Understanding the contact characteristics in single or multi-layer graphene devices: The impact of defects (carbon vacancies) and the asymmetric transportation behavior

Process induced variation of contact resistance (R
_c
) in Ti/graphene (single or multi-layer) devices is investigated physically and electrically. It is proposed that the increased R
_c
can be attributed to the carbon defects (vacancies) created during SPUTTER process, which is evidenced by Raman spectra. For the first time, the asymmetric behaviors in hole and electron transportation regions in different metal/SLG devices are experimentally understood using scanning Kelvin probe microscopy technique. The asymmetric behavior in hole and electron transportation regions is attributed to the transition from p-p-p (n-p-n) to p-n-p (n-n-n) junction under the modulation of V
_bg
.