(Invited) Vertical Field Effect Transistor Based on Graphene/Transition Metal Dichalcogenide Van Der Waals Heterostructure

Abstract

Owing to the absence of a band gap, the direct applicability of graphene for transistors and logic devices is still limited. Recent studies have shown that the van der Waals heterostructure of graphene and other two-dimensional (2D) materials can be a great candidate to overcome these issues. The layered structure of these materials exhibits no direct chemical bonding between the different layers. Thus the vdW junction between these materials is, in principle, free from lattice mismatch issues and interface reactions. With these advantages, these vdW heterostructures has been demonstrated vertical field effect transistor (FET) operations as well as large current ON-OFF ratio [1]. However, realization of a large ON-OFF ratio simultaneously with a large ON current density has been still challenging. Here we fabricated exfoliated-graphene/MoS2/Ti vertical heterostructure including vdW interface built from freshly cleaved surface of graphene and MoS2 by using mechanical exfoliation and dry transfer method (Fig. 1(a)). The vertical transport of the device measured under the modulation of back gate voltage V G exhibits large modulation of its conductance (Fig. 1(b)). We obtained large current ON-OFF ratio of 105 as well as large ON current density of 104 A/cm2 [2]. Such large current modulation is attributed to the modulation of the Schottky barrier at graphene/MoS2 interface as schematically depicted in the right panel of Fig. 1(b). At the V G = +50 V, the Fermi level of the graphene is highest; therefore the Schottky barrier height at graphene/MoS2 junction is lowest. The reduction of Schottky barrier height makes device to be low resistance ON state. On the contrary, at the V G= -50 V, the Fermi level of the graphene is lowest; therefore the Schottky barrier height is highest. Here current flow in the device is greatly suppressed and works as OFF state. The importance of Schottky-barrier modulation on the performance of the transistor could be confirmed by the plot between the current ON-OFF ratio ION/IOFF vs. modulation of barrier height Δφ=φ max-φ min; these data are obtained from the series of devices with different thickness of MoS2 (Fig. 1(c)); results reveal correlation between two values [3]. These results suggest that even larger current modulation can be possible if one could have larger modulation of Schottky barrier height. For this purpose, we compared MoS2, MoSe2, and α-MoTe2 as transition metal dichalcogenide (TMD) materials in graphene/TMD/Ti heterostructure; these materials have different electron affinity and thus provide systematic change of band alignment to graphene (Fig. 1(d)). We show that the transport in the graphene/TMD junction is sensitive to the band alignment [4]. The V G dependence of the current revealed systematic change from MoS2 to MoTe2 (Fig. 1(e)). We demonstrated large current ON-OFF ratio and ON current density on MoSe2-based heterostructure; the value is comparable to that of MoS2-based device. These comparisons suggest that further improvement may be possible by optimizing device structure. We believe that our fabricated exfoliated-graphene/TMD/metal vertical heterostructure reveal superior performance to other existing graphene-based vertical transistors and present an important advance toward electronics applications.