Atomic-Scale Structural and Chemical Characterization of Hexagonal Boron Nitride Layers Synthesized at the Wafer-Scale with Monolayer Thickness Control

Hexagonal boron nitride (h-BN) is a promising twodimensional insulator with a large band gap and low density of charged impurities that is isostructural and isoelectronic with graphene. Here we report the chemical and atomic-scale structure of CVD-grown waferscale (∼25 cm) h-BN sheets ranging in thickness from 1 to 20 monolayers. Atomic-scale images of h-BN on Au and graphene/Au substrates obtained by scanning tunneling microscopy reveal high h-BN crystalline quality in monolayer samples. Further characterization of 1− 20 monolayer samples indicates uniform thickness for wafer-scale areas; this thickness control is a result of precise control of the precursor flow rate, deposition temperature and pressure. Raman and infrared spectroscopy indicate the presence of B−N bonds and reveal a linear dependence of thickness with growth time. X-ray photoelectron spectroscopy shows the film stoichiometry, and the B/N atom ratio in our films is 1 ± 0.6% across the range of thicknesses. Electrical current transport in metal/insulator/metal (Au/h-BN/Au) heterostructures indicates that our CVD-grown h-BN films can act as excellent tunnel barriers with a high hard-breakdown field strength. Our results suggest that large-area h-BN films are structurally, chemically and electronically uniform over the wafer scale, opening the door to pervasive application as a dielectric in layered nanoelectronic and nanophotonic heterostructures. ■ INTRODUCTION Since the isolation of graphene on an insulating substrate, a variety of other layered materials have been isolated and characterized, and have opened up an exciting field of research. However, the electronic quality of two-dimensional (2D) active layers in devices is highly sensitive to their immediate environment owing to the large surface to volume ratio. Therefore, a crystalline 2D material that can serve the role of an insulating substrate, encapsulating layer or gate dielectric is highly desirable in device applications. Hexagonal boron nitride (h-BN) has emerged as a promising material for these applications. It has been observed experimentally that encapsulating other 2D materials in h-BN not only enhances the performance of devices but also extends their durability to long time-scales. However, most previous studies have used mechanically exfoliated h-BN derived from bulk crystals, which does not allow for control over layer thickness and sample lateral size. Although numerous reports about chemical vapor deposition (CVD) synthesis of h-BN already exist ranging from low pressure CVD (LPCVD) for monolayer growth to ambient pressure CVD (APCVD) for multilayer growth, most of these approaches do not yield precise thickness control from the monolayer scale to thick multilayers over large areas. Many previous experiments have reported on use of solid ammoniaborane as the boron and nitrogen source. The sublimation of a solid source gives poor control of precursor flow rate and partial pressure in the growth chamber over large length scales. Further, monolayer to sub 2 nm growth of h-BN is a process catalyzed by the Cu surface and requires low growth pressures. However, the catalytic activity of Cu surface is diminished after Received: January 15, 2017 Revised: May 21, 2017 Published: May 22, 2017 Article