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Influence of the gate dielectric on the morphological and electronic structure of pentacene films for transistor applications

The structural and transport properties of pentacene thin film transistors are reported, showing the influence of the deposition temperature, the deposition rate and the substrate on the structural and transport properties of oTFTs. The structure and morphology of pentacene films on thermal oxide and plasma CVD silicon nitride were compared by x-ray diffraction measurements and atomic force microscopy. There is a clear correlation between the morphology and the structural properties of the highly polycrystalline films on the two dielectrics. In the case of silicon nitride the roughness of the film has a distinct influence on the morphology and the structural properties, whereas the films on thermal oxide are in general highly ordered independent of the deposition conditions. The ordered films exhibit a thin film and a crystalline bulk phase, and the crystalline bulk phase fraction increases with the deposition temperature and the film thickness. We find that careful control of the deposition conditions give virtually identical films on the oxide and nitride dielectrics. To study the electronic properties we have realized inverted staggered transistors. The mobility of the TFTs is correlated with the morphology and the structural properties of the films, and increases with the size of the crystals. The TFTs exhibit very similar mobilities of ~0.4 cm2/Vs and on/off ratios >108 on thermal oxide and flat silicon nitride. The impact of the dielectric on the device parameters of mobility, threshold voltage and sub threshold voltage slope are discussed. Bias stress experiments are performed to investigate the stability of the TFTs, and to gain understanding of the transport mechanisms of thermally evaporated pentacene TFTs.