  Hexagonal boron nitride (h-BN) is a promising two-dimensional 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 wafer-scale (~25 cm2) h-BN sheets ranging in thickness
from 1-20 monolayers. Atomic-scale images of h-BN on Au and graphene/Au
substrates obtained by scanning tunneling microscopy (STM) 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 (XPS) 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.
