Low-energy electron-loss spectroscopy of GaSe and InSe

Low-energy electron-loss spectra (LEELS) of GaSe and InSe have been measured for what we believe to be the first time. Both crystals produced very similar spectra, indicating the similarity of the electronic structures of these compounds. Ten loss peaks were observed in the second derivative of the LEELS spectrum. Two of these are associated with collective excitations of the valence electrons, namely, the bulk and surface plasmons. The peaks related to the excitation of Ga 3d (In 4d) core electrons have three final states in the conduction band. The other five peaks are explained by transitions from the Se 3d core level and the valence band, whose final states coincide with those for the Ga 3d (In 4d) level. The spectrum from the ${\mathrm{Ar}}^{+}$-ion-sputtered surface showed bulk and surface plasmon losses for a metal, i.e., Ga in GaSe and In in InSe, respectively. This result indicates that the surface of these crystals is changed to a metallic state by ${\mathrm{Ar}}^{+}$-ion sputtering.