[en] The structural and optical properties of hydrogenated amorphous silicon (a-Si:H)/nanocrystalline silicon (nc-Si:H) thin films, grown from high argon/hydrogen diluted silane by plasma-enhanced chemical vapor deposition (PE-CVD) using high rf power, were investigated. The growth rate decreased with increasing hydrogen flow rate and decreasing rf power, and increased slightly with increasing substrate temperature. For all samples deposited without hydrogen addition, SiH stretching was predominant as detected by infrared spectroscopy, whereas with hydrogen addition, the SiH intensity became lower than that of SiH₂ stretching. Moreover, the hydrogen content was found to decrease with increasing hydrogen flow rate and substrate temperature. Raman scattering clearly showed the nanocrystallites formed in the amorphous silicon matrix in the thin films deposited with hydrogen addition, and these films had higher optical band gaps. There was no direct relation between the hydrogen content and the optical band gap or edge width parameter due to the heterogeneity of the grown thin films. However, IR and Raman scattering spectra were found to be closely related to the microstructure of the grown thin films, and may provide clear information on the quality or heterogeneity of the films deposited using high rf power and high argon/hydrogen dilution. In addition, the smooth surface morphology and adherent stable structure of the grown thin films make them suitable for application in electronic devices

[en] The diffusion properties of Cu, Cu/titanium nitride (TiN) and Cu/TiN/Ti metallization on GaAs, including as-deposited film and others annealed at 350-550 deg. C, were investigated and compared. Data obtained from X-ray diffractometry, resistivity measurements, scanning electron microscopy, energy dispersive spectrometer and Auger electron spectroscopy indicated that in the as-deposited Cu/GaAs structure, copper diffused into GaAs substrate, and a diffusion barrier was required to block the fast diffusion. For the Cu/TiN/GaAs structure, the columnar grain structure of TiN films provided paths for diffusion at higher temperatures above 450 deg. C. The Cu/TiN/Ti films on GaAs substrate were very stable up to 550 deg. C without any interfacial interaction. These results show that a TiN/Ti composite film forms a good diffusion barrier for copper metallization with GaAs

[en] The authors report a novel ultra high-gain, low-noise AlGaAs/GaAs separate-absorption-multiplication (SAM) Avalanche Photo-Diode (APD) using an effective mass filtering (EMF) superlattice absorption layer to enhance the values of multiplication gain and electron/hole impact ionization rate ratio (α/β). The results yield a maximum multiplication gain factor of 6,000 and α/β equal to 25 at an electric field strength of 1.25 x 10⁶ V/cm. The APD has a peak quantum efficiency of 72% and responsivity of 0.45 A/W at 0.8 μm wavelength

[en] Epitaxial (0 0 1) NiMn/NiFe (NiMn at bottom) films were grown on Cu buffer layer by using e-beam evaporation at room temperature. After annealing at 275 deg. C, NiMn layers became an ordered phase, and showed an exchange field of 78 Oe for 15 nm NiFe. During annealing, Cu diffused into NiMn, and hence affected the exchange field

[en] New barrier layer, etch stop and hardmask films, including hydrogenated amorphous a-SiC_x:H (SiC), a-SiC_xO_y:H (SiCO), and a-SiC_xN_y:H (SiCN) films with a dielectric constant (k) approximately 4.3, are produced using the plasma-enhanced chemical vapor deposition technique. The chemical and structural nature, and mechanical properties of these films are characterized using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and nano-indentation. The leakage current density and breakdown electric field are investigated by a mercury probe on a metal-insulator-semiconductor structure. The properties of the studied films indicate that they are potential candidates as barrier layer, etch stop and hardmask films for the advanced interconnect technology. The SiC film shows a high leakage current density (1.3x10^-7 A/cm² at 1.0 MV/cm) and low breakdown field (1.2 MV/cm at 1.0x10^-6 A/cm²). Considering the mechanical and electrical properties requirements of the interconnect process, SiCN might be a good choice, but the N content may result in via poison problem. The low leakage current (1.2x10^-9 A/cm² at 1.0 MV/cm), high breakdown field (3.1 MV/cm at 1.0x10^-6 A/cm²), and relative high hardness (5.7 GPa) of the SiCO film indicates a good candidate as a barrier layer, etch stop, or hardmask

[en] The stress intensity factor corresponding to a radial crack emanating from the internal boundary of a tube is calculated by the boundary collocation method. Uniform external tension on the outer boundary is chosen as the applied load. By using superposition, it can be shown that this solution is identical with that of a uniform internal pressure acting in the tube and on the crack surfaces. The present results calculated by this method coincide very well with the existing solution in most cases. For small wall ratios, this method gives better results. (author)

[en] Ferritin synthesis is a dramatic example of mRNA repression: excess iron causes recruitment of ferritin mRNA, increasing synthesis ≤ 40 x. Using the iron-storing tadpole red cell as a simple and accessible model, isolated poly(A+) RNA directed the synthesis of ferritin and globin in cell-free extracts from wheat germ (WG); in contrast, ferritin mRNA was specifically repressed (72%) in extracts from rabbit reticulocytes (RR) as it is in vivo. Translatable and hybridizable ferritin mRNA did not enter polysomes of RR in contrast to globin mRNA and to both ferritin and globin mRNA in WG. Single-sequence mRNA, uncapped, was prepared in vitro for both a ferritin M chain and a globin beta chain; both were translated with similar efficiency in RR (164 +/- 66) x 10^-3 and (205 +/- 144) x 10^-3 cpm [³H]leucine/h/μg RNA for ferritin and globin, respectively). Ferritin with the expected immunoreactivity and mobility in SDS gel electrophoresis was obtained. The results suggest that ferritin mRNA repression may require a cap or factors present in poly(A+) RNA and that repression can be released in WG but not in RR

[en] Mesocosm experiments were conducted to examine belowground responses of Phragmites australis and Suaeda salsa to the combined stresses of increased salinity and water depth. The results demonstrated that: (1) belowground biomass of P. australis responded negatively to both increased salinity and water depth, whereas belowground biomass of S. salsa mainly responded negatively to increased water depth; (2) belowground biomass of S. salsa negatively responded to increased water depth more strongly than that of P. australis, thus S. salsa might disappear before P. australis in wetlands experiencing prolonged water-logging; (3) P. australis and S. salsa responded to increased salinity and/or water-logging by shifting their resource allocations towards aboveground biomass; (4) belowground biomass of P. australis tended to have more negative responses to increased salinity and water depth at deeper versus shallower soil depths, hindering P. australis from utilizing resources in deeper soil; in contrast, belowground biomass of S. salsa tended to have more negative responses to increased water depth at shallower versus deeper soil depths, greatly decreasing the overall root density and thereby increasing the chance of uprooting disturbance to S. salsa. These responses would accelerate sediment loss due to compromised sediment-binding abilities of P. australis and S. salsa, leading to an adverse positive feedback between environmental changes associated with sea level rise and performance of P. australis and S. salsa, resulting in faster deterioration of coastal wetlands than might otherwise be expected. (author)

[en] Spacecraft measurements over the past two sunspot cycles have shown that the average strength of the interplanetary magnetic field (IMF) undergoes surprisingly modest long-term variation, unlike the total magnetic flux observed on the Sun. Our attempt to model the IMF during sunspot cycle 21, based on a current-free extrapolation of the observed photospheric field out to a fixed source surface where the field lines become radial, yields calculated IMF intensities which vary by an order of magnitude and which are far too low near sunspot minimum. We obtain much better agreement with a model containing both heliospheric sheet currents, which deflect polar flux toward the ecliptic, and volume currents, which maintain a residual latitudinal gradient in the IMF intensity. In order to match the observed IMF intensity levels, however, the measured photospheric fields had to be scaled up by approximately a factor of 2. Our composite model has the following main consequences: (1) The source of the radial component of the IMF may be represented to a first approximation by the dipole component of the photospheric field. (2) The radial IMF intensity is strongest in the direction of the dipole axis, which is aligned with the Sun's rotation axis near sunspot minimum but tilts toward the ecliptic near sunspot maximum. (3) The average strength of the photospheric field above latitude 55⁰ is of order 10 G around sunspot minimum. copyright American Geophysical Union 1988

[en] In low-dielectric-constant (low-k) materials, the dielectric constant is reduced through a reduction in electronic polarization or through the introduction of porosity. For ultra-low-k (k value less than 2.4) materials, introduction of porosity is a common method. However, this reduces the mechanical strength of the materials. In this work, two types (carbon-based and silica-based) of spin-on porous ultra-low-k materials are studied. The mechanical properties, including hardness, Young's modulus, stress, stress hysteresis, and adhesion properties of the films are investigated. Compared with the dense low-k materials, the hardness and Young's modulus of the ultra-low-k films are low. The residual stress for the ultra-low-k films on bare Si wafer at room temperature is less than 100 MPa. The stress decreases from tensile to compressive with the increasing temperature up to 430 deg. C, and returns to the initial value as temperature decreasing to room temperature. Scotch tape test, Stud pull test, and chemical mechanical polishing (CMP) check are used for adhesion characterization. No peeling is found between the ultra-low-k materials and the underlying layer SiC after Scotch tape test