Scaling of Island Densities in Multilayer Growth and an Application to α-Sexithiophene Films

Abstract

Abstract Kinetic Monte Carlo simulations of thin-film deposition are performed with film–substrate interactions slightly stronger than film–film interactions and with layer-dependent Ehrlich–Schwoebel (ES) barriers for molecules to cross step edges. Simulations with rapid changes from weak to strong ES barriers in the first few layers represent the crossover from layer-by-layer (LBL) growth to rapid roughening observed in ultrathin films of aligned organic molecules such as α-sexithiophene (6T) and diindenoperylene (DIP) deposited on SiOx; moreover, the shapes of wide islands formed in the first and second layers resemble those of the 6T films. However, weak ES barriers recently predicted by a mean-field approach for that material are ruled out by the simulations. When densities of 10–3 islands per lattice site or less are obtained, these densities follow relations with the model parameters as predicted by submonolayer growth theories, despite the strong interlayer fluxes which those theories do not account for. This feature allows extrapolations to the much smaller island densities of 6T films, in which the relations 2ES + EB = 2.1 ± 0.4 eV and 2EF + EB = 1.8 ± 0.3 eV are estimated for the activation energies of molecule hopping on the substrate (ES), on upper film layers (EF), and of molecule bonding with lateral neighbors (EB). From the same extrapolations, we obtain characteristic frequencies of hopping to neighboring sites ν ∼ 1021–1028 s–1 in the first layer and ∼1021–1026 s–1 in the second layer. These results suggest that individual energy parameters are on the order of a few tenths of eV. We propose to interpret these values by relating activation energies and contact areas between neighboring molecules with a rod-like shape: first, large EB is a consequence of large contact areas of neighboring standing-up molecules on islands; second, standing-up molecules on terraces have large diffusion lengths between two orientational changes, so ES and EF account for breaking bonds of the laying-down orientation during these changes, while the anomalously large ν incorporates those diffusion lengths. These possible interpretations may motivate additional experimental and theoretical studies of thin films of rod-like molecules.