Nucleation and growth of ultrathin pentacene films on silicon dioxide:: effect of deposition rate and substrate temperature

We report on a study of pentacene thin-films grown by high vacuum deposition on silicon dioxide, using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The nucleation density of pentacene islands on SiO2 is found to rise as the deposition rate is increased but decreases at higher substrate temperature. This observation is consistent with the predictions of the rate-equation formalism for homogeneous nucleation. Hence, by controlling the two deposition parameters we are able to tune the mean size of the pentacene islands, as well as the grain boundary density in the first pentacene layer. The deposition rate and the substrate temperature also affect the subsequent growth of the pentacene islands, which can be satisfactorily described by a capture zone model under given conditions of deposition. From a structural point of view, the use of selected area electron diffraction (SAED) reveals the crystalline nature of the pentacene islands. For pentacene films grown at the substrate temperature of 353 K (80degreesC), the observed SAED pattern matches well the calculated patterns obtained for the bulk triclinic structure. (C) 2004 Elsevier B.V. All rights reserved.