2-DIMENSIONAL INDIRECT EXCITONS IN THE LAYER-TYPE TRICHALCOGENIDE ZRS3

Optical-absorption spectra have been measured near the indirect-gap ed ge for quasi-one-dimensional ZrS3 single crystals with chains parallel to the b axis. The temperature evolution of polarized spectra allows the identification of two indirect parallel gaps (E parallel to b) at 2.165 and 2.062 eV with an exciton binding energy of 10 meV and one in direct perpendicular gap (E perpendicular to b) at 2.091 eV with an ex citon binding energy of 31 meV. While the band-structure model based o n the molecular-orbital scheme of the (S-2)(2-) molecule is confirmed, the shape analysis of different structures of the absorption coeffici ent indicates clearly a two-dimensional indirect-band-gap character.