Exceptional Thermochemical Stability of Graphene/N-Polar GaN for Remote Epitaxy for Exfoliation and Device Fabrication

Graphene (Gr) has drawn great attention as an epitaxial template for production of single-crystalline, freestanding semiconductor membranes through remote epitaxy. This emerging epitaxy facilitates the growth of single-crystalline epi-layer membranes, whose crystallographic properties follow to that of the underlying wafer across a Gr gap layer.[1] The lattice transparency of Gr allows the overlying layer to resemble the crystallinity of the mother substrate while permitting the effortless release of the grown film using sticky tape- and/or metal stressor-assisted exfoliation techniques, a result of non-chemical bonds at the interface.[1,2] Notably, the mother substrate can be reused for other subsequent epitaxial growths, given the damage- and contamination-free exfoliation, which is contrast to the damage and performance degradation caused by laser/chemical lift-off. Various material systems, such as compound semiconductors, complex oxides, halide perovskites, etc., have been extensively studied for remote epitaxy.[3] However, metal–organic vapor deposition (MOCVD)-based GaN remote homoepitaxy remains unaccomplished due to the poor thermochemical stability of graphene/Ga-polar GaN substrates.[4] III–nitrides (i.e., GaN, AlN, and AlGaN) are conventionally grown using MOCVD at high temperatures under a hydrogen atmosphere for commercial purposes. However, the harsh conditions required for MOCVD growth of III–nitride potentially lead to thermochemical decomposition of graphene and underlying substrate, making it difficult to take the full advantages of remote epitaxy. Thus, the remote homoepitaxy of GaN and successful exfoliation of GaN overlayer have remained as challenges.<br/><br/>In this presentation, we report the N-polar remote homoepitaxy of GaN using single-layer graphene (SLG). MOCVD yields N-polar micro-crystals (μCs) and Ga-polar thin films are yielded on graphene-coated N- and Ga-polar substrates, respectively. However, only the μCs overlayer is gently separated from the mother substrate via the adhesive tape-assisted exfoliation method, confirming the successful remote homoepitaxy. We investigate the stability of GaN substrates of Ga- and N-polarity for remote epitaxy and demonstrate only the SLG-coated N-polar GaN substrates allow the remote epitaxy. The N-polar remote homoepitaxy is confirmed by Raman spectroscopy, electron microscopy analyses, and overlayer delamination tests. The successful remote epitaxy is contingent on SLG’s stability, correlated with the decomposition of GaN substrates. Complementary molecular dynamics (MD) simulations and annealing experiments confirm that the thermochemical stability of graphene is dependent on the polarity of GaN due to varying hydrogen reaction behaviors at high temperatures. Our findings elucidate why graphene’s thermochemical stability varies depending on substrate polarity and present an opportunity to prepare N-polar GaN that is releasable from the substrate through MOCVD-based remote epitaxy. Lastly, we apply the successful N-polar remote homoepitaxy of μCs to grow flexible light-emitting diodes (LEDs), consisting of p–n junction μCs with InGaN heterostructures for fabricating deformable LEDs.<br/><br/><b>References</b><br/>(1) Kim, Y. et al. Remote epitaxy through graphene enables two-dimensional material-based layer transfer. <i>Nature </i><b>2017</b>, <i>544</i>, 340.<br/>(2) Jeong, J. et al. Remote heteroepitaxy of GaN microrod heterostructures for deformable light-emitting diodes and wafer recycle. <i>Sci. Adv. </i><b>2020</b>, <i>6</i>, eaaz5180.<br/>(3) Kim, H. et al. Remote epitaxy. <i>Nat. Rev. Methods Primers </i><b>2022</b>, <i>2</i>, 40.<br/>(4) Park, J. H. et al. Influence of Temperature-Dependent Substrate Decomposition on Graphene for Separable GaN Growth. <i>Adv. Mater. Interfaces </i><b>2019</b>, <i>6</i>, 1900821.