RF Transport Electromagnetic Properties of Graphene from DC to 110 MHz
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The paper describes measurement of RF transport electromagnetic properties of CVD graphene over the DC to 110 MHz frequency range at room temperature. Graphene on Si/SiO2 substrate was mounted in a shielded four terminal-pair (4TP) adaptor which enabled direct connection to a calibrated precision impedance analyser for measurements. Good agreement is observed for the DC four-probe resistance and the 4TP resistance at 40 Hz, both yielding R ~104 Ohms. In general the apparent graphene channel electromagnetic properties are found to be strongly influenced by quantum contact effects, such as resistance and capacitance, particularly at DC and low frequencies f < 1 MHz. A phenomenological lumped-parameter equivalent circuit model is presented which matches the frequency response of the graphene 4TP impedance device over approximately seven decades of the frequency range of the applied transport alternating current. Based on this model, the intrinsic graphene channel resistance is found to be RG = 2.2 Ohms or sheet resistance of 3.85 Ohms/sq, which is frequency independent, with each contact impedance being RC = 51.6 Ohms and CC = 1.2 nF. These results suggest that our RF 4TP method may be more accurate and reliable than the conventional DC four-probe method for measuring the intrinsic sheet resistance of single-atom thick materials such as graphene. This may be significant for the production and optimisation of graphene for solar-cells and touch-screen displays where sheet resistance (combined with its optical transparency) figures-of-merit play an important role, particularly in comparison with the values of the current material of choice, indium tin oxide (ITO), for such applications.
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