Charging efficiency and lifetime of image-bound electrons on a dielectric surface

Abstract

The surface charge generated on an Al0.24Ga0.76As∕GaAs quantum well sample by electron bombardment was monitored by measuring the change in the conductivity of the channel. Upon turning off the electron bombardment the surface charge on adsorbed layers of xenon and water at 8K decays in room temperature darkness with a lifetime τ=0.30±0.02s. The average charging efficiency, μ0, defined as the ratio of the charge collected by the surface to the beam current times the charging time, is μ0≃0.001. Surface charging proves to be an effective method for contactless gating of field effect devices.

Phonon-mediated desorption of image-bound electrons from dielectric surfaces

“…For most dielectrics of practical interest more than two phonons are required implying that for these materials the desorption time for an image-bound electron may be in fact rather long. Indeed, in an ingenious experiment, using a field-effect transistor set-up with the gate replaced by an externally provided electron surface charge, Biasini and coworkers 25,26 determined the desorption time for an electron on a GaAs surface. They obtained 0.48 s which is rather long indeed but not unexpected, from our point of view, because the energy difference between the lowest two image states of GaAs, obtained from the dynamically corrected image potential, is ∆E 12 = −0.152 eV implying more than 5 phonons to be necessary for that transition ( ω D = 0.03 eV for GaAs), which makes it accordingly unlikely.…”

“…In principle, the device of Biasini and coworkers 25,26 would also allow to determine the electron sticking coefficient, making it a promising tool for a quantitative experimental investigation of physisorption of electrons specifically at GaAs surfaces. The empirical data about τ e and s e are however sparse in general.…”

“…Interesting in this respect are also electronegative dielectric structures used in electron emitting devices such as cesium-doped silicon oxide films [22][23][24] and GaAs-based heterostructures. [25][26][27] In contrast to intrinsic surface states, 28 originating either from the abrupt disappearance of the periodic lattice potential or unsaturated bonds at the surface, image states are not localized at the edge but typically a few Å in front of the solid. An external electron approaching the solid from the vacuum with a kinetic energy below the lowest unoccupied intrinsic electron state of the surface may thus get trapped ͑adsorbed͒ in these states provided it can get rid of its excess energy.…”

“…They should thus support image states. Interesting in this respect are also electro-negative dielectric structures used in electron emitting devices, such as, cesium-doped silicon oxide films [22][23][24] and GaAs-based heterostructures [25][26][27] .…”

Physisorption of an electron in deep surface potentials off a dielectric surface

“…In addition, it is relevant in other physical contexts as well. For instance, (i) in elec-tron emitters, such as cesium-doped silicon oxide films with negative electron affinity, electron emission via image states reduces the operational voltage considerably, 17 (ii) in gallium arsenide based heterostructures surface charging can be used for the contactless gating of field devices, 18 and (iii) for the alkaline earth oxides, studied in the field of heterogeneous catalysis, [19][20][21][22] the electronic surface states provide the environment for catalytic reactions. Some situations encompass electronic transitions from bulk to surface states, as it is the case for electron emitters, while for others, the electron does not penetrate into the bulk and the electron kinetics takes only place in surface states.…”