1, CEA Grenoble, Grenoble, , France
3, LNCMI, Grenoble, , France
Chalcogenide phase change materials (PCMs), such as Ge-Sb-Te alloys, have outstanding properties, which have led to their successful use for a long time in optical memories (e.g., DVD-RAM or CD-RW) and, more recently, in non-volatile resistive memories with phase change random access memories. The latter are the most promising candidates to replace the current FLASH memories at CMOS technology nodes under 28 nm . The main feature of PCMs are fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties. Controlling their crystallization, however, is a challenge. In the abundant literature devoted to interface and size effects on PCM crystallization, mostly on thin films (in the sub-100 nm thickness range) capped with various materials, no interface effect was reported for film thicknesses above 30 nm. Recently, we established a completely new picture on the crystallization of relatively thick (100 nm) films of prototypical Ge2Sb2Te5 (GST) and GeTe alloys. We revealed the impact of interfaces on their crystallization mechanism by scanning transmission electron microscopy (STEM) images that were obtained at room temperature after various annealing treatments . We demonstrated that the crystallization temperatures of GST and GeTe thin films can significantly vary as a function of their surface state. Whereas GST and GeTe PCM alloys have been, and are still, the subject of a huge number of publications during the last decades, their crystallization mechanisms in thin films are not yet fully understood. In that context, acquiring STEM images in situ during annealing was required to get a full understanding of the crystallization process.
We will present new results on the crystallization steps of GeTe thin films at a nanometer scale obtained by combining in situ annealing with STEM cross section imaging of the films. Thanks to a novel sample preparation technique, we were able for the first time to observe directly in the microscope the effect of interfaces on the crystallization process of amorphous GeTe thin films . Surface-oxidation promotes heterogeneous nucleation at the oxidized upper surface of the film. By contrast, perfectly capped, and thus non-oxidized films, crystallize at a significantly higher temperature through volume nucleation. We will also discuss the origin of surface nucleation in oxidized GeTe films. The latest findings demonstrate that one can control the stability of the amorphous phase of PCMs by controlling their interfaces, opening the route to new 3D memory cell architecture with improved data retention.
 P. Noé et al., accepted manuscript, Topical review in Sem. Sc. And Tech. (2017). http://iopscience.iop.org/article/10.1088/1361-6641/aa7c25
 P. Noé et al., Acta Mater. 110, 142 (2016).
 R. Berthier et al., J. Appl. Phys. 122, 115304 (2017).