Electron beam induced alteration of LiF thin films monitored by low energy electron transmission

Abstract

Low energy electron transmission (LEET) spectra are measured for 2–20 nm films of LiF deposited on Mo ribbon. Energy position of the target current onset and the LEET spectrum features are used as sensitive indicators of the layer charging. It is shown that the film, when irradiated by a 9 eV electron beam (i.e. below the desorption threshold of 12 eV), becomes negatively charged and a dose of ca. 1 μC mm −2 already produces a layer potential shift of −0.2 V. Sample irradiation with more energetic electrons (with energies above the desorption threshold) results in positive charging of the layer, for example, for a 90 eV electron beam, a dose of 1.8 μC mm −2 is sufficient to generate a well detectable layer potential shift of +0.2 V. For higher doses, splitting of the low energy peak is observed, which indicates the formation of potential patches. By prolonged electron beam irradiation, the potential spread in the range from −2.2 to +2 V is generated in the layers studied. Such charging may substantially contribute to the energy spread of emitted ions in electron-stimulated desorption (ESD) processes. The obtained results indicate that strong electric fields are generated in the layer–substrate system during electron bombardment. This induces an electron tunnelling between the layer and the substrate, thanks to which the secondary-electron emission coefficient of the probed system always differs from unity under the quasi-stationary potential drop across the layer.