Effects of an external electric field on the electronic properties of MSi2PxAsy monolayers and homojunctions: a first-principles study.

Using first-principles density functional theory (DFT), we systematically investigate the influence of a perpendicular external electric field (Eext) on the electronic properties of monolayer and homojunction structures of MSi2PxAsy (M = Mo, W; x + y = 4). Our results demonstrate that an external electric field of approximately 0.7 V Å-1 induces a direct-to-indirect bandgap transition, followed by a semiconductor-to-metal phase transition in the monolayer. The Rashba spin splitting around the Γ point, as captured by a derived k·p Hamiltonian, exhibits a tunable magnitude that depends linearly on the Eext, with a maximum coefficient of 0.08 eV Å. Moreover, the Eext leads to a significant redistribution of charge density and a rearrangement of the electronic structure, resulting in the formation of a type-II band alignment in the homojunction. These findings reveal the strongly tunable electronic and spin-dependent behavior of MSi2PxAsy monolayers under external fields, suggesting their potential for application in flexible electronic, spintronic, and optoelectronic devices, particularly those utilizing field-driven transitions.