Publications

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The thickening behavior of aluminum scandium nitride (Al_0.88Sc_0.12N) films grown on Si(111) substrates has been investigated experimentally using X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy, and residual stress measurement. Al_0.88Sc_0.12N films were grown with thicknesses spanning 14 nm to 1.1 um. TEM analysis shows that the argon sputter etch used to remove the native oxide prior to deposition produced an amorphous, oxygen-rich surface, preventing epitaxial growth. XRD analysis of the films show that the A1ScN(002) orientation improves as the films thicken and the XRD A1ScN(002) rocking curve full width half maximum decreases to 1.34 q for the 1.1 pm thick film. XRD analysis shows that the unit cell is expanded in both the a- and c-axes by Sc doping; the a-axis lattice parameter was measured to be 3.172 ± 0.007 A and the c-axis lattice parameter was measured to be 5.000 ± 0.001 A, representing 1.96% and 0.44% expansions over aluminum nitride lattice parameters, respectively. The grain size and roughness increase as the film thickness increases. A stress gradient forms through the film; the residual stress grows more tensile as the film thickens, from -1.24 GPa to +8.5MPa.

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We discuss chemical, structural, and ellipsometry characterization of low temperature epitaxial Si. While low temperature growth is not ideal, we are still able to prepare crystalline Si to cap functional atomic precision devices.

Tin-germanium alloys are increasingly of interest as optoelectronic and thermoelectric materials as well as materials for Li/Na ion battery electrodes. However, the lattice incompatibility of bulk Sn and Ge makes creating such alloys challenging. By exploiting the unique strain tolerance of nanosized crystals, we have developed a facile synthetic method for homogeneous SnxGe1-x alloy nanocrystals with composition varying from essentially pure Ge to 95% Sn while still maintaining the cubic structure.

The different rate-limiting processes underlying ignition and self-propagating reactions in Al/Pt multilayers are examined through experiments and analytical modeling. Freestanding, ∼1.6 μm-thick Al/Pt multilayers of varied stoichiometries and nanometer-scale layer thicknesses ignite at temperatures below the melting point of both reactants (and eutectics) demonstrating that initiation occurs via solid-state mixing. Equimolar multilayers exhibit the lowest ignition temperatures when comparing structures having a specific bilayer thickness. An activation energy of 76.6 kJ/mol at. associated with solid state mass transport is determined from the model analysis of ignition. High speed videography shows that equimolar Al/Pt multilayers undergo the most rapid self-sustained reactions with wavefront speeds as large as 73 m/s. Al- and Pt-rich multilayers react at reduced rates (as low as 0.3 m/s), consistent with reduced heat of reaction and lower adiabatic temperatures. An analytical model that accounts for key thermodynamic properties, preliminary mixing along interfaces, thermal transport, and mass diffusion is used to predict the wavefront speed dependencies on bilayer thickness. Good fits to experimental data provide estimates for activation energy (51 kJ/mol at.) associated with mass transport subject to high heating rates and thermal diffusion coefficient of premixed interfacial volumes (2.8 × 10-6 m2/s). Pt dissolution into molten Al is identified as a rate-limiting step underlying high temperature propagating reactions in Al/Pt multilayers.

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We present measurements of Josephson junctions containing three magnetic layers with noncollinear magnetizations. The junctions are of the form S/F′/N/F/N/F″/S, where S is superconducting Nb, F′ is either a thin Ni or Permalloy layer with in-plane magnetization, N is the normal metal Cu, F is a synthetic antiferromagnet with magnetization perpendicular to the plane, composed of Pd/Co multilayers on either side of a thin Ru spacer, and F″ is a thin Ni layer with in-plane magnetization. The supercurrent in these junctions decays more slowly as a function of the F-layer thickness than for similar spin-singlet junctions not containing the F′ and F″ layers. The slower decay is the prime signature that the supercurrent in the central part of these junctions is carried by spin-triplet pairs. The junctions containing F′= Permalloy are suitable for future experiments where either the amplitude of the critical current or the ground-state phase difference across the junction is controlled by changing the relative orientations of the magnetizations of the F′ and F″ layers.