二维六方氮化硼的制备及其光电子器件研究进展(特邀)

Research Progress on Fabrication of Two-dimensional Hexagonal Boron Nitride and Its Optoelectronic Devices(Invited)

六方氮化硼是一种具有宽带隙和二维层状结构的代表性材料,拥有优异的物理化学稳定性和高热导率等独特特性。其表面原子级平整、无悬挂键和带电杂质,被公认为是作为其它低维材料衬底的理想选择。另一方面,六方氮化硼对深紫外波段的强吸收和中红外波段的双曲特性,使其成为了制备高性能深紫外和中红外探测器的重要材料。本文首先介绍了六方氮化硼的晶体结构和材料特性,然后将材料制备方法分为“自上而下”和“自下而上”两类,系统阐述了六方氮化硼的制备现状,接着从衬底/介质层/钝化层、隧穿层和吸收层三方面重点回顾了六方氮化硼在光电子器件领域的研究进展。最后,基于六方氮化硼的研究现状,分析了其所面临的一些挑战和机遇。

Boron Nitride(BN)isⅢ-Ⅴbinary compound crystal material formed by B and N atoms in 1∶1 stoichiometric ratio,which primarily comprises four major crystallographic structures:wurtzite BN(w-BN),cubic BN(c-BN),rhombohedral BN(r-BN),and hexagonal BN(h-BN).In w-BN and c-BN,the B and N atoms undergo sp3 hybridization to form tetrahedral structures.Conversely,in r-BN and h-BN,these atoms exhibit sp2 hybridization within the plane,resulting in the formation of hexagonal ring structures.These structures are similar to those of graphene,with layers stacked upon each other through weak van der Waals(vdWs)forces,forming bulk materials.The spacing between the basal planes and the in-plane lattice constants of the two crystal structures are identical,with the difference being in the stacking order of the basal planes.h-BN is a representative material with wide band gap and twodimensional(2D)layered structure,possessing unique properties such as excellent physicochemical stability and high thermal conductivity.It is considered as a novel 2D functional material with significant application potential in electronics,photonics,energy catalysis,and surface protection.Controllable preparation of large-area,high-quality h-BN is a challenging and active research direction.In this paper,recent research results are presented by categorizing the main ways of preparing h-BN into two broad categories:“top-down”exfoliation and“bottom-up”growth methods.The“top-down”method refers to the exfoliation of large-sized or bulk materials into monolayers to nanosheets ranging from monolayers to layers by different material preparation methods,such as mechanical exfoliation,liquid-phase exfoliation,and electrochemical exfoliation.In this process,the interlayer vdWs forces within the 2D materials are primarily disrupted by the application of external forces.The“bottom-up”method refers to the direct preparation of nanosheets by chemical synthesis using atoms,ions or molecules,which is mainly implemented by chemical vapor deposition,physical vapor deposition and molecular beam epitaxy.For h-BN,on the one hand,controlling nucleation presents a certain level of difficulty,making it challenging to grow large single crystals from individual nuclei.On the other hand,the crystal structure of h-BN exhibits three-fold symmetry,leading to the easy formation of antiparallel structures and twins during epitaxial growth on most substrates.Currently,chemical vapor deposition stands as the predominant method for growing large-area,high-quality h-BN.Ongoing research into the development of new technologies for the controllable production of large area,high quality materials is key to promoting the industrialization and application of h-BN.The aim is to improve yield while ensuring structural integrity and performance consistency of the target materials,and to precisely control the size,number of layers and other parameters of the materials to meet the needs of different application scenarios.Following the study of the structure,the properties and the synthetic preparation of the 2D h-BN,this paper reviews the research progress of h-BN in optoelectronic devices by focusing on three aspects,namely,substrate/dielectric layer,tunneling layer and absorber layer.Firstly,h-BN possesses unique intrinsic properties such as a wide bandgap,atomically smooth surface,absence of dangling bonds and surface trap states,and chemical inertness.These outstanding properties make h-BN highly suitable for serving as key functional layers such as gate dielectrics,passivation layers,and substrates in electronic and optoelectronic devices.Secondly,the wide band gap and large electron affinity of h-BN result in the formation of a higher tunneling potential barrier in heterostructures,which can effectively suppress the occurrence of direct tunneling current,thereby enhancing the on-state resistance of the devices.Several research results show that the transport mechanism is dominated by Fowler-Nordheim(FN)tunneling when thin layer of h-BN is used as the tunneling layer.In addition,h-BN is an indirect bandgap material,but due to its unique flat electronic band structure,2D h-BN exhibits distinctive properties in light-matter interactions.The highly localized electrons in h-BN result in a higher effective electron density involved in the Ultraviolet(UV)light absorption process,giving 2D h-BN an inherent UV light response capability.h-BN also has natural hyperbolic properties in the mid-infrared wavelength range.The hyperbolicity gives rise to a unique physical property,i.e.,the sign of the dielectric constant along the in-plane direction is opposite to that of the dielectric constant along the out-of-plane direction.The directional propagation of hyperbolic phonon polaritons is confined within sub-wavelength dimensions,similar to behavior in metals.Based on this inherent structural advantage in the mid-infrared,h-BN has become a potential material for detecting mid-infrared signals.Large-area,high-quality,low-cost material synthesis,damage-free transfer onto any substrates,and compatibility with traditional CMOS processes represent common challenges that 2D materials encounter on the path to industrialization,which are equally true for h-BN.Furthermore,direct growth of additional 2D materials on h-BN offers potential for significant process simplification and enhanced device performance.Finally,this paper delves into the prevailing research landscape of h-BN,highlighting the challenges and opportunities it faces.