Peter E. Vanier

New features of the temperature dependence of photoconductivity in plasma-deposited hydrogenated amorphous silicon alloys

The temperature and flux dependences of photoconductivity have been investigated for plasma‐deposited hydrogenated amorphous silicon alloys produced under a variety of processing conditions. In undoped films, new features such as thermal quenching and supralinearity are observed. Such behavior is critically dependent on the position of the Fermi level, and is not observed in alloys doped by the addition to the plasma of PH3, B2H6, O2+N2 mixtures, or air. Interpretation of the data is based on a model of competing recombination centers.

Securing special nuclear material: Recent advances in neutron detection and their role in nonproliferation

Neutron detection is an integral part of the global effort to prevent the proliferation of special nuclear material (SNM). Applications relying on neutron-detection technology range from traditional nuclear nonproliferation objectives, such as safeguarding material and verifying stockpile reductions, to the interdiction of SNM—a goal that has recently risen in priority to a level on par with traditional missions. Large multinational programs targeting interdiction and safeguards have deployed radiation-detection assets across the globe. In parallel with these deployments of commercially available technology, significant research and development has been directed toward the creation of next-generation assets. Neutron-detection technology plays a prominent role because of the capability of neutrons to penetrate materials that readily absorb gamma rays and the unique fission signatures neutrons possess. One particularly acute technology-development challenge results from dwindling supplies of H3e, partially ...

A study of hydrogenated amorphous silicon deposited by rf glow discharge in silane‐hydrogen mixtures

The rf glow discharge deposition of hydrogenated amorphous silicon from silane‐hydrogen mixtures has been studied as a function of silane fraction. Discharge processes were investigated using optical emission spectroscopy and mass spectrometry. The deposited films were characterized using UV and IR spectroscopy, dark conductivity, and photoconductivity. The dilution of silane in hydrogen resulted in an increase in deposition rate by as much as an order of magnitude over the rate for pure silane, for constant values of total flow rate, pressure, substrate temperature, and rf power. The dependence of deposition rate on silane fraction is attributed to changes in the electron energy distribution in the plasma. The hydrogen content and optical gap of the films are independent of the silane fraction. Photoconductivity values, corrected for slight variations of the Fermi level, increase with silane dilution.

Residual strains in amorphous silicon films measured by x‐ray double crystal topography

A new application of double crystal topography using selected diffraction from portions of bent crystals was used to determine the curvature of single crystal Si wafers coated with hydrogenated amorphous Si (a‐Si:H) films. This direct imaging method allowed measurements of the radii of curvature R of bent crystals over a range ≂0.2–1000 m. Using this procedure two different series of a‐Si:H films were measured to compare the internal strains and monitor these as a function of film thickness. The curvature of the sample crystals was shown to be inversely proportional to the thickness of the film. The internal stress was found to be independent of film thickness, but only depended on the processing conditions.

Mass spectrometric studies of impurities in silane and their effects on the electronic properties of hydrogenated amorphous silicon

The concentration of volatile impurities in silane (SiH4) and disilane (Si2H6) used to prepare hydrogenated amorphous silicon (a‐Si:H) thin films by glow‐discharge deposition has been measured by modulated molecular beam mass spectrometry with a sensitivity of 1 ppm. From dark conductivity measurements on these films the location of the Fermi energy EF was determined. The Fermi levels of a‐Si:H films were progressively lower with increasing concentrations of chlorine‐containing molecules (mostly HCl and SiH3Cl) in the silane, indicating that these species can act as weak p‐type dopants in a‐Si:H. Oxygen‐containing species (mostly SiH3OSiH3) were also detected in silane. It is clear from these results that large variations in electronic properties of glow discharge a‐Si:H can be attributed to differences in purity of silane used in the deposition.

Infrared quenching of photoconductivity and the study of gap states in hydrogenated amorphous silicon alloys

Infrared quenching of photoconductivity has been investigated for plasma‐deposited a‐Si:H alloys. The temperature and spectral dependences of the quenching are consistent with a two‐level model of competing recombination centers. The infrared quenching spectrum shows that this effect occurs for photons having energies in the range 0.4 eV<hνir <1.6 eV. The low energy threshold is found at Et = 0.41 eV for T = 100 K. This threshold shifts slightly with temperature in a manner expected for the quasi‐Fermi level for trapped holes.

Improvements in coded aperture thermal neutron imaging

A new thermal neutron imaging system has been constructed, based on a 20-cm x 17-cm He-3 position-sensitive detector with spatial resolution better than 1 mm. New compact custom-designed position-decoding electronics are employed, as well as high-precision cadmium masks with Modified Uniformly Redundant Array patterns. Fast Fourier Transform algorithms are incorporated into the deconvolution software to provide rapid conversion of shadowgrams into real images. The system demonstrates the principles for locating sources of thermal neutrons by a stand-off technique, as well as visualizing the shapes of nearby sources. The data acquisition time could potentially be reduced two orders of magnitude by building larger detectors.

Study of gap states in a–Si:H alloys by measurements of photoconductivity and spectral response of MIS solar cells

A picture of the density of gap states n (E) in glow discharge a–Si:H is constructed using four different kinds of transport measurement on a large number of samples. The minimum in n (E) lies 0.4 eV below Ec, rather than in the middle of the gap. A distribution of fast recombination centers lies at mid‐gap, and two sets of hole traps lie between mid‐gap and the valence band. Modifications in n (E) have been studied by the effects of selected impurities on the conversion efficiency and spectral response of MIS and pin solar cells.

Factors affecting light-induced excess conductivity in doping-modulated amorphous silicon superlattices

Doping‐modulated amorphous silicon semiconducting films which exhibit the phenomenon of light‐induced excess conductivity (LEC) have been made by silane glow discharge in a single‐chamber system. This phenomenon shows a strong dependence on substrate temperature and process gas composition. The LEC effect decreases for very small and very large layer thickness. There also seems to be an optimum defect density for producing large effects.

Calibration and testing of a large-area fast-neutron directional detector

We have developed a new directional fast-neutron detector based on double proton recoil in two separated planes of plastic scintillators with continuous position-sensitive readout in one of two dimensions. This method allows the energy spectrum of the neutrons to be measured by a combination of peak amplitude in the first plane and time of flight to the second plane. The planes are made up of 100-cm long, 10-cm high paddles with photomultipliers at both ends, so that the location of an event along the paddle can be estimated from the time delay between the optical pulses detected at the two ends. The direction of the scattered neutron can be estimated from the locations of two time-correlated events in the two planes, and the energy lost in the first scattering event can be estimated from the pulse amplitude in the first plane. The direction of the incident neutron can then be determined to lie on a cone whose angle is determined by the kinematic equations. The superposition of many such cones generates an image that indicates the presence of a localized source. Setting upper and lower limits on time of flight and energy allows discrimination between gamma rays, muons and neutrons. Monte Carlo simulations were performed to determine factors affecting the expected angular resolution and efficiency. These models show that this design has a lower energy limit for useful directional events at about 250 keV, because lower energy neutrons are likely to scatter more than once in the first plane.