Orientation Control of Standing Epitaxial Pentacene Monolayers Using Surface Steps and In-plane Band Dispersion Analysis
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0965-S06-19
Orientation Control of Standing Epitaxial Pentacene Monolayers Using Surface Steps and Inplane Band Dispersion Analysis by Angle Resolved Photoelectron Spectroscopy Tadamasa Suzuki1, Toshihiro Shimada1, Keiji Ueno2, Susumu Ikeda3, Koichiro Saiki3, and Tetsuya Hasegawa1 1
Department of Chemistry, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
2
Department of Chemistry, Saitama University, 255, Shimokubo, Sakura-ku, Saitama, 338-8570, Japan
3
Department of Complexity Sciences and Engineering, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, 277-8583, Japan
INTRODUCTION Pentacene is one of the most important organic semiconductors for thin film transistor (TFT) applications due to its high carrier mobility in thin films. Anisotropy in the effective mass [1, 2] and interaction between carrier transport and phonons [3] are fundamental properties which affect the device performance directly through anisotropic and temperature-dependent carrier mobilities. Although many studies using photoelectron spectroscopy have been reported in relation to those issues, the measured pentacene was in a gas phase[4], polycrystalline monolayers grown on SiO2[5] or in the monolayer adsorbate form on metal or graphite surfaces[6, 7]. All of them are not suitable to derive the band structure of the films actually used in TFTs directly. Angle resolved PES (ARPES) measurement of singly crystalline pentacene with its molecular long axis perpendicular to the surface (“standing orientation”) is awaited to extend those studies to give useful information for the actual device designs. Several groups including us found that epitaxial growth of pentacene in the standing orientation occurs on hydrogen-terminated Si(111)[8] and Bi film formed on Si(111)[9], but the epitaxial film is composed of domains with six equivalent in-plane crystal orientations, which still hinders the precise analysis of the in-plane band dispersion. Since vicinal surfaces of single crystals such as Si(111) have regularly spaced steps with atomic order height, they are often used as a template of controlling growth of organic thin films [10, 11] in a similar manner with graphoepitaxy [12]. Larger periodic structure formed by step bunching can also be used for this purpose [13]. In this paper, we present the in-plane orientation control of the epitaxial pentacene domains by using surface steps on Si (111) as a template and ARPES analysis of in-plane band dispersions of the pentacene monolayer film.
EXPERIMENT We tried to find a surface satisfying the following conditions: (1) suitable for the epitaxial growth of pentacene with “standing orientation”, and (2) with surface steps whose spacing and height are controllable by the preparation procedure. We focused on step-bunching of vicinally-cut Si (111) surfaces because the height and the spacing of the periodic structure can be controlled by carefully tuning the annealing and cooling conditions[14]. Since clean surfaces of Si(111) is not suitable for the epitaxial growth of penta
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