Electron transport in two-dimensional monolayer semiconductorsβ-ZrNCl andβ-HfNCl: a first-principles study

Two-dimensional (2D) transition metal nitride halides (TMNHs), such asβ-ZrNCl andβ-HfNCl, demonstrate promising electronic properties for next-generation nanoelectronics, though their intrinsic charge transport mechanisms remain to be fully elucidated. This study investigates phonon-limited carrier mobility in monolayerβ-ZrNCl andβ-HfNCl using first-principles calculations combined with Boltzmann transport theory. The results indicate that both materials exhibit indirect bandgap semiconductor characteristics, with bandgaps of 1.91 eV (β-ZrNCl) and 2.26 eV (β-HfNCl), and exhibit isotropic electron transport but markedly reduced hole mobility attributable to heavier effective mass and intervalley scattering. Electron mobilities reach 45.53 cm²V^-1s^-1(β-ZrNCl) and 28.13 cm²V^-1s^-1(β-HfNCl), while hole mobilities remain below 4 cm²V^-1s^-1at room temperature. Crucially, acoustic deformation potential theory significantly overestimates the room-temperature mobility by disregarding optical phonon contributions, particularly the dominant scattering from out-of-plane optical (ZO) phonons. Comprehensive analysis demonstrates significant optical phonons involvement in room-temperature carrier scattering, with ZO phonons proving decisive in monolayerβ-ZrNCl, while transverse optical and longitudinal optical phonons prevail in monolayerβ-HfNCl. These results highlight the critical importance of electron-phonon coupling in transport properties and suggest that substrate-induced phonon engineering could be a viable strategy for enhancing mobility. The study advances fundamental understanding of charge transport in 2D TMNHs and provides essential design principles for their implementation in high-performance electronic devices.