A photoemission spectroscopic study of the interface formation in organic thin film transistors
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A photoemission spectroscopic study of the interface formation in organic thin film transistors Neil J. Watkins, Serkan Zorba, Li Yan and Yongli Gao Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627 U.S.A. ABSTRACT Pentacene is widely used in organic thin film transistors due to relatively large mobilities. It has been suggested that the functional behavior in these organic thin film transistors occurs within the first few molecular layers of the device at the interfaces between the organic and the metals and dielectrics used in fabrication of the thin film transistors. This makes understanding the electronic behavior of the interfaces involved in these devices critical. In order to better understand these interfaces we investigated the interface formation using photoemission spectroscopy to examine layer by layer growth of pentacene on Au and Ag and vice versa. We observed indications of dipole formation at the interfaces between the metals and organics for organic on metal deposition. On the other hand, more complex material intermixing takes place during metal on organic deposition and as a result, the electronic structure of the interface differs from that of organic on metal deposition. INTRODUCTION In the past few years, the performance of organic thin-film transistors has improved considerably due, in part, to use of pentacene as an active material [1-3]. Pentacene is of use in these organic based devices due to its relatively high mobility [3-9]. Pentacene based devices have exhibited, at room temperature, performance comparable to hydrogenated amorphous silicon devices: on/off current ratios at 10 V in the range of 106, room temperature mobilities as high as 2.7 cm /Vs, and operating voltages as low as 5 volts [8,9]. At low temperatures, pentacene device mobilities are as high as 10 cm /Vs [9]. Superconducting behavior has been observed in single crystal pentacene field effect transistor devices [10]. Fractional quantum Hall measurements have also been made with these same device structures [11]. It is generally believed that device performance is heavily influenced by the characteristics of the interfaces between the organic and the metals and dielectrics used in fabrication of the thin-film transistors [8,12,13]. For instance, it has been shown that the morphology of pentacene films affects device mobilities. Thin film transistors can be built using device geometries such as staggered and staggered-inverted. These different geometries require that the electrodes and dielectric be deposited or grown in reversed orders. This variety makes understanding any differences in the interface properties that may result from different orders of growth of interest. We have examined the formation of interfaces between pentacene and Au and Ag. EXPERIMENTAL DETAIL Photoemission measurements were made using two techniques. X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) studies were performed using a VG ESCA Lab system equipped with an unfiltered He
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