Chemical order in Ge_xAs_ySe_1-x-y glasses probed by high resolution X-ray photoelectron spectroscopy

We have measured high-resolution x-ray photoelectron spectra of Ge _xAs_ySe_1-x-y glasses with a mean coordination number (MCN) from 2.2 to 2.78. The valence band spectra showed that a number of Se-Se-Se trimers can be found in Se-rich samples, whilst multiband features induced by phase separation can be observed in extremely Se-poor samples. When the Ge, As, and Se 3d spectra were decomposed into several doublets, which correspond, respectively, to different chemical environments, the perfect AsSe_3/2 pyramidal and GeSe_4/2 tetrahedral structures in Se-rich samples gradually evolved into defect structures, including As-As and Ge-Ge homopolar bonds, with increasing Ge and As concentrations. Two transition-like features were found at MCN = 2.5 and 2.64-2.72 that correspond first to the disappearance of Se-chains in the glass network and, subsequently, destruction of the perfect GeSe_4/2 tetrahedral structures, respectively.