A comparative simulation study on the band-to-band (BTB) tunneling current of van der Waals heterojunctions consisting of transition metal dichalcogenides has been performed using a method based on the non-equilibrium Green function combined with a tight-binding approximation. For a longer channel, a dip structure of low tunneling transmission appears on the transmission function from the source electrode to the drain electrode. This dip structure appears in the middle of the transport window and originates in the anti-crossing gap of the band-structure. It is found that the BTB tunneling current is strongly enhanced when a satellite valley (Q-valley) is located within the transport window. This enhancement is attributed to the Q-valley assisted BTB tunneling.
We propose an equivalent model of simple form suitable for using in a computer program. The model reduces the order of device Hamiltonian and reproduces a narrow transport window of a target band-structure calculated by an atomistic model. We implemented the model with the adaptive moment estimation and the automatic differentiation technique by using an end-to-end open-source platform for machine learning. We tested the equivalent model on semiconducting nanoribbon and nanowire structures, and confirmed that it correctly reproduces not only the band-structures but also the ballistic transmission functions calculated by the non-equilibrium Green's function method.
Analytical formula of the transmission function of the inter-layer intra-band tunneling is derived for coupled narrow two-dimensional materials. Analytical models of the intra-band tunneling conductance G, the transmission function of the inter-layer band-to-band tunneling, and the maximum band-to-band tunneling current Imax, are also obtained. G and Imax are shown to exhibit different characteristics depending on the channel length.
Intra-layer band-to-band tunneling transmission function T(E) through monolayer transition metal dichalcogenides is calculated using the nonequilibrium Green function method combined with the tight-binding approximation. We focus on the differences in T(E) according to structures (nanosheet and nanoribbon) or materials (MoS2, WS2, MoSe2, WSe2, MoTe2, and WTe2). We find T(E) of the nanoribbon structure becomes much lower than that of the nanosheet structure due to the indirect transition and the small spatial overlap of the wave functions at the conduction band (CB) and valence band (VB) edges. In the nanosheet structure, the material dependence of T(E) is shown to be understood in terms of the tunneling mass and the bandgap energy. In the nanoribbon structure, MoTe2 and WTe2 show large T(E) due to the large spatial overlap of the wave functions at the CB bottom and VB top.
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