Growth process control of pentacene thin films and its application in full organic TFTs
Publication from Materials
4TH IEEE Internat. Conf. on Polymers and Adhesives in Microelectronics and Photonics, 219-224, (2004) , 12/2004
Due to its outstanding carrier transport capabilities pentacene is a prominent candidate for the active semiconducting layer in organic thin film transistors. This compound crystallizes in a layered structure with herringbone arrangement within each layer. Pentacene appears in several polymorphic structures, which differ basically by their c-axis lengths, meaning that the angle at which the molecules adsorb relative to the substrate changes from phase to phase. Obviously the interaction of the π-electron systems between adjacent molecules depends strongly on the stacking nature of the molecules. It has been argued, that a smaller angle between the molecular axis and the surface normal results in a larger orbital overlap which is expected to give better carrier transport properties. Therefore it is of major interest to clarify and control the growth conditions for the different phases. We have synthesized high-quality pentacene epitaxial thin films under different growth conditions and investigated them by atomic force, scanning electron and polarization microscopy. The aim was to identify the critical growth parameters with respect to surface quality, thickness and crystallinity of the thin films and with respect to homogeneity, size and shape of grains, ordering, substrate dependence, morphology and phase formation of the polycrystalline thin films. The polymorphic phase identification and the macrostructure of the films were determined by Xray diffraction, whereas micro-structural differences, small impurity concentrations and the temperature dependence of the structure were investigated by Raman microscopy. It turned out, that the growth process, the film quality and/or the phase formation are highly sensitive to the deposition rate, the choice of the substrate material, the substrate temperature, the film thickness and the purity of the source material. We observed a clear power-law dependence between deposition rate and grain size. In order to draw a bow between the structural and the electronic properties of pentacene, thin film transistors were fabricated based on the different polymorphic phases and different dielectric and electrode materials. Organic thin film transistors (OTFT) have great potential in electronic applications. For example, they find use in sensors and a variety of simple and cheap circuits. Organic electronics focus on implementation fields, where low costs, large area production and the use of flexible and even biological substrates are essential. It turned out, that the transistor characteristics are strongly correlated with the morphological as well as the structural parameters of the active layer, with the integrity and the dielectric properties of the insulator and with the contact resistance characteristics of the electrodes. We found a threshold-like dependence of the mobility on grain size, which can be described by a field-activated transport mechanism. Careful control of the growth process results in devices with high hole mobilities comparable to amorphous silicon, low leakage currents, high on-off ratios and reproducible trapping dynamics. Organic dielectrics enable the use of simple processing techniques, such as spin- or dipcoating by being available in liquid or soluble form. As direct application of the former results, we present in this work organic TFTs with pentacene as an active layer, organic dielectric materials and a comparison of organic and inorganic source/drain contact materials. For structuring of the source/drain contacts shadow masks and inkjetprinting were applied and both methods are compared by the electrical characterisation and determination of the charge carrier mobility of field effect transistors. OTFTs with inkjet-printed electrical contacts show contact properties comparable to devices with inorganic electrodes, and the field-effect mobility of the all organic devices are in the same order of magnitude.