Brian W. Robertson

A class of boron-rich solid-state neutron detectors

Real-time solid-state neutron detectors have been fabricated from semiconducting boron–carbon alloys, deposited by plasma-enhanced chemical vapor deposition. Single neutrons were detected and signals induced by gamma rays were determined to be insignificant. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron–carbon alloys with only the natural isotopic abundance of 10B. Devices made of thicker boron–carbon alloy layers enriched in 10B could lead to increased detection efficiency and active diodes could use the inherent micron scale spatial resolution, increasing the range of possible applications.

Boron carbide/n-silicon carbide heterojunction diodes

The fabrication, initial structural characterization, and diode measurements are reported for a boron carbide/silicon carbide heterojunction diode. Current–voltage curves are obtained for operation at temperatures from 24 to 351 °C. Plasma-enhanced chemical-vapor deposition (PECVD) -deposited undoped boron carbide material is highly crystalline and consists of a variety of polytypes of boron carbide (BC) with crystal sizes as large as 110 nm. Crystal phases are similar to those for PECVD BC on Si but only partially match known boron and boron-rich BC phases.

Nanostructures of Sm-Co on Cr thin films

The nanostructures of Sm-Co on Cr thin films prepared by dc magnetron sputtering were investigated by high resolution transmission electron microscopy (HRTEM) and diffraction techniques. HRTEM micrographs show that the crystallites in the Sm-Co films, as revealed by the lattice fringes, are distributed discontinuously in the matrix. The matrix is amorphous as indicated by the microdiffraction study. The size of the crystallites is in the range of 2/spl sim/5 nanometers. The volume fraction of the crystallites in the film decreases from 91% to 54% as the argon pressure is increased from 5 mTorr to 30 mTorr. Micrographs recorded in a bright field transmission electron microscope (TEM) with a defocus of a few micrometers reveal grain-like structures of about 25 nm in some but not all films. This grain-structure is found to be inherited from the Cr underlayer. >

Evidence for multiple polytypes of semiconducting boron carbide (C2B10) from electronic structure

Boron carbides fabricated via plasma enhanced chemical vapour deposition from different isomeric source compounds with the same C2B10H12 closo-icosahedral structure result in materials with very different direct (optical) band gaps. This provides compelling evidence for the existence of multiple polytypes of C2B10 boron carbide and is consistent with electron diffraction results.

Semiconducting boron-rich neutron detectors

Semiconducting boron-rich boron-carbon alloys have been deposited by plasma-enhanced chemical vapor deposition. Heterojunction diodes made with 276nm thick nanocrystalline layers of these alloys have been used as real-time solid-state neutron detectors. Individual neutrons were detected and signals induced by gamma rays were determined to be insignificant. Linearity of detection was demonstrated over more than two orders of magnitude in flux. The neutron detection performance was unaffected by > 1 x 1015 neutrons / cm2. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron carbon alloys with only the natural isotopic abundance of 10B. Devices made of thicker boron carbon alloy layers enriched in 10B could lead to increased detection efficiency.

High resolution electron microscopy and nano-probe study of CoSm/Cr films

The crystal structure of the crystallites in CoSm thin films deposited on Cr underlayer was studied by nanodiffraction and high resolution electron microscopy (HREM). It was found that the crystallites have a closed-packed structure. Some nanodiffraction patterns taken from different crystallites using a two nanometer probe can be indexed by two layer stacking AB (HCP structure), three layer stacking ABC (FCC structure), and four layer stacking ABAC (double hexagonal structure), suggesting that a particular local stacking mode could exist. [112~0] HREM images confirmed that stacking sequence changes within one crystallite. In local regions, random stacking, and unit cells of two layer stacking ABAC were found.

Microstructure of the Cr underlayer and its effect on Sm-Co//Cr thin films

Sm‐Co film is a potential candidate for the future high density recording media of 10 Gb/in2 which requires bit sizes of the order of 300 nm and grain sizes of about 10 nm. This article investigates the microstructure of the Cr underlayer in Sm‐Co thin films and its effect on Sm‐Co thin films prepared by the dc magnetron sputtering technique. The grain size of the Cr underlayer is found to be about 25 nm. Grains with small angle misorientation usually form local agglomerates. Studies by transmission electron microscopy (TEM) bright field images at different defocus settings and by high resolution electron microscopy indicate that a large proportion of the grain boundaries have gaps. The gap width (about 1–3 nm) varies from place to place. The Sm‐Co films deposited on the Cr underlayer inherit similar gaps at positions adjacent to the grain boundary gaps of the Cr underlayer. Such gaps produce grain‐like contrast of about 25 nm in TEM bright field images. However, such contrast becomes weak as the thicknes...

Ultraviolet and electron radiation induced fragmentation of adsorbed ferrocene

From thermal desorption spectroscopy we find that ferrocene, Fe(C5H5)2, adsorbs and desorbs associatively on Ag(100). Photoemission results indicate that the initially adsorbed surface species closely resembles that of molecular ferrocene. The shift in photoemission binding energies relative to the gas phase is largely independent of the molecular orbital. We find that ultraviolet light does lead to partial fragmentation of the ferrocene and that the molecular fragments are much more strongly bound to the surface than the associatively adsorbed ferrocene. Since fragmentation occurs only in the presence of incident radiation, selective area deposition from this class of molecules is possible. Using a focused electron beam in a scanning transmission electron microscope, we show that selective area deposition of features with resolution of a few hundred angstroms is readily achieved.

Magneto-optical and structural properties of nanocrystalline MnBi-based films

Abstract In this paper we discuss recent research on MnBi and MnBiX films where X denotes alloying elements whose purpose is to modify the properties of the parent compound in beneficial ways. While MnBi has a large Kerr response, large uniaxial anisotropy and can be grown in thin-film form with its easy axis normal to the film plane, it also has large grain sizes and a high-temperature structural instability near the Curie point, both of which are detrimental to magneto-optic recording applications. We report systematic studies of Al doping which show that it does not enhance the Kerr response nor eliminate the high temperature structural instability. Al does, however, promote small grain sizes which are required for a low noise magneto-optical recording medium. Preliminary results are reported on sputtered samples with a number of other dopants, and some of these have promising properties.

Magnetoresistance in boron carbide junctions

Photoemission and electric transport properties of ferromagnet–insulator–ferromagnet junctions with boron carbide (C2B10) dielectric barrier are presented. Using a non-oxide barrier confidence avoids oxidation of the interfaces with the ferromagnetic layers. Photoemission confirms chemical abruptness of the interface. Magnetoresistance ratios reaching 50% are observed at low temperatures, and large nonlinearity in the current–voltage curves show that impurities in the junctions play a key role.