristics, while N-polar domains exhibited hexagonally faceted morphology and were highly conductive.     

Weather effects on returns: Evidence from the Korean stock market

In this study, we attempted to determine whether a relationship exists between stock returns and the weather variables of temperature, humidity, and cloud cover in the Korean stock market. We delineated three key implications with regard to weather effects. First, after the 1997 financial crisis, the presence of a weather effect disappeared. Second, the inclusion of weather variables helps to model the GJR-GARCH process in the conditional variance. Third, the interaction effects of weather variables fully demonstrate the weather effect, but the interaction effects also vanished after the crisis. Overall, the findings of this study indicate that the weather effect was weakened as the result of heightened market efficiency.