Growth of hexagonal boron nitride for deep ultraviolet photonics, quantum emitters and van der Waals substrates

Lead Research Organisation: University of Nottingham
Department Name: Sch of Physics & Astronomy

Abstract

Hexagonal boron nitride (hBN) is currently attracting international attention due to its technological potential for deep ultraviolet (UV) photonics, single photon emission and its incorporation into van der Waals (vdW) heterostructures. hBN is a layered material in which strong covalent bonds between boron and nitrogen atoms stabilise a planar honeycomb atomic arrangement. In its bulk form hBN consists of many such planes stacked on top of each other, and, like graphite, layers of hBN with thickness down to a single monolayer can be exfoliated from bulk crystals. There have been many demonstrations showing that exfoliated hBN layers can be combined with other layered materials, for example graphene, to form 'van der Waals heterostructures' in which hBN acts as a tunnel barrier, substrate or gate dielectric.

The interest in hBN has motivated many groups to explore the growth of thin films and monolayers of hBN using various techniques, but it has proved difficult to reproduce the optical and electrical properties of the highest-quality mm-scale bulk hBN crystals, which are grown by our Project Partners in Tsukuba (Japan). In a recent breakthrough, we demonstrated that hBN can be grown using high-temperature molecular beam epitaxy (HT-MBE) and that layers grown using this technique have unprecedented optical quality with strong luminescence in the deep UV region with a photon energy, for monolayer thickness, of 6.08 eV. This high photon energy offers the prospect of solid-state devices emitting light in the UV-C range, which is known to be relevant to water purification and surface sterilisation. In collaboration with Australian academics, we have also shown that single photon emitters can be formed in our hBN material. In addition, we have demonstrated the growth of lateral heterostructures of graphene and boron nitride, in which the composition varies within a single monolayer. These structures are predicted to have novel electronic and magnetic properties.

In order to build on our promising early results and realise the technological potential of hBN, we now propose to advance our understanding of the relevant growth mechanisms and explore, both in Nottingham and through our network of international collaborations, the technological opportunities provided by high quality hBN monolayers and thin films. Our hypothesis is that HT-MBE provides a route to the scalable growth of high-quality hBN layers, which have the potential for technological exploitation in the areas of deep UV photonics, single photon sources and vdW heterostructures, as well as the exploration of the electronic properties of hBN edge states and lateral heterojunctions.

In our research programme we will investigate and optimise HT-MBE growth of hBN. In addition, we will explore doping of hBN and the formation of simple optoelectronic devices, as well as the growth of hBN-based alloys of BNC and BNSi as a route to the spontaneous formation of phase separated nanostructures and band gap engineering. In addition, we will establish the relationship between growth parameters and the formation of carbon-induced single photon emitters in hBN. To determine the potential of HT-MBE-grown hBN for deep UV photonics, we will fabricate prototype devices operating at UV-C wavelengths. We will also utilise epitaxial hBN to study the formation and structure of lateral hBN/graphene heterojunctions and investigate the emergence of novel electronic and magnetic effects in these structures due to electron-electron interactions. An important further objective is to demonstrate the scalable growth of hBN on large area substrates, which are commercially available, for example sapphire and silicon carbide, so that the hBN layers are compatible with processing and fabrication techniques, which are used widely in industry.
 
Description We are investigating high-temperature molecular beam epitaxy (HT-MBE) as a route to the scalable growth of high-quality hexagonal boron nitride (hBN) layers, which have the potential for technological exploitation in the areas of deep ultra-violet (UV) photonics, single photon sources and van der Waals (vdW) heterostructures, as well as the exploration of the electronic properties of hBN edge states and lateral heterojunctions formed with graphene. An overarching objective is to improve our understanding and control of the growth process of hBN and extend our current approach to include epitaxy on technologically-relevant wafers (up to 3 inch diameter), including substrates such as sapphire and silicon carbide. A further requirement for the technological exploitation of hBN is the introduction of dopants and we will investigate the use of group IV and other elements for this application. This also includes the possibility of hBN band structure engineering, through alloy formation, and novel in-plane heterostructures, through segregation of materials within one atomically thick layer. Our research programme combines an investigation of the hBN growth process itself with an exploration of the functional properties of the grown material with the overall aim of determining the inter-relationship between, growth, structure and function (electronic, optical and magnetic properties). We have investigated and optimised HT-MBE growth of hBN. We have explored hBN doping with carbon (C) during HT-MBE growth of BCN alloys focussing on: bandgap engineering; phase separation and carbon-induced single photon emitters. We have utilised epitaxial hBN and graphene layers in vdW heterostructures and study the formation, structure and electronic properties of monolayer-thick
lateral hBN/graphene heterojunctions.
Exploitation Route Hexagonal boron nitride have the potential for technological exploitation in the areas of deep ultra-violet (UV) photonics, single photon sources and van der Waals (vdW) heterostructures, as well as the monolayer-thick lateral heterojunctions.
Sectors Education

Electronics

Energy

 
Description Our project has demonstrated for the first time worldwide the potential and capabilities of HT-MBE for epitaxy of high-quality graphene and hBN layers. It has formed the base for the further exploration of the HT-MBE technology for epitaxy of graphene, hBN and other materials by several research groups around the world. In our current project we are actively using developed HT-MBE for the doping of hBN layers and for the growth of hBN-graphene lateral heterostructures.
Sector Education,Electronics,Energy
Impact Types Economic