Vacancy ordering as driving mechanism of metal insulator transition in GeSbTe phase change materials

Phase change materials (PCMs) are employed in non-volatile random access memory for the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical properties. The most interesting compounds exhibiting PCM properties are Te-based alloys, typified by Ge-Sb-Te (GST), as they exhibit the ability to switch between phases on the nanosecond time scale.

In addition to the amorphous-to-crystalline transition experimental results on polycrystalline GST films evidenced a metal insulator transition (MIT) of Anderson type, which has been attributed to the degree of ordering in the crystalline phase [T. Siegrist et al., Nat. Mater. 2011, 10, 202]. Zhanget al. [Nat. Mater. 2012, 11, 952] proposed that the MIT in GST relates to the ordering of localized vacancy clusters into ordered layers, as the spatial separation and ordering of Ge, Sb, and vacancies lower the total energy of the system.

 

Figure 1: a) STEM micrograph of epitaxial GST; vacancy layers (dashed green lines) are highlighted. b) Superimposed ω-2θ scans for crystalline GST without (blue curve) and with vacancy layer peak (VLp) (red curve), arising from vacancy layers periodicity. c) Resistivity as a function of temperature corresponding to each ω-2θ scan in b). Negative (blue empty squares) and positive slope (red triangles) indicate insulating and metallic behavior of the corresponding GST samples, respectively

In this study we were able to experimentally correlate the resistivity change with the vacancy ordering in the GST. For this purpose, the fabrication of a highly ordered single-crystalline material was necessary to facilitate the direct experimental observation of the vacancy layers. We succeeded in fabricating GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular beam epitaxy, thermal annealing and application of femtosecond laser pulses, indication of the fact that the ordering in GST occurs regardless of the fabrication method. By means of x-ray diffraction we assessed the degree of vacancy ordering in the material and directly correlated it with the MIT through low temperature electrical measurements. In addition we have shown that is possible produce GST with different degree of ordering, gaining a large range of resistivity. The latter result might allow the realization of ordered GST based cells with larger programming windows.

 

 

 

References

T. Siegrist et al., Nat. Mater. 2011, 10, 202

Zhang et al. Nat. Mater. 2012, 11, 952

1 Autor V. Bragaglia , F. Arciprete , W. Zhang , A. M. Mio , E. Zallo , K. Perumal , A. Giussani , S. Cecchi , J. E. Boschker , H. Riechert , S. Privitera , E. Rimini , R. Mazzarello , R. Calarco
Titel

Metal-insulator transition driven by vacancy ordering in GeSbTe phase change materials

Source Sci. Rep. , 6 , 23843 ( 2016 )
DOI : 10.1038/srep23843 | Download: PDF | 2754 Cite : Bibtex RIS
V. Bragaglia, F. Arciprete, W. Zhang, A. M. Mio, E. Zallo, K. Perumal, A. Giussani, S. Cecchi, J. E. Boschker, H. Riechert, S. Privitera, E. Rimini, R. Mazzarello, and R. Calarco