Metal-Insulator Transition Driven by Vacancy Ordering in GeSbTe Phase Change Materials.

Valeria Bragaglia,Emanuele Rimini,Jos Emiel Boschker,Henning Riechert,Raffaella Calarco,Karthick Perumal,Wei Zhang,Eugenio Zallo,Fabrizio Arciprete,Stefania Privitera,Stefano Cecchi,Riccardo Mazzarello,Alessandro Giussani,Antonio Massimiliano Mio

doi:10.1038/srep23843

Abstract

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of 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. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows.

Highlights

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties
We first focus on the structural characteristics of highly textured GeSbTe alloys (GST) with optimized stacking
Peaks at Qz = 2.00, 4.01 Å−1 are attributed to the Si substrate while two narrow (Qz = 1.81, 3.61 Å−1) peaks are ascribed to the GST epilayer

Summary

Results and Discussion

We first focus on the structural characteristics of highly textured GST with optimized stacking. Note that the crystallization into ordered crystalline phase may proceed via a two-step process, involving the initial formation of a disordered cubic phase and the subsequent ordering of the vacancies, as indicated by the ab initio molecular dynamics simulations of crystallization of GST in the presence of a two dimensional crystalline template[15] These results unequivocally show that vacancy ordering is achieved by crystallization of an a-GST film deposited on a crystalline substrate by either thermal annealing or application of short laser pulses. The GST annealed at 170 °C for 1 h (filled blue squares), the as grown highly ordered GST with both cubic and rhombohedral stacking (filled orange squares), the GST in hexagonal phase obtained by annealing at 270 °C for 1 h (filled blue triangles) all show a positive temperature coefficient slope (metallic behaviour) with systematically decreasing resistivity This indicates a progressive increase in vacancy ordering, as given by the XRD intensity ratio (IVLp/IGST) between the VLp and the GST (00.30) peak. Employing ordered GST might offer benefits similar to chalcogenide superlattices[17] and allow the realization of cells with larger programming windows

Methods

Author Contributions

Additional Information