Graphene field-effect transistors with source/drain contacts made of metals that can be easily oxidized such as ferromagnetic metals often display a double dip structure in the transfer characteristics because of charge density depinning at the contacts. Generally, transfer characteristics of field-effect transistors show no dependence on the length of the source/drain contacts because charge carrier injection occurs mainly at the edges of the contact. However, the shape of the transfer characteristics of devices fabricated using Ni contacts is found to be dependent on the length of the contact. This peculiar behavior was attributed to charge carrier injection from near the center of the contacts. This is because of oxygen diffusion and the resultant formation of an interfacial oxide layer of non-uniform thickness. The observed contact length dependent transfer characteristics were reproduced using a model calculation that includes charge carrier injection from the center of the electrode and subsequent charge transport underneath the metal contact.
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