Near-surface Charge Research Articles

Electron transport to positive centers in GaAs

The origin of neutral muonium defect centers in semi-insulating GaAs has been studied using muon spin rotation/relaxation techniques employing alternating electric fields. This technique prevents the accumulation of near-surface charges which may screen the external field. Suppression of the bond-centered muonium signal with electric field suggests that muonium formation proceeds via transport of excess electrons from the ionization track to the muon. The characteristic field of about 5 kV/cm may correspond to an electric field induced ionization of the muon donor impurity center.

Near-surface electronic structure on InAs(100) modified with self-assembled monolayers of alkanethiols

Surface chemical modification is used to eliminate the problem of high surface recombination velocity and control surface band bending on InAs(100). Alkanethiols, RSH; R=CH3(CH2)n, both neat and in ethanolic solutions, are used to passivate this surface against oxidation, as characterized by Raman scattering and x-ray photoelectron spectroscopy of the adsorbate-covered and bare surfaces. The magnitude of the interfacial band bending is obtained by analysis of Raman scattering from the unscreened longitudinal optical phonon, which arises from the near-surface charge accumulation region. Removing the native oxide with a Br2:CH3OH chemomechanical etch reduces the surface band bending, but atmospheric oxidation increases band bending to its original level over several hours. In contrast, alkanethiol passivation of InAs(100) prevents band bending for periods of up to several weeks.

Report on the round table session on: Near surface analysis and charge exchange studies with high-resolution spectrographs

This note is a report on the round table session on near-surface analysis and charge exchange studies with high-resolution spectrographs, with contributions from: H.D. Carstanjen, Max-Planck Institut für Metallforschung, Stuttgart, Germany (see separate contribution to these proceedings, H.D. Carstanjen, Overview of existing high resolution spectrographs.). P. Sigmund, Physics Department, Odense University, Denmark (see separate contribution to these proceedings, P. Sigmund, L. Glazov, Energy loss and charge exchange: Statistics and atomistics.). W.M. Arnoldbik, Debye Institute, Utrecht University, The Netherlands G. Dollinger, Technical University Munich, Germany K. Kimura, Department of Engineering Science, Kyoto University, Japan O. Kruse, Forschungszentrum Rossendorf, Dresden, Germany T. Gustafsson, Dept. of Physics and Astronomy, Rutgers University, Piscataway, USA I. Yamada, Dept of Ion Beam Engineering, Kyoto University, Japan R. Behrisch, Max Planck Institut für Plasmaphysik, Garching, Germany

Electron-beam-induced charging of phosphors for low voltage display applications

Shifts of the surface potential were measured for several commercial phosphors by monitoring secondary and Auger electron spectra during electron beam irradiation at energies between 0.5 and 5 keV. The insulating phosphors charge strongly negative above 2 keV, whereas the conductive ones do not. This charging behavior is exaggerated after prolonged beam exposure. Measurements using two coincident electron beams to control the surface potential demonstrate that aging-enhanced surface charging decreases the phosphor cathodoluminescence. The dependence of the phosphor surface potential on the voltage applied to the metal plate on which they are deposited was also determined. These data suggest the presence of substantial space charge in the phosphors, even at low electron beam energies where no beam-induced shifts of the surface potentials are seen. We suggest that the electric fields due to these near-surface space charge regions are crucial in altering secondary emission and in establishing a steady state charge balance in the case where no currents can flow from the irradiated phosphor surface to the conductive backplane.

Photoconduction in Vacuum-Evaporated Films of Phenothiazine

Abstract Steady-state photoconduction characteristics have been measured for the system vacuum-evaporated film of phenothiazine contained between aluminum semitransparent electrodes. Three spectral regions of the action spectrum are distinguished: (i) (200–300) nm with near-surface charge generation and its subsequent injection into the bulk; (ii) (300–370) nm dominated by the surface injected current; and (iii) > 370 nm characterized by a superposition of the surface and bulk generated carriers. A weak feature between 400 and 420 nm corresponds to the low energy charge-transfer excitons suggested previously to yield a large photoconduction peak at ≌ 410 nm in phenothiazine single crystal.

Spectral distribution of the photoconductivity of cadmium diphosphide

The spectral distribution of the photoconductivity of CdP2 crystals has been studied in the range 0.3–1.5 μm. The intrinsic photoconductivity has been found to be localized in the region of the near-surface space charge. The role of the natural superlattice in the photoconductivity is considered.

Photon-charge effect in conductors

A new so-called photon-charge effect in conductors is reported. Electromagnetic radiation falling on the surface of a conductor causes a redistribution of near-surface charges in the double layer accompanied by a corresponding change of the electrostatic Coulomb field in the double layer. The effect is explained in terms of the force exerted by an electromagnetic wave crossing the conductor's boundary. The effect has been observed and studied in various metals and semiconductors. The magnitude of the effect is shown to be extremely sensitive to the surface properties of the sample.

Effect of the surface charge on the defect transport kinetics through interfaces of ionic solids

Solutions of the Poisson equation were derived for ionic solids in the case when a localized surface charge or a near-surface charge, produced as a result of segregation, is present. Explicit analytical expressions for the electrical potential distributions are obtained and the values of the kinetic rate constant characterizing the quasi-stationary transport of defects through the surface layer against the electrical barrier are defined. Chromium-doped nickel monoxide was taken as an example as the segregation and chemisorption model is relatively well known. The effect of the surface electrical barrier and resulting limitations for gas-solid kinetics are discussed.

Chemical modification of near-surface charge trapping in polymers

The surfaces of three polyolefin films, low-density polyethylene, high-density polyethylene, and polypropylene, were chemically modified by etching with permanganic acid, and low-density polyethylene by etching with chromic acid and by staining with chlorosulphonic acid. Polytetrafluoroethylene (Teflon) was treated with a sodium naphthalene complex in glycol ether. The surfaces were examined with a scanning electron microscope, and chemical groups identified by attenuated total reflectance infrared measurements. The surfaces were then corona charged, and the charge stability and trap distributions determined by the methods of thermally stimulated charge decay and thermally stimulated current, respectively. Scanning electron micrographs revealed a nodular surface structure in polyolefins etched with permanganic acid, a rough surface without nodules in low-density polyethylene etched with chromic acid, isolated corroded surface spots in low-density polyethylene stained with chlorosulphonic acid, and a web-like surface in etched Teflon. Permanganic acid removed amorphous and defective regions of the polymer surfaces, exposing more crystalline regions and crystalline-amorphous boundaries, both of which provided trapping sites, producing better charge storers than the virgin equivalents. Lifetimes, and hence usable temperatures in device application, were increased for both negative and positive charge. The other acids reduced the lifetimes in low-density polyethylene. Etching made Teflon as good a positive charge storer as the virgin negatively charged material, a result important to practical applications. While attenuated total reflectance measurements showed the presence of various chemical groups, charge stability seemed to depend more on surface morphology than on which active groups were added to the polymers by the etchants and staining agent.