Theoretical Model for Negative Differential Conductance in 2D Semiconductor Monolayers

Abstract

A simple theoretical model of electron heating in a system with two valleys is applied for the first time to describe 2D semiconductor monolayers of the MoS2 and WS2 types. The model is demonstrated to describe sufficiently well the available experimental data on the negative differential conductance effect in a WS2 monolayer. It confirms a possibility to fabricate Gunn diodes of a new generation based on the structures concerned. Such diodes are capable of generating frequencies of an order of 10 GHz and higher, which makes them attractive for many practical applications. A carbon atomic monolayer, graphene, was obtained for the first time in 2004. It was found to be semimetal [1]. Therefore, its parameters can hardly be used to implement essentially different states "0" and "1", which became a principal obstacle on the way to the creation of the graphene-based hardware components for new electronics. Numerous attempts to induce semiconductor-like properties in graphene (by the hydrogenation, creation of nanostripes, inserting defects, and so forth; see work [2] and references therein) turned out unsuccessful from the viewpoint of further practical applications.However, during last years, other monolayers with semiconducting properties (MoS 2 , WSe 2 , and other chalcogenides of transition metals, black phosphorus, and others; see, e.g., works [3,4]) were intensively synthesized and studied. The most known from this class of materials are the MoS 2 and WS 2 monolayers. They are direct-band semiconductors with the band gap widths ≈ 1.7 and 1.8 eV, respectively. The extrema of the conduction and valence bands are located at points and ′ of the hexagonal Brillouin zone [5], as it takes place in graphene.The results of calculations carried out from the first principles, by using the density functional method demonstrated that the conduction band spectrum of those materials includes a lateral extremum (the -valley) with energies by approximately 0.2 and 0.08 eV larger than the band bottom energy, which is located in the direction from the points and ′ to the Brillouin zone center Γ (Fig. 1). The energy spectrum near those two extrema is parabolic. The presence of two subbands in the conduction bandlower (denoted by subscript 1) and upper (denoted by subscript 2) ones, for which the effective-mass relation 1 < 2 for two-dimensional electrons is obeyed [5] -gives us grounds to expect that the effect of negative differential conductance, which is associated with the filling by field-heated electrons of the higher val-

2D semiconductor structures as a basis for new high-tech devices (Review)

“…Other types of 2D structures, for which the theoretical model of electron heating in the system with two valleys was adapted in [19] for the first time to describe 2D semiconductor monolayers of the MoS 2 and WS 2 types, can well describe available experimental data. It confirms the possibility to create a new generation of Gunn diodes on the basis of those structures.…”

Vadym Yevhenovych Lashkaryov, the Outstanding Ukrainian Physicist of the 20th Century, the Discoverer of the p-n Junction in Semiconductors (Dedicated to the 120th Anniversary of his Birthday)

“…Borzyak, and V.I. Lyashenko) "one of the founders of physical science in Ukraine" in the years before World War II [1]. The following clarification has to be made.…”