Innovative GeSe_1-xTe_x Chalcogenide Thin Films—From Bonding Mechanism to Phase-Change Material Applications

By exploiting their strong optical and resistivity contrast between the amorphous (a-) and the crystalline (c-) phase, chalcogenide phase-change materials (PCMs), such as canonical Ge₂Sb₂Te₅ alloy, have made possible to envisage a wide range of application in particular for data storage technology [1]. The difference in bonding mechanism between the a-phase, which is a covalent bonded (CB) structure, and the c-phase, which is governed by the recently introduced concept of “metavalent bonding” (MVB), is responsible for their large property contrast [2]. Many efforts have been devoted to shed light on this unconventional chemical bond, in order to accelerate the discovery of new PCMs with tailored properties for multiple applications. GeSe_1-xTe_x alloys, forming a solid solution along the GeSe-GeTe pseudo-binary line, arouse particular interest since they bridge the gap between GeSe, which is a CB material in both its a- and c-phases, and GeTe that is, in its c-state, a prototypical MVB material characterized by outstanding electronic properties such as a high electronic polarizability. Indeed, despite GeTe and GeSe are both IV-VI chalcogenide compounds with an isoelectronic structure, the moderate Peierls distortion in rhombohedral (rh-) c-GeTe (characterized by 3 short and 3 long Ge-Te bonds) enables the formation of MVB, which instead collapses in orthorhombic (orth-) c-GeSe due to the too strong local distortion. In addition to being a model system for fundamental studies, the unprecedented high thermal stability of a-phase of GeSe_1-xTe_x films, makes these materials extremely promising for embedded PCM memory applications, for which a data retention at high temperature is crucially needed [3].<br/>A-GeSe_1-xTe_x thin films were deposited by industrial magnetron co-sputtering on silicon and silicon oxide substrate. Their crystallization was monitored by four-point probe resistivity measurement during annealing. The crystallization temperature increases with decreasing Te content, reaching very high a-phase stability. Moreover, the resistivity contrast between a- and c-phase is extremely large for all the studied compositions, except in the case of GeSe. By means of room-temperature X-ray diffraction, Raman spectroscopy and X-ray Absorption Fine Structure on c-GeSe_1-xTe_x films, we identified an orth-structure in GeSe and a rho-MV bonded phase, isostructural to GeTe, in the x range [0.16-1]. This is an unexpected result since, according to the GeSe-GeTe phase diagram, the domain of the rh-phase is restricted to x range [0.48-1], and a covalent-bonded hexagonal phase is stable for x ranging from 0.14 to 0.42 [4]. However, it is precisely the persistence of the rh-structure in our thin films down to x=0.16, even if metastable, that allows the formation of MVB giving to the material such unprecedented resistivity contrast upon crystallization. The replacing of a few percent of Se atoms by Te in the covalently bonded Ge-Se network transforms the latter in a MVB material with a unique portfolio of properties. The high thermal stability of GeSe is combined with the wide resistivity contrast provided by MV bonded GeTe in an alloy that remains homogenous after crystallization, thanks to the complete solubility of Te in GeSe. Besides giving the opportunity to explore the transition between CB and MVB, their exceptional combination of properties makes Se-rich GeSe_1-xTe_x alloys an extremely promising candidate for integration in phase-change devices requiring high data retention.<br/>[1] P. Noé et al., Semicond. Sci. Technol. 33 (2018) 013002. [2] a) J.-Y. Raty et al., Adv. Mater. 2019, 31, 1806280 ; b) Kooi, B. J., Wuttig, M., Adv. Mater. 2020, 32, 1908302. [3] M. Tomelleri et al., Phys. Status Solidi RRL, 2020, 15, 22000451. [4] H. Wiedemeier, P. A. Siemers, High Temp. Sci. 1984, 17, 395.