  We report a systematic study of the bias-dependent contact resistance in
rubrene single-crystal field-effect transistors with Ni, Co, Cu, Au, and Pt
electrodes. We show that the reproducibility in the values of contact
resistance strongly depends on the metal, ranging from a factor of two for Ni
to more than three orders of magnitude for Au. Surprisingly, FETs with Ni, Co,
and Cu contacts exhibits an unexpected reproducibility of the bias-dependent
differential conductance of the contacts, once this has been normalized to the
value measured at zero bias. This reproducibility may enable the study of
microscopic carrier injection processes into organic semiconductors.
