Phonon Absorption by D − Band Tails in Ge:Sb

Uniaxial stress dependence of acoustical phonon absorption in intermediately doped Ge:Sb has been studied using heat pulse technique. Abruptly decreasing LA and FTA phonon scattering in stress interval of 3–7 · 10 ⁸ dyn/cm ² with further saturation up to 1.9 · 10 ⁹ dyn/cm ² was observed. Results obtained from phonon measurements correlate with conductivity activation energy behavior on stress. Acoustical transparency stress dependence is assumed to be connected with phonon assisted electron transitions from impurity ground state D ⁰ to D^– band.     

Controlled Nucleation of GaN Nanowires Grown with Molecular Beam Epitaxy

The location of GaN nanowires is controlled with essentially perfect selectivity using patterned SiN_x prior to molecular beam epitaxy growth. Nanowire growth is uniform within mask openings and absent on the mask surface for over 95% of the usable area of a 76 mm diameter substrate. The diameters of the resulting nanowires are controlled by the size of the mask openings. Openings of approximately 500 nm or less produce single nanowires with symmetrically faceted tips.     

Observations of supersonic free shear layers

Visual spreading rates of turbulent shear layers with at least one stream supersonic were measured using Schlieren photography. The experiments were done at a variety of Mach number-gas combinations. The spreading rates are correlated with a compressibility-effect parameter called the convective Mach number. It is found that for supersonic values of the convective Mach number, the spreading rate is about one quarter that of an incompressible layer at the same velocity and density ratio. The results are compared with other experimental and theoretical results. This work is being supported by a research grant from the Rockwell International Corporation Trust.     

Segregation at Special Boundaries in MgO

Segregation of Ca to three high-angle boundaries in MgO was analyzed using scanning transmission electron microscopy. Two boundaries were special [001] twist boundaries (Σ=5 (Σ=36.9°), Σ=17 (Σ=28.5°)) and one was nonspecial. The concentration of Ca at the special boundaries was approximately half that at the nonspecial boundary.     

High current pressure contacts to Ag pads on thin filmsuperconductors

High current, low resistance, nonmagnetic, and nondestructive pressure contacts to Ag pads on YBa₂Cu₃O_7-δ (YBCO) thin film superconductors were developed in this study. The contact resistance reported here includes the resistance of the current lead/Ag pad interface, the Ag pad/YBCO interface, and the bulk resistance of the contact material. This total contact resistance is the relevant parameter which determines power dissipation during critical-current measurements. It was found that regardless of the optimization of the Ag pad/YBCO interface through annealing, a pressure contact can yield a lower total resistance than a soldered contact. The lowest resistance obtained with pressure contacts was 3 μΩ (for a 2×4 mm ² contact). These contacts may be useful for many different high temperature superconductor (HTS) studies where high-current contacts with low heating are needed     

Catalyst-free growth of GaN nanowires

We have grown GaN and AlGaN nanowires on Si (111) substrates with gassource molecular beam epitaxy (MBE). No metal catalysts were used. The nanowires displayed a number of interesting materials properties, including room-temperature luminescence intensity greater than that of free-standing HVPE-grown GaN, relaxed lattice parameters, and the tendency of nanowires dispersed in solvents to align in response to electric fields. The wires were well separated, 50–250 nm in diameter, and grew to lengths ranging from 2 μm to 7 μm. Transmission electron microscopy indicated that the wires were free of defects, unlike the surrounding matrix layer.