Sputtering and annealing effect of sapphire substrate for an orientation of lead phthalocyanine films

Abstract

We examined the property of lead phthalocyanine (PbPc) films in relation to Ar ion sputtered and/or annealed sapphire (0 0 0 1) faces prepared by thermal deposition. The film structure and morphology were investigated by X-ray diffraction and atomic force microscopy, respectively. The interaction between the film and the substrate was evaluated from the viewpoint of the surface energy. The PbPc films consisted of the monoclinic form, in which the molecular planes stacked along the c-axis. On the all substrates, the c-axis was oriented parallel to the substrate surface and the a-axis perpendicular to the surface. The film on a sputtered sapphire possessed a highly oriented a-axis with the step and terrace structure at the surface of each crystal, and the c-axis direction had a wide distribution within the surface plane of the substrate. On an annealed sapphire, however, there were two types of a-axis orientation in the film. One is oriented to the (0 0 0 1) face and the other is to the polished face. The c-axis direction within the surface plane was not random on the annealed sapphire. This result suggests a strong relationship between the in-plane film axes and the substrate axes. We determined that the molecular orientation was dependent on the surface treatment where the ratio of the adhesion force to the cohesion force is derived from the hydrogen bond force. We concluded that the sputtering treatment was effective in yielding a highly oriented PbPc film through the formation of a film-substrate interface possessing an appropriate balance between adhesion and cohesion forces.