  The structural stability and electronic states of GaSe monolayer with
trigonal-antiprismatic (AP) structure, which is a recently discovered new
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.
