First-principles study on the stability and electronic structure of monolayer GaSe with trigonal-antiprismatic structure

The structural stability and electronic states of GaSe monolayer with trigonal-antiprismatic (AP) structure, which is a recently discovered polymorph, were studied by first-principles calculations. The AP-phase GaSe monolayer was found stable, and the differences in energy and lattice constant were small when compared to those calculated for a GaSe monolayer with conventional trigonal-prismatic (P) structure which was found to be the ground state. Moreover, it was revealed that the relative stability of P-phase and AP-phase GaSe monolayers reverses under tensile strain. These calculation results provide insight into the formation mechanism of AP-phase GaSe monolayers in epitaxially grown GaSe thin films.