Atomic Scale Assessment of Graphene-Substrate Interactions, Grain Boundaries and Materials for Heterostructures
Graphene is an atomically-thin honeycomb lattice of sp2-bonded carbon atoms with a linear, low-energy band structure. Despite its exceptional electronic properties, the primary challenges to development of graphene for device applications are wafer-scale synthesis methods and graphene-substrate interactions. Chemical vapor deposition (CVD) growth of graphene on Cu foil provides one path to wafer-scale graphene. Typical graphene CVD on Cu yields rotationally misoriented graphene domains that form grain boundaries (GBs) when these domains merge. These graphene GBs strongly perturb the local graphene electronic structure.
These GBs lead to localized states and decrease the local work function, leading to p-n-p and p-p-p (p < p) potential barriers at the GBs that act scatter charge carriers. The effects of the GBs decay over a length ~ 1 nm. Graphene-substrate interactions are critical in determining key properties such as carrier mobility. Graphene deposited in UHV on GaAs(110), InAs(110), and Si(111) – 7x7 surfaces exhibits an electronic semitransparency effect in which the substrate electronic structure is observable ‘through’ the graphene by scanning tunneling microscopy (STM). The mechanical force of the STM tip leads to a reduction of the graphene-substrate spacing, which induces the observed semitransparency. Transport experiments and STM studies of graphene on hexagonal boron nitride (h-BN) show that it is an ideal substrate for graphene. However, a full understanding of the growth mechanisms for CVD growth of h-BN on Cu foil is lacking.
The chamber pressure during the growth step has a dramatic effect on the morphology, chemical structure, and growth rate of the resulting h-BN. Experiments varying the chamber pressure for h-BN synthesis clearly shows that h-BN growth by low pressure CVD (LPCVD) yields more planar, uniform h-BN than that obtained by atmospheric pressure CVD (APCVD). Understanding the perturbative effects of GBs on the electronic properties of graphene and the interactions between graphene and its substrate are critical to device development. Furthermore, understanding the role of pressure in the CVD growth of h-BN will further the development of flexible graphene and transition metal dichalcogenide-based electronics and enable the growth of their heterostructured combinations.