Hot Electrons and Hot Phonons in 2D Semiconductors

The dynamics of hot carriers in the energy bands of semiconductors has long been exploited in electronic and optoelectronic technology. By combining different semiconductors in periodic superlattices, artificial bandstructures can moreover be implemented, a technology that has long been exploited. The recent emergence of atomically thin two-dimensional (2D) materials, and the capacity to stack them in multilayered “van-der Waals heterostructures”, has now opened up the possibility of extending traditional concepts of band-engineering to a whole new class of structures. By combining the best of these qualities from different 2D materials, the potential exists to enhance the performance of semiconductor technology well beyond its present capabilities. While the band engineering of 2D materials therefore offers enormous promise for the design of tailored electronic systems, realizing this potential requires a proper understanding of hot-carrier dynamics in the artificial bands of their heterostructures. However, this problem of high-field (&gt; kV/cm) transport typically bears little relation to its low-field counterpart. Instead, it is necessary to consider a strong coupling of the electronic and phononic systems, which arises as both are driven far from equilibrium. In this presentation, I review some of the work that we have been doing in recent years to explore the details of hot-carrier transport in 2D semiconductors (notably graphene and the transition-metal dichalcogenides and trichalcogenides) and their heterostructures (graphene and hexagonal bornon nitride). Using a strategy of fast (ns) electrical pulsing, we drive the carriers to large electric fields (as high as 100 kV/cm) while minimizing effects of self-heating (i.e., parasitic energy flow from the channel to surrounding dielectric layers of the device). Some of our key observations include the demonstration of non-substrate limited velocity saturation in graphene, negative differential conductance in the transition-metal dichalcogenides, and re-entrant metallic and semiconducting states in the Moire bands of graphene/h-BN heterostructures. Overall, the picture is one of rich physics arising from the interaction of hot electrons and hot phonons in these materials.<br/><br/>H. Ramamoorthy, R. Somphonsane, J. Radice, G. He, C.-P. Kwan, and J. P. Bird, ““Freeing” graphene from its substrate: Observing intrinsic velocity saturation with rapid electrical pulsing”, Nano Letters vol. 16, 399 (2016).<br/>G. He, J. Nathawat, C.-P. Kwan, H. Ramamoorthy, R. Somphonsane, M. Zhao, K. Ghosh, U. Singisetti, N. Perea-López, C. Zhou, A. L. Elías, M. Terrones, Y. Gong, X. Zhang, R. Vajtai, P. M. Ajayan, D. K. Ferry, and J. P. Bird, "Negative Differential Conductance & Hot-Carrier Avalanching in Monolayer WS2 FETs", Scientific Reports vol. 7, 11256 (2017).<br/>J. Nathawat, K. K. H. Smithe, C. D. English, S. Yin, R. Dixit, M. Randle, N. Arabchigavkani, B. Barut, K. He, E. Pop, and J. P. Bird, "Transient hot-carrier dynamics and intrinsic velocity saturation in monolayer MoS2", Physical Review Materials vol. 4, 014002 (2020).<br/>A. Mohammadzadeh, S. Baraghani, S. Yin, F. Kargar, J. P. Bird, and A. A. Balandin, "Evidence for a thermally driven charge-density- wave transition in 1T-TaS2 thin-film devices: Prospects for GHz switching speed", Applied Physics Letters vol. 118, 093102 (2021).<br/><br/>Acknowledgement: I would like to acknowledge the contributions of my many collaborators, including: N. Aoki, D. K. Ferry, J. Nathawat, J. E. Han, G. He, I. Mansaray, H. Ramamoorthy, U. Singisetti, and R. Somphonsane,