An improved electron transmission method for measuring electron trapping cross sections at the surface of dielectric films

We investigate several problems inherent in the low energy electron transmission (LEET) technique for measuring cross sections σCT for charge trapping, by submonolayer (ML) quantities of a target molecule deposited onto the surface of a dielectric film. In particular we see that the energy of the incident electron beam while charging the film was poorly defined in the original method. Furthermore, we demonstrate that interactions between trapped charges and the metallic substrate set a limit of about 100 mV for the maximum surface potential that should be allowed in absolute measurements of CT cross sections. To surmount these problems, we show how the surface potential ΔV generated by electrons of specific incident energy can be obtained rapidly, by monitoring the relative transmitted current Ir at a single reference energy EREF, rather than by recording multiple LEET spectra (or injection curves). The approach allows several measurements at different incident electron energies to be made on a single film and increases markedly the rate of data acquisition. The overall uncertainty in these measurements has an upper limit of ∼50%.