Effects of an external electric field on the electronic properties of MSi ₂ P _<i>x</i> As _<i>y</i> 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 (<i>E</i>_ext) on the electronic properties of monolayer and homojunction structures of MSi₂P_<i>x</i>As_<i>y</i> (M = Mo, W; <i>x</i> + <i>y</i> = 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 <i>Γ</i> point, as captured by a derived <i>k</i>·<i>p</i> Hamiltonian, exhibits a tunable magnitude that depends linearly on the <i>E</i>_ext, with a maximum coefficient of 0.08 eV Å. Moreover, the <i>E</i>_ext 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 MSi₂P_<i>x</i>As_<i>y</i> monolayers under external fields, suggesting their potential for application in flexible electronic, spintronic, and optoelectronic devices, particularly those utilizing field-driven transitions.