Real-time x-ray diffraction analysis of GaN remote epitaxy on sapphire substrates covered with directly grown graphene

Remote epitaxy is a thin-film growth technique in which a material is epitaxially grown on a substrate through a two-dimensional (2D) interlayer. The weak van der Waals (vdW) interaction enables easy exfoliation of the grown film, facilitating flexible and transferable devices. Moreover, remote epitaxy allows strain in the grown material to relax spontaneously, even in lattice-mismatched systems, enabling the growth of high-quality heteroepitaxial thin films. In this study, we investigate the role of 2D materials in strain relaxation by examining the lattice deformation of GaN crystals during growth on c-plane sapphire, with and without 2D interlayers, using real-time x-ray diffraction. Significant differences are observed in the initial strain magnitude: the highest on bare sapphire, the lowest on graphene/sapphire, and an intermediate level on hexagonal boron nitride (h-BN)/sapphire. These results confirm that weak vdW interactions promote early-stage lattice relaxation. Additionally, graphene without polar bonds is found to be more effective for strain relaxation than h-BN, which has polar bonds. The insights gained from this study on the initial growth process of GaN are expected to contribute to the advancement of ultrathin, highly flexible GaN film production techniques.