Pentacene thin film transistors on inorganic dielectrics: Morphology, structural properties, and electronic transport

The structural and transport properties of evaporated pentacene organic thin film transistors (TFTs) are reported, and they show the influence of the deposition conditions with different inorganic dielectrics. Dielectrics compatible with large area fabrication were explored to facilitate low cost electronics on glass or flexible plastic substrates. X-ray diffraction and atomic force microscopy show a clear correlation between the morphology and the structure of the highly polycrystalline films for all dielectrics investigated. The roughness of the dielectric has a distinct influence on the morphology and the structural properties, whereas the films on smooth thermal oxide are in general highly ordered and independent of the deposition conditions. The ordered films exhibit a “thin film” and a bulk phase, and the bulk phase volume fraction increases with the deposition temperature and the film thickness. Careful control of the deposition conditions gives virtually identical films on thermal oxide and silicon nitride dielectrics. The electronic properties of inverted staggered transistors show that the TFT mobility is correlated with the morphology and structure of the films. The TFTs exhibit very similar mobilities of ∼0.4 cm2/Vs and on/off ratios >108 on thermal oxide and silicon nitride. The impact of the dielectric on the device parameters of mobility, threshold voltage, and subthreshold voltage slope is discussed. Temperature dependent measurements of the mobility were performed to study the influence of traps on electronic transport. Bias stress experiments were carried out to investigate the stability of the TFTs, and to gain understanding of the transport mechanisms of thermally evaporated pentacene TFTs.