In-plane Structure and Polymorphism of Pentacene Thin Films
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0965-S12-52
In-Plane Structure and Polymorphism of Pentacene Thin Films Toshiyuki Kakudate1, Noriyuki Yoshimoto1, and Yoshio Saito2 1 Iwate University, Morioka, 020-8551, Japan 2 Kyoto Institute of Technology, Kyoto, 606-8585, Japan
ABSTRACT The structure and polymorphism of pentacene thin films on SiO2 substrate were investigated by grazing incidence x-ray diffractometry (GIXD). The in-plane GIXD patterns were successfully obtained from vacuum deposited ultra-thin films of a few monolayers thick. The two-dimensional lattice constants were determined for both thin-film and bulk phases from the observed GIXD patterns. Considering the obtained unit cell parameters, the mechanism of the transformation between polymorphs was discussed using the classical nucleation theory. INTRODUCTION Pentacene is a representative organic semiconductor that shows promise for use in organic thin film transistors (OTFTs) [1-5]. At least three different polymorphic modifications of pentacene have been identified so far, which are characterized by interplanar d001 spacings of 1.41, 1.45 and 1.51 nm [6-8]. The vacuum deposition process generally yields two of the polymorphs, a thin-film phase and a bulk phase, in vapor-deposited thin films [7, 8]. Although the bulk phase is considered as the stable phase at around room temperature [9], the thin-film phase preferentially occurs in vapor-deposited process. The appearance of polymorphs depends on the growth conditions such as substrate temperature and film thickness [10]. Because carrier transport properties of organic thin films depend on π-orbital overlap that is sensitive to the molecular packing in the crystal, difference in polymorphs brings different conducting properties [11, 12]. Therefore, investigation and control of the polymorphism and film structures are demanded to improve device performance of OTFTs. In this study, the structure and polymorphism of pentacene thin films on SiO2 substrates were investigated by grazing incidence x-ray diffractometry (GIXD). Two-dimensional lattice constants of the polymorphs were determined from observed GIXD patterns, and the consistency of determined interplanar d11 spacings of the polymorphs was found. Furthermore, the equilibrium shapes were deduced from the lattice constants and reported surface energy values [13]. Finally, the mechanism of the transformation was discussed using the classical nucleation theory.
EXPERIMENT Pentacene powder (98%, Tokyo Kasei Co.) was sublimated from K-cell-type crucible. The substrates used were silicon wafers having a 300 nm insulating layer of thermally grown silicon dioxide on the top. The substrate temperatures (Ts) and the deposition rate were 20 or 50 °C and 0.01 nm/s, respectively. Final film thicknesses and the base pressure were 1 nm to 100 nm and 1 × 10-4 Pa, respectively. The deposited thin films were thermally treated at 100 °C for 24 hours under pressure of 1 × 10-4 Pa. Characterization of the films were done by using x-ray diffractometers (Rigaku Co., ATX-G and synchrotron radiation at the BL13
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