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

六方氮化硼(HBN)因其在深紫外光(UV)光子学、单光子发射以及将其引入范德华(VDW)异质结中的技术潜力而受到国际关注。HBN是一种层状材料，其中硼和氮原子之间的强烈共价键稳定了平面蜂窝原子排列。在其大块形式中，hBN由许多这样的平面相互堆叠组成，并且，像石墨一样，厚度小到单一单层的hBN层可以从大块晶体中剥离。已经有许多实验表明，剥离的hBN层可以与其他层状材料结合，例如石墨烯，形成‘van der Waals异质结’，其中hBN起隧道势垒、衬底或栅电介质的作用。对hBN的兴趣促使许多团队使用各种技术探索hBN薄膜和单层的生长，但事实证明，很难再现由我们的项目合作伙伴在日本筑波生长的最高质量的毫米级块状hBN晶体的光学和电学性能。在最近的一项突破中，我们证明了可以使用高温分子束外延(HT-MBE)生长hBN，并且使用该技术生长的薄膜具有前所未有的光学质量，在深紫外区具有强烈的发光，单分子层厚度的光子能量为6.08 eV。这种高光子能量为固态设备提供了在UV-C范围内发光的前景，众所周知，这与水净化和表面杀菌有关。在与澳大利亚学者的合作中，我们还证明了我们的hBN材料中可以形成单光子发射体。此外，我们还展示了石墨烯和氮化硼的横向异质结构的生长，其中组成在一个单分子层内变化。这些结构被预测具有新颖的电子和磁性。为了在我们有希望的早期成果的基础上再接再厉，实现hBN的技术潜力，我们现在提议加深我们对相关生长机制的理解，并通过我们在诺丁汉和我们的国际合作网络，探索高质量hBN单层和薄膜提供的技术机会。我们的假设是，HT-MBE为高质量hBN层的可伸缩生长提供了一条途径，在深紫外光子学、单光子源和VDW异质结领域具有潜在的技术开发潜力，以及对hBN边态和横向异质结的电子性质的探索。在我们的研究计划中，我们将调查和优化hBN的高温分子束外延生长。此外，我们还将探索hBN的掺杂和简单光电子器件的形成，以及hBN基BNC和BNSi合金的生长，作为自发形成相分离纳米结构和带隙工程的途径。此外，我们还将建立生长参数与hBN中碳诱导单光子发射体的形成之间的关系。为了确定高温分子束外延生长的hBN在深紫外光子学方面的潜力，我们将制作工作在UV-C波长的原型器件。我们还将利用外延hBN来研究横向hBN/石墨烯异质结的形成和结构，并研究由于电子-电子相互作用而在这些结构中出现的新的电子和磁效应。另一个重要的目标是展示hBN在商业上可获得的大面积衬底上的可扩展生长，例如蓝宝石和碳化硅，以便hBN层与工业上广泛使用的工艺和制造技术兼容。

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.