Understanding High-Field Electron Transport Properties of Monolayer Transition Metal Dichalcogenides and Strain Effects

TL;DR

This study combines first-principles calculations and Monte Carlo simulations to analyze high-field electron transport in monolayer transition metal dichalcogenides, revealing how strain and valley effects influence mobility and negative differential mobility.

Contribution

It provides a detailed understanding of high-field transport mechanisms in MX2 monolayers, highlighting the effects of strain and valley transfer on electron velocity and mobility, which was previously underexplored.

Findings

WS2 has the highest peak velocity at the lowest electric field.

Strain increases the peak velocity by enhancing scattering energy.

Most MX2 exhibit negative differential mobility after reaching peak velocity.

Abstract

Monolayer transition metal dichalcogenides (MX2) are promising candidates for future electronics. Although the transport properties (e.g. mobility) at low electric field have been widely studied, there are limited studies on high-field properties, which are important for many applications. Particularly, there is lack of understanding of the physical origins underlying the property differences across different MX2. Here by combining first-principles calculations with Monte Carlo simulations, we study the high-field electron transport in defects-free unstrained and tensilely strained MX2 (M=Mo, W and X=S, Se). We find that WS2 has the highest peak velocity (due to its smallest effective mass) that can be reached at the lowest electric field (owing to its highest mobility). Strain can increase the peak velocity by increasing the scattering energy. After reaching the peak velocity, most MX2 demonstrates negative differential mobility (NDM). WS2 shows the largest NDM among unstrained MX2 due to the strongest effect of electron transfer from the low-energy small-mass valley to the high-energy large-mass valley. The tensile strain increases the valley separation, which on one hand suppresses the electron transfer in WS2, on the other hand allows the electrons to access the non-parabolic band region of the low-energy valley. The latter effect leads to an NDM for electrons in the low-energy valley, which can significantly increase the overall NDM at moderate strain. The valley-separation induced NDM in the low-energy valley is found to be a general phenomenon. Our work unveils the physical factors underlying the differences in high-field transport properties of different MX2, and also identifies the most promising candidate as well as effective approach for further improvement.