Diffusion-limited submonolayer pentacene thin film growth on hydrogen-passivated Si(1Â 1Â 1) substrates

Growth of ultrathin pentacene films is investigated as a function of coverage by atomic force microscopy. Initially, pentacene grows as monolayer fractal islands and evolves into compact islands before coalescence. Stabilization factors, against diffusion-limited-aggregation in terms of interaction between islands and interlayer monomer transport, are proposed to explain the shape transition. Simulations based on a simple model of heterogeneous film growth are found to agree with experimental observations. The role of surface diffusion in island shape transition is revealed by a comparison between pentacene growth on the hydrogen terminated and oxidized Si substrates.