Effect of Germanium Surface Orientation on Graphene Chemical Vapor Deposition and Graphene-Induced Germanium Nanofaceting

The synthesis of graphene directly on Ge and on Ge deposited on Si provides a scalable route toward integrating graphene onto conventional semiconductors. Here, we elucidate the effect of Ge surface orientation on graphene island formation, strain in large-area graphene films, and nanofaceting of Ge below graphene by comparing the growth on Ge(001), Ge(011), Ge(111), Ge(112) (on which graphene growth has not been previously reported), Ge(001)-6 degrees, and Ge(001)-9 degrees via chemical vapor deposition. During the initial growth, islands ranging from compact hexagons and ovals to anisotropic ribbons are formed, depending on the Ge surface orientation. When the growth time and methane concentration are increased, the islands coalesce to form continuous films with low defect density on each substrate. Below graphene, the Ge surface is atomically flat on Ge(011) and Ge(111) but forms hill-and-valley nanofacets on the other surface orientations. We extract a partial phase diagram of the graphene-Ge interface and find that graphene modifies the Ge surface stability. Regardless of surface topography and orientation, graphene is compressively strained by 0.1-0.4%. Nanofaceting is suppressed when a fast growth rate is used; however, graphene also becomes more defective. These results provide insight into engineering mixed-dimensional heterostructures based on graphene and Ge for hybrid group-IV technologies.