A poly(styrene-co-methyl methacrylate)/room-temperature sputtered hafnium oxide bi-layer dielectrics as gate insulator for a low voltage organic thin-film transistors

In this study, we present a low voltage pentacene organic thin film transistor (OTFT) with poly(styrene-co-methyl methacrylate) grafted hafnium oxide (PS-r-PMMA/HfO_x) as gate dielectrics. The HfO_x was sputtered at room temperature to approach low temperature and meet low cost requirements of organic electronics. The thickness of the PS-r-PMMA can be controlled to be extremely thin (<10 nm). Consequently, the gate overdriving voltage as low as 3 V was achieved for an OTFT. Furthermore, by applying PS-r-PMMA on top of the HfO_x surface, the traps on the HfO _x surface and possible pinholes of the HfO_x can be passivated. Therefore, the gate dielectric properties in terms of interface states and leakage current were improved. The leakage current reduced to approximately two orders of magnitude and the interface states reduced to nearly one order of magnitude. The reduction of the interface state density was observed by capacitance-voltage measurement. As a result, the subthreshold swing and on/off current ratio were improved significantly from 1.01 V/dec. and 10³ to 0.41 V/dec. and 10⁵, respectively.