Thin-Film Phase of Pentacene Film Formed on KCl by Vacuum Deposition

Abstract

On KCl substrates, vacuum-deposited pentacene films are found to exhibit the so-called thin-film phase with 1.54 nm d-spacing perpendicular to the substrates when the film thickness is smaller than 30 nm. The films have in-plane unit cell sizes of a=0.59 nm, b=0.75 nm and Γ=90°, which are almost the same as the values reported previously for the thin-film phase on SiO2/Si. The pentacene films formed on KCl show good epitaxy being composed of monomolecular layers with an approximately 1.5 nm step height. These films are oriented in the two epitaxial angles of 0 and 32° with respect to the a-axis of KCl. From the point-on-line epitaxy, a very small misfit value is found for the combination of the KCl(020) substrate and the thin-film phase (120) so that the a- or b-axes of the thin-film phase should orient making an angle of 32° from the a-axis of KCl, which explains exactly the observed 32° orientation. However, the 0° orientation can be explained not from the point-on-line epitaxy, but is suspected to be due to the nucleation controlled at surface steps along the a-axis of KCl.