掺杂对金属-MoS2界面性质调制的第一性原理研究

First principles investigation of the tuning in metal-MoS_2 interface induced by doping

采用基于密度泛函理论的第一性原理赝势平面波方法,计算了卤族元素掺杂对金属-MoS_2界面性质的影响,包括缺陷形成能、电子能带结构、差分电荷密度以及电荷布居分布.计算结果表明:卤族元素原子倾向于占据单层MoS_2表面的S原子位置;对于单层MoS_2而言,卤族元素的掺杂将在禁带中引入杂质能级以及导致费米能级位置的移动.对于金属-MoS_2界面体系,结合Schottky-Mott模型,证明了卤族元素的掺杂可以有效地调制金属-MoS_2界面间的肖特基势垒高度.发现F和Cl原子的掺杂将会降低体系的肖特基势垒高度.相比之下,Br和I原子的掺杂却增大了体系的肖特基势垒高度.通过差分电荷密度和布居分布的分析,阐明了肖特基势垒高度的被调制是因为电荷转移形成的界面偶极矩的作用导致.研究结果解释了相关实验现象,并给二维材料的器件化应用提供了调节手段.

Two-dimensional （2D） materials have shown great potential for electronic and optoelectronic applications. Among the 2D materials, molybdenum disulfide （MoS2） has received great attention in the transition metal dichalcogenides family. Unlike graphene, 2D MoS2 can exhibit semiconducting properties and its band gap is tunable with thickness. A demonstration of a single-layer MoS2 based field-effect transistor （FET） with a high on/off current ratio （about 10s） has aroused the considerable interest. Although 2D MoS2 exhibits fascinating intrinsic properties for electronics, the contact may limit the device performance severely. In a real device such as FET, semiconducting 2D MoS2 needs contact with a metal electrode, and a Schottky barrier is always formed at the semiconductor-metal interface. The formation of low-resistance contact is a challenge, which is important for achieving high ＂on＂ current, large photoresponse and high-frequency operation. Therefore, understanding and tuning the interfaces formed between metals and 2D MoS2 is critical to controlling the contact resistance. In this work, some efforts have been made to investigate the 2D MoS2- metal interface in order to reduce the Schottky barrier height. By using the first-principles calculations based on density function theory, we investigate the effects of halogen doping-on metal-MoS2 interface, including the formation energy of defect, electronic structure, charge difference, and population. All calculations are performed using the ultrasoft pseudopotential plane wave method implemented in the CASTEP code. We use the generalized gradient approximation for the exchange and correlation potential as proposed by Perdew-Burke-Ernzerhof. Firstly, we calculate the formation energy to find the thermodynamically stable positions for the halogen elements located in 2D MoS2. It is shown that the halogen elements tend to occupy the S site of a MoS2 monolayer. Meanwhile, for the MoS2 monolayer, the halogen doping may introduce the defect level into the forbidden gap and make the Fermi level shift. For the metal-MoS2 interface, halogen doping can modulate its Schottky barrier height effectively in terms of Schottky-Mott model. This is because the Schottky barrier height at the metal-semiconductor interface depends on the difference between the Fermi level and the band edge position of the semiconductor. At the metal-MoS2 interface, the Fermi level is partially pinned as a result of the interface dipole formation and the production of the gap states. Therefore, using different metals with different work functions cannot modify the Schottky barrier height effectively. Here we demonstrate that F and Cl doping can reduce the Schottky barrier height, while Br and I doping can increase it. According to the results of the differential charge density analysis, we can ascribe the tuning of Schottky barrier height to the influence of the dipole caused by the charge transfer among t^le interfaces. This study can explain the relevant experimental results very well and provide a potential route to achieving low-resistance contact in the future applications of 2D materials.