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

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 cm(2) V(-1)s(-1) for MoS2 and to 12 040 cm(2) V(-1)s(-1) for WS2 when temperature decreases to 77 K and carrier concentration is around 5 x 10(12) cm(-2). In the case of holes, best mobility values were 9320 cm2V-1s-1 and 13290 cm2 V-1s-1, reached at similar temperature and carrier concentration conditions while at room temperature these fall to 80 cm(2) V(-1)s(-1) and 150 cm(2) V(-1)s(-1) 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.