Optical spectra and electronic structure of crystalline and glassy Ge(S,Se)2

Dielectric-function spectra from 1.5 to 5.8 eV are reported for evaporated ($g$-)Ge${\mathrm{Se}}_2$ films, ($c$-)Ge${\mathrm{Se}}_2$ single crystals, and evaporated Ag-doped photoactivated Ge${\mathrm{Se}}_2$ films. $c$-Ge${\mathrm{Se}}_2$ is dichroic, yielding two spectra that we relate to the components of the dielectric tensor parallel and perpendicular to the-GeSe-chains in this material. The dielectric function of $g$-Ge${\mathrm{Se}}_2$ shows two broad peaks, which closely resemble the $c$-Ge${\mathrm{Se}}_2$ spectra. Except for fine structure, the $g$-Ge${\mathrm{Se}}_2$ spectrum can be synthesized from the $c$-Ge${\mathrm{Se}}_2$ spectra in the effective-medium approximation. All structures are described by a simple model consisting of two narrow (∼ 1 eV) valence bands and a lower conduction band. Spectra for Ag-doped films have only a single broad peak in ${\epsilon }_2$. These results point to the existence of medium-range order in $g$-Ge${\mathrm{Se}}_2$, in agreement with a recent theory which ascribes much greater order to the glass than is commonly expected on the basis of "random" network models.