Mark A. Hoffbauer

Fabrication of high density, high aspect-ratio polyimide nanofilters.

A novel fabrication process produces high porosity polymer nanofilters with smooth, uniform, and straight pores with high aspect ratios. The process utilizes electron beam lithography and energetic neutral atom beam lithography and epitaxy techniques. The method has the potential to produce a new generation of high-precision, very-high-porosity, biocompatible filters with pore sizes down to 100nm.

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.

EFFECT OF SURFACES ON THE PERFORMANCE OF CdZnTe DETECTORS

Surface processing plays a major role in manufacturing CdZnTe semiconductor devices used for radiation detection. We are conducting a thorough, systematic study of surfaces and contacts and their effect on charge transport and signal formation in CdZnTe devices. We are investigating wet chemical processing techniques as well as treatment of surfaces with energetic neutral atoms. Our goal is to develop and implement improved surface treatment methods and device manufacturing techniques for large-volume CdZnTe detectors. In addition, we will determine how surfaces and electrical contacts affect the performance of CdZnTe devices used for radiation detection. In this paper, we will show how surface electronic properties influence carrier transport and signal formation in devices designed to simulate coplanar grid detectors. By altering the surface using a wet chemical process, we will show that charge collection is significantly effected by the conductivity of the surface.

PERFORMANCE OF CDZNTE DETECTORS PASSIVATED WITH ENERGETIC OXYGEN ATOMS

Abstract Noise caused by surface-leakage current can degrade the performance of CdZnTe spectrometers, particularly devices with closely spaced contacts such as coplanar grid detectors. In order to reduce surface leakage, we are treating CdZnTe detector surfaces with energetic, neutral oxygen atoms. Energetic oxygen atoms react with the surface to form a resistive oxide layer. Because the reaction is effective at room temperature, deleterious heating of the substrate is avoided. In most cases, leakage current and noise are shown to decrease significantly after treatment. The effect of the treatment on the performance of coplanar grid detectors is presented.

Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection

How long does a neutron live? Unlike the proton, whose lifetime is longer than the age of the universe, a free neutron decays with a lifetime of about 15 minutes. Measuring the exact lifetime of neutrons is surprisingly tricky; putting them in a container and monitoring their decay can lead to errors because some neutrons will be lost owing to interactions with the container walls. To overcome this problem, Pattie et al. measured the lifetime in a trap where ultracold polarized neutrons were levitated by magnetic fields, precluding interactions with the trap walls (see the Perspective by Mumm). This more precise determination of the neutron lifetime will aid our understanding of how the first nuclei formed after the Big Bang. Science, this issue p. 627; see also p. 605 Ultracold polarized neutrons are levitated in a trap to measure their lifetime with reduced systematic uncertainty. The precise value of the mean neutron lifetime, τn, plays an important role in nuclear and particle physics and cosmology. It is used to predict the ratio of protons to helium atoms in the primordial universe and to search for physics beyond the Standard Model of particle physics. We eliminated loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls. As a result of this approach and the use of an in situ neutron detector, the lifetime reported here [877.7 ± 0.7 (stat) +0.4/–0.2 (sys) seconds] does not require corrections larger than the quoted uncertainties.

The structure and morphology of Ag film growth on Cu(110)

Abstract Low-energy electron diffraction (LEED), Auger electron spectrometry (AES), and workfunction change measurements have been used to characterize the structure and morphology of ultrathin Ag films grown on Cu(110). The LEED data show pronounced differences in diffraction arrays for multilayer deposition at 120 and ⩾ 300 K that are, respectively, consistent with disordered layering and layer-cluster growth morphologies deduced from AES. Following the initial growth of Ag in-registry with the substrate troughs, a split c(2 × 4) monolayer structure is observed that to first order is interpreted in terms of a distorted Ag(111) overlayer with a seventh-order superlattice periodicity normal to the substrate troughs. At 120 K, postmonolayer deposition reconstructs the monolayer template, producing a p(8 × 1) → p(8 × 3) → p( 8 7 × 3 ) sequence of diffraction features. These data are consistent with development of a heavily corrugated and strained Ag(110) film that grows from a configuration where the first atomic layer of Ag is captured in the Cu troughs. Multilayer coverages at ⩾ 300 K give LEED features that are indicative of clusters, which gradually coalesce to produce {111} microfacets in a geometry that is consistent with the 120 K structural observations. In agreement with trends seen in bulk binary properties of Ag and Cu, the film development appears to be driven by dominant Ag-Ag bonding as modulated by the row-trough substrate corrugation. The clustering is viewed as the most energetically favorable way to relieve the interfacial strain and attain a bulk Ag film in the absence of low-temperature kinetic limitations.

Growth of oxide layers on gallium arsenide with a high kinetic energy atomic oxygen beam

Oxide layers have been formed on (110) and (100) GaAs wafers by exposure to a high kinetic energy beam of atomic O and characterized using x‐ray photoemission spectroscopy (with Ar+ ion sputter profiling) and Raman spectroscopy. Photoemission shows the reacted layer, ∼500 A thick, to be uniform in composition and fully oxidized. Raman spectroscopy shows that the substrate is not appreciably disordered during oxidation and in some cases no free‐elemental As is present at the oxide‐substrate interface at a detectable level.

A multilayer surface detector for ultracold neutrons

Abstract A multilayer surface detector for ultracold neutrons (UCNs) is described. The top 10 B layer is exposed to vacuum and directly captures UCNs. The ZnS:Ag layer beneath the 10 B layer is a few microns thick, which is sufficient to detect the charged particles from the 10 B(n,α) 7 Li neutron-capture reaction, while thin enough that ample light due to α and 7 Li escapes for detection by photomultiplier tubes. A 100-nm thick 10 B layer gives high UCN detection efficiency, as determined by the mean UCN kinetic energy, detector materials, and other parameters. Low background, including negligible sensitivity to ambient neutrons, has also been verified through pulse-shape analysis and comparison with other existing 3 He and 10 B detectors. This type of detector has been configured in different ways for UCN flux monitoring, development of UCN guides and neutron lifetime research.

In-rich InGaN thin films: Progress on growth, compositional uniformity, and doping for device applications

A number of In-rich InGaN films with In contents in the 20–40% range have been grown at moderately low temperatures on sapphire and silicon substrates at high growth rates using a versatile molecular beam epitaxy-type technology that utilizes an energetic beam of N atoms called energetic neutral atom beam lithography and epitaxy to overcome reaction barriers in the group III-nitride system. Extensive characterization results on the crystalline, optical, and electrical properties of the In-rich InGaN materials are reported. It was found that N-rich growth conditions are required to produce materials that have excellent crystallinity, uniform compositions, and bright band edge photoluminescence. For In-rich InGaN growth on sapphire, electrical transport measurements show reasonably low carrier concentrations and high mobilities. Successful p-type doping of In-rich InGaN with ∼20% and ∼40% In contents is demonstrated, and preliminary results on the formation of a p–n junction are reported. For In-rich InGaN ...

A specialized torsion balance designed to measure the absolute flux density of hyperthermal molecular beams containing reactive species

A torsion balance is described that has been used to measure the absolute flux density of seeded hyperthermal molecular beams containing reactive and/or condensable species with uncertainties of approximately ±5%. The balance is ultrahigh‐vacuum compatible and can be used in corrosive environments. A specially designed beam stop mounted on the torsion balance lever arm, traps the incoming beam and allows only completely thermalized molecules to exit. The thermalized molecules exit the beam stop in equal numbers per unit time in opposite directions, ensuring that the force exerted on the beam stop by the exiting molecules is zero. The balance was suspended from a 25‐μm‐diam gold‐coated tungsten wire that had a torsion constant of 6.04×10−8 N m/rad and a period of slightly larger than 400 s. Absolute molecular beam flux density measurements were made using both the torsion balance and the effusive method for a variety of pure and seeded molecular beams. The beams were composed of gases that could easily be ...