第ⅥA主族元素掺杂Mg-MoS_(2)异质结势垒高度调控

Barrier height control of the Mg-MoS_(2) heterojunction doped with group VIA elements

在纳米尺寸的薄膜场效应晶体管中,源极、漏极(金属材料)与有源层(半导体材料)之间的肖特基势垒是制约器件发展的关键因素之一.本文采用密度泛函理论的第一性原理赝势平面波方法,通过第Ⅵ主族元素对二硫化钼(MoS_(2))的硫原子进行替位掺杂,电子结构分析表明,氧的替位掺杂可以显著降低MoS_(2)的带隙值.选择功函数值较低的金属Mg,构建氧掺杂Mg-MoS_(2)异质结,研究发现,界面位置的氧掺杂可以使该异质结由肖特基接触变为欧姆接触.分析结果表明,欧姆接触的形成原因主要来自3个方面:(1)氧的掺杂增大了MoS_(2)的电子亲合能;(2)未掺杂的Mg-MoS_(2)异质结禁带中存在金属诱导间隙态,使费米能级被钉扎在禁带中靠近导带底的位置,界面氧掺杂降低了金属诱导间隙态在费米能级附近的强度,使费米能级的钉扎效应减弱而进入导带;(3)界面氧掺杂时,界面电荷转移减少,电偶极矩对Mg-MoS_(2)异质结相对能级改变的影响减小.本文的研究结果为金属-半导体界面的肖特基势垒高度调控提供了一定的理论指导.

Because of its excellent optical and electrical properties,monolayer MoS_(2)has great potential for thin-film transistor applications.However,as the device size continues to shrink,the Schottky barrier between the metal and the monolayer MoS_(2)becomes the main factor limiting the device current.Therefore,the Schottky barrier at the metal-monolayer MoS_(2)interface must be reduced.Doping is a primary way of adjusting the Schottky barrier height.Considering that O,Se,Te,and S are group VIA elements,if the S atom is replaced by another group VIA atom,an impurity level will not be introduced into the band gap.This result benefits Schottky barrier height adjustment.In this paper,on the basis of the first-principles pseudopotential plane-wave method of density functional theory,the electronic structure of S atoms in doped MoS_(2)substituted by group VIA elements is investigated.The results show that compared with selenium and tellurium doping,the doping of O reduces the band gap of MoS_(2),mainly because of the larger difference in atomic radius between oxygen and sulfur,resulting in a more severe lattice distortion.On this basis,Mg with a lower work function value was selected to construct an O-doped Mg-MoS_(2)heterojunction structure.O-doping at the interface position is found to form ohmic contact,while doping at non-interface positions enlarges the Schottky barrier.An analysis of the state density and difference charge density shows that ohmic contact formation occurs mainly for three reasons:First,the doping of oxygen increases the electron affinity of MoS_(2);second,the presence of metal-induced gap states(MIGSs)pins the Fermi level in the forbidden band near the bottom of the conduction band,and when interfacial O is doped,the strength of MIGSs near the Fermi level can be reduced,weakening the pinning effect;third,the reduction of the interfacial charge transfer reduces the effect of the electric dipole moment on the relative energy level change when Mg and MoS_(2)are in contact.The results of this study contribute to a more comprehensive understanding of the electronic structure of the heterojunction interface formed by the contact between two-dimensional materials and metals and provide a theoretical basis for the subsequent device design and optimization of two-dimensional materials.