  Metal monochalcogenide GaSe is a classic layered semiconductor that has
received increasing research interest due to its highly tunable electronic and
optical properties for ultrathin electronics applications. Despite intense
research efforts, a systematic understanding of the layer-dependent electronic
and optical properties of GaSe remains to be established, and there appear
significant discrepancies between different experiments. We have performed GW
plus Bethe-Salpeter equation (BSE) calculations for few-layer and bulk GaSe,
aiming at understanding the effects of interlayer coupling and dielectric
screening on excited state properties of GaSe, and how the electronic and
optical properties evolve from strongly two-dimensional (2D) like to
intermediate thick layers, and to three-dimensional (3D) bulk character. Using
a new definition of the exciton binding energy, we are able to calculate the
binding energies of all excitonic states. Our results reveal an interesting
correlation between the binding energy of an exciton and the spread of its wave
function in the real and momentum spaces. We find that the existence of
(nearly) parallel valence and conduction bands facilitates the formation of
excitonic states that spread out in the momentum space. Thus, these excitons
tend to be more localized in real space and have large exciton binding
energies. The interlayer coupling substantially suppresses the Mexican-hat-like
dispersion of the top valence band seen in monolayer system, explaining the
greatly enhanced photoluminescence (PL) as layer thickness increases. Our
results also help resolve apparent discrepancies between different experiments.
After including the quasiparticle and excitonic effects as well the optical
activities of excitons, our results compare well with available experimental
results.
