Abnormally weak intervalley electron scattering in MoS2 monolayer: insights from the matching between electron and phonon bands

TL;DR

This study reveals that the abnormal decrease in electron mobility from monolayer to bulk MoS2 is due to differences in scattering mechanisms, which are governed by the matching between electronic band structures and phonon spectra, not just their densities.

Contribution

It introduces the phonon-energy-resolved matching function to identify intra- and inter-valley scattering channels, providing new insights into electron-phonon interactions in layered semiconductors.

Findings

Monolayer MoS2 exhibits stronger intravalley but weaker intervalley scattering compared to bulk.

Matching between electronic bands and phonon spectra determines scattering channels, not just their densities.

Multiple valleys can have weak intervalley scattering if scattering channels are limited or valleys are far apart in reciprocal space.

Abstract

It is known that carrier mobility in layered semiconductors generally increases from two-dimension (2D) to three-dimension due to suppressed scattering channels resulting from decreased densities of electron and phonon states. In this work, we find an abnormal decrease of electron mobility from monolayer to bulk MoS2. By carefully analyzing the scattering mechanisms, we can attribute such abnormality to the stronger intravalley scattering in the monolayer but weaker intervalley scattering caused by less intervalley scattering channels and weaker corresponding electron-phonon couplings compared to the bulk case. We show that, it is the matching between electronic band structure and phonon spectrum rather than their densities of electronic and phonon states that determines scattering channels. We propose, for the first time, the phonon-energy-resolved matching function to identify the intra- and inter-valley scattering channels. Furthermore, we show that multiple valleys do not necessarily lead to strong intervalley scattering if: (1) the scattering channels, which can be explicitly captured by the distribution of the matching function, are few due to the small matching between the corresponding electron and phonon bands; and/or (2) the multiple valleys are far apart in the reciprocal space and composed of out-of-plane orbitals so that the corresponding electron-phonon coupling strengths are weak. Consequently, the searching scope of high-mobility 2D materials can be reasonably enlarged using the matching function as useful guidance with the help of band edge orbital analysis.