# 摘要

In this Letter, we use first-principles simulations to demonstrate the absence of Fermi-level pinning when graphene is in contact with transition metal dichalcogenides (TMDs). We find that formation of either an ohmic or Schottky contact is possible. Then we show that, due to the shallow density of states around its Fermi level, the work function of graphene can be tuned by ion adsorption. Finally we combine work function tuning of graphene and an ideal contact between graphene and TMDs to propose an ionic barristor design that can tune the work function of graphene with a much wider margin than current barristor designs, achieving a dynamic switching among p-type ohmic contact, Schottky contact, and n-type ohmic contact in one device.

# 前言

Single layered transition-metal dichalcogenides (TMDs) have received great research attention because of their outstanding mechanical and electrical properties which show great potential in transistor engineering.[1,2] However, recent device studies have found that real devices behaviors are far inferior to the theoretical expectations.[1,3,4] One of the major limiting aspects of the device performance is the contact between metal and the TMD, which is very sensitive to scaling.[4] It has been confirmed by both experimental[5−8] and theoretical[4,9−11] studies that a partial Fermi level pinning exists between the metal and the TMD, which will always pin the work function of metals to the gap states of TMDs induced by a metal-TMD interaction, resulting in high contact resistance and inability to establish p-type contact.

Previous theoretical studies have revealed the cause of the partial Fermi level pinning.[9,10] Apart from extrinsic and material imperfection (intrinsic defects) reasons, the cause of such partial Fermi level pinning lies in the charge transfer and chemical bonding at the interface resulting in metal work function modification and interface gap state formation. The high reactivity of the dangling bonds on metal surface causes strong orbital overlapping and hybridization between states of chalcogen and metal atoms on the surface, resulting in semicovalent or covalent bonds.[9]

Graphene, another 2D material in the spotlight, possesses semimetallic electric properties and molecular mechanical properties.[12,13] Graphene has strong bonding in only two dimensions, which behaves similarly to the TMDs family. In absence of interlayer dangling bonds, graphene can form weak interlayer bonding with TMD compared to metal, thus providing a compatible and therefore less reactive interface. If this assumption is true, the Fermi level of graphene will not be modified by the contact and thus partial Fermi level pinning will not happen. In this case, as the work function of graphene is above the conduction band edge of group 4 TMD such as ZrS2,[14] an n-type ohmic contact should be observed for the graphene−ZrS2 interface. As the Fermi level of graphene lies in the middle of the band gap for both MoS2 and WSe2, they will form a Schottky contact when brought into contact.

The “barristor” is a novel device structure that makes use of the Schottky contact.[15] Different from traditional Schottky field effect transistors, which fix the Schottky barrier height and function by tuning the thickness of the Schottky barrier and hence controlling tunneling current,[16] barristors change the Schottky barrier height to achieve logic functionality. Graphene, being a single layered material, has a far lower density of states compared to metals. Thus, the graphene work function is very susceptible to a wide variety of tuning methods[12,13] and is typically chosen to play the metal contact role in barristor designs. The tuning methods include static field effect,[15,17] ionic liquid polarization,[18] and so forth. The tuning margin of current barristor designs is about $0.2 \rm{eV}$.

“势垒晶体管（barristor）”是一种利用肖特基接触的新型器件结构[15]。不同于传统肖特基场效应晶体管，即通过固定肖特基势垒的高度，调节肖特基势垒的厚度从而控制隧穿电流[16]，势垒晶体管通过改变肖特基势垒的高度来实现逻辑功能。石墨烯是一种单层材料，其态密度远低于金属。石墨烯功函数非常容易受到各种调谐方法的影响[12,13]，因此，在设计势垒晶体管时通常选择石墨烯作为金属接触。调节方法包括静态场效应[15,17]、离子液体极化[18]等。当前势垒晶体管设计的调节极限大约为 $0.2 \rm{eV}$。

In this paper we use first-principles calculations with density functional theory to study the contact behavior of graphene and TMD, in which we show that no gap state is formed and hence there is no Fermi level pinning between TMD and graphene. As typical semiconducting TMDs, MoS2, and WSe2 are chosen because they show the most promising potential for transistor applications and hence have been studied most actively. ZrS2 is also chosen to further demonstrate the absence of any kind of Fermi level pinning, because it is predicted to have ohmic contact with graphene.[10] Then we show that by inducing Li atoms or PF6 groups onto graphene, the work function of graphene may be tuned up and down to a very wide margin, without substantial changes to its band structure. Such a wide range can enable graphene to establish both n-type and p-type ohmic contact with a variety of semiconductors. Finally we use the graphene−TMD system as an example to propose a design of an ionic barristor. By exploiting the reversible nature of ionic adsorption and desorption, one side of graphene is used to form a high quality contact with the TMD, and the other side is used to reversibly tune the work function of graphene to establish and break an ohmic contact, realizing logic functionality.

# 结论

In conclusion, we have demonstrated separately that Fermi level pinning is absent between TMD and graphene and that ion adsorption is able to change the work function of graphene from $3.30$ to $6.18 \rm{eV}$. Then by putting them in contact, we have shown that both n-type and p-type ohmic contact can be achieved by tuning the work function of graphene with ion adsorption. Then we proposed a design of the ionic barristor, which can be switched among three states, being off (Schottky contact), n-type ohmic contact and p-type ohmic contact. This Fermi level “unpinning” phenomenon between graphene and the TMD is due to the van der Waals interaction between them. In this sense, the Fermi level of graphene is not pinned to all other families of semiconductors that interact only weakly with graphene, in addition to the TMDs. This predicted ohmic contact between metals and semiconductors in contact with only van der Waals interaction is subject to experimental demonstration and its applications are open to a limitless possibility of creative device designs.