George P. Lamaze

Conduction processes in boron- and nitrogen-doped diamond-like carbon films prepared by mass-separated ion beam deposition

Abstract Boron- and nitrogen-doped diamond-like amorphous carbon (DLC) films were prepared by alternating direct deposition of low energy mass-separated 12C+ and dopant ions. Concentration vs. depth profiles for N and B dopants were determined by neutron depth profiling. The measured current-voltage characteristics of these films, which were deposited on polished stainless steel, are explained best by Frenkel-Poole emission for high electric fields. Two different trap states Φ1 and Φ2 were found to contribute to the conduction process. At low electric fields our results suggest that conduction is due to variable-range hopping via localized states at the Fermi level. The doped DLC films show a higher electrical conductivity, indicative of an increased density of localized states, rather than a shift in the Fermi level. A diode-like device was prepared, but the measured I-V curves did not indicate that a p-n junction had formed. DLC/Si heterojunctions were also prepared and their current-voltage characteristics are presented and discussed.

EFFECT OF STOICHIOMETRY ON THE PHASES PRESENT IN BORON NITRIDE THIN FILMS

Boron nitride thin films were deposited by ion‐assisted evaporation and characterized by neutron depth profiling (NDP), a nondestructive method for the compositional analysis of solids. The phases present in the films were determined by infrared spectroscopy. Examination of the data presented here and comparison with the work of other authors revealed that stoichiometric or nearly stoichiometric films contained the greatest amount of the cubic phase. This led to the proposition that film stoichiometry is one of the factors that stabilize cubic boron nitride in boron nitride thin films. A shift in the position of the cubic boron nitride infrared absorption was also observed by the present authors which was related to film stoichiometry. Discussion of the various techniques commonly used to determine the stoichiometry of boron nitride thin films emphasized the need for all stoichiometry measurements to be made using the same characterization method in order for all results to be compared with confidence. Th...

Neutron Transmission of Single-Crystal Sapphire

Thermal and cold neutron transmission time-of-flight measurements have been taken on blocks of single-crystal sapphire. The transmission characteristics of single-crystal sapphire are not altered by irradiation for a number of years within a beam port at a reactor. Cooling the filter is only useful for the production of beams with the longest wavelengths. Even for poorer grade sapphire there is no degradation of the neutron transmission characteristics.

Characterization of a cold neutron beam from a curved guide

Abstract A supermirror guide that includes both straight sections and curved channels has been installed at a cold neutron source facility. A description of the beam line and the beam characteristics are given. The transmitted neutron wavelength spectrum and the beam current density have been measured. Factors that affect the spectrum are discussed. Though the final straight guide section improves the spatial uniformity of the beam intensity at the guide exit, the spatial-angular correlations caused by the curved channels persist. These correlations are observed in the beam divergence measurements at various positions along the beam line using neutron imaging plate technology, and are explained by ray projections. The divergence of the transmitted beam is determined both by the critical angle of the guide and the collimation beyond the guide.

Analysis of cubic boron nitride thin films by neutron depth profiling

Abstract Cubic boron nitride (c-BN) thin films are of interest in such diverse areas as semiconducting devices, wear resistant coatings, and low friction coatings, owing to the exceptional electrical and mechanical properties of that material. In this work, the boron to nitrogen ratio of thin films produced by physical vapor deposition (PVD) was determined and compared with the crystal phase of BN thin films. Thin films (approximately 0.1 μm) of c-BN were deposited onto heated silicon substrates by electron beam evaporation of boron with concurrent nitrogen and argon ion bombardment. The films were characterized by IR spectroscopy. The stoichiometry of the BN layer was determined by neutron depth profiling (NDP) using the nuclear reactions 10 B(n,α) 7 Li and 14 N (n,p) 14 C. Boron-to-nitrogen ratios were determined with uncertainties of 2%–3%. Shifts in IR spectra were observed over a narrow range of boron-to-nitrogen ratios near unity. Stoichiometric results are compared with the crystal phase. Details of the NDP analysis are presented.

Mass assay and uniformity tests of boron targets by neutron beam methods

Abstract The areal densities (g/cm2) of enriched boron and lithium fluoride targets were compared to those of reference deposits from the Central Bureau for Nuclear Measurements (CBNM, Geel, Belgium) by charged-particle counting at the NIST Research Reactor. The uniformity of the target thickness was also investigated by scans with a small-diameter, intense neutron beam. The neutron uniformity measurements agree satisfactorily with previously reported interferometric results and no pattern of uniformity variation except the expected, small radial dependence was found. In a separate series of measurements, the boron areal density ratios of a number of the deposits were determined using a new double ionization chamber. The new double ionization chamber permits a comparison of two targets directly without a beam monitor and without sensitivity to the exact placement of the samples. The new device needs only a very simple data acquisition system and requires less neutron beam intensity than the previously employed systems with arrays of surface barrier detectors. The ionization chamber counting losses from the combined effects of self-absorption and sub-threshold pulse-height formation were determined by comparison with ratio measurements made by the conventional surface barrier detector technique.

Modeling detector response for neutron depth profiling

Abstract In Neutron Depth Profiling (NDP), inferences about the concentration profile of an element in a material are based on the energy spectrum of charged particles emitted due to specific nuclear reactions. The detector response function relates the depth of emission to the expected energy spectrum of the emitted particles. Here, the detector response function is modeled for arbitrary source and detector geometries based on a model for the stopping power of the material, energy straggling, multiple scattering and random detector measurement error. At the NIST Cold Neutron Research Facility, a NDP spectrum was collected for a diamond-like carbon (DLC) sample doped with boron. A vertical slit was placed in front of the detector for collimation. Based on the computed detector response function, a model for the depth profile of boron is fit to the observed NDP spectrum. The contribution of straggling to overall variability was increased by multiplying the Bohr Model prediction by a ramp factor. The adjustable parameter in the ramp was selected to give the best agreement between the fitted profile and the expected shape of the profile. The expected shape is determined from experimental process control measurements.

Neutron depth profiling technique and facilities

Abstract Neutron depth profiling (NDP) is a nondestructive technique for the measurement of isotopic concentrations as a function of depth. Using neutrons to probe the first few micrometers of solid surfaces, NDP has become an important tool in the nondestructive analyses for several technologically important elements. The National Institute of Standards and Technology (NIST) has operated a dedicated NDP facility (I) since 1982 using thermal neutrons. Recently, a second, more versatile instrument (2) with improved detection limits was installed as part of the Cold Neutron Research Facility (CNRF) at NIST.

A HIGH EFFICIENCY DEVICE FOR THE DETECTION OF RADIOACTIVE XENON ISOTOPES

Abstract A detector has been designed for the detection of sub-Becquerel amounts of the radioactive xenon isotopes 131mXe and 133Xe. High efficiency has been obtained by the condensation of the sample directly onto the surfaces of two closely spaced silicon surface-barrier detectors. The measurement of the 131mXe is based on the detection of the conversion electrons. The 133Xe measurement is obtained by detection of the 81 keV photon in a NaI detector that nearly surrounds the silicon detectors. The minimum detectable activity for 133Xe is 0.015 Bq. For a 131mXe sample with no 133Xe present, the minimum detectable activity is about 0.003 Bq.

Use of neutron beams for chemical analysis at NIST

At the National Institute of Standards and Technology, there are two techniques for chemical analysis that use neutron beams from the reactor for target irradiation: neutron depth profiling (NDP) and prompt γ-ray activation analysis (PGAA). There are two facilities for each technique, one equipped with a thermal neutron beam and the other, with a cold neutron beam. In addition, focused beams of cold neutrons will be used to measure the two-dimensional element distributions by PGAA and three-dimensional distributions by NDP. This paper includes a brief description of the facilities, the measurement capabilities of each, some recent applications of NDP and PGAA, and neutron focusing as applied to these techniques.