High-Temperature Molecular Beam Epitaxy of Hexagonal Boron Nitride and hBN-Based Lateral Heterostructures

There has been a surge of interest in hexagonal boron nitride (hBN) due to its technological potential for deep ultraviolet (DUV) photonics, single photon emitters (SPEs) and through its incorporation into van der Waals (vdW) two-dimensional (2D) heterostructures as either a substrate, tunnel barrier or capping layer.<br/><br/>We have developed high-temperature molecular beam epitaxy (HT-MBE) of hexagonal boron nitride with the thicknesses up to 70 nm at growth temperatures from 1100^oC to 1700^oC using high-temperature sublimation and e-beam MBE sources for boron and nitrogen RF-plasma sources. By growing hBN on highly oriented pyrolytic graphite (HOPG) substrates, we have produced monolayer and few-layer thick boron nitride with atomically flat hBN surfaces, which are essential for 2D and DUV applications. The hBN coverage can be reproducibly controlled by the growth time, substrate temperature and boron to nitrogen flux ratios. We will discuss our recent measurements of a direct optical energy gap of ~6.1 eV [1] and electronic band gap of ~6.8 eV [2] in single monolayer hBN.<br/><br/>We will present data demonstrating that the single-photon emitters in hBN are related to carbon (C) incorporation [3] and will discuss different C-doping techniques that we are currently using in HT-MBE.<br/><br/>In addition to the conventional hBN–based vertical vdW heterostructures, recent studies worldwide have focused on the development of novel 2D lateral heterostructures. These vdW lateral heterostructures consist of multiple connected monolayer-thick materials within the same atomic plane and can have novel and unique transport and optical properties. Integration of graphene and hBN in lateral heterostructures can provide a route to engineer the material properties by quantum confinement of electrons or introduction of novel electronic and magnetic states at the interface. Whereas vertical 2D heterostructures can be produced by epitaxy or by exfoliating and stacking of 2D layers, lateral 2D heterostructures can only feasibly be produced by an epitaxial growth process.<br/><br/>We have demonstrated that lateral heterojunctions of hBN and graphene can be grown <i>in-situ</i> using HT-MBE. We used different types of the MBE carbon sources, including a sublimation carbon source, atomic carbon source and e-beam carbon source, to grow graphene monolayers on hBN substrates and hBN epitaxial layers at temperatures between 1000^oC and 1700^oC.<br/><br/>We will discuss HT-MBE of graphene monolayers in crystallographically oriented hBN trenches, formed <i>ex-situ</i> by catalytic Ni-nanoparticle etching. High-resolution atomic force microscopy (AFM) reveals that graphene monolayers grow epitaxially from the hBN etched trench edges, and merge to form a graphene nanoribbon network.<br/><br/>We will also discuss <i>in-situ</i> HT-MBE of monolayer-thick lateral hBN-graphene heterostructures. Monolayer-thick hBN, grown at a higher temperature of ~1400^oC, exhibited armchair edges, minimal aggregate presence, and a reduced but significant width of the hBN monolayer sheet. We have shown that in HT-MBE graphene grows preferentially laterally from the edges of hBN in a mode of step-flow growth, which results in the formation of lateral hBN-graphene heterojunctions. The graphene and hBN regions show a clear epitaxial relationship through the alignment of atomic lattices. Sequential HT-MBE growth of hBN, graphene and a second cycle of hBN growth resulted in the formation of monolayer-thick lateral hBN–graphene–hBN heterostructures, in which a strip of graphene is laterally embedded between monolayers of hBN.<br/><br/><b>References</b><br/>[1] C. Elias et al., <i>Nat. Commun.</i> <b>10</b> 2639 (2019).<br/>[2] R.J.P. Roman et al., <i>2D Mater.</i> <b>8</b> 044001 (2021).<br/>[3] N. Mendelson et al., <i>Nat. Mater.</i> <b>20</b> 321 (2020).