Oligothiophenes thin films grown by SuMBD: the influence of kinetic energy on sub-monolayer formation
M. Tonezzer1,2,3, T. Toccoli2, E. Rigo4, P. Bettotti4, L. Pavesi4, and S. Iannotta2,5
1 Dipartimento di Fisica, Università di Trento, Via Sommarive 14, I-38100 Povo Trento, Italy
2 IFN-CNR, Via alla Cascata 56/C, 38100 Povo di Trento, Italy
3 TASC INFM-CNR National Laboratory, S.S. 14 km163,5, I-34012 Basovizza Trieste, Italy
4 Laboratorio Nanoscienze, Dipartimento di Fisica, Università di Trento, Via Sommarive 14, I-38100 Povo Trento, Italy
5 IMEM-CNR, Parco Area delle Scienze, 37/a 43100 Parma, Italy
A critical question in realizing organic devices with good performance, is nowadays the limited ability to control their growth, a factor that strongly affects in particular structure, morphology and physical/chemical properties in the solid state and at the interfaces. Deposition methods based on vacuum sublimation give the best results in terms of film quality, but it is still far from the desirable performance. Typical growth approaches typically control thermodynamic parameters at equilibrium, so they can hardly drive towards single phase structures organized at the needed length scale and in particular produce the layer-by-layer growth. Supersonic Molecular Beam Deposition (SuMBD) has shown to be uniquely suitable to approach such question, achieving unprecedented control on morphology and structure. This technique, based on the realization of supersonic beams made of organics seeded in lighter carrier gasses, permits to overcome some thermodynamic limitations and opens to new perspectives in realizing structures controlled at different length scales. In this work we investigate the sub-monolayer growth of sexi-tiophene by SuMBD as a function of substrate temperature, surface hydrofobicity and kinetic energy of the supersonic seeded beam. We show the key role played by the kinetic energy of the impinging molecules in controlling and determining the morphology right from the early stages. The possibility to control the energetic properties of the precursor by SuMBD permits indeed to realize highly ordered thin films. We report the morphological (AFM) characterizations showing the high quality of the films deposited by SuMBD (large grain dimensions, terraced structures) as a function of the growth conditions. Furthermore we report the electrical characterization of field effect transistors realized with these organic films, showing state-of-art electrical properties with optimal carrier mobility that can be useful for different kinds of applications.