Structural, electronic, and transport properties of quintuple atomic Janus monolayers Ga2SX2 ( X= O, S, Se, Te): First-principles predictions

Two-dimensional Janus structures with vertical intrinsic electric fields exhibit many interesting physical properties that are not possible with symmetric materials. We systematically investigate the structural, electronic, and transport properties of quintuple-layer atomic Janus ${\mathrm{Ga}}_{2}\mathrm{S}{X}_{2}$ ($X=$ O, S, Se, Te) monolayers using the first-principles calculations. The stability of the Janus structures is evaluated via the analysis of their phonon dispersion curves, cohesive and formation energies, and elastic constants. The existence of a vertical internal electric field due to the lack of mirror symmetry has resulted in a vacuum level difference between the two sides of the investigated Janus structures. At the ground state, while the Janus ${\mathrm{Ga}}_{2}\mathrm{S}{\mathrm{O}}_{2}$ is metallic, the other three configurations (${\mathrm{Ga}}_{2}{\mathrm{S}}_{3}, {\mathrm{Ga}}_{2}\mathrm{S}{\mathrm{Se}}_{2}$, and ${\mathrm{Ga}}_{2}\mathrm{S}{\mathrm{Te}}_{2}$) are semiconductors with indirect band gaps. The electronic properties of ${\mathrm{Ga}}_{2}\mathrm{S}{X}_{2}$ monolayers can be altered by strain engineering. In particular, the indirect--direct band gap and semiconductor-metal phase transitions were observed when the biaxial strain was introduced. The carrier mobilities of the semiconducting Janus monolayers are directional anisotropic due to their anisotropy of the deformation potential constant. It is found that all three semiconducting Janus monolayers have high electron mobility, up to 930.34 ${\mathrm{cm}}^{2}$/V s, which is suitable for applications in next-generation electronic devices.