  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.
