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Aluminum multicharged ion generation from femtosecond laser ablation is studied. A Ti:sapphire laser (wavelength 800 nm, pulse width ~100 fs, and maximum laser fluence of 7.6 J/cm2) is used. Ion yield and energy distribution of each charge state are measured. A linear relationship between the ion charge state and the equivalent acceleration energy of the individual ion species is observed and is attributed to the presence of an electric field within the plasma-vacuum boundary that accelerates the ions. The ion energy distribution follows a shifted Coulomb-Boltzmann distribution. For Al1+ and Al2+, the ion energy distributions have two components; the faster one can be attributed to multiphoton laser ionization, while the slower one is possibly due to collisional processes. Ion extraction from the plasma is increased with an applied external electric field, which is interpreted to be due to the retrograde motion of the plasma edge as a result of the external electric field. Multicharged ion generation by femtosecond laser ablation is compared to previously reported ion generation with nanosecond laser ablation and is shown to require significantly lower laser fluence and generates higher charge states and more energetic ions. [ABSTRACT FROM AUTHOR]

We report on a new technique for obtaining off-Hugoniot pressure vs. density data for solid metals compressed to extreme pressure by a magnetically driven liner implosion on the Z-machine (Z) at Sandia National Laboratories. In our experiments, the liner comprises inner and outer metal tubes. The inner tube is composed of a sample material (e.g., Ta and Cu) whose compressed state is to be inferred. The outer tube is composed of Al and serves as the current carrying cathode. Another aluminum liner at much larger radius serves as the anode. A shaped current pulse quasi-isentropically compresses the sample as it implodes. The iterative method used to infer pressure vs. density requires two velocity measurements. Photonic Doppler velocimetry probes measure the implosion velocity of the free (inner) surface of the sample material and the explosion velocity of the anode free (outer) surface. These two velocities are used in conjunction with magnetohydrodynamic simulation and mathematical optimization to obtain the current driving the liner implosion, and to infer pressure and density in the sample through maximum compression. This new equation of state calibration technique is illustrated using a simulated experiment with a Cu sample. Monte Carlo uncertainty quantification of synthetic data establishes convergence criteria for experiments. Results are presented from experiments with Al/Ta, Al/Cu, and Al liners. Symmetric liner implosion with quasi-isentropic compression to peak pressure ∼1000GPa is achieved in all cases. These experiments exhibit unexpectedly softer behavior above 200GPa, which we conjecture is related to differences in the actual and modeled properties of aluminum. [ABSTRACT FROM AUTHOR]

Aluminium nitride (AlN) is a promising material for challenging sensor applications such as process monitoring in harsh environments (e.g., turbine exhaust), due to its piezoelectric properties, its high temperature stability and good thermal match to silicon. Basically, the operational temperature of piezoelectric materials is limited by the increase of the leakage current as well as by enhanced diffusion effects in the material at elevated temperatures. This work focuses on the characterization of aluminum nitride thin films after post deposition annealings up to temperatures of 1000 °C in harsh environments. For this purpose, thin film samples were temperature loaded for 2 h in pure nitrogen and oxygen gas atmospheres and characterized with respect to the film stress and the leakage current behaviour. The X-ray diffraction results show that AlN thin films are chemically stable in oxygen atmospheres for 2 h at annealing temperatures of up to 900 °C. At 1000 °C, a 100 nm thick AlN layer oxidizes completely. For nitrogen, the layer is stable up to 1000 °C. The activation energy of the samples was determined from leakage current measurements at different sample temperatures, in the range between 25 and 300 °C. Up to an annealing temperature of 700 °C, the leakage current in the thin film is dominated by Poole-Frenkel behavior, while at higher annealing temperatures, a mixture of different leakage current mechanisms is observed. [ABSTRACT FROM AUTHOR]

Core-shell-structured nanoalloy particles with an Al-dominated interior covered by few Yb monolayers have been fabricated using a vapor-aggregation method involving magnetron sputtering. The radially segregated structure of the Yb-Al nanoparticles has been disclosed by "on-the-fly" photoelectron spectroscopy monitoring of the nanoparticle beam in Yb 4f and Al 2p electron binding energy regions. Both, the binding energy values and the electron microscopy images taken on the deposited nanoparticles, allow estimating their dimensions to be in the 5-10 nm range. The photoelectron spectroscopy results suggest that in these nanoparticles no trivalent Yb - the typical case for the macroscopic Yb-Al alloy - is present. The oxidation of preformed Yb-Al nanoparticles was successfully attempted, leading to the appearance of divalent Yb surface oxide - in contrast to the bulk macroscopic Yb which is trivalent in the oxide. Our results suggest that at intermediate oxygen exposures "sandwich-like" nanoparticles of YbO/Yb/Al were synthesized. At higher O2 exposures, the oxygen seems to penetrate all the way to the Yb-Al interface. The results of the present study have to be considered when photonic applications of Yb-doped garnet nanoparticles are planned. [ABSTRACT FROM AUTHOR]

This investigation examines the extraordinary light transmission through periodic hexagonally ordered aluminum hole arrays on silicon substrates. The holes are circular and the arrays with various hole diameters were fabricated. The transmission spectra through the hole arrays were measured as a function of the incident angle of the light beam. It is observed that the transmission peak splits into two when the diameter of the hole is close to half of the lattice constant. At larger incident angles, the six degenerate (1,0) Al/Si surface plasmon modes split into four or three modes depending on the symmetry axis, and even higher order modes can be identified for small hole diameters. [ABSTRACT FROM AUTHOR]

We employ molecular dynamics simulations with a first principles-based many body potential to characterize the exothermic alloying reactions of nanostructured Ni/Al multilayers induced by shock loading. We introduce a novel technique that captures both the initial shock transit as well as the subsequent longer-time-scale Ni3Al alloy formation. Initially, the softer Al layers are shock heated to a higher temperature than the harder Ni layers as a result of a series of shock reflections from the impedance-mismatched interfaces. Once initiated, the highly exothermic alloying reactions can propagate in a self-sustained manner by mass and thermal diffusion. We also characterize the role of voids on the initiation of alloying. The interaction of the shock wave with the voids leads not only to significant local heating (hot spots) but also directly aids the intermixing between Al and Ni; both of these phenomena contribute to a significant acceleration of the alloying reactions. [ABSTRACT FROM AUTHOR]

In order to improve the magnetic and electrical properties of low resistance alumina based magnetic tunnel junctions, an off-axis method of sputtering has been investigated. It is shown that the tunnel magnetoresistance ratio can be greatly increased when there is an offset between the target and the wafer axes during the deposition of the ultrathin aluminum layer (off-axis sputtering) prior to its natural oxidation. The ferromagnetic coupling between the pinned and the free layer through the alumina barrier is also reduced compared to a classical on-axis deposition. This observation is interpreted as an improvement of the barrier quality, reducing both the roughness and the pinholes density. We assume that when the Al layer is sputtered off-axis, the magnetic and aluminum layers are protected from energetic neutralized Ar atoms bombardment. [ABSTRACT FROM AUTHOR]

Diluted magnetic semiconductor Ce doped Si films with p-type conduction by means of codoping of Al were fabricated with low-temperature molecular beam epitaxy. The uniformly distributed Ce as a magnetic ion is responsible for the disappearance of ferromagnetic nature. Therefore, the magnetotransport properties of the films were investigated if they have any p-f interaction between f electron in Ce and carrier in this DMS sample. The samples with lower carrier concentration (2 ×1019 cm-3 at 80 K) showed variable range hopping (VRH) conduction and VRH induced magnetoresistance (MR) was recognized. Samples with higher carrier concentration (7 ×1020 cm-3 at 80 K) showed a metallic conduction and a positive MR. The positive MR is attributed to the quantum correction on the conductivity due to the p-f exchange interaction induced spin splitting of the valence band. [ABSTRACT FROM AUTHOR]

GaN-based Schottky barrier diodes (SBDs) with single-crystal Al barriers grown by plasmaassisted molecular beam epitaxy are fabricated. Examined using in-situ reflection high-energy electron diffractions, ex-situ high-resolution x-ray diffractions, and high-resolution transmission electron microscopy, it is determined that epitaxial Al grows with its [111] axis coincident with the [0001] axis of the GaN substrate without rotation. In fabricated SBDs, a 0.2V barrier height enhancement and 2 orders of magnitude reduction in leakage current are observed in single crystal Al/GaN SBDs compared to conventional thermal deposited Al/GaN SBDs. The strain induced piezoelectric field is determined to be the major source of the observed device performance enhancements. [ABSTRACT FROM AUTHOR]

The authors present a detailed investigation of defects generated during catastrophic optical damage (COD) in high-power 650 nm AlGaInP lasers using microphotoluminescence (μ-PL) mapping, focused ion beam (FIB) microscopy, and deep-etching techniques. High-resolution μ-PL images demonstrated that during COD, nonradiative dark line defects (DLDs) originate from the front mirror of the laser and propagate in several branches into the laser perpendicular to the output facet. Furthermore, FIB microscopy identified the epitaxial layers affected by COD, revealing that DLDs are confined to the active region. In addition, deep etching confirmed that these defects have a noncrystalline nature. [ABSTRACT FROM AUTHOR]