In-situ Dynamic SPM Studies of Organic Semiconductor Thin Film Growth on Test Patterns

The physics of organic field effect transistors is strongly correlated with the organization of the organic semiconductor at multiple length scales. In the case of molecular semiconductors, which are deposited on device test patterns by high- or ultra-high vacuum sublimation, this organization arises from the nucleation anf growth phenomena occurring at timescales which are often not easily accessed by standard ex-situ characterization. We use in-situ dynamic scanning force microscopy to study the early stages of growth of a conjugated oligomer semiconductor, viz. sexithienyl (T6), on a technologically-relevant substrate, silicon oxide/silicon wafer and on test patterns with Au electrodes. In an ultra-high vacuum chamber a Knudsen cell produces a thermal beam of T6 molecules directly under the tip of a variable temperature SPM. Non-contact AFM images of the same sample area is acquired at different times to be mounted into a movie depicting the growth of the ultra-thin film. The coexistence of quasi-layer-by-layer and 3D growth modes, the latter promoted by heteronucleation on surface defects, is observed. By performing the experiment at different temperatures, we extract the relevant molecular energy barriers of T6 thin film growth: desorption energy (0.53 eV), layer-dependent diffusional barriers (0.15-0.20 eV), Erlich-Schwoebel barrier (0.070 eV). The latter is cause of the transition from layer-by-layer to island growth, which appears to be universal occurrence in organic semiconductor growth with important implications in the charge transport in organic field effect transistors.