Investigation of electrical contact resistances in graphene-based devices by Kelvin Probe Force Microscopy
Graphene is a carbon atom-thick layer with outstanding mechanical and electrical properties. It has been envisioned as a promising candidate for its use as a conductive medium in polymers, a flexible, transparent and highly conductive electrode in optoelectronic devices and as the conductive channel of high frequency transistors. In the pathway to use graphene and its exceptional properties in such applications there are still open questions to be answered. One key issue always present for the fabrication of devices is that of the electrical contacting of the conductive channel. Such is the case for graphene where it is still not clarified how the oxygen content in Functionalized Graphene Sheets (FGS), the lithography process or the contact design in graphene produced by Chemical Vapor Deposition (CVD) impact the contact resistance between the metal electrode and the graphene channel. This understanding is however of utmost importance since contact resistances significantly affect the conductivity of electronic devices. Hence, the aim of this work is the characterization of electrical contacts in graphene-based devices on ambient conditions. For a better understanding of the electrical contacts, information on the submicrometer scale is advantageous, therefore Kelvin Probe Force Microscopy (KPFM) was used as the main characterization technique. Since an improvement is expected in the resolution of KPFM by detecting the force gradient instead of the electrostatic force itself, phase modulation is implemented in the existing system and its performance is examined. A feature of less than 20 nm and 80 mV in surface potential variation could be clearly resolved in ambient conditions. In the following section, it is shown that with decreasing oxygen content in FGS, the transport mechanism of the charge carriers has a transition from predominantly hopping to predominantly diffusive transport, along with a reduction of the sheet resistance from > 400 kΩ/□ to < 10 kΩ/□. At the same time it is reported that the contact resistance changes from nonlinear, Schottky-type behavior with high resistance (> 100 kΩµm) to linear, ohmic behavior with low contact resistance (~ 1 kΩµm). In the last part of this work, the influence of the fabrication process and contact design in the contact resistance of CVD graphene is investigated. It is determined that optical lithography systematically produced devices with contact resistances up to an order magnitude larger (>> 1 kΩµm) than e-beam lithography (< 1 kΩµm). It is determined that this is caused by a 3 - 4 nm thick residual layer from the optical lithography process which is present between graphene and the metal electrode. An elegant solution to prevent the effect of this residual layer in contact resistance is the use of novel one dimensional contacts instead of the conventional two dimensional contacts. In the former type of contact the charge carriers transit the metal/graphene interface not vertically but horizontally. It can be shown that such novel type of contact design, even with the use of optical lithography, can reach contact resistances lower than 200 Ωµm.
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