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 structure of chalcogenide glasses Ge&lt;em&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/em&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;92-&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt; (&lt;em&gt;x&lt;/em&gt;=24%, 26.67%, 29.6%, 32% and 36%) at a fixed Ga atomic content of 8% has been studied by high-resolution X-ray photoelectron spectroscopy and Raman scattering spectra. In order to quantify the evolution of the different structural units in Ge&lt;em&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/em&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;92-&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt; glasses, the number of double peaks in the Ge 3&lt;em&gt;d&lt;/em&gt;, Ga 3&lt;em&gt;d&lt;/em&gt; and S 2&lt;em&gt;p&lt;/em&gt; spectra were determined by iterative fitting method, their 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 were thus achieved. On the other hand, Raman scattering spectra of Ge&lt;em&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/em&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;92-&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt; glass were decomposed into multiple Gaussians based on the structural units. We used 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 was found that the network structure of glass network is mainly formed by S atom bridging the tetrahedral structure of GeS&lt;sub&gt;4&lt;/sub&gt;and GaS&lt;sub&gt;4&lt;/sub&gt;. The S chains or rings structural units are formed in Ge&lt;sub&gt;24&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;68&lt;/sub&gt; 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&lt;sub&gt;26.67&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;65.33&lt;/sub&gt; glass. The Ge-Ge homopolar bonds in the ethane-like structure S&lt;sub&gt;3&lt;/sub&gt;Ge-GeS&lt;sub&gt;3&lt;/sub&gt; and the M-M (Ge-Ge, Ga-Ga or Ge-Ga) homopolar bonds in the S&lt;sub&gt;3&lt;/sub&gt;Ge/Ga-Ga/GeS&lt;sub&gt;3&lt;/sub&gt; structure simultaneous appear in the Ge&lt;sub&gt;29.6&lt;/sub&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;62.4&lt;/sub&gt; glass, and the number of structures increased gradually with the increase of Ge content. This is mainly due to the insufficient content of S atoms in the Ge-Ga-S glasses. Once S atoms are lacking, the excess Ge and Ga atoms can only combine with themselves to form the homopolar bond M-M. It can be concluded that, firstly, Ge and Ga atoms appear mainly in the form of 4 coordination, while S atoms appear mainly in the form of 2 coordination in the chalcogenide glasses of Ge&lt;em&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/em&gt;Ga&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;92-&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt;. Secondly, the existence of M-M bond leads to the separation of nanophase, and the ordering degree of glass network structure is reduced.