Alexander H. Mueller

Low-temperature growth of crystalline GaN films using energetic neutral atomic-beam lithography/epitaxy

Crystalline and polycrystalline gallium nitride films have been grown on bare c-axis-oriented sapphire at low temperatures (100 °C to 500 °C) using energetic neutral atom-beam lithography/epitaxy. Surface chemistry is activated by exposing substrates to nitrogen atoms with kinetic energies between 0.5 and 5.0 eV and a simultaneous flux of Ga metal, allowing low-temperature growth of GaN thin films. The as-grown GaN films show semiconducting properties, a high degree of crystallinity, and excellent epitaxial alignment. This method of low-temperature nitride film growth opens opportunities for integrating novel substrate materials with group III nitride technologies.

Kläui Ligand Thin Films for Rapid Plutonium Analysis by Alpha Spectrometry

As part of a nuclear forensics capability, rapid and effective methods to analyze for plutonium and other actinide metals are needed. A key requirement of these methods is that they afford a high chemical yield while still providing isotopic information necessary for forensic evaluation. Toward this objective, a new method for binding plutonium for analysis by alpha spectrometry has been developed. Thin films of Kläui-type tripodal oxygen donor ligands were prepared by spin-casting solutions onto glass substrates. Three different ligands were evaluated for plutonium binding, and the best results were obtained using the ethyl-substituted complex Na[Cp*Co(P(O)(OEt)2)3], which bound 80-88% of the dissolved Pu under equilibrium conditions. The thin films are simple and inexpensive to prepare and exhibit excellent alpha spectral resolution, having line widths of ~33 keV. The method has been successfully applied to analyze for plutonium in both an archived nuclear debris sample and a certified environmental soil sample. The results obtained from the soil analysis are in good agreement with the certified values, demonstrating the effectiveness of the method for rapid plutonium analysis.

Characterization of the thin-film NbN superconductor for single-photon detection by transport measurements

Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA(Dated: May 21, 2013)The fabrication of high-quality thin superconducting films is essential for single-photon detectors. Theirdevice performance is crucially a ected by their material parameters, thus requiring reliable and nondestructivecharacterization methods after the fabrication and patterning processes. Important material parameters to knoware the resistivity, superconducting transition temperature, relaxation time of quasiparticles, and uniformity ofpatterned wires. In this work, we characterize micro-patterned thin NbN films by using transport measurementsin magnetic fields. We show that from the instability of vortex motion at high currents in the flux-flow state ofthe IV characteristic, the inelastic life time of quasiparticles can be determined to be about 2 ns. Additionally,from the depinning transition of vortices at low currents, as a function of magnetic field, the size distribution ofgrains can be extracted. This size distribution is found to be in agreement with the film morphology obtainedfrom scanning electron microscopy and high-resolution transmission electron microscopy images.

Innovative approach to nanoscale device fabrication and low-temperature nitride film growth

Energetic neutral beam lithography/epitaxy (ENABLE) was used for etching very high-aspect-ratio nanoscale structures into polymers and for growing templated AlN films at low temperatures. Various methods were used for masking polymeric films for selective etching by energetic oxygen atoms to fabricate sub-100 nm structures with aspect ratios exceeding 35:1. ENABLE was also utilized for low-temperature growth of AlN into previously etched polymer templates to directly form AlN wires. By taking advantage of the unique processing capabilities of ENABLE, new opportunities for making delicate nanostructures are made possible.

Electrocaloric refrigerator using electrohydrodynamic flows in dielectric fluids

Heat switches are a key enabling element of efficient refrigerators that are based on the electrocaloric effect. We demonstrate a new concept for a heat switch that is based on micro-scale electrohydrodynamic (EHD) flows in thin layers of dielectric fluids. In this device, convective flow of the fluid is controlled by applying an electric field across the fluid layer. This creates a heat switch that can be cycled between a “closed” state with efficient convective heat transport and an “open” state with less efficient conductive heat transport. Substantial switching of the thermal transport coefficient was achieved in 500 μm thick layers of commercial hydrofluoroethers and bias voltages of typically 390 V. The efficacy of the heat switch varied by almost four orders of magnitude for different biasing schemes. The highest efficacy was achieved by biasing a patterned strip electrode and using a planar ground electrode. A preliminary experiment found a thermal conductivity contrast of 4.7±1.1 for the switch in the closed vs. open state. We also characterize the electrocaloric response of commercial multilayer ceramic chip capacitors and show that they can serve as serve as a useful surrogate material for first-generation electrocaloric refrigerators until higher performing multilayer structures of ferroelectric polymers are available.