Intrinsic electron transport in monolayer MoSi2N4 and WSi2N4

We systematically studied the intrinsic electron transport of the recently fabricated two-dimensional (2D) monolayer MoSi2N4 and WSi2N4 (belong to the MA2Z4 family, where M is a transition metal; A is Si or Ge; and Z is N, P, or As) by using the Boltzmann transport theory with the scattering rates calculated from first-principles calculations. It is found that both materials have moderate room temperature electron mobility of 87 cm2/V s for MoSi2N4 and 119 cm2/V s for WSi2N4. Our detailed analysis shows that their electron mobility difference is attributed to the different average effective mass. Moreover, by comparing studies with monolayer MoS2 and WS2, we reveal that the polar optical phonon pattern of MoSi2N4 and WSi2N4 is governed by the antiparallel silicon–nitrogen bond oscillation, different from MoS2 and WS2, in which is governed by the antiparallel transition metal–sulfur bond oscillation. This feature is arising from the large electronegativity difference between N and Si. Nitrogen has the fourth largest electronegativity, so it always tends to attract plenty of electrons from Si and thus Si has a large Born effective charge, thereby resulting in strong Fröhlich interaction. We further found that all the 2D semiconducting MA2Z4 have large Born effective charges, thereby indicating a generally strong electron–phonon coupling strength.