Stable chalcogenide Ge-Sb-Te heterostructures with minimal Ge segregation

Abstract

Matching of various chalcogenide films shows the advantage of delivering multilayer heterostructures whose physical properties can be tuned with respect to the ones of the constituent single films. In this work, (Ge-Sb-Te)-based heterostructures were deposited by radio frequency sputtering on Si(100) substrates and annealed up to 400 °C. The as-deposited and annealed samples were studied by means of X-ray fluorescence, X-ray diffraction, scanning transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy. The heterostructures, combining thermally stable thin layers (i. e. Ge-rich Ge_5.5Sb₂Te₅, Ge) and films exhibiting fast switching dynamics (i. e. Sb₂Te₃), show, on the one side, higher crystallization-onset temperatures than the standard Ge₂Sb₂Te₅ alloy and, on the other side, none to minimal Ge-segregation.

Keywords: Crystallization; GST; Ge; Ge segregation; Ge-rich; PCM; Sb2Te3; Sputtering.

Figure 1

X-ray GID (ω − 2θ) scans on (bottom) sample A and (top) sample B. The scans during annealing (T = 30/400 °C, black color) and after annealing (RT, brown yellow color) were acquired for 25’ and 3 h, respectively. The dotted lines indicate the peak positions for the experimental GID (ω − 2θ) scan of c-GST225 (blue color) and Ge (orange color); the continuous red lines indicate the peak positions for simulated GID (ω − 2θ) scan of t-GST124.

Figure 2

GID (ω − 2θ) scans on (left) sample D and (right) sample E. The scans during annealing (T = 30/400 °C, black color) and after annealing (RT, brown yellow color) were acquired for 25’ and 3 h, respectively. Dotted lines indicate the positions of the GID (ω − 2θ) scan peaks for crystalline Sb₂Te₃ (green color), c-GST225 (blue color), t-GST225 (purple color) and Ge (orange color).

Figure 3

(a) HAADF STEM image of the ML heterostructure of sample D after annealing at 400 °C (substrate on the left, capping layer on the right of the ML heterostructure). (b) Ge EELS map obtained from Ge L-edge at 1217 eV. (c) Sb EELS map obtained from Sb M-edge at 528 eV. (d) Te EELS map obtained from Te M-edge at 572 eV. (e) Composite map of Ge, Sb and Te EELS maps.

Figure 4

Raman spectra of sample D, E and single layer Ge_5.5Sb₂Te₅ @ RT after annealing at 400 °C.

Figure 5

(a) HAADF STEM image of the ML heterostructure forming sample E, as-grown (substrate on the left, capping layer on the right of the ML heterostructure). (b) Ge EELS map obtained from the Ge L-edge at 1217 eV. (c) Sb EELS map obtained from the Sb M-edge at 528 eV. (d) Te EELS map obtained from the Te M-edge at 572 eV. (e) Composite map of the Ge, Sb and Te EELS maps.