Strain-induced localization as a universal mechanism toward high mobility in 2D semiconductors

Two-dimensional (2D) semiconductors with atomic thickness have shown great potential for application in high-performance microelectronic devices due to the lack of short channel effects, but their carrier mobility is often limited by strong electron-phonon coupling (EPC) compared to that of three-dimensional (3D) semiconductors. Particularly, the hole mobility in 2D materials is typically much lower than the electron mobility, creating a critical bottleneck for realizing complementary electronics. In this work, we propose and validate a universal strategy to significantly enhance mobility by the localization of out-of-plane degrees of freedom, either electron orbitals or phonon modes, to weaken EPC. Using first-principles calculations and phonon-energy-resolved analysis, we demonstrate this mechanism in monolayer black phosphorous (MBP) and monolayer black arsenic (MBAs) under in-plane tensile strains, with a focus on overcoming the longstanding limitation in hole transport. In MBP, where the valence band maximum (VBM) is dominated by out-of-plane p_z orbitals, strain-induced orbital localization weakens EPC from longitudinal acoustic (LA) and out-of-plane optical (ZO) phonons. In MBAs, where out-of-plane phonons dominate scattering, strain-induced vibrational localization suppresses EPC contributions from out-of-plane acoustic (ZA) and optical (ZO) modes. Consequently, the hole mobilities of strained MBP and MBAs can exceed 900 cm² V^-1 s^-1 at room temperature, rivaling the electron mobility of silicon. Furthermore, low-energy phonon scattering in strained MBP becomes nearly negligible, leading to an ultrahigh low-temperature mobility exceeding 10⁷ cm² V^-1 s^-1. This work establishes strain-induced localization as a universal and powerful design principle for engineering high-mobility 2D semiconductors where out-of-plane degrees of freedom control carrier transport.