Electric-field-modulated spectra of orpiment (As₂S₃) near the energy gap

Abstract

Transverse electroabsorption and electroreflectance spectra of As₂S₃ single crystals have been studied at low temperatures in the vicinity of the indirect and direct gaps and compared with the spectra of the isomorphic compound As₂Se_3. Although the general features of the spectra are the same in both materials, their dependence on the polarization of light and on the orientation of the field reveals significant differences which point to a reduced coupling in As₂S₃ of the chalcogen lone-pair p-states along the stacking axis b of the layers and along the a axis in the layer plane. Above 50 K, where the gap begins to decrease (dE_g/dT = −0·4 meV K⁻¹), the spectra broaden rapidly, probably due to the rising population of low-energy optical phonons (rigid-layer modes) which affect the fine splitting by interlayer coupling.