Electron-Beam-Deposited Thin Polymer Films: Electrical Properties vs Bombarding Current

Abstract

Polymer films about 150 Å thick were deposited on glass substrates by electron bombardment of tetramethyltetraphenyltrisiloxane (DC704). They were sandwiched between evaporated aluminum electrodes, the top one semitransparent. The capacitance, conductance, and photoconductance of the sandwiches were measured at room temperature as a function of the electron bombarding current which formed the polymer. The polymer thickness was obtained independently from Christy's formula for the rate of formation. The dielectric constant, calculated from the capacitance and thickness, increased with bombarding current up to our maximum current density of 4 mA/cm2, starting from the value 2.6 measured for the bulk liquid DC704. After exposure of the film to air, the values fell to 2.6 regardless of bombarding current. The conductance, which showed an exponential dependence on square root of electric field, was interpreted as field-assisted thermal ionization of donors heavily compensated by acceptors. It increased as the cube of bombarding current, and fell several orders of magnitude on exposure to air. The photocurrent, which increased with photon energy at fixed voltage bias, showed a threshold which fell from 3.5 to 2 eV with increasing bombarding current and a proportionality constant which increased. It was not measured before air exposure. Our conclusion from these results is that with increasing bombarding current, the polymer undergoes an increase in both crosslinking bonds (which contribute to the carrier mobility) and dangling bonds (which function as donors or acceptors). Exposure to air drastically reduces the density of dangling bonds, with donor radicals more susceptible to oxidation than acceptors, but does not much affect the crosslinking.