2, CEA Grenoble, Grenoble, , France
3, CEA Grenoble, Grenoble, , France
4, LNCMI, Grenoble, , France
The outstanding properties of chalcogenide phase change materials (PCMs) have led to their successful application in optical memories for a long time and, more recently, in phase change random access memories (PCRAMs) [1,2]. PCMs feature fast and reversible phase transformations between crystalline and amorphous states which have different transport and optical properties. A prototypical PCM is GeTe which thin films crystallize at ≈ 230 °C, governed by “homogeneous” nucleation in the volume of the film [1,2,3]. However, when GeTe is surface-oxidized, its crystallization occurs in a completely different manner: the crystallization is heterogeneous, starts already at ≈ 180 °C, and is slower than that of non-oxidized GeTe thin films. Although this effect has already been reported, a detailed comprehension of its origin at the nanoscale is still missing [3,4]. Here, we will show why surface-oxidation affects the crystallization mechanism of GeTe thin films using advanced scanning transmission electron microscopy (STEM) techniques, energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS).
Aiming at understanding the origin of heterogeneous crystallization in surface-oxidized amorphous GeTe layers, a surface-oxidized 100 nm GeTe film was annealed under Ar atmosphere. The crystallization was followed by optical reflectivity at 670 nm and quenched just after the first sharp optical increase corresponding to heterogeneous crystallization of the surface of the film [3,4]. Subsequent analysis of the film by STEM imaging revealed that only the top 20 nm of the film was crystallized while the bottom of the film remained amorphous. EDS mapping and XPS depth profiling showed how oxidation affects the chemical composition of the film. Using nanobeam precession electron diffraction (NPED) mapping, we identified the different crystallographic structures present near the sample surface. Furthermore, spectrum imaging diffraction (SIdiff) mapping allowed for the direct correlation of chemical and crystallographic information in different regions of the film. Taking all results together, we found that surface-oxidation causes, e.g., separation of the GeTe phase and formation of crystalline species other than GeTe. In conclusion, the presented results will provide clues for elucidating the puzzling aspects of the effect of surface-oxidation on the crystallization of GeTe thin films, opening up novel strategies for interface engineering in order to master crystallization of PCM thin films.
 P. Noé et al., accepted manuscript, Topical review in Sem. Sc. And Tech. (2017). http://iopscience.iop.org/article/10.1088/1361-6641/aa7c25
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