PhD defense: Jakob Meyer-Holdt
Experimental methods for implementing graphene contacts to finite bandgap semiconductor
Present Ph.D. thesis describes my work on implementing graphene as electrical contact to finite bandgap semiconductors. Different transistor architectures, types of graphene and finite bandgap semiconductors have been employed. The device planned from the beginning of my Ph.D. fellowship was a graphene-C60 monolayer-graphene vertical transistor named the Carbon Burger. The fabrication of such device proved increasingly difficult to achieve and many experimental methods to handle graphene were implemented and improved in attempt to fabricate the Carbon Burger. In the end, a device platform for molecular electronics with parallel CVD graphene bottom electrodes with SiO2 passivation was successfully fabricated and electronically characterized. A functioning Carbon Burger was not achieved. Along the work on the Carbon Burger, the scope was broadened and focus was put on implementing graphene contacts to semiconductor nanowires, more specifically, epitaxially grown InAs nanowires. First, we tried a top down method where CVD graphene was deposited on substrate supported InAs nanowires followed by selective graphene ashing to define graphene electrodes. While electrical contact between the nanowires and graphene was achieved, the contact resistance was higher (>100 kΩ) than what is achieved for optimum metal contacts (~5 kΩ). We therefore developed a method to directly grow InAs nanowires on graphitic flakes. This was achieved by using silver seed particles for epitaxial growth of InAs nanowires on graphitic flakes. An added benefit of our growth experiment is that the entire growth system comprising graphitic flakes and nanowires can be transferred to TEM grids for a full TEM analysis of the growth system as a whole. While time did not allow for an electronic characterization of the graphene-nanowire system, the epitaxial interface may provide the ultimate electronic contact between the graphene and nanowires.