•Lower Al incorporation in InAlAs and InGaAs was found in thin (<2 nm) strained layers.•A full QCL structure was analyzed via APT to investigate solid-state diffusion.•Al overshoot in the thin InAlAs ...layers converged modeled and experimental wavelength.
In order to conduction-band engineer quantum cascade lasers (QCLs) for emission wavelength, specifically QCLs emitting at 4.6 µm, precise control over layer thicknesses and compositions is required. In this study, Al/Ga incorporation in a strain-balanced InAlAs/InGaAs superlattice (SL) on InP is characterized as a function of layer thickness, keeping the composition constant, using high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and Atom Probe Tomography (APT). A SL structure was grown on an InP substrate by organometallic vapor phase epitaxy (OMVPE) at a temperature of 605 °C and a reactor pressure of 100 torr, with 5 sec interruption time between layers. The full growth thickness obtained from STEM images was used to calibrate the reconstruction of the atom probe data. From the APT results it was found that the Al and Ga incorporation in thin layers (<2 nm) is significantly lower (down by as much as 10–15% for 1 nm-thick layers) than the targeted composition, which results in a red-shifted emission wavelength when compared to theoretical-model calculations. This result is further shown to be self-consistent at these compositions when compared to a full QCL structure. By introducing an Al overshoot of 5% in the OMVPE gas phase Al/In ratio for the thinnest barrier layers of a full QCL structure, the emission wavelength was reduced from 4.8 µm to 4.6 µm, in accordance with the modeled wavelength for the design target.
The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. ...This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Creep rupture behavior of semi-solid cast 7075-T6 Al alloy Mahathaninwong, N.; Zhou, Y.; Babcock, S.E. ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
10/2012, Letnik:
556
Journal Article
Recenzirano
The creep rupture behavior of semi-solid cast 7075-T6 Al alloy produced by the Gas Induced Semi-Solid (GISS) process was investigated and compared to that of commercial 7075-T651 Al alloy. The ...semi-solid cast 7075-T6 Al alloy displayed lower minimum creep rate and longer creep rupture time than the commercial 7075-T651 Al alloy. On the basis of their stress exponent, n, values of 6.3, dislocation creep was seemingly the predominant mechanism controlling the creep deformation of both alloys. The creep rupture time of the semi-solid cast 7075-T6 Al alloy was distinctly longer than that of the commercial 7075-T651 Al alloy at stress regimes of 120–140MPa. This difference was attributed to the lower precipitate coarsening and higher precipitate density in the semi-solid cast alloy. Creep cavities predominately controlled the creep rupture of the semi-solid cast 7075-T6 Al alloy despite the appearance of precipitate coarsening. The commercial 7075-T651 Al alloy creep rupture behavior was controlled by the combination of rapid precipitate coarsening and creep cavities. However, de-cohesion between insoluble particles and the matrix is evidently accelerated with increasing stress to 180MPa, leading to cavity propagation and resulting in the convergence of creep rupture time in the semi-solid cast 7075-T6 Al alloy to that of the commercial 7075-T651 Al alloy.
Defect reduction in heteroepitaxial growth of GaSb and InAs was achieved using nano-patterned GaAs substrates generated by block copolymer lithography (BCL). The lattice-mismatched growth of both ...GaSb and InAs on nano-patterned GaAs templates exhibited defect mitigation with few observed threading dislocations, but the growth behavior and surface morphology of the resultant films were very different for the two systems. The strain relaxation process proceeds through the relaxation of the nanoscopic, highly lattice-mismatched islands prior to film coalescence. The growth of GaSb on GaAs templates yielded a smooth, coalesced film whereas large islands were observed for the growth of InAs on such templates. The difference in surface morphology was described in terms of the interaction of adatom surface diffusion and the strain-dependent attachment of these atoms with the growing islands. The morphology can be altered through changes in growth temperature and rate. Nanolithographic patterning can be used as a general technique for integration of lattice-mismatched materials.
The defect-trapping effectiveness of an InAsP metamorphic buffer layer (MBL) design was investigated by studying the light-emission characteristics of InAs quantum wells grown on the MBL, along with ...structural characterization through SIMS and TEM measurements. Using a fixed recipe for the MBL, several separate confinement heterostructures (SCHs) were studied using 8-band k.p simulations, HRXRD, SEM, optical microscope and variable-temperature photoluminescence. Room temperature PL was observed at wavelengths near 3 mu m, although the PL intensity drops significantly for samples with wavelengths longer than 2.85 mu m. Laser operation was achieved at 77 K at a wavelength of 2.45 mu m and threshold as low as 290 A/cm super(2).
The bicrystals were composed of square-plate grains that were 125-375 p,m on a side and 50-180 p,m thick. ...the grain boundaries were on a bulk scale. (For the case of exactly coincident lattice ...sites, 2-1 = fraction of lattice sites in one crystal that are coincident with lattice sites in the other crystal. Because of the orthorhombic lattice, the low-energy misorientations for YBa2Cu307„s are associated with NCSL misorientations. 1 is used in the same way to describe NCSL misorientations.) It is possible that the superconducting properties of the boundary correlate with the 2 value and/or the deviation from the exact low-energy misorientation, Дер7,10. ...there are clearly a number of other possible crystallographic and processing-related origins for the observed variety in properties to be investigated in future experiments. To develop useful Jct properties in larger magnetic fields this transition from a Josephson junction to a flux-pinning grain-boundary characteristic must be made.
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