In semiconductors in the high excitation density limit, inter-particle correlations and exchange forces increase to a point where the thermal and Fermi pressure are overcome. In this limit, electrons and holes condense to form a two component liquid phase. This new phase is determined by strong-coupling and quantum correlations and best described as a degenerate Fermi-liquid. An adequate description of this exotic state of matter lies at the intersection of plasma, solid-state, and quantum condensed matter physics. Many of the properties of this condensed phase would find great use in the semiconductor applications, e.g. high-speed optoelectronic transistors, semimetal conductivity, broadband light emission and amplification, high droplet mobility. However, due to material parameters the observation of EHL state is mostly limited to cryogenic temperatures, and thus practical applications are hindered. The critical temperature "T" under which EHL exists (gas-liquid transition), is empirically found to be approximately one tenth of the exciton binding energy "E_b ". Since most semiconductors exhibit high dielectric screening (ϵ≳10), typical values for E_b range from 1-100 meV, therefore the EHL state is observed typically below 100K. Semiconductors with higher binding energies are being explored to reach ever-higher values of T. With Eb=80 meV, diamond represents the current state-of-the-art material to show EHL at TL=165 K.
Reaching room temperature condensation will require a significant reduction in the dielectric screening. In that regard atomically thin 2D materials, provides a significant opportunity to push condensation to room temperature values, due to their reduced dimensionality and weakened material screening. These materials exhibit significantly increased excitonic binding energies as well as substantially high electron and hole effective masses, both of which favor condensation.
Here we by using time resolved and steady state photoluminescence, differential absorption, and Raman spectroscopies, we investigated the formation and dissipation dynamics of EHL in 2D MoS2. We will discuss the roll of electronic and structural properties of the single layer materials in hosting such high excitation density states.
Kenan Gundogdu, "Carrier dynamics in optically excited Fermi degenerate states in atomically thin TMDC semiconductors (Conference Presentation)," Proc. SPIE 10724, Physical Chemistry of Semiconductor Materials and Interfaces XVII, 1072411 (Presented at SPIE Nanoscience + Engineering: August 23, 2018; Published: 17 September 2018); https://doi.org/10.1117/12.2321206.5836016901001.
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