Browsing by Author "Gong, Y."
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Item Negative Differential Conductance & Hot-Carrier Avalanching in Monolayer WS2 FETs(Springer Nature, 2017) He, G.; Nathawat, J.; Kwan, C.-P.; Ramamoorthy, H.; Somphonsane, R.; Zhao, M.; Ghosh, K.; Singisetti, U.; Perea-López, N.; Zhou, C.; Elías, A.L.; Terrones, M.; Gong, Y.; Zhang, X.; Vajtai, R.; Ajayan, P.M.; Ferry, D.K.; Bird, J.P.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.Item Optical study of local strain related disordering in CVD-grown MoSe2ᅠmonolayers(AIP Publishing, 2016) Krustok, J.; Raadik, T.; Jaaniso, R.; Kiisk, V.; Sildos, I.; Marandi, M.; Komsa, H.-P.; Li, B.; Zhang, X.; Gong, Y.; Ajayan, P.M.We present temperature dependent micro-photoluminescence and room temperature photoreflectance spectroscopy studies on aged MoSe2 monolayers with high surface roughness. A0 and B0 exciton bands were detected at 1.512 eV and 1.72 eV, respectively, which are 50–70 meV lower than those commonly reported for high-quality samples. It is shown that the difference can be accounted for using a model of localized excitons for disordered MoSe2monolayers where the optical band gap energy fluctuations could be caused by random distribution of local tensile strain due to surface roughness. The density of localized exciton states is found to follow the Lorentzian shape, where the peak of this distribution is about 70 meV from the energy of delocalized states.