FIRST-PRINCIPLES INVESTIGATION OF THE LATTICE-DYNAMICS OF EPSILON-GASE

We have calculated the bulk-phonon dispersion of the layered semicondu ctor epsilon-GaSe using ab initio methods. In contrast to previous mod el calculations, our results show good agreement both with the phonon frequencies from neutron-scattering experiments as well as with the ph onon-dispersion curves of the GaSe(0001) surface measured with inelast ic He-atom scattering. We have also performed the calculation of the e lastic constants C-13 and C-33 and the linear combination C-11 + C-12, in good agreement with experimental values. Furthermore, we have eval uated the equilibrium structure and the surface phonon frequencies at the (K) over bar point of the GaSe(0001) surface. We observed no surfa ce relaxations, and only small differences between bulk and surface fr equencies, supporting the fact that only weak interactions exist betwe en the layers. [S0163-1829(98)02107-9].