Structural properties of grain boundary in graphene grown on germanium substrates with different orientations

The direct synthesis of graphene with high-quality on semiconducting germanium (Ge) substrates has been developed recently, which has provided a promising way to integrate graphene with semiconductors for the application of electronic devices. However, the defects such as grain boundaries (GBs) introduced during the growth process have a significant influence on the crystalline quality of graphene and the performance of related electronic devices. Therefore, the investigation of the formation of GBs in graphene grown on a Ge substrate is essential for optimizing the crystalline quality of graphene. Herein, the formation mechanism and microstructure of GBs in graphene grown on Ge (110), Ge (001), and Ge (111) substrates via a chemical vapor deposition method are revealed. Ex situ atomic force microscopy is utilized to monitor the evolution of graphene domains. It is found that a single crystalline graphene film without GBs is formed on Ge (110), while polycrystalline graphene films with GBs are grown on Ge (001) and Ge (111) substrates, as suggested by transmission electron microscopy and x-ray photoelectron spectroscopy measurements. Our work may motivate the future exploration in improving the crystalline quality of graphene grown on a semiconducting substrate and the performance of associated electronic devices.