Investigation of graphene as electrode in n-type OFETs and its use in nanometric devices

This work aims to investigate the use of CVD-graphene as electrode material in nanometric channel n-type Organic Field Effect Transistors (OFETs) based on thermally evaporated thin films of the perylene-3,4,9,10-tetracarboxylic acid diimide derivatives (PDIF-CN2 and PDI8-CN2). We firstly explored the electrical response of nano devices with standard bottom-contact/ distributed bottom gate architecture. By a thorough comparison with short channel transistors made with gold electrodes, output characteristics of the graphene-based devices suggests that SCLC contribution is suppressed. Moreover, current on/off ratios independent of the channel length (L) and enhanced response for high longitudinal biases are demonstrated for (L) down to 140 nm. Further advances have been reached by the use of a proper device architecture for nano devices with patterned local gate tracks and an ultra-thin films (8nm) of Hafnium Dioxide as high-k gate dielectric. The largely improved gate modulation results in a proper output currents saturation for channel length down to 200nm, with supply biases of few volts. Through impedance spectroscopy, overlap capacitances and the overall AC response of CVD-graphene electrodes have been investigated as well. The cut-off frequency of the nanodevice has been indirectly evaluated considering the DC transconductance and the measured overlap capacitance of the graphene electrodes. Values of the order of 150 kHz has been obtained for channel lengths of 200nm. Lastly, the organic/graphene interfaces and their injection and extraction phenomena have been further investigated in micrometric architectures. In particular, the problem of contact resistances have been analyzed via Scanning Kelvin Probe Force Microscopy (SKPFM) and the energetics of the interfaces has been reconstructed by the analysis of UV Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS).