Structural and electronic properties of monolayer group III-VII compounds: A first-principle study

Recently, single-layer group IV-VI compounds, which are isoelectronic counterparts to layered group V semiconductor, have attracted great interest for their tunable mechanical, electronic, and optoelectronic properties. Herein, by using atomic transmutation and first-principles calculations, we report a systematical study of monolayer group III-VII MX (M = Al, Ga, In, and X = Cl, Br, I) compounds. Three structural symmetries of lattices Pn21 m, Pma2 and P-3m1, named as alpha, beta, and gamma phases correspondingly, are considered. All the binary systems are energetically stable and the relative stability of the three phases are related with their chemical compositions. We find that most of the group III-VII compounds are semiconductors with a wide range of band gaps ranging from 0.37 to 3.58 eV. The electronic band dispersion can also be largely tuned by changing the atomic geometry and constituent element. For all of alpha phase and most of gamma phase, the compounds are indirect band semiconductor with band gaps in the UV-light region. In the beta phase, the AlBr, AlI, GaCl, GaBr, and GaI are direct band gap semiconductors with the gap near visible light. Moreover, some of compounds have sharp van Hove singularity near the valence band edge and hence show electronic instabilities to ferromagnetic and half-metallic states under small hole doping. The unique characteristics of group III-VII binary monolayers can be suitable for high-performance device applications in nanoelectronics, optoelectronics and spintronics.