Impact of surface modification by addition of self-assembled monolayer for carrier transport of quaterrylene thin films

Quaterrylene field-effect transistors (FETs) were formed on a silicon oxide (SiO₂) layer and on an octadecyltrichlorosilane self-assembled monolayer (OTS-SAM). To elucidate the transport mechanisms in the respective devices, we examined the dependence of carrier mobility on film thickness and temperature. On the OTS surface, a marked increase in the carrier mobility was observed in the initial layers, indicating that the accumulated carriers were distributed closer to the interface than were those on the SiO₂ surface. Moreover, the carrier transport in the respective devices exhibited distinct behaviors in the low temperature range, particularly in the initial layers. On the SiO₂ surface the carrier mobility depended strongly on temperature; the value drastically declined with the decreasing temperature from 300 K down to 60 K. On the OTS surface, the carrier mobility showed temperature-independent transport below 210 K. This maintenance of the carrier transport at low temperatures was caused by the termination of the trap-state density near the interface. These results clearly reveal that the OTS treatment effectively helped improve the interface properties because of a reduction in the density of the carrier traps, dramatically facilitating the carrier transport in the initial layers.