Graphene-assisted spontaneous relaxation and direct CVD growth of graphene on III-V substrate

Abstract

Although conventional homoepitaxy forms high-quality epitaxial layers, the limited set of material systems for commercially available wafers restricts the range of materials that can be grown homoepitaxially. At the same time, conventional heteroepitaxy of lattice-mismatched systems produces dislocations above a critical strain energy to release the accumulated strain energy as the film thickness increases. The formation of dislocations, which severely degrade electronic/photonic device performances, is fundamentally unavoidable in highly lattice-mismatched epitaxy. This thesis reports a unique mechanism of relaxing misfit strain in heteroepitaxial films that can enable effective lattice engineering. We have observed that heteroepitaxy on graphene-coated substrates allows for spontaneous relaxation of misfit strain owing to the slippery graphene surface while achieving single-crystalline films by reading the atomic potential from the substrate. This spontaneous relaxation technique could transform the monolithic integration of largely lattice-mismatched systems by covering a wide range of the misfit spectrum to enhance and broaden the functionality of semiconductor devices for advanced electronics and photonics. We also noticed the defective areas caused by graphene transfer process, which were observed at the interface of graphene-coated substrates and epitaxial films, clearly degraded the quality of grown epilayers. In order to solve this problem, we developed a CVD growth process that directly coats III-V semiconductor substrates with graphene having full coverage and thickness that is close to monolayer. This novel graphene growth process can further improve the materials quality and performance of lattice-mismatched systems that utilize the spontaneous relaxation mechanism.