Structural stability of monolayer GaSe with trigonal-antiprismatic structure studied by first-principles calculations

Gallium Selenide (GaSe) is a layered post-transition metal monochalcogenide with a 2 eV band gap. We have recently succeeded in growing epitaxial GaSe(0001) thin films on Ge(111) substrates by molecular beam epitaxy (MBE) and revealed that a new polymorph of GaSe with trigonal-antiprismatic (AP) structure exists near the GaSe(0001)/Ge(111) interface [1]. In this study [2], the structural stability and electronic states of AP-phase GaSe monolayer were studied by first-principles calculations. While conventional trigonal-prismatic (P)-phase is the most stable phase at equilibrium lattice constants, the AP-phase GaSe become more stable than the P-phase GaSe as the in-plane tensile strain increases. In contrast to MoS₂ for which the P- and AP-phase (octahedral) MoS₂ have very different band structures, P- and AP-phase GaSe have similar band structures. This is due to the absence of changes in the angles between bonds between nearest neighbors. The difference in band structure and optical properties due to the differences in crystal symmetry will be discussed.