Schottky-Junction TMD-Based Monomaterial Field-Effect Transistor
Authors: Gity Farzan , Hurley P. K. , Ansari Lida 
Published: May 2020

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted extensive interest as this class of layered materials exhibit electronic properties from semimetals to semiconductors. In addition, their properties may significantly change when moving from bulk to ultra-thin films due to strong spin-orbit coupling (SOC) [1-3]. These properties open up new opportunities in bandgap engineering for future electronic and photonic devices.
Our recent study of platinum diselenide (PtSe2) films demonstrates that PtSe2 has distinct thickness-dependent electronic structures and physical properties [4]. Although the bulk crystal is a semimetal with an indirect overlap of the conduction and valence bands, monolayer PtSe2 has been revealed to be a semiconductor [4-6]. The possibility of making semimetal hetero-dimensional junctions with uniform chemical bonding at the interface promises the possibility of fabricating ideal Schottky barriers closely mirroring the behaviour of an ideal junction [7,8]. As a result, it is proposed to consider ‘thick’ (semimetallic) PtSe2 as the source and drain contact regions and ‘thin’ (semiconducting) PtSe2 region as the channel (gated) region. The electronic structure of the PtSe2-based Schottky barrier transistor is determined based on density functional theory (DFT). The DFT Hamiltonian is used to determine electrode self-energies to describe ‘semi-infinite’ electrodes achieved by periodic extension of the 3D semimetallic and the 2D semiconducting regions at distances away from the junctions, implemented in the Atomistix ToolKit (ATK) [9,10]. An explicit device region encompassing the junction regions is then treated by adding the energy-dependent complex self-energies to the device region Hamiltonian. Including the self-energies explicitly opens the system by application of the boundary conditions suitable for non-equilibrium Green's function (NEGF) using the self-energies calculated for semi-infinite electrodes. In directions parallel to the interface, periodic boundary conditions are applied leading to a 3D region in direct contact to a 2D thin film (see Fig. 1(a)).
This structure is expected to improve the contact resistance. This structure could also be beneficial for the performance of short-channel devices due to the resonance states originated at the thick-thin interface [8]. Based on the semimetal-to-semiconductor transition, where the value of the induced gap is controlled by varying the film thickness, a Schottky barrier transistor is designed. Ab-initio calculations combined with the NEGF formalism for charge transport determine the device current-voltage characteristics. Output characteristic of a back-gated structure (inset of Fig. 1(b)) shows that the proposed device is OFF at VGS = -0.5 V while at VGS = 1.5 V it is at ON state with ION/IOFF > 108. This current modulation confirms the potential application of such monomaterial field effect transistor with no doping for low-power electronics. In this study, we are also investigating the energy resolved local density of states (LDoS) and charge transfer at the semimetal-gated region interface which provides insight into the physics of the performance of the devices.
Another advantage of the proposed FET structure is that it does not require any doping at any of the thick (metallic) or thin (channel) regions, which makes the fabrication considerably more feasible. However, vacancies have shown to influence the band structure [4]; hence, such point defects are considered in the structure, and their impact on the device performance will be presented.

References

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Figure 1