Two forms of nanoscale order in amorphous Ge_xSe_1-x alloys

We analyze the evolution of nanoscale order in amorphous Ge _xSe_1-x alloys using fluctuation electron microscopy. We identify two distinct structural signatures that behave independently as a function of composition. The strong signature of order at scattering vectors k ∼ 0.30 and 0.55 Å^-1 in Ge-rich alloys (x > 0.40) diminishes rapidly in Se-rich compositions. However, a second signature of order at scattering vector k ∼ 0.15 Å^-1 appears only for compositions in the middle range x = 0.30-0.53. We interpret that structural ordering occurs among pure Ge tetrahedra or among GeSe₄ tetrahedra, respectively, in amorphous Ge_xSe_1-x.