Optoelectronic Switching of Porphyrin Coated Si Nanowire Field Effect Transistors
Master Thesis, TU Dresden, December 2012
Switching characteristics of Porphyrin coated silicon nanowire field effect transistors (Si NW FETs) have been demonstrated by the analysis of a full current switching period under light illumination. Si Schottky barrier (SB) NW FETs were fabricated using a bottom-up approach and their ambipolar features have been characterized. Two different photo-switchable molecules, Azobenzene and Porphyrin, are successfully functionalized on the Si surface. To confirm molecular functionalization on the surface, ATR-IR spectroscopy and AFM and SEM imaging were analyzed. From the absorptionspectrum, suitable frequency of light for Porphyrin is decided. Current switching of Porphyrin covered devices is measured near threshold region on the time domain. Switching speed, on/off ratio and off-current level were characterized according to the concentration change of Porphyrin. As the coverage concentration increases, the devices show fast switching due to low resistance of Porphyrin and oxide layer by increasing charge transfer through Porphyrin and oxide. If Porphyrin layer is too thick to conduct the light irradiation, on/off ratio is reduced. Because bare Si show an irreversible increase of photoconductivity by light illumination, full coverage of Porphyrin on Si NW can reduce the irreversible change. The energy transfer and electrical mechanism of concentration-dependency of current switching has been discussed as well.