Monte Carlo study of noise velocity fluctuations and microscopic carrier transport in monolayer transition metal dichalcogenides

The microscopic transport properties of electrons and holes in MoS2, MoSe2, WS2 and WSe2 are studied by means of an ensemble Monte Carlo simulator. Moreover, instantaneous carrier velocity fluctuations are analyzed in depth from a microscopic point of view, including their power spectral density, which is directly related to thermal noise in the GHz range. Results show important differences in the mean free path and momentum relaxation time for electrons and holes: valence band transport has, in general, a more ballistic character, particularly for tungsten based transition metal dichalcogenides. Electrons (especially in MoSe2 and WSe2) have faster momentum relaxation times in the whole electric field range under study. Transport in subsidiary valleys is relevant for electrons, while in the case of holes it only has a minor influence, being noteworthy only for WS2 at high electric fields. Holes present larger decay times of the velocity fluctuations correlation function: the faster decorrelation in the conduction band is associated to a larger scattering activity, a general trend observed in all the materials under study. The results indicate that it is to be expected a much larger thermal noise for p-type devices in the GHz range as compared to n-type devices: materials containing selenium have reduced levels of noise generated by velocity fluctuations, especially for electrons.