Structure and Morphology of Tetracene Thin Films on Hydrogen-Terminated Si(001)
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Structure and Morphology of Tetracene Thin Films on Hydrogen-Terminated Si(001) X. R. Qin, A. Tersigni, J. Shi, and D. T. Jiang Department of Physics, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
ABSTRACT Scanning tunneling microscopy (STM), atomic force microscopy (AFM) and near-edge x-ray absorption fine structure (NEXAFS) have been used to study the structure of tetracene films on hydrogen-passivated Si(001). A distinct growth morphology change that occurs around a few monolayers of film thickness was characterized. This coverage-dependent film structural phase transition leads to a molecularly ordered film structure commensurate with the crystalline substrate. INTRODUCTION Small aromatic molecules are attracting increasing attention because of their potential applications in future organic electronics [1-3]. Tetracene is a planar aromatic molecule (C18H12) consisting of four fused-benzene rings. The bulk crystalline structure of tetracene is layered herringbone molecular packing on the ab plane [4]. Tetracene thin films prepared with vacuumsublimation have already been used to fabricate field-effect transistors (FETs) with nonoptimal film connectivity limiting the carrier mobility [5], and tetracene films have also displayed impressive properties in organic light-emitting FETs [6]. In order to achieve optimal material properties for these devices, the control of the film growth based on a good understanding is required. Studies aimed on increasing the mobility in tetracene thin films on silicon dioxide reported that the film growth is in favor of forming 3D islands with a granular structure [7]. Similar structural results were indicated by grazing incidence X-ray diffraction study that polycrystalline structures with polymorphs of different upright orientations of the molecules were found [8]. Recently a “layer-by-layer” growth, similar to that in the pentacene film growth [9-11], has been realized for tetracene on H/Si(001) substrates with good film connectivity and significantly improved grain size [12, 13]. In this paper, we report a combined multi-technique structural characterization on the tetracene films. We show that the film of a few monolayers coverage on H/Si(001)-2x1 possesses apparent epitaixial domains in which molecules are in an upright standing configuration with a bulk-kind herringbone lattice. Also, the average molecular tilting angle with respect to the surface first increases with the coverage in forming the expitaxial domains and stabilizes around the bulk tetracene value in further growth. We associate the molecular-level structural information to a distinct growth morphological change that occurs around a few mononalyers of thickness. EXPERIMENTAL DETAILS The film growth and in situ scanning tunneling microscopy (STM) studies were conducted in a two-chamber ultrahigh vacuum (UHV) system, with base pressure ~ 1x10-10 Torr
for the STM (home-made) chamber and ~ 3x10-9 Torr for the growth chamber, respectively. In the STM chamber, Si(001) wafers were cleaned by therm
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