Two dimensional BN an atomically thin insulator, substrate, and encapsulation layer from growth to application
Air Force Research Laboratory, Sensors Directorate, Wright Patterson AFB, OH
Since free standing graphene was found in 2004, there has been an explosion of research on atomically thin two dimensional (2D) materials based isolated sheets of layered van der Waals solids. The spectacular electrical and thermal transport properties of graphene generated a great deal of hype making it a heavily researched material for ultra-high-speed electronics; however, strong interaction with conventional 3D substrates and the lack of a band gap has proven to degrade properties and limit its usefulness in these devices. In response, a great deal of effort has gone into preserving graphene’s properties on a substrate using vdW buffer layers and forming a band gap, by doping, alloying, cutting, and electric fields, with limited success. As a consequence, attention has been steadily shifting beyond graphene to semiconducting 2D materials like MoS2 and phosphorene. However, like graphene these alternative 2D materials are all surface, so they also suffer from degraded properties due to interactions with 3D substrates, encapsulation layers, and molecules absorbed from the ambient atmosphere. 2D BN has been widely accepted as the best substrate and encapsulation layer for preserving the properties of graphene and other 2D materials. Recent, reports have shown 10x increase in graphene and 100x increase in MoS2 mobility when BN is used. These improvements are due to ability to create atomically smooth, clean, and abrupt interfaces with the BN surface. Similar in structure to graphene, sp2-bonded BN forms a honeycomb lattice with strong in-plane and weak out-of-plane bonding. In BN the symmetric covalent sp2 C-C bonds of graphene are replaced with partially ionic sp2 B-N bonds giving rises to a large direct band gap (6eV). The structural (atomically smooth and flat) and electronic (insulator) properties of BN make it ideally suited to complement graphene and other 2D materials.
At this point much of the work on 2D BN and 2D heterostructures with BN has involved the use of micron sized flakes exfoliated from bulk materials due to the limits of current thin film growth technology. In this talk I will cover some of the challenges and limitation to growing high quality 2D BN and heterostructures and describe how we have been successful overcoming a few of these issues using chemical vapor deposition (CVD). I will then present a few applications of our CVD grown BN to 2D devices and heterostructures.