The properties and structure of Ge Se Te glasses and thin films

Abstract Ge 30 Se 70 − x Te x bulk glasses and thin films for x = 0, 10 and 20 were prepared. The properties and structure were characterized by differential scanning calorimetry (DSC), Raman spectroscopy and also by 77 Se and 125 Te MAS NMR spectroscopy. The optical properties were also measured. The absence of crystalline phases was verified by XRD, while DSC indicated that the glasses were not phase separated. Although the glass transition temperature of both Ge 30 Se 70 glass and thin film is the same, different ΔC p reveal a higher disorder of the thin film. Both the glass transition temperature and ΔC p of Te glasses as well as crystallization temperatures decrease when Te increases due to increasing GeTe 2/2 instead of GeSe 4/2 structural units thereby reducing 3D interconnection of the glassy network. Based on the 77 Se MAS NMR results, the short-range order model was determined as a suitable structural model for Ge 30 Se 70 glass. 77 Se and 125 Te MAS NMR confirmed that Ge in alloys with Te only reaches oxidation state + II in agreement with Ge Te phase diagram and therefore no GeTe 4/2 tetrahedra can be present in these glasses and/or films. Raman spectroscopy confirmed that the short-range order of glasses and films is basically the same. The refractive index of thin films increases with tellurium content. The optical band gap of the thin films strongly decreases from 2.02 eV to 1.36 eV with increasing Te content and simultaneously the increasing structural disorder and width of the band tails increase. Reduction of the oxidation state of germanium to Ge II caused by a reaction with tellurium ultimately leads to a reduction in 3D connectivity of the non-crystalline matrix and the loss of the glass-forming ability of the Ge Se Te system.