有机分子吸附和衬底调控锗烯的电子结构

Effects of organic molecule adsorption and substrate on electronic structure of germanene

锗基集成电子学的发展潜力源于其极高的载流子迁移率以及与现有的硅基和锗基半导体工业的兼容性,而锗烯微小带隙能带特点极大程度地阻碍其应用.因此,在不降低载流子迁移率的情况下,打开一个相当大的带隙是其应用于逻辑电路中首先要解决的问题.本文采用范德瓦耳斯力修正的密度泛函理论计算方法,研究了电场作用下有机分子吸附和衬底对锗烯原子结构和电学性质的影响.研究结果表明,有机分子吸附和衬底通过弱相互作用破坏了锗烯亚晶格的对称性,从而在狄拉克点上打开了相当大的带隙.苯/锗烯和六氟苯/锗烯体系均在K点打开了带隙.当使用表面完全氢化的锗烯(锗烷HGeH)衬底时,苯/锗烯/HGeH和六氟苯/锗烯/HGeH体系的带隙可进一步变宽,带隙值分别为0.152和0.105 eV.在外电场作用下,上述锗烯体系可实现大范围的近似线性可调谐带隙.更重要的是,载流子迁移率在很大程度上得以保留.本文提出了一种有效的可调控锗烯带隙的设计方法,为锗烯在场效应管和其他纳米电子学器件中的应用提供了重要的理论指导.

The development potential of germanene-based integrated electronics originates from its high carrier mobility and compatibility with the existing silicon-based and germanium-based semiconductor industry.However,the small band gap energy band(Dirac point)of germanene greatly impedes its application.Thus,it is necessary to open a sizeable band gap without reducing the carrier mobility for the application in logic circuits.In this study,the effects of organic molecule(benzene or hexafluorobenzene)adsorption and substrate on the atomic structures and electronic properties of germanene under an external electric field are investigated by using density functional theory calculations with van der Waals correction.For benzene/germanene and hexafluorobenzene/germanene systems,four different adsorption sites are considered,with the center of the organic molecules lying directly atop the upper or lower Ge atoms of germanene,in the Ge-Ge bridge center,and on the central hollow ring.Meanwhile,different molecular orientations at each adsorption site are also considered.Thus,there are eight high-symmetry adsorption configurations of the systems,respectively.According to the adsorption energy,we can determine the most stable atomic structures of the above systems.The results show that the organic molecule adsorption can induce the larger buckling height in germanene.Both the adsorption energy and interlayer distance indicate that there is no chemical bond between the organic molecules and germanene. Mulliken population analysis shows that a charge redistribution in the two sublatticesin germanene exists since benzene is an electron donor molecule and hexafluorobenzene is an electron acceptormolecule. As a result, the benzene/germanene system exhibits a relatively large band gap (0.036 eV), whilehexafluorobenzene/germanene system displays a small band gap (0.005 eV). Under external electric field,germanene with organic molecule adsorption can exhibit a wide range of linear tunable band gaps, which ismerely determined by the strength of electric field regardless of its direction. The charge transfer among organicmolecules and two sublattices in germanene gradually rises with the increasing the strength of electric field,resulting in the electron density around the sublattices in germanene unequally distributed. Thus, according tothe tight-binding model, a larger band gap at the K-point is opened. When germanane (fully hydrogenatedgermanene HGeH) substrate is applied, the band gaps further widen, where the band gap of benzene/germanene/germanane system can increase to 0.152 eV, and that of hexafluorobenzene/germanene/germananesystem can reach 0.105 eV. The sizable band gap in germanene is created due to the symmetry of twosublattices in germanene destroyed by the dual effects of organic molecule adsorption and substrate. Note thatboth of organic molecules and substrate are found to non-covalently functionalize the germanene. As thestrength of the negative electric field increases, the band gaps can be further modulated effectively. Surprisingly,the band gaps of the above systems can be closed, and reopened under a critical electric field. These features areattributed to the build-in electric field due to the interlayer charge transfer of the systems, which breaks theequivalence between the two sublattices of germanene. More importantly, the high carrier mobility ingermanene is still retained to a large extent. These results provide effective and reversible routes to engineeringthe band gap of germanene for the applications of germanene to field-effect transistor and other nanoelectronicdevices.