van der Waals Epitaxy of Mechanically Separable AlN with Enhanced Crystalline Quality by Using a Double-Buffer-Layer Insertion

Aluminum nitride (AlN) stands as a cornerstone material in modern electronic and deep ultraviolet optoelectronic devices. However, the lack of homogeneous substrate makes the heteroepitaxy on foreign substrates mainstream but poses significant challenges in lattice and thermal mismatch. The van der Waals epitaxy (vdWE) of AlN on graphene essentially solves these challenges via the weak interfacial adhesion, while the structural destruction of graphene for improving the AlN nucleation results in the strong interfacial interaction and difficulty for epilayer separation. Here, a double-buffer-layer insertion is designed for the vdWE of AlN, in which the AlN epilayer is featured with enhanced crystalline quality and mechanically separable ability. The optimal growth procedure achieves a coalescent AlN surface with a low roughness of 0.77 nm, and the crystalline quality is enhanced by 33.8% with an extremely low stress of 0.11 GPa. Since the graphene is intactly separated with the AlN epilayer, the flexible photodetector based on the graphene–AlN junction is fabricated, showing a responsivity of 65.9 A W^–1, and it could maintain 61.9% of its initial performance as the bending curvature radius is 3.09 cm. The present strategy for AlN vdWE not only optimizes its nucleation characteristics but also promotes flexible applications for advanced devices.