Temperature-dependent high-angle electron scattering from a phase-separated amorphous Ge-Te thin film

This paper describes the results of experiments and simulations on the thermal diffuse scattering of 200 keV electrons from an amorphous Ge₄₈Te₅₂ thin film using high-angle annular-dark-field (HAADF) imaging in a scanning transmission electron microscope (STEM). This imaging techniques shows that the film is phase separated on a approx. 10-20 nm length scale into Te-rich and Ge-rich amorphous domains. The average high-angle-scattered intensity increases as the temperature increases from 106 to 300 K. The contrast between the two domains decreases over this same range indicating that, in addition to the atomic-number (Z) effects on contrast for a given temperature, the temperature-dependent vibrational properties and, hence, phonon-scattering behavior are different for the two different elements in this specimen. Simulations of the HAADF intensity based on an Einstein model using a Debye expression for the mean-square vibrational amplitude give qualitative agreement where temperature-dependent contrast variations are due to differences in atomic number, Debye temperature, and atomic-scattering factor.