Influence of The Dielectric on The Growth and Performance of Pentacene Thin Film Transistors
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INFLUENCE OF THE DIELECTRIC ON THE GROWTH AND PERFORMANCE OF PENTACENE THIN FILM TRANSISTORS D. KNIPP, R. A. STREET, B. KRUSOR, J. HO, R. B. APTE Xerox PARC, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT The electronic transport and the device properties of pentacene thin film transistors are reported, showing the influence of the dielectric on the structural and transport properties. The structure and morphology of pentacene films on organic and inorganic dielectrics were compared by x-ray diffraction measurements and atomic force microscopy. For the investigated dielectrics we observed a clear correlation between the morphology and the structural properties of the highly polycrystalline films. In the case of inorganic dielectrics the roughness of the dielectric has a distinct influence on the morphology and the structural properties, whereas the films on organic dielectrics bonding between pentacene and the dielectric may have an influence on the growth mechanism. We find that careful control of the deposition conditions give films with similar transport properties on organic and inorganic dielectrics. To study the electronic properties we have realized inverted staggered transistors. The TFTs exhibit mobilities of ~0.4 cm2/Vs and on/off ratios of 108 on organic and inorganic dielectrics. The influence of the dielectric on the device mobility, threshold voltage and sub-threshold voltage slope are discussed. INTRODUCTION Single crystal transistors based on small molecules have been reported with very low levels of impurity and electronic properties close to the intrinsic transport limit [1, 2]. Pentacene single crystals with hole mobilities up to 3.2cm2/Vs at room temperature have been demonstrated [1]. However, the size of the single crystal substrates is relatively small (1-10mm) in comparison to large area and/or low cost substrates because the crystals were grown by vapor phase deposition [1-3]. Thermal evaporation of aromatic hydrocarbons might be an interesting alternative, because the process facilitates scaling of the process on large areas, as it has been demonstrated for small molecule based OLED displays [4]. Thermally evaporated pentacene is polycrystalline with large crystals, usually in the range of a few microns [5, 6]. The mobility of polycrystalline pentacene transistors at room temperature is 0.3 cm2/Vs - 1 cm2/Vs. Despite enormous progress in the fabrication of thin film transistors (TFTs) with very good properties [4, 5], the transport mechanism of thermally evaporated polycrystalline pentacene TFTs, which are usually realized in bottom gate configuration, is not completely understood. Furthermore, most of the OTFTs so far are realized on thermal oxide coated crystalline wafers. Thermal oxide on silicon wafers is a favorable substrate for the growth of pentacene, but for the intended applications silicon substrates are too small and expensive. Attractive large area compatible dielectrics are sputtered or plasma deposited silicon nitride or silicon oxide. More favorable are organic or polym
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