Carrier mobility and high-field velocity in 2D transition metal dichalcogenides: degeneracy and screening

Abstract

The effect of degeneracy and the impact of free-carrier screening on a low-field mobility and a high-field drift velocity in MoS2 and WS2 are explored using an in-house ensemble Monte Carlo simulator. Electron low field mobility increases to 8400 cm2/Vs for MoS2 and to 12040 cm2/Vs for WS2 when temperature decreases to 77 K and carrier concentration is around 5cdot10^12cm^-2. In the case of holes, best mobility values were 9320 cm2/Vs and 13290 cm2/Vs, reached at 77 K, while at room temperature these fall to 80 cm2/Vs and 150 cm2/Vs for MoS2 and WS2, respectively. The carrier screening effect plays a major role at low fields, and low and intermediate temperatures, where a combination of large occupancy of primary valleys and carrier-phonon interactions dominated by relatively low energy exchange processes results in an enhanced screening of intrinsic scattering. For electrons, degeneracy yields to transport in secondary valleys, which plays an important role in the decrease of the low field mobility at high concentrations and/or at room temperature. The high-field drift velocity is not much affected by carrier screening because of an increased carrier scattering with surface optical polar phonons, favouring larger phonon wavevector interactions with small dielectric function values.

2D MoS2 under switching field conditions: Study of high-frequency noise velocity fluctuations

“…In this work, we analyze the noise generated by instantaneous velocity fluctuations in MoS2 (the most prominent TMD) during the transient regime that takes place after abrupt switching electric field conditions (both from low-to-high and high-to-low values) using an in-house ensemble Monte Carlo (EMC) material simulator [18,19]. The simulations are performed in the reciprocal space, which is equivalent to considering an infinitely large 2D material sheet, thus avoiding any issue relative to edge effects or modifications of the band structure due to small size constrictions.…”

“…Lett. 123, 052105 (2023), and may be found at http://dx.doi.org/10.1063/5.0152078 from the substrate, degeneracy, and the screening of free carriers [18,19]. Undesired impurities and defects, which are known to be a notorious source of degradation in experimental samples due to the relatively immature degree of development of TMD fabrication technology, are not considered in this work in order to provide the intrinsic noise performance of MoS2 in a best-case scenario.…”