Pressure-driven selective sublimation and electron-beam-induced sputtering during high vacuum heating observed by...

In situ heating transmission electron microscopy (TEM) enables real-time observation of microstructural evolution. However, interpretation can be hindered by artifacts arising from electron beam irradiation and the high-vacuum environment. In this study, (i) the vacuum effect was identified through an investigation of bulk surface behavior after ex situ heat treatment under atmospheric air, atmospheric argon, and high-vacuum (∼10^-5 Pa) conditions, and (ii) the behavior of GeTe thin foils under different electron-beam conditions was examined using in situ heating TEM. Through these approaches, the electron-beam effects and vacuum effects were systematically investigated. As a result of bulk heat treatment performed at 450 ℃, oxidation occurred under an air atmosphere, Ge-Te interdiffusion and GeTe formation were observed under an Ar atmosphere, and under high-vacuum conditions, Te sublimation occurred, leading to the formation of a Ge-rich columnar structure. In situ CTEM revealed rapid electron-beam-induced sputtering of GeTe at 300 ℃, which led to hole formation in the electron-beam-irradiated regions under both 200 kV and 80 kV conditions. In contrast, sputtering was suppressed in in situ STEM using a low probe current. However, upon heating to 325 ℃, selective sublimation of Te and the consequent agglomeration of Ge occurred, leading to changes in stoichiometry and the formation of Ge nanocrystals. These results indicate that, for reliable in situ heating TEM analysis of GeTe, the beam current density should be minimized, and the selected temperature must be lower than the lowest sublimation onset temperature among the constituent elements under the vacuum conditions of the instrument.