Structure of Ge_<i>x</i>Ga₈S_{92–<i>x</i>} glasses studied by high-resolution X-ray photoelectron spectroscopy and Raman scattering

Abstract

In this paper, the structures of chalcogenide glasses Ge_<i>x</i>Ga₈S_{92–<i>x</i>} (<i>x</i> = 24%, 26.67%, 29.6%, 32% and 36%) at a fixed Ga atomic content of 8% are studied by high-resolution X-ray photoelectron spectroscopy and Raman scattering spectra. In order to quantify the evolutions of the different structural units in Ge_<i>x</i>Ga₈S_{92–<i>x</i>} glasses, the number of double peaks in the Ge 3d, Ga 3d and S 2p spectra are determined by iterative fitting method, the binding energy and the full width at half maximum of each peak, and the relative ratio of the integral area of each decomposed peak to that of the whole area of the X-ray photoelectron spectroscopy are thus achieved. On the other hand, the Raman scattering spectra of Ge_<i>x</i>Ga₈S_{92–<i>x</i>} glass are decomposed into multiple Gaussians based on the structural units. We use the iterative method to simulate the position of peak center, full width at half maximum, and height of each Raman peak. By analyzing the evolution of each unit structure in the glasses, it is found that the network structure of glass network is mainly formed by S atom bridging the tetrahedral structure of GeS₄ and GaS₄. The S chains or rings structural units are formed in Ge₂₄Ga₈S₆₈ glass, indicating that S atoms are in excess in the chemical composition of the glass, so there are enough S atoms around Ge and Ga atoms, forming heteropolar Ge—S and Ga—S bonds. With the gradual increase of Ge content, S chains or rings structure units rapidly disappear in Ge_26.67Ga₈S_65.33 glass. The Ge—Ge homopolar bonds in the ethane-like structure S₃Ge—GeS₃ and the <i>M</i>—<i>M</i> (Ge—Ge, Ga—Ga or Ge—Ga) homopolar bonds in the S₃Ge/Ga—Ga/GeS₃ structure simultaneous appear in the Ge_29.6Ga₈S_62.4 glass, and the number of structures increases gradually with the increase of Ge content. This is mainly due to the insufficient number of S atoms in the Ge-Ga-S glass. Once S atoms are lacking, the excess Ge and Ga atoms can only combine with themselves to form the homopolar bond <i>M</i>—<i>M</i>. It can be concluded below. Firstly, Ge and Ga atoms appear mainly in the form of 4-coordination, while S atoms occur mainly in the form of 2-coordination in the chalcogenide glasses of Ge_<i>x</i>Ga₈S_{92–<i>x</i>}. Secondly, the existence of <i>M</i>—<i>M</i> bond leads the nanophase to separate, and the ordering degree of glass network structure to decrease .