Growth, morphology and scaling of pentacene thin films on oxidized and H-terminated silicon substrates

Organic/inorganic interfaces play a crucial role in the currently growing field of molecular electronics. Even though huge progress has been achieved in the understanding of electronic transport in conjugated molecular materials, a complete theory is still missing to model nucleation, growth and film morphology of organic materials deposited on inorganic substrates.; We have deposited ultra-thin pentacene films on oxidized and H-terminated Si substrates and have monitored the film evolution, confirming a layer-by-layer growth, from 0 to 2 ML with AFM and synchrotron X-ray diffraction. AFM images confirmed the formation of a closed first layer. However, X-ray reflectivity indicates that its density is only 75% of what would be expected from a bulk-like closed packed layer. The nucleation density of pentacene islands on the oxidized Si substrate (0.7μm−1) is two orders of magnitude higher than that on the H-terminated Si surface (0.007 μm−1).; It was demonstrated that in the submonolayer regime (0.1–0.5 ML) the pentacene island size distribution scales through a universal scaling function independent of the coverage. This scaling behavior can be modeled applying a model that has been used in the past for conventional inorganic molecular beam epitaxy. A best fit of the scaling function suggests a critical island size of 4 molecules to be the smallest stable island for the oxidized substrates.; The island-island correlation length remained constant in the range of 0.1–0.5 ML as expected for films in the aggregation regime. Scaling of the structure factor was also demonstrated, The power law decay of the structure factor indicates a fractal dimension, d _f of 1.67 for the pentacene islands on oxidized Si substrates, close to the known 5/3 value for DLA. It was also demonstrated that the structure factor can be decomposed into a product of two functions: a single island form factor and a structure factor of the island distribution.; In summary, the applicability of existing diffusion models to two-dimensional organic thin films deposited by OMBD, has been demonstrated opening the doors to a better understanding of the growth dynamics in organic thin film science.