PERFORMANCES OF SEXITHIOPHENE BASED THIN-FILM TRANSISTOR USING SELF-ASSEMBLED MONOLAYERS

摘要

High performance thin-film transistors (TFT) made of conducting oligomers are obtained when the organic films are well ordered at a molecular level. Highly ordered films are obtained provided that oligomers have a sufficient mobility on the substrate surface during film formation. One possible way to fulfill such a condition is to evaporate oligomers on heated substrates [1,2]. In this work, we suggest that a high surface mobility is obtained by a chemical functionalization of the silicon dioxide surface, and the corresponding improvements of the TFT performances are evidenced. A self-assembled monolayer of octadecyltrichlorosilane (OTS) was deposited on the SiO_2 by chemisorption from solution before the evaporation of sexithiophene film. Room temperature current-voltage measurements indicate that the presence of the OTS monolayer improves TFT performances : threshold voltage is decreased, subthreshold slope is decreased, a high current ratio I_(on)/I_(off) is obtained for a reduced gate voltage excursion, the field-effect mobility is slightly increased. We have also fabricated and characterized a nanometer scale organic FET (gate length = 50 nm) made of 6T films and only with a self-assembled monolayer as the insulating film between the degenerated silicon substrate (gate) and the conducting channel (no thick SiO_2, we call it ? oxide-free ? organic FET). Performances of this nanometer size organic FETs are the following : subthreshold slope of 0.35V/dec, threshold voltage of-1.3V, effective mobility of 2 × 10~(-4) cm~2/V.s.