Passivation of Germanium by Graphene for Stable Graphene/Germanium Heterostructure Devices

Graphene grown directly on Ge via chemical vapor deposition (CVD) can passivate the underlying Ge surface, preventing its oxidation in ambient air for at least months. However, the factors that govern oxidation of Ge coated with graphene have not been elucidated. We investigate the effect of graphene synthesis parameters and Ge surface orientation on passivation of Ge and correlate these data with the density and type of defects in graphene. Oxidation of Ge can be reduced by increasing the H2:CH4 flux ratio or decreasing the growth rate, which decrease the density of atomic-scale defects, such as point defects and grain boundaries, in graphene. Oxidation of graphene is concomitant with oxidation of Ge and occurs more readily when the density of atomic-scale defects is relatively high. Passivation of Ge, however, depends more strongly on Ge surface orientation, as Ge(110) oxidizes significantly less than Ge(001) or Ge(111), even at the same graphene defect density. These results provide a pathway for engineering high-quality graphene films on Ge, which may enable improved passivation of Ge and direct integration of graphene-based or hybrid graphene/Ge heterostructure devices on conventional semiconductor platforms.