Multistep Crystallization of Ge‐Rich GST Unveiled by In Situ synchrotron X‐ray diffraction and (scanning) transmission electron microscopy

Abstract

Among many phase change materials, Ge‐rich GeSbTe (GST) alloys are of considerable interest due to their high thermal stability, a specification required for the next generation of embedded digital memories. This stability results from the fact that these alloys do not crystallize congruently but experience phase separation forming Ge and GST‐225 nanocrystals upon crystallization. However, the details of the crystallization process remain unclear. Combining in situ X‐ray diffraction studies during isothermal annealing and ex situ (scanning) transmission electron microscopy ((S)TEM) observations, the successive phases through which these alloys crystallize are identified. At low temperature (310 °C), the homogeneous amorphous material undergoes phase separation during wherein small regions of different Ge contents are formed. After a long incubation time, Pnma GeTe embryos first crystallize and trigger the heterogeneous crystallization of the Ge cubic phase. While the Ge phase progressively builds up through the addition of new small Ge crystals, cubic GeTe forms. At this point, the microstructure ceases to evolve, and Sb is still dispersed and contained within some remaining amorphous matrix surrounding Ge and GeTe crystals. Higher annealing temperatures (typically 400 °C) are needed to force Sb to diffuse and get incorporated into the GeTe grains to form cubic Ge2Sb2Te5.

Local structure effects of carbon-doping on the phase change material Ge2Sb2Te5

“…17,21,24,32,[42][43][44] Alternatively, this behavior may be attributed to an intermediate separating phase, such as pure Ge, which has been detected briefly before GST crystallization. 20,45 Overall, it is difficult to compare GST crystallization mechanisms across literature, since reported crystallization behavior depends on temperature and time spent at that temperature, film thickness, film substrate, and capping layer (often TiN). 20,21,31,44,45 In the context of the literature, our findings imply a more complex transition from short-to long-range order upon crystallization for GST and GST-C. A decoupling of the short-, medium-, and long-range order of phase change materials has been explored in computational studies 17,42,43 and may be key to reconciling the many conflicting reports regarding the structure and performance of GST and GST-C.…”

“…20,45 Overall, it is difficult to compare GST crystallization mechanisms across literature, since reported crystallization behavior depends on temperature and time spent at that temperature, film thickness, film substrate, and capping layer (often TiN). 20,21,31,44,45 In the context of the literature, our findings imply a more complex transition from short-to long-range order upon crystallization for GST and GST-C. A decoupling of the short-, medium-, and long-range order of phase change materials has been explored in computational studies 17,42,43 and may be key to reconciling the many conflicting reports regarding the structure and performance of GST and GST-C. Certainly, further work consisting of a ensemble of complementary characterization techniques such as in situ XAS, pair distribution function (PDF) analysis, XRD, Raman spectroscopy, and transmission electron microscopy (TEM) could fully explore the structure of GST across length scales, and each structural feature's effect on bulk properties.…”

“…47,48 As for long-range structure, in addition to diffraction studies, TEM can reveal spatially resolved information, such as the size, morphology, and distribution of crystalline and amorphous domains. 15,20,23,24,45,49…”

“…17,21,24,32,42–44 Alternatively, this behavior may be attributed to an intermediate separating phase, such as pure Ge, which has been detected briefly before GST crystallization. 20,45 Overall, it is difficult to compare GST crystallization mechanisms across literature, since reported crystallization behavior depends on temperature and time spent at that temperature, film thickness, film substrate, and capping layer (often TiN). 20,21,31,44,45…”

“…20,45 Overall, it is difficult to compare GST crystallization mechanisms across literature, since reported crystallization behavior depends on temperature and time spent at that temperature, film thickness, film substrate, and capping layer (often TiN). 20,21,31,44,45…”