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.
Highlights
Transferred-electron effects have long been exploited in conventional semiconductors, with the most well-known example being provided by the Gunn effect[3, 4] In this phenomenon, the scattering of hot electrons from a central, high-mobility, valley, to a nearby one with heavier effective mass, gives rise to negative differential conductance and to the formation of traveling high-field domains
Negative differential conductance was observed, and was found to exhibit characteristics typical of the Gunn effect in more-traditional semiconductors. Reminiscent of those materials, the negative differential conductance was accompanied by the observation of pronounced hysteresis in the transistor characteristics, behavior that is consistent with the excitation of hot carriers from the lighter-mass K valleys into the heavier-mass T valleys
Observation of the negative differential conductance was found to be sensitive to thermal annealing, a result that we have explained within a scenario in which the annealing relaxes unintentional strain within the transition-metal dichalcogenide channel
Summary
Transferred-electron effects have long been exploited in conventional semiconductors, with the most well-known example being provided by the Gunn effect[3, 4] In this phenomenon, the scattering of hot electrons from a central, high-mobility, valley, to a nearby one with heavier effective mass, gives rise to negative differential conductance and to the formation of traveling high-field domains. The periodic repetition of this process, on a time scale determined by the saturated drift velocity and by the anode-cathode separation, results in the emission of (microwave) radiation from the device, a phenomenon that has been exploited for some five decades in compact, inexpensive microwave sources[4] The purpose of this Article is to demonstrate the possibility of exploiting the specific features of the band structure of transition-metal dichalcogenides, to realize novel transferred-electron effects. By applying a sufficiently large electric field, it should be possible to drive carriers up the K valleys, allowing them to eventually scatter into the lower-mobility T valleys (Fig. 1(c)) This should give rise to a region of negative slope (Fig. 1(d)) in the velocity-field (vd-ε) characteristic, and lead in turn to negative differential conductance and to high-field domain formation (see the insets to Fig. 1(d)). The high saturation velocity associated with this material (and with other transition-metal dichalcogenides6, 7) bodes well for the prospect of extending the operation of these sources to frequencies approaching the terahertz range (i.e. ≥10 GHz)
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