Size-induced evolution of optical properties in gallium selenide thin layers

Abstract

Thin layers of gallium selenide (GaSe) have been fabricated on Si:SiO2 substrates by micromechanical exfoliation. Size-induced effects in GaSe thin layers of various thickness have been revealed. The photoluminescence maximum associated with direct free exciton transition shifts to higher energies with reduction of GaSe crystal thickness. When going from the bulk GaSe crystal down to 6 tetralayers, the bandgap value increases from 2 eV (620 nm) to 2.17 eV (571 nm). No photoluminescence signal has been detected for GaSe layers thinner than 6 TL. At the same time, the Raman spectra of thin GaSe flakes demonstrate the features corresponding to Ga2Se3, Ga2O3 and other compounds. The appearance of these lines is attributed to thermal and photo-induced oxidation of GaSe layers. For the thin flakes, a remarkable modification of the photoluminescence spectra shape takes place. Presumably, it is a consequence of the electronic structure evolution, as the top of the valence band has undergone changes and has flattened for 8 TL.