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Notes: Nanocomposites of Al-In, Al-Pb, and Zn-Pb have been prepared and characterized using rapid quenching techniques and the nature of superconducting transitions in them has been studied by resistivity measurements. The precipitated second phases (In and Pb) have particle sizes (d) of a few tens of nanometers such that ξ0≥d≥dmin, where ξ0 is the superconducting zero temperature coherence length and dmin is the minimum particle size that supports superconductivity. The onset of superconductivity generally starts in samples with d∼ξ0 and progressively other grains with d≤ξ0 become superconducting. We suggest that the proximity effect of the matrix plays a significant role. In an Al-In system, even with 40 wt.% In, the zero resistivity state is obtained at T∼1.33 times the Tc of Al. But in Al-Pb and Zn-Pb, the zero resistivity state is obtained at T∼4 and 5 times the Tc of Al and Zn with only 10–15 wt % Pb, respectively.

Notes: Single-crystal GexSi1−x/Si superlattices have been successfully fabricated using remote plasma-enhanced chemical vapor deposition, a novel low-temperature thin-film growth technique. Reflection high-energy electron diffraction, cross-sectional transmission electron microscopy (XTEM), plan view TEM, x-ray-diffraction, and secondary-ion mass spectroscopy techniques have been applied to study the crystallographic properties of the superlattice structures. Arrays of dislocation lines, which are either parallel or perpendicular to each other, have been observed in the superlattices for those cases in which the total layer thickness exceeds the critical layer thickness. The location, orientation, and Burgers vectors of the misfit dislocation lines have been analyzed. Possible mechanisms of the generation of the misfit dislocations are also discussed.

Notes: Thermal processing of InP at temperatures above 500 °C is indispensable in the growth and device fabrication of InGaAsP alloy semiconductors for optoelectronic and microwave applications. Incongruous loss of P at these temperatures creates native defects and their complexes. The presence of such defects modifies the electrical and optical properties of the material resulting in poor device performance. In addition, native defects play a significant role in dopant diffusion which is a topic of current interest. We have measured deep-level photoluminescence (PL) on undoped InP after heat treatments at 500 and 550 °C in an open-tube processing system in different protective environments of powder InP, and Sn-InP melt together with an InP cover. In this paper we shall present the PL results which have bearing on the question of defects. We find that (1) the Sn-InP melt provides better protection in preserving the overall luminescence in InP; (2) the deep-level PL related to defects has at least two components in the virgin samples, viz., MnIn, and band C, which is a native defect complex related to VP; (3) a new defect appears in samples heated in a P-deficient environment; and (4) the enhancement in the deep-level luminescence intensity after heat treatment can be attributed to the excess defect concentrations existing under nonequilibrium conditions of an open-tube processing environment.

Notes: The chemical and electronic aspects of a GaAs (100) surface passivated by selenium sulfide (SeS2) have been investigated by x-ray photoelectron spectroscopy and photoluminescence. It has been observed that this treatment gives rise to an arsenic selenide (As2Se3) terminated surface. No S—GaAs bonds were observed. The remarkable electronic properties and the formation of the chemically and thermally stable As2Se3 phase reveals the successful passivation. Passivation of GaAs in single step and identification of a single selenium species on the surface are considered to be the major advantages of using SeS2.

Notes: Boron diffusion in polycrystalline Si-on-single crystal Si systems has been studied by secondary ion mass spectrometry. The extrapolated B-diffusion profiles in polycrystalline Si and in the single crystal Si substrate reveal a discontinuity at the polycrystalline Si-single crystal Si interface. The discontinuity in the B profiles is believed to occur due to the blockage of B-defect complexes by the interfacial oxide between polycrystalline Si and the single-crystal Si substrate, as well as the immobility of these defect complexes in single crystal Si. The B in the implant peak region above the B solid solubility limit is found to be immobile in single crystal Si during annealing due to the formation of electrically inactive B-defect complexes. In polycrystalline Si, however, our results show that the B in the peak region spreads out more rapidly than in single crystal Si possibly due to the diffusion of B-defect complexes along grain boundaries. The B-defect complexes are electrically inactive as determined by spreading resistance analysis. If the B concentration is lowered below the solid solubility limit, either by decreasing the dose or by raising the anneal temperature, no discontinuity is observed in the B profile across the polycrystalline Si-single crystal Si interface.

Notes: The acquisition of viscous magnetization has been measured on a single domain magnetite sample in the temperature range 80 to 330 K and in applied fields up to 1540 Oe. Based on Néel's single domain theory, we have calculated the field and temperature dependence of viscous acquisition by numerical two-dimensional integration, using experimentally determined volume and switching field distributions. Predicted and experimental viscosity coefficients do not match, with especially strong discrepancies below the Verwey transition. The experimental acquisition coefficient is larger below the transition than at room temperature, even though the corresponding switching field distributions are nearly identical. Thus we cannot explain single domain viscosity by switching field and volume distributions alone, and we suspect that stress plays a prominent role.

Notes: We report on a modified Kroemer's analysis [Appl. Phys. Lett. 36, 295 (1980)] for the determination of the band offset ΔEc of a single quantum well from a carrier profile obtained by capacitance-voltage measurement. The procedure is applied to a pseudomorphic GaAs/InGaAs/GaAs strained layer structure.

Notes: Homoepitaxial Si films were deposited using an ArF excimer laser beam parallel to the substrate to photodissociate Si2H6 at low temperatures (250–350 °C) under laser intensity and Si2H6 partial pressure conditions that result in low initial photofragment concentrations (〈 1013 cm−3). Total pressure and flow conditions were chosen such that there is little secondary photolysis of the initial photofragments. The deposition yield of solid Si from photoexcited Si2H6 is estimated to be 0.20±0.04, indicating that in order for film growth to result solely from the primary products in ArF laser (193 nm) photolysis of Si2H6, a sticking coefficient ≥0.6 must be assigned to the dominant growth precursor. Growth rates vary linearly with laser intensity and Si2H6 partial pressure over a range of 1–15 mJ/cm2 pulse and 5–40 mTorr, respectively. Increasing the distance between the laser-beam axis and the silicon substrate results in a reduction of the growth rate that can be explained by gas-phase chemical reaction of the growth precursors as they diffuse to the substrate. Epitaxy is maintained for temperatures above the threshold for thermal decomposition of surface (SiH2)n chains at ∼250 °C, and for temperatures below the onset of Si2H6 pyrolysis and rapid thermal desorption of surface H2 at ∼350 °C.

Notes: The structural features of turbulence at the free surface of a channel flow have been experimentally investigated. The experiments were conducted in a horizontal channel of large aspect ratio in the (depth based) Reynolds number range of 2800–8800. The results indicate that the persistent structures on the free surface can be classified as upwellings, downdrafts, and spiral eddies. Upwellings are shown to be related to the bursts originating in the sheared region at the channel bottom and the eddies are seen to be generated at the edges of the upwellings. The eddies often merge if rotating in the same direction, and form "pairs" if rotating in opposite directions—though there are occasional mergers of such counter-rotating ones. The spiral eddies decay slowly and are sometimes annihilated by fresh upwellings. The population densities and the persistence times of the various structures were measured for different flow conditions. The resulting data show that the physical parameters characterizing the structures at the interface, scale with a mix of inner (wall shear stress and viscosity) and outer variables. Measurement of the streamwise and spanwise velocities at the free-surface were made by particle imaging velocimetry (PIV) and the surface normal velocity near the free-surface estimated by continuity. The results indicate that the upwellings and spiral eddy regions would be expected to dominate scalar transport rates at high Prandtl/Schmidt numbers. The one-dimensional energy spectra of the flow field at the free-surface compare well with direct numerical simulations and show a region with −5/3 slope at low wave numbers. This experimentally confirms a previous result regarding the two-dimensionality of turbulence near the free surface, based on numerical simulations by Pan and Banerjee [Phys. Fluids 7, 1649 (1995)]. © 1998 American Institute of Physics.

Notes: A method for particle image velocimetry (PIV) is presented which improves upon the accuracy, computational efficiency and dynamic range (i.e., the difference between the largest and smallest resolvable particle displacement vectors) of conventional PIV techniques. The technique is applied to free-surface turbulence to resolve energy spectra for motions with a wide dynamic range. The methodology—based on multi-grid image processing algorithms for rigid body motion analysis, estimates the displacement vectors at discrete particle locations. The essence of this technique is to estimate large scale motions from image intensity patterns of low spatial frequencies and small scale motions from intensity patterns of high spatial frequencies. Cross-correlation between a pair of time separated particle images is implemented by the hierarchical computational scheme of Burt ["Fast filter transforms for image processing," Int. J. Comput. Vision 16, 20 (1981)]. Each image is convolved with a series of band-pass filters and subsampled to obtain a set of images progressively decreasing in resolution and size. A coarse estimate of the displacement field obtained from pairs of lower resolution images are used to obtain more accurate estimates at the next (finer) level. Processing starts at the level of lowest resolution and stops at the highest resolution level, which contains the original image pair. Due to subsampling of low resolution images, the match template size can be kept constant for all stages of computation, thus eliminating the dependence of the largest resolvable displacement on the size of match template. In the present work, the search area at each level is kept constant at 3×3 pixels and the match template size at 5×5 pixels for all levels of computation. The algorithm has been implemented using simple thresholding based on the confidence level of an estimated displacement vector, as suggested by Anandan ["A computational framework and an algorithm for measurement of visual motion," Int. J. Comput. Vision, 2, 283, (1987)]. However, the confidence-level-based smoothing technique for rigid body motions (continuous velocity fields) could not be applied to displacement estimates obtained at discrete points i.e., the particle locations. Instead, smoothing was performed over the area covered by each particle. The algorithm has been tested against direct numerical simulations of turbulent flows when the flow field is known and particle images have been generated from these with the addition of noise. Both the accuracy of motion estimation and the computation time are seen to improve as compared to conventional PIV methods. Finally, video images taken of particle motion on the free-surface of a channel flow have been used to determine the capabilities of the technique in an experimental study. The resulting spectra show a quasi-two-dimensional character of the free-surface turbulent flow field, which corresponds well with the direct numerical simulations. © 1998 American Institute of Physics.