Determination of scattering time and of valley occupation in transition-metal dichalcogenides doped by field effect

The transition-metal dichalcogenides have attracted a lot of attention as a possible stepping-stone towardatomically thin and flexible field-effect transistors. One key parameter to describe the charge transport is the timebetween two successive scattering events—the transport scattering time. In a recent report, we have shown thatit is possible to use density functional theory to obtain the band structure of two-dimensional semiconductorsin the presence of field effect doping. Here, we report a simple method to extract the scattering time from theexperimental conductivity and from the knowledge of the band structure. We apply our approach to monolayersand multilayers of MoS$_2$, MoSe$_2$, MoTe$_2$, WS$_2$, and WSe$_2$ in the presence of a gate. In WS$_2$, for which accuratemeasurements of mobility have been published, we find that the scattering time is inversely proportional to thedensity of states at the Fermi level. Finally, we show that it is possible to identify the critical doping at whichdifferent valleys start to be occupied from the doping dependence of the conductivity