Radiation Induced Damage in GaAs Particle Detectors

The motivation for investigating the use of GaAs as a material for detecting particles in experiments for High Energy Physics (HEP) arose from its perceived resistance to radiation damage. This is a vital requirement for detector materials that are to be used in experiments at future accelerators where the radiation environments would exclude all but the most radiation resistant of detector types.Comment: 5 pages. PS file only - original in WORD Also available at http://ppewww.ph.gla.ac.uk/preprints/97/06

Erratum: “Seed layer technique for high quality epitaxial manganite films” [AIP Advances 6, 085109 (2016)]

No abstract available

Characterisation of low pressure VPE GaAs diodes before and after 24 GeV/c proton irradiation

GaAs Schottky diode detectors have been fabricated upon Low Pressure Vapour Phase Epitaxial GaAs. The devices were characterised before and after a $1.25 barrier height was measured, via two electrical methods, to be$0.81\pm0.005$and$0.85\pm0.01$~eV and a space charge density of$2.8 \pm 0.2 \times 10^{14}$~cm$^{-3}$was determined. The current was greater than that expected for an ideal barrier with the excess attributed to generation current from the bulk. The charge collection efficiency, determined from front alpha illumination and 60 keV gamma irradiation, was inexcess of 95$^{o}$~C, increased from 90~nA to 1500~nA due to radiation induced generation centres. Deep levels were showed to be present using capacitance techniques. The charge collection of the device determined from front alpha illumination fell to$32\pm5$% at a reverse bias of 200V.GaAs Schottky diode particle detectors have been fabricated upon low-pressure vapour-phase epitaxial GaAs. The devices were characterised with both electrical and charge collection techniques. The height of the TiGaAs barrier used was determined via two electrical methods to be (0.81±0.005) and (0.85±0.01) eV. The current density was greater than that expected for an ideal Schottky barrier and the excess current was attributed to generation current in the bulk of the material. A space charge density of (2.8±0.2)×10 14 cm −3 was determined from capacitance voltage characterisation. The charge collection efficiency was determined from front alpha illumination and 60 keV gamma irradiation to be greater than 95% at a reverse bias of 50 V. The diodes were characterised after an exposure to a radiation fluence of 1.25×10 14 24 GeV/ c protons cm −2 . The reverse current measured at 20°C increased from 90 to 1500nA at an applied reverse bias of 200 V due to the radiation induced creation of extra generation centres. The capacitance measurements showed a dependence upon the test signal frequency which is a characteristic of deep levels. The capacitance measured at 5 V reverse bias with a test frequency of 100 Hz fell with radiation from 300 to 40pF due to the removal of measurable free carriers. The charge collection of the device determined from front alpha illumination also fell to (32±5)% at a reverse bias of 200 V

Ballistic-electron-emission spectroscopy of Al<SUB>x</SUB>Ga<SUB>1-x</SUB>As/GaAs heterostructures: conduction-band offsets, transport mechanisms, and band-structure effects

We report an extensive investigation of semiconductor band-structure effects in single-barrier AlxGa1-xAs/GaAs heterostructures using ballistic-electron-emission spectroscopy (BEES). The transport mechanisms in these single-barrier structures were studied systematically as a function of temperature and Al composition over the full compositional range (0&#8804;x&#8804;1). The initial (&#915;) BEES thresholds for AlxGa1-xAs single barriers with 0&#8804;x&#8804;0.42 were extracted using a model which includes the complete transmission probability of the metal-semiconductor interface and the semiconductor heterostructure. Band offsets measured by BEES are in good agreement with previous measurements by other techniques which demonstrates the accuracy of this technique. BEES measurements at 77 K give the same band-offset values as at room temperature. When a reverse bias is applied to the heterostructures, the BEES thresholds shift to lower voltages in good agreement with the expected bias-induced band-bending. In the indirect band-gap regime (x&gt;0.45), spectra show a weak ballistic-electron-emission microscopy current contribution due to intervalley scattering through AlxGa1-xAs X valley states. Low-temperature spectra show a marked reduction in this intervalley current component, indicating that intervalley phonon scattering at the GaAs/AlxGa1-xAs interface produces a significant fraction of this X valley current. A comparison of the BEES thresholds with the expected composition dependence of the AlxGa1-xAs &#915;, L, and X points yields good agreement over the entire composition range

Photocatalytic hydrogen production using ethanol as sacrificial agent from gas and liquid phases on reduced graphene oxide-TiO2- Pt nanocomposites

ABSTRACT: Various strategies such as heterostructuring, crystal/textural modifications and band gap engineering, have been applied to the improvement of the photocatalytic activity of Titania for hydrogen production from water splitting. In this work deposited Pt on TiO2 is used as electron trap to suppress charge recombination. To reinforce this effect, composites with graphene oxide (GO) have been prepared, exhibiting promising photocatalytic performance for both hydrogen generation and the degradation of ethanol added as hole scavenger. Photocatalytic reactions were conducted in gas and liquid phases.N/

Recent results on GaAs detectors - 137

The present understanding of the charge collection in GaAs detectors with respect to the materials used and its processing are discussed. The radiation induced degradation of the charge collection efficiency and the leakage current of the detectors are summarised. The status of strip and pixel detectors for the ATLAS experiment are reported along with the latest results from GaAs X-ray detectors for non-high energy physics applications.Comment: 7 pages. 4 postscript figures + 1 postscript preprint logo + 1 LaTeX file + 1 style file. Also available at http://ppewww.ph.gla.ac.uk/preprints/97/05

Imaging and spectroscopy of single InAs self-assembled quantum dots using ballistic electron emission microscopy

Single InAs self-assembled quantum dots buried spatially beneath a Au/GaAs interface are probed for the first time using the imaging and spectroscopic modes of ballistic electron emission microscopy (BEEM). BEEM images show enhanced current through each dot. Spectra taken with the tip positioned on a dot show shifted current thresholds when compared with the off dot spectra, which are essentially the same as those of Au on bulk GaAs. Shifts in the γ and L conduction band thresholds are attributed to strain in the GaAs cap layer. Fine structure below the γ threshold is consistent with resonant tunneling through zero-dimensional states within the quantum dots

The application of a Trous wave filtering and Monte Carlo analysis on SECIS 2001 solar eclipse observations

8000 images of the Solar corona were captured during the June 2001 total Solar eclipse. New software for the alignment of the images and an automated technique for detecting intensity oscillations using multi scale wavelet analysis were developed. Large areas of the images covered by the Moon and the upper corona were scanned for oscillations and the statistical properties of the atmospheric effects were determined. The a Trous wavelet transform was used for noise reduction and Monte Carlo analysis as a significance test of the detections. The effectiveness of those techniques is discussed in detail.Comment: 17 pages, 8 figures, accepted by Solar Physics Journal for publication in Topical Issue: "Frontiers in Solar Image Processing

Helper T cell IL-2 production is limited by negative feedback and STAT-dependent cytokine signals

Although required for many fundamental immune processes, ranging from self-tolerance to pathogen immunity, interleukin (IL)-2 production is transient, and the mechanisms underlying this brevity remain unclear. These studies reveal that helper T cell IL-2 production is limited by a classic negative feedback loop that functions autonomously or in collaboration with other common γ chain (IL-4 and IL-7) and IL-6/IL-12 family cytokines (IL-12 and IL-27). Consistent with this model for cytokine-dependent regulation, they also demonstrate that the inhibitory effect can be mediated by several signal transducer and activator of transcription (STAT) family transcription factors, namely STAT5, STAT4, and STAT6. Collectively, these findings establish that IL-2 production is limited by a network of autocrine and paracrine signals that are readily available during acute inflammatory responses and, thus, provide a cellular and molecular basis for its transient pattern of expression