In situ transmission electron microscopy study of the crystallization of $Ge2Sb2Te5$

Crystallization of amorphous $Ge2Sb2Te5$ films (10, 40, and 70 nm thick) was studied by in situ heating in a transmission electron microscope (TEM). Electron irradiation-induced crystallization is possible at room temperature using a 400 kV electron beam where the reciprocal of the incubation time for crystallization scales linearly with the current density during electron irradiation. Without electron-beam exposure, crystallization starts at 130 °C. Using a 200 kV beam, crystallization also occurred in the temperature interval between 70 and 130 °C. In principle, electron irradiation always affects the crystallization kinetics, strongly promoting nucleation and probably not hampering growth. At 130 °C without electron-beam exposure, 400 nm diameter colonies of 10–20 nm grains develop in the 40 and 70 nm thick films showing clear symmetric bending contour contrast. These spherulites prefer to have in their center the 〈111〉 zone axis of the $Fm3̄m$ structure perpendicular to the surface of the film and show a typical tilt variation of ±10°. At 340 °C, the transition from the metastable to the stable trigonal $(P3̄m1)$ crystal structure takes place. Fast and excessive grain growth occurs with the [0001] axis perpendicular to the film surface of the film. Also shown is that oxidation of the $Ge2Sb2Te5$ film strongly influences its crystallization; its critical temperature decreases from 130 to 35 °C.