Fabrication and Characterization of Nanometer Scale Organic Electronic Devices: Application to Field-Effect Transistors

This thesis deals with the fabrication and electrical characterization of organic electronic devices with critical dimensions down to the sub 100-nm scale. The applications include organic field-effect transistors (OFETs) as well as molecular devices where the interelectrode separation is defined by the thickness of a self-assembled molecular layer. Electrodes are prepared using several techniques such as shadow mask evaporation, photolithography, electron-beam lithography (EBL), nanoimprint lithography (NIL) and breakdown of nanowires by controlled electromigration, depending on the application and the range of interelectrode separation. In the case of OFETs, the channel length of the devices is in the range from 70 µm down to 50 nm. Dihexylquaterthiophene (DH4T) is used as active material and is deposited from solution by drop casting, spin coating and self-assembly. In the case of OFETs, a different behavior was found when comparing results on devices produced by drop casting and spin coating. In particular with respect to the presence, or not, of a saturation region at high drain-source voltage. The output characteristics of almost all of the spin-coated devices presented saturation at high drain-source voltages while for all of the drop-cast devices the drain-source current did not saturate. Non-saturating output characteristics at short channel lengths were initially interpreted as resulting from the overlap of the depletion regions next to the metal contacts. Nevertheless, the observation of this effect at long channel lengths for devices containing relatively thick organic semiconductor layers suggested that this is the result of the current flowing through the bulk of the organic semiconductor in addition to the current through the accumulation layer at the semiconductor-insulator interface. In order to account for this effect, the model for the output characteristics of FETs in the gradual channel approximation was modified to take into account the contribution of the mobility of the bulk of the organic semiconductor to the total channel mobility. The proposed model also incorporates the metal-semiconductor contact resistance because at short channel lengths it becomes comparable to the channel resistance. This model satisfactorily fits the results for all the fabricated devices. An increase in the field-effect mobility was expected for devices with channel lengths in the nanometer scale caused by the increase of material homogeneity and reduction of grain boundaries in the channel at this lengthscale. Nevertheless, the field-effect mobility was found to decrease with decreasing channel length. This decrease is not due to a contact resistance limitation, as suggested in the literature, but it is most probably the result of incomplete filling of the channels for short channel length devices. Additionally, gate voltage modulation in molecular electronic devices, where the channel length is defined by the thickness of a self-assembled DH4T layer sandwiched between gold electrodes, is demonstrated