Obtaining glasses in the extremely crystallizing Ge–Sb–Te phase change material

Abstract

Using thermal co-evaporation techniques, we show that various glassy compositions can be obtained along the Ge x Sb x Te 100-2x join in the ternary Ge-Sb-Te system which is known to display dramatic crystallization tendencies. Earlier attempts to produce bulk glasses have been limited to Sb-poor compositions close to the eutectic GeTe 6 . Results indicate a weak variation of T g with composition x and also a thermal stability that is weak for most systems, and especially for compositions close to the domain where Ge 2 Sb 2 Te 5 can be formed. Our results are put in perspective with other network-forming chalcogenides and the T g variation suggests the preferential formation of Te-Sb-Te, at variance with isochemical compounds such as Ge-Sb-Se. Data are discussed within the framework of topological based approaches which not only indicate that optimal glass formation is achieved for compositions satisfying the Maxwell stability criterion but also predict a flexible to rigid transition at x = 8.5%. Most of our glasses could be formed around this composition.

Phase-change materials based on amorphous equichalcogenides

“…These crystallization peaks, however, are important to understand phase-change behaviour of equichalcogenide thin films giving an idea on what local arrangements are favored in the bulk, even if the full-scale crystallization cannot occur due to different steric constraints. The peaks look very similar to those obtained for the crystallization of other GST films 46 , 56 . Third, the glass transition is not visible, although we can speculate from the DSC curves behaviour in Fig.…”

“…3 a) as the difference between the crystallization peak temperature ( T c = 341.3 °C) and the onset of glass transition temperature ( T g on = 214.9 °C), this bulk material looks quite attractive for molding and fiber-drawing applications. The obtained value of Dietzel criterion is ~ 70 K on average higher than for binary Ge–Te 48 and ~ 100 K higher than for ternary Ge–Sb–Te 46 glass systems. This enables certain applications (in meta-optics or waveguides) of the proposed material, which are not possible or hindered for the conventional GST-based PCMs due to a high crystallization affinity right above T g .…”

“…According to these values the Ge 15 Sb 40 S 15 Se 15 Te 15 glass can be classified as narrow-bandgap semiconductor similar to 45 . It is also on the lower side of E g values reported for Ge–Te or Ge–Sb–Te glass systems 46 . Advantage of equichalcogenide glass is the possibility to tune the optical gap in wider than pure GST ranges by changing S and Se concentration 39 .…”

Glass transition, topology, and elastic models of Se‐based glasses

“…This result has been established from a combination of ab initio MD simulations and MD-based constraint counting algorithms (Figure 16) along the Ge x Sb x Te 1−2x join, thus permitting one to identify an isostatic condition (flexible to rigid transition) for x c = 8.5%. 134 Close to this composition and as already anticipated from what is known in Ge-Se on optimal glass-forming tendency, 146 glasses can be produced in this highly crystallizing Ge-Sb-Te ternary alloy, 147 and now extend the glass-forming domain that was previously limited to Sb poor compositions close to the GeTe 6 eutectic (cyan-colored region in Figure 18) and has also been used to develop a successful OTS device. 23 Since the first OTS material composition As 30 Ge 10 Te 48 Si 12 announced by Ovshinsky et al, 18 who launched the field of commercializing chalcogenidebased memory devices, many other materials have now emerged, including those of GeTe 6 , 23 amorphous GeSe 24 and GeS 25 thin films.…”

Alteration of structural, electronic, and vibrational properties of amorphous GeTe by selenium substitution: An experimentally constrained density functional study

“…We first note that for GeTe, the calculated band structure reproduces rather well experimental results obtained from Xray photoemission spectroscopy (XPS 60,61 ), and the band gap is found to be of about 0.7 eV, consistently with previous DFT calculations 62 which are close to the experimental estimates (0.75-0.85, see Refs. [63][64][65]. Early XPS studies 60,66,67 have emphasized the important difference in density of states of amorphous and crystalline GeTe that lead to a semi-or p-type extrinsic semiconducting behavior, respectively.…”