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Multilayers composed of many thin films of PbTe and Pb1-xSnxTe on BaF2 substrates were grown epitaxially by hot-wall-vapor deposition. In order to investigate the fraction of the total misfit (2.5x10-3 at x=O, 12) accommodated by misfit strain we have performed strain measurements on these superlattices by two different X-ray diffractometer techniques. We also report on substrate induced strain due to different thermal expansion coefficients of films and substrate. For film thicknesses smaller than 300 nm there is clear evidence for almost complete accommodation of lattice mismatch by misfit strain. Below room temperature the substrate induces a tensile strain which is comparable to that of the misfit strain.
First 405 GHz and 212 GHz solar flare observations were obtained during short campaigns while the new solar submillimeter-wave telescope (SST) was still undergoing adjustments at the CASLEO El Leoncito observatory in the Argentina Andes. We show here preliminary results for a large X1.1 class X-ray event occurred on 2000 March 22, which exhibited a small submm-w continuum response to the slow (minutes) bulk flare emission, and numerous subsecond spikes (100-300 ms), the brightest spikes reaching about 180 and 50 s.f.u. at 405 and 212 GHz, respectively.
We derive zphot for sources in the entire (~0.4 deg2) H-HDF-N field with the EAzY code, based on PSF-matched broad-band (U band to IRAC 4.5 μm) photometry. Our catalog consists of a total of 131,678 sources. We find σNMAD = 0.029 for non-X-ray sources. We also classify each object as a star or galaxy through SED fitting. Furthermore, we match our catalog with the 2 Ms CDF-N main X-ray catalog. For the 462 matched non-stellar X-ray sources, we improve their zphot quality (σNMAD = 0.035) by adding three additional AGN templates. We make our photometry and zphot catalog publicly available.
A computer-controlled xyz dispensing system called the Biological Architecture Tool (BAT) has been extensively tested in the creation of multilayered and three-dimensional biological objects: tissue scaffolds and plain and patterned cellular-array slides. The BAT dispensing system has proven its versatility and reliability in tissue engineering and biological experiments. The potential employments of modified versions of the xyz dispensers for in vivo minimally invasive surgery and other in vitro aspects of biological and medical research are discussed.
For negative charging electrophotographic applications a three layer heterostructure (Al/n+-a-Si:H/a-Si:H/a-SiC:H) has been investigated. To some extent a separation of the function of charge storage by a high band gap amorphous silicon carbon alloy and of that of photogeneration of carriers and of sufficient photosensitivity in an intrinsic amorphous silicon film has been achieved. Due to a low injection of electrons from the surface into the a-SiC:H and a small thermal generation rate of carriers within the a- Si:H film, the dark decay of the negative surface potential is dominated by space charge limited currents of holes. Thus a considerable difference in surface potential for negative and positive charging occurs, which is related to the difference in electron and hole mobility in a-Si:H. Since there is no remarkable barrier at the heterointerface a-Si:H/a-SiC:H an excellent injection of photogenerated holes into the a-SiC:H can be provided.
Doping of a-Si:H and a-SiC:H has been performed by plasma enhanced boron migration from thin (5–30 nm) Si0.2B0.8 layers on top of the substrate into the growing i-film. Transport coefficients for this plasma assisted diffusion (PAD) evaluated from SIMS profiles (D* = 10−15 … 10 cm2/s for a-Si:H, 10−15 cm2/s for a:SiC:H) by far exceed those for thermal diffusion after film deposition (10−21 cm2/s). Boron profiles are strongly governed by deposition parameters like substrate temperature, types and energies of radicals growing the film. Boron migration can be modelled assuming particle extraction from the network to the surface by bombardment of ions and atoms and high surface mobility of BHx radicals as a result from interaction with the plasma. Temperature dependent dark conductivity measurements show doping efficiencies comparable to B2H6 gas phase doping, but leaving plasma parameters optimized for i-film Leposition unchanged. Thin a-SiC:H layers are effectively p+-doped without affecting transmission, as no layer is optically detectable after PAD.
Time-of-flight (TOF) and charge collection measurements are evaluated to determine electron transport quality of a-Si1−x Gex:H for 0 ≤ x ≤ 0.3. The drift mobility data are used to calculate the tail state distribution at the conduction band, which turns out to be of hybrid structure (flat linear followed by a steep exponential decay). By incorporation of Ge additional localization introduced by chemical disorder broadens the band tail. The dangling bond density, calculated from electron µDTE products, also dramatically increases. Both effects contribute to the drop of photoelectronic quality of a-Si1−xGex: H alloys.
In addition it is shown that the extended state mobility deduced from TOF experiments reflects a tunneling transport mechanism in localized states above a dominant transport level that separates states with high tunnel probability from states where carriers propagate via thermal release from and capture into localized states.
Ambipolar diffusion lengths in a-SiGe:H and a-SiC:H have been analyzed by Steady State Photocarrier Grating Technique. Diffusion lengths and photoconductivity of a-Si:H are considerably affected by alloying. Photoconductivity in a-SiGe:H can be improved by special deposition methods, diffusion lengths, however turn out to remain nearly unchanged. The comparison of diffusion lengths and -ho~oconductivity yields hole mobilities in a-Si1-xGex:H of 10-2cm2 /Vs for O≤×≤O.2. For a-Si:H changes in photoconductivity by generation of defects (light soaking) result in noticeable changes in diffusion lengths, whereas different photoconductivities in a-SiGe:H caused by different deposition methods end up in the same diffusion lengths. Consequently the improvement of a-SiGe:H photoconductivity by changes in deposition condition is by far a much more complex process than only decrease of density of midgap states.
a-Si1−xCx:H superlattice structures were fabricated by photo-CVD and glow discharge deposition. The compositional abruptness of the heterojunction has been confirmed by X-ray diffraction and Auger electron spectroscopy. The optical bandgap of amorphous silicon-based superlattices increases as the well layer thickness decreases. The existence of quantized levels in a-Si:H wells is demonstrated by the observation of resonant tunneling current through the three-barrier two-well structure.
The electronic structure of the Pu-based superconductor PuCoGa5 and the Pauli paramagnet UCoGa5 is investigated using photoemission spectroscopy. The photoemission data of PuCoGa5 reveal features at the Fermi energy EF and about 1-1.5 eV below EF indicative of itinerant and localized f-electrons, respectively. Angle-resolved spectra of UCoGa5 show two peaks at similar energies that are highly dispersive, providing evidence for itinerant character of the f-electrons in this material. A comparison of the PuCoGa5 and UCoGa5 data to the spectra of α-Pu and δ-Pu serves to place PuCoGa5 within the context of the more general electronic structure problem in elemental Pu.
The energy distribution of localized states above the Fermi level in undoped a-Si:H and a-SiGe:H has been determined by phase shift analysis of modulated photocurrents. (1) A peak in the DOS with 0.56 – 0.65 eV activation energy has been found, reflecting the D− -state of isolated dangling bonds. A second peak with 0.35 eV activation energy has been detected which is attributed to the T3+-state of T3+-T3− -pairs or to the antibonding state of weak Si-Si bonds, respectively. Strong illumination raises the lower peak and quenches the upper one supporting a shallow state - deep state conversion model. (2) From temperature-dependent measurements a shift of the dominant electron transport path into the tail with decreasing temperature - associated with tail state hopping - has been obtained.
Evolution of stress during interfacial reactions between thin films of nickel and (001) silicon substrates has been determined at temperatures ranging from 250 to 325° C by a levered optical beam reflection technique. This technique allows accurate and in situ measurement of composite specimen curvature, thereby providing a continuous measure of stress during ongoing interfacial reactions. In addition, microstructure and phase morphology associated with these reactions have been determined by x-ray diffraction and transmission electron microscopy. Initially, Ni2Si forms and grows at the original Ni/Si interface, accompanied by an increasing net compressive stress in the composite film. Subsequently, NiSi forms and grows at the Ni2Si/Si interface, accompanied by an increasing net tensile stress in the composite film. The initial compressive stress is attributed to localized formation and growth of Ni2Si into the adjacent silicon substrate, whereas the subsequent tensile stress is attributed to formation and growth of NiSi into the adjacent Ni2Si layer. Time required to effect transition from compressive to tensile stress decreases and magnitude of both stresses decreases with increasing temperature, due to thermally activated reaction processes.
Modulated primary photocurrent (MPC) studies on pin structures show spatial variations of the gap state distribution across the i-layer that can be correlated with Fermi level shifts by band bending towards interfaces. These results as well as reverse bias annealing effects are explained in terms of the defect pool model. It is demonstrated that MPC measurements are basically identical to TOF measurements with clear advantages in the post-transit time regime.
Optical and transport studies of both cb- and vb-tail states in a-Si1−xGex:H such as subband absorption (PDS), instationary photocurrent experiments (TOF, PTS) for electrons and holes, Modulated Photocurrent Spectroscopy (MPS), and Raman scattering have been performed. The main consequences of Ge-alloying into the a-Si:H network are i) an increase in cb-tail state density at the conduction band edge and in the exponential cb- tail even for small x (O<x<0.3), accompanied by ii) a rise in deep cb-tail and midgap states which to some extent can be reduced by appropriate deposition methods; iii) at the valence band side up to x≈0.3 the tail seems not to be affected at all and for 0.3<x<0.9 the vb-tail obviously can be kept similar to that of a-Si:H (Evo≈(50–60) meV). Halfwidths of Raman TO-like modes point to the existence of a rigid Si-network in O<x<0.3 in which Ge is incorporated and to a transition at x>0.35 into a Si-Ge compound structure with maximum disorder at x≈0.5.
Gap state profiles in a-Si:H around the Fermi energy have been determined by phase shift analysis of modulated photocurrents. Measurements on different samples in various light-soaked and annealed states show a general strong correlation of the defect structure with the Fermi level position, with a minimum in the DOS at Ef and a peak above Ef. A model of the defect structure involving a peak of D+ states above Ef is outlined that accounts for the observed correlations.
The optoelectronic properties of SimGen strained layer superlattices (SLS's) depend strongly on the structural perfection. We used double crystal and triple axis x-ray diffractometry to characterize the structural properties of short period Si9Ge6 SLS's grown on about lμm thick step-graded SiGe alloy buffers. As grown SLS's and samples annealed subsequently at 550°C, 650°C and 780°C for 60 mmn were investigated. Precise strain data were extracted from two-dimensional reciprocal space maps around (004) and (224) reciprocal lattice points. These data were used as refined input parameters for the dynamical simulation of the integrated intensity along the qll direction. Annealing causes interdiffusion as indicated by the decreasing superlattice (SL)-satellite peak intensities and by the change of the Si/Ge thickness ratio. However, the full width at half maximum of the SL satellite peaks does not change significantly with annealing up to 650°C. The in-plane SL lattice constant in both samples is increased only slighty by annealing (< 9×10−3 Å). Consequently the interface intermixing due to interdiffusion is the main cause for the shift of the luminescence energy to higher values in these annealed samples.
From transmission experiments in the mid infrared region the optical interband transitions between electric subbands in the valence and conduction band of the PbTe wells embedded in Pb1−xEuxTe barriers (x = 2.6% to 3.0%) were determined and compared with calculations based on the envelope function approximation. The frequency dependence of the absorption constant α was calculated, taking the in-plane dispersion of the multiquantum well band structure into account. The frequency dependence of the refractive index n(ω) was derived from α(ω) using a Kramers-Kronig transformation and it exhibits several extrema which coincide with the onset of the various interband transitions. Experimentally the onset of these transitions was also observed in photoconductivity experiments. The changes in n(ω) are as large as 15 % and therefore important for the optical confinement in quantum well lasers grown with these materials.
A dense network structure with low defect and void densities in a-SiGe:H alloys are realized by remote deposition conditions (triode reactor configuration, H2-dilution of SiH4+GeH4 gas mixture). The material properties are characterized by optical transmission, photothermal deflection spectroscopy (PDS), scanning electron microscopy (SEM), and space charge limited currents (SCLC). A high refractive index n∞. (λ->∞) and a fine surface structure in SEM images indicate an optical dense network, moreover the defect density determined by SCLC is reduced in comparison to films deposited in a conventional diode reactor. The substrate temperature TS, one of the key parameters for high quality low gap material, sensitively influences the optical and electronic properties of the samples. The optimum substrate temperature (TS≍=225°C) is found to be lower than for the deposition of a-Si:H (TS≍250°C).
The remote plasma deposition process was studied by optical emission spectroscopy using Ne and N2 to detect He-metastables. a- Ge:H was prepared and its optoelectronic and structural properties were characterized. AM 1.5 photoconductivities around 10−6(Ωcm)−1 were obtained in intrinsic material with the Fermi level position lying near midgap. However, even in the best film the defect density is higher than 1017cm−3.