Negative Differential Conductance & Hot-Carrier Avalanching in Monolayer WS2 FETs

Abstract

The high field phenomena of inter-valley transfer and avalanching breakdown have long been exploited in devices based on conventional semiconductors. In this Article, we demonstrate the manifestation of these effects in atomically-thin WS2 field-effect transistors. The negative differential conductance exhibits all of the features familiar from discussions of this phenomenon in bulk semiconductors, including hysteresis in the transistor characteristics and increased noise that is indicative of travelling high-field domains. It is also found to be sensitive to thermal annealing, a result that we attribute to the influence of strain on the energy separation of the different valleys involved in hot-electron transfer. This idea is supported by the results of ensemble Monte Carlo simulations, which highlight the sensitivity of the negative differential conductance to the equilibrium populations of the different valleys. At high drain currents (>10 μA/μm) avalanching breakdown is also observed, and is attributed to trap-assisted inverse Auger scattering. This mechanism is not normally relevant in conventional semiconductors, but is possible in WS2 due to the narrow width of its energy bands. The various results presented here suggest that WS2 exhibits strong potential for use in hot-electron devices, including compact high-frequency sources and photonic detectors.

Theoretical Model for Negative Differential Conductance in 2D Semiconductor Monolayers

“…At Δ = 0.25 eV, the indicated values provide a good agreement with the experimental curve and correlate with the data given for WS 2 in the literature (see work [10]). Note that the ratio between the effective masses in the -and -valleys (in accordance with the data presented in work [9], this parameter equals 0.32 0 and 0.75 0 , respectively) is somewhat lower, but the given values are relevant only in vicinities of the extrema and do not make allowance for the mass increase with the energy in the -valley.…”

“…2 that, starting from a certain threshold value Δ ≈ 0.15 eV, the dependence ( ) acquires a maximum, and the ratio 2 / begins to grow at the fields corresponding to this maximum (this phenomenon corresponds to the intensive filling of the upper valley in the conduction band by heated electrons), which also qualitatively corresponds to the results of work [10]. At lower Δ -values, as is shown in Fig.…”

“…Finally, the negative differential conductance has been experimentally revealed quite recently in WS 2 monolayers [10]. It was shown that if the monolayer is unstrained, the effect does not take place owing to a small energy distance between the valleys, Δ ≈ 0.08 eV, because, at room temperature, electrons begin to fill the -valley at minimum values of the fields between the gate and the drain.…”

“…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- [10] ley characterized by a higher effective mass, can take place in 2D monolayers of the WS 2 or MoS 2 type [6]. Note that this effect has already been observed in "traditional" quantum heterostructures [7].…”

“…For the parameter values corresponding to the system that was studied in work [10] ( = 4 × 10 6 cm/s and = 5 m), Fig. 3.…”