The oxidation behavior of Ge(001) and Ge(110) surfaces underneath the chemical vapor deposition (CVD)-grown graphene films has been investigated experimentally and interpreted on the basis of ab initio calculations. Freshly grown samples were exposed to air for more than 7 months and periodically monitored by X-ray photoelectron spectroscopy, scanning electron microscopy, and Raman spectroscopy. The oxidation of Ge(110) started with incubation time of several days, during which the oxidation rate was supposedly exponential. After an ultrathin oxide grew, the oxidation continued with a slow but constant rate. No incubation was detected for Ge(001). The oxide thickness was initially proportional to the square root of time. After 2 weeks, the rate saturated at a value fivefold higher than that for Ge(110). We argue that after the initial phase, the oxidation is limited by the diffusion of oxidizing species through atomic-size openings at graphene domain boundaries and is influenced by the areal density and by the structural quality of the boundaries, whereby the latter determines the initial behavior. Prolonged exposure affected the surface topography and reduced the strain in graphene. In the last step, both the air-exposed samples were annealed in vacuum at 850 °C. This removed oxygen from the substrate and restored the samples to their initial state. These findings might constitute an important step toward further optimization of graphene grown on Ge.
Keywords: Ge(001); Ge(110); chemical vapor deposition; graphene; oxidation.