Large-Area Hexagonal Boron Nitride Layers by Chemical Vapor Deposition: Growth and Applications for Substrates, Encapsulation, and Membranes

Abstract

ConspectusHexagonal boron nitride (hBN) has emerged as a promising two-dimensional (2D) material because of its unique optical properties in the deep-UV region, mechanical robustness, thermal stability, and chemical inertness. Ultrathin hBN layers have gained significant scientific attention for various applications, including nanoelectronics, photonics, single photon emission, anticorrosion, and membranes. For example, the carrier mobility of graphene- and MoS2-based transistors can be improved by using an hBN encapsulation layer, which protects graphene or MoS2 from air and/or screens the charge trap site of a substrate. Moreover, deep-UV emitters and detectors have been developed on the basis of the large bandgap of hBN (∼6 eV), which exhibits a sharp absorption at approximately 200 nm. Additionally, oxidation of metal surfaces can be prevented by hBN encapsulation, and proton transport can be facilitated by hBN membranes with low gas permeability. Wafer-scale growth of hBN films is crucial to enable their industrial-scale applications. In this regard, chemical vapor deposition (CVD) is a promising method in which scalable high-quality films can be grown at reasonable cost. To date, considerable efforts have been made to develop continuous hBN thin films with high crystallinity, from those with large grains to single-crystal ones, and to realize thickness control of hBN films by CVD. However, the growth of wafer-scale high-crystallinity hBN films with precise thickness control has not been reported yet. The hBN growth is significantly affected by the substrate, in particular the type of metals, because the intrinsic solubilities of boron and nitrogen depend on the type of metal; moreover, control of the grain size and thickness of hBN is difficult. Although growth mechanisms for various substrates have been proposed using various control experiments, a precise growth mechanism has not yet been established through systematic studies. Thus, a deeper understanding of the CVD-based growth of hBN is critical.In this Account, state-of-the-art strategies adopted for growing wafer-scale, highly crystalline hBN are summarized, followed by the proposed mechanisms of hBN growth on catalytic substrates. Furthermore, various applications of the hBN thin films fabricated in our laboratory are demonstrated, including a dielectric layer, an encapsulation layer, a wrapping layer of gold nanoparticles for surface enhanced Raman scattering, a proton-exchange membrane, a template for growth of other 2D materials or nanomaterials, and a platform of fabricating in-plane heterostructures. Finally, the inherent challenges are summarized, and future research directions for the facile CVD-based growth of single-crystal hBN are proposed, including the development of a transfer method that is effective for various applications.