Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides

Abstract

Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (R). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS and MoTe by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS and MoTe, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS. Our results further imply that metal work functions have very little influence on contact properties of 2D-material-based devices. Moreover, we found that R is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chemical doping into the 2D materials. These findings provide a practical guideline for depinning Fermi level at the 2D interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.

Defect Dominated Charge Transport and Fermi Level Pinning in MoS2/Metal Contacts

“…Recent experimental 20,61 and theoretical studies 60,62,63 have found a similar discrepancy. According to these studies, the Fermi level is partly pinned as a result of two interface effects: first, due to a metal work function modification resulting from a dipole formation at the interface, and second, by the introduction of gap states due to the weaker Mo–S bonding induced by interface metal–S interactions at the interface.…”

“…According to these studies, the Fermi level is partly pinned as a result of two interface effects: first, due to a metal work function modification resulting from a dipole formation at the interface, and second, by the introduction of gap states due to the weaker Mo–S bonding induced by interface metal–S interactions at the interface. 62 To introduce Fermi level pinning into the Schottky–Mott rule, a pinning factor ( S ) and a charge neutrality level (ϕ CNL ) are added to eq 1(61,64,65) S is defined as S = d ϕ B / d ϕ M and can vary from 1 for an unpinned interface to 0 for a strongly pinned interface. b is the y -intercept of the ϕ B versus ϕ M graph, which is related to the ϕ CNL ashere, ϕ CNL is the energy at which the interface is electroneutral (see Figure 4a).…”

Formation and morphological evolution of self-similar 3D nanostructures on weakly interacting substrates

“…Such approaches may, for example, be relevant for achieving 2D metal film growth on 2D crystals, including graphene and Mo 2 S, and thereby synthesize low-resistivity electrical contacts on nanoelectronic devices [4,5,[7][8][9][10][11][12][13][14] or fabricate catalytic devices that exhibit enhanced turnover frequencies [6].…”

“…A notable example is the deposition of metal films on twodimensional (2D) crystals (e.g., graphene and MoS 2 ) [4][5][6] for which the tendency toward the formation of 3D agglomerates imposes technological obstacles for the use of 2D materials in a wide range of switching and, in some cases, catalytic devices [4][5][6][7][8][9][10][11][12][13][14]. Thus, understanding atomistic mechanisms that govern 3D island formation and shape evolution is a key step toward controlling film morphology and, by extension, the functionality of devices based on weakly interacting film/substrate materials systems.…”

The ambipolar transport behavior of WSe2 transistors and its analogue circuits

“…Among them, tungsten diselenide (WSe 2 ), which consists of one layer of W atoms sandwiched between two layers of Se atoms, has many excellent properties providing potential applications, including valley-based electronics 15,16 , spin-electronics, and optoelectronics 17,18 . More significantly, different from the unipolar n-type semiconductor MoS 2 with the presence of sulfur vacancy and the strong Fermi level pinning near the conduction band [19][20][21] , WSe 2 as an ambipolar semiconductor has been demonstrated as having Fermi level effectively shifting between the valence band and the conduction band under application of an external field [22][23][24] . Recently, the optical and hole dominant transport properties of exfoliated WSe 2 have been explored [24][25][26][27] .…”