A.H. Weiss

Electrodeposition and Characterization of SnS Thin Films

A new room‐temperature electrodeposition technique was devised to synthesize thin films on indium tin oxidecoated glass slides. This technique is based on a nonaqueous ethylene glycol bath containing anhydrous and elemental sulfur. Three types of electrosyntheses, namely, potentiostatic, galvanostatic, and pulse modes, are discussed and their relative merits compared. A wide variety of characterization techniques were employed to develop a self‐consistent and complementary picture of the morphology, composition, and photoactivity of the thin films. These included scanning electron microscopy, x‐ray diffractometry, electron probe microanalyses, Auger electron spectroscopy, x‐ray photoelectron spectroscopy, optical analyses, and voltammetry. The photoactivity of these films was evaluated using photoelectrochemical techniques. Finally, the dark and photocorrosion behavior of these films are discussed with the aid of Pourbaix diagrams.

Apparatus for positron annihilation‐induced Auger electron spectroscopy

The apparatus used in the first direct measurement of energy spectra of positron annihilation‐induced Auger electrons is described. The apparatus consists of a magnetically guided low‐energy positron beam, a UHV surface preparation and characterization chamber, and an energy spectrometer. The spectrometer includes a trochoidal monochromator and a microchannel plate detector. A permanent magnet is mounted behind the sample to produce a field gradient which redirects the outgoing Auger electrons along the spectrometer axis. The combination of trochoidal monochromator and permanent magnet permits the measurement of the total kinetic energy of Auger electrons with an effective angular acceptance of ∼2π. The large angular acceptance and single particle detection capability of this spectrometer make it possible to perform Auger measurements using extremely low incident beam currents (∼10−15 A), and may make it useful in other low signal experiments. The spectrometer response function is modeled and compared to ...

Novel x-ray diffractometer for liquid surface studies

An x‐ray diffractometer for studying the structure of the liquid–vapor interface is described. It is designed to permit reflectivity and scattering studies from liquid surfaces for angles varying from grazing incidence, below the critical angle for total external reflection up to angles ∼3° using a rotating anode x‐ray generator. In principle the diffractometer system can be used to study both the density profile normal to the surface and in‐plane structural features. The former is determined by deviations of the measured reflectivity from the Fresnel law of classical optics and the latter from nonspecular scattering. Results obtained using this spectrometer to measure the density profile normal to the surface of water and a liquid crystal are presented.

Depth profile analysis of surfaces produced by annealing ultra-thin films of Au deposited on Si(100)

Abstract The surface formed when Au is deposited on a low-temperature (173 K) Si(100) substrate was studied as a function of the thickness of deposited Au utilizing positron-annihilation induced Auger electron spectroscopy (PAES). The concentration of Au as a function of depth has been the subject of some controversy for this system. Ion sputter depth profiles obtained using PAES indicate that the Au concentration is ∼ 100% to a depth of ∼ 1ML and then decreases continuously to ∼0% at depths of 13 and 28 A for initial Au depositions of 5 and 10 A, respectively. A comparison of PAES and EAES results indicate that PAES, because of its ability to selectively probe the topmost atomic layer, can be used to obtain significantly higher depth resolution in ion sputter depth profiles than is possible using EAES.

Rare gas moderated electrostatic positron beam

An electrostatically guided positron beam which is variable in energy from 0 to 60 keV is described. A frozen inert gas (Ar, Kr, and Xe) is used as the positron moderator. The efficiency, e, defined as the ratio of the number of slow positrons at the target to the number of positrons emitted by the source has been measured; the values obtained for Ar, Kr, and Xe were e∼ 2.5×10−4, 1.6×10−4, and 1.1×10−4, respectively. We estimate a source/moderator efficiency of em∼7.6×10−4, 4.7×10−4, and 3.0×10−4 for Ar, Kr, and Xe, respectively, which includes source effects, grid transmission, geometrical losses, and beam transport. These values are comparable to the best reported values for thin metal foil moderators. The measured em is specific to the electrostatic system; significantly higher values for the same moderator should be attainable in magnetically guided beams, where higher transmission for moderators with large energy spreads is usually obtained.

Elimination of the secondary electron background in Auger electron spectroscopy using low energy positron excitation

Data is presented which demonstrate that positron annihilation‐induced auger electron spectroscopy (PAES) can be used to eliminate secondary electrons in the energy range of Auger electrons. In PAES the core–hole excitations necessary for Auger electron spectroscopy (AES) are created by annihilation of core electrons using low energy (<25 eV) positrons. Secondary electrons cannot be created through collisional processes with energies in excess of an energy Ek≊Ep (the primary beam energy). Spectra obtained for a Cu (110) surface demonstrate that Auger signals can be obtained with peak‐to‐background ratios in excess of 50:1 (more than two orders of magnitude improvement over conventional electron excited Auger methods). PAES spectra obtained from Cs‐covered Cu surfaces are used to place upper limits on the relative contribution of gamma‐ray induced secondary electrons to the PAES background. Implications of the high signal‐to‐background ratios obtained using PAES for increasing the elemental sensitivity and reducing beam damage of AES are discussed.Data is presented which demonstrate that positron annihilation‐induced auger electron spectroscopy (PAES) can be used to eliminate secondary electrons in the energy range of Auger electrons. In PAES the core–hole excitations necessary for Auger electron spectroscopy (AES) are created by annihilation of core electrons using low energy (<25 eV) positrons. Secondary electrons cannot be created through collisional processes with energies in excess of an energy Ek≊Ep (the primary beam energy). Spectra obtained for a Cu (110) surface demonstrate that Auger signals can be obtained with peak‐to‐background ratios in excess of 50:1 (more than two orders of magnitude improvement over conventional electron excited Auger methods). PAES spectra obtained from Cs‐covered Cu surfaces are used to place upper limits on the relative contribution of gamma‐ray induced secondary electrons to the PAES background. Implications of the high signal‐to‐background ratios obtained using PAES for increasing the elemental sensitivity and...

Auger line shape measurements using positron annihilation induced Auger electron spectroscopy

Abstract Conventional methods of Auger electron spectroscopy (AES) make use of energetic electron or photon beams to create the core-hole excitations that lead to the Auger transition. The energetic beams result in a large secondary electron background in the Auger peak region. In positron annihilation induced Auger electron spectroscopy (PAES), the core holes are created by matter-antimatter annihilation and not through collisional ionization. Measurements are reviewed which indicate that PAES can eliminate the secondary electron background by the use of very low (∼10 eV) positron beam energies and that PAES has greatly increased surface selectivity due to the trapping of positrons in surface state prior to annihilation. A new PAES spectrometer has been developed in our laboratory with an energy resolution which is an order of magnitude better than previous PAES spectrometers. The high-resolution PAES system has been used to measure the Auger M 2,3 VV line shape from a clean polycrystalline Cu surface. The atomic-like Auger M 2 VV and M 3 VV features are clearly resolved. Differences observed between the PAES spectra and spectra obtained using electron induced Auger spectroscopy are discussed.

Theory versus experiment in low-energy positron diffraction by Cu{111}

Abstract Four experimental spectra measured in a low-energy positron diffraction (LEPD) experiment on Cu{111} are satisfactorily matched by intensity calculations. The calculations were carried out with a computer program developed for LEED (low-energy electron diffraction) but using a potential consisting of negative Coulomb contribution, no exchange term and the correlation correction used normally for electrons. The present experimental data are not refined enough to show that positrons do not feel an exchange potential.

Stability of Se passivation layers on Si(001) surfaces characterized by time-of-flight positron annihilation induced Auger electron spectroscopy

The stability of the selenium passivation layer on the Si(001) surface was investigated using a nondestructive surface-sensitive technique: time-of-flight positron annihilation induced Auger electron spectroscopy. After 10days of exposure in the air, the Se passivation layer was observed to incorporate some oxygen but to remain largely intact. Part of the adsorbed oxygen was desorbed during annealing up to 400°C in the ultrahigh-vacuum environment. However, some oxygen remained on the surface until high-temperature annealing at 1030°C. We posit that the oxygen that remained after the low-temperature anneals was chemisorbed on the Si surface at defects in the Se passivation layer. The Se passivation layer was stable up to an annealing temperature of ∼800°C before desorbing from the surface. The stability of the Se-passivated Si(001) surface is attributed to the saturation of the Si dangling bonds on the surface and to the strong Se–Si bonds.

Positronium emission from metal surfaces

Positronium (Ps, a positron–electron bound pair) emission as a result of slow positron (30–1000 eV) bombardment of Cu, Al, Ni, and Ag surfaces has been investigated as a function of incident positron energy, sample temperature, and surface contamination. The fraction of 30 eV positrons that were reemited as Ps at 380 K with <5% surface contamination ranged from 0.43 to 0.47. The emission fraction is found to increase with both increasing surface contamination and incident positron energy. Both thermally activated and nonthermally activated Ps emission is suggested by the data. A one‐step thermal activation model applied to the data yields activation energies of 0.44±0.04, 0.46±0.04 and 0.75±0.05 eV for Al(100), Ag(polycrystalline), and Ni(100), respectively.