Inverse Janus design of two-dimensional Rashba semiconductors

The search for optimal Rashba semiconductors with large Rashba constants, strong electric field responses, and potential thermoelectric properties is pivotal for spin field-effect transistors (SFETs) and Rashba thermoelectric devices. Herein, we employ first-principles calculations to explore the intrinsic Rashba spin splitting in a series of two-dimensional (2D) $XY{Z}₂$ (X, $Y=\mathrm{Si}$, Ge, Sn; $X\ensuremath{\ne}Y$; $Z=\mathrm{P}$, As, Sb, Bi) monolayers via unnatural inverse Janus structural design. Instead of common Janus-type Rashba systems, the $\mathrm{SiSn}{\mathrm{Sb}}₂$ and $\mathrm{GeSn}{\mathrm{Sb}}₂$ monolayers within inverse Janus structures are first predicted as ideal Rashba systems with isolated spin-splitting bands near the Fermi level, and the Rashba constants ${\ensuremath{\alpha}}_{\mathrm{R}}$ are calculated as 0.94 and $1.27\phantom{\rule{0.16em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\AA{}$, respectively. More importantly, the Rashba effect in such $\mathrm{SiSn}{\mathrm{Sb}}₂$ and $\mathrm{GeSn}{\mathrm{Sb}}₂$ monolayers can be more efficiently modulated by the external electric field compared to the biaxial or uniaxial strain, especially with $\mathrm{GeSn}{\mathrm{Sb}}₂$ monolayer exhibiting a strong electric field response rate of $1.34\phantom{\rule{0.16em}{0ex}}\mathrm{e}{\AA{}}²$, leading to a short channel length, $L=64\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. Additionally, owing to the inapplicability of work function and potential energy in assessing built-in electric field $({E}_in)$ in inverse Janus $\mathrm{SiSn}{\mathrm{Sb}}₂$ and $\mathrm{GeSn}{\mathrm{Sb}}₂$ structures, we further propose an effective method to characterize ${E}_in$ through a view of fundamental charge transfer to approximately quantize the ${\ensuremath{\alpha}}_{\mathrm{R}}$ and its variation under an external electric field. Our work not only proposes the $\mathrm{GeSn}{\mathrm{Sb}}₂$ monolayer acting as a promising multifunctional material for potential applications in SFETs and Rashba thermoelectric devices but also inspires future research to introduce Rashba spin splitting in 2D materials through inverse Janus design.