2, Centre National de la Recherche Scientifique (CNRS), Marseille, , France
3, synchrotron SOLEIL, Gif-sur-Yvette, , France
4, ID01/ESRF, The European Synchrotron, Grenoble, , France
Temperature is a key thermodynamic parameter that drives many phase transformations in functional films. Phase transitions in phase change materials (PCM) are deeply studied to develop Phase Change Random Access Memory (PCRAM)1. The use of PCMs for non-volatile data storage relies on their physical properties exhibiting a reversible temperature induced transition between amorphous and crystalline phases with distinctly different electrical and optical properties. Typically the amorphous (resp. crystalline) phase possesses low (resp. high) optical reflectivity, low (resp. high) mass density and very high (resp. low) electrical resistivity. Such phase transitions are usually studied by in-situ techniques as X-ray diffraction and reflectivity (XRD, XRR), wafer curvature for stress study and sheet resistance (Rs). These techniques are often used separately, thus the correlation between all the physical properties is often ambiguous. GeTe is a prototypical PCM particularly interesting due to its large resistivity window, high crystallization speed and better stability in amorphous state1. Its large density change upon crystallization raises, however, important issues because of the large mechanical stresses occurring during memory cycling. Furthermore its crystallization process is not still fully understood.
We developed 2 combined experiments (at DiffAbs beamline-Synchrotron SOLEIL-France) to characterize in-situ and simultaneously electrical, mechanical and structural changes occurring during phase transitions of thin or nanostructured PCM films: i) combined wafer curvature, XRR and XRD performed simultaneously2,3 to follow the concomitant stress buildup during the phase transition as well as the structural properties (structure, lattice parameter, grain size, thickness, density), the strain and phase segregation occurring in crystalline phases; ii) combined in-situ Rs measurements and X-ray scattering techniques (XRD and XRR)4 allowing to simultaneously follow both structural and electrical parameters. Both experiments were applied to study the crystallization process of GeTe alloy as a function of 1D confinement, by performing experiments during heating of thin layers down to 5 nm. This contribution will focus on the interest of such coupling for the fundamental understanding of 3 key phenomena governing the reliability of PCRAMs: i) the increase of crystallization temperature with increasing confinement considering the role of both nucleation and oxidation; ii) phase segregation5 and iii) stress buildup upon crystallization and further heating6.
This study has been funded by ANR-SESAME ANR-15-CE24-002; the authors thank P.Noé -CEA-LETI-France for providing samples.
1 V.L. Deringer et al. Adv. Funct. Mater. 25 (2015) 6343–6359
2 C. Rivero et al. APL 87 (2005) 041904
3 T. Ouled-Khachroum et al. Thin Solid Films 617 (2016) 44–47
4 M. Putero et al. J. Appl. Cryst. 44 (2011) 858–864
5 M. Putero et al. APL 108 (2016) 101909
6 M.S. Amara et al. To be submitted