Istituto dei materiali per l'elettronica ed il magnetismo     
Toccoli T., Wu Y., Zhang J., Koch N., Tonezzer M., Iannotta S., Rudolf P. High quality pentacene sub-monolayer by supersonic molecular beam deposition. In: ECOSS26 - 26th European Conference on Surface Science (Parma, 30 Aug - 4 Sept 2009). Abstract, p. 178. European Physical Society, 2009.
The performances of organic devices depend by the degree of order achieved during the growth process starting from the quality of the first layer. Supersonic molecular beam deposition (SuMBD) has been adopted as a new method to control the pentacene thin film growth [Y. Yu et al. Phys. Rev. Lett. 98 (2007) 076601]. The morphology and crystallinity in the SuMBD grown pentacene first monolayer on the SiO2 surface can be controlled by kinetic energy and momentum of the impinging molecules. The range of kinetic energy accessible with this method (from thermal up to more than ten eV) overcomes some limitation present in conventional vacuum deposition techniques thus increasing order and crystallinity. Here we shall emphasizes the significant differences between pentacene (sub-)monolayer growth by thermal sublimation and by SuMBD: with increasing Ek, the island structure changes from fractal to non fractal, and islands with smooth borders and increased coalescence are observed; the morphology evolves towards higher density of smaller islands and the island size uniformity is enhanced by correlated island growth. The growth, interpreted by the diffusion mediated model, shows the critical nucleus changing from 3 to 2 pentacene for Ek>5-6 eV. By using transverse shear microcopy, we demonstrate that islands grown by SuMBDare single crystalline while those deposited by thermal sublimation have a polycrystalline structure. Very large single crystal grains (~ 10microns) can be obtained in pentacene monolayers grown with Ek=6.4 eV. We report also the characterization of top contact field effect transistors realized with different kinetic energies, showing the improved performances due to the high quality films grown by SuMBD.
Subject Supersonic Molecular Beam Deposition

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