The Complete Scenario of the Thermal Crystallization of Ge-Rich GST Alloys

Germanium enrichment of GeSbTe (Ge-rich GST or GGST) alloys appears to be a promising way of increasing their crystallization temperature and thus enabling their integration as PCMs in embedded systems. However, the crystallization stages of these highly non-stoichiometric alloys - a key phenomenon in their programming, but also in their degradation within memory cells - remain incomplete to date.<br/>While numerous experiments show that crystallization of amorphous GGST layers ultimately leads to the formation of a two-phase material in which pure Ge and GST-225 crystalline phases coexist, the order of appearance of these two phases and the details of this crystallization are still debated. Moreover, the observation that Ge crystallizes at temperatures of around 310-330°C, well below the homogeneous crystallization temperature of bulk pure Ge (380-400°C), merits explanation.<br/>To answer these questions, we carried out a campaign of XRD measurements at the synchrotron during 5 isothermal anneals (in situ) of GGST samples at low temperature (310 - 340°C). These isothermal anneals enable the mechanisms involved to be studied in detail over a wide time interval (several hours), and their energetic and kinetic characteristics to be decoupled, which is impossible when measurements are carried out during ramping-up. These results were complemented by analyses of samples annealed ex-situ at higher temperatures (up to 500°C). The characteristic parameters of the different phases present, such as grain sizes and weight fractions, were then extracted from the Rietveld analysis of the spectra obtained during annealing.<br/>However, the many cubic GST phases that can be formed have extremely close lattice parameters, making them indistinguishable by XRD. For this reason, we complemented these analyses with structural and chemical analyses of in situ and ex-situ annealed samples using advanced transmission electron microscopy techniques such as STEM-HAADF and STEM-EELS. All these analyses were performed on extremely thin (&lt; 25 nm) fresh lamella obtained by means of a new advanced approach using Focused Ion Beam (FIB) to avoid too much overlapping of the grains and mistakenly assigning them “exotic” phases.<br/>We were thus able to formally identify the successive phases through which these alloys crystallize and identify the mechanisms underlying their formation.<br/>At low temperatures (&lt; 350°C), the homogeneous amorphous material undergoes phase separation, during which small regions of varying Ge content are formed (STEM-HAADF). After ripening, orthorhombic GeTe embryos (SG <i>Pnma</i>) are formed, triggering heterogeneous crystallization of the cubic Ge phase. The growth of this phase is totally dominated by nucleation (5 nm grains), and its kinetics limited by the production, via phase separation, of regions of ad hoc size and stoichiometry. In parallel, the <i>Pnma</i> GeTe phase transits to its cubic GeTe phase. At this point, the microstructure ceases to evolve over time. Sb is still dispersed and contained in an amorphous residue with a composition close to that of Sb-Te. Higher annealing temperatures (&gt; 400°C) are required to force Sb to take part in crystallization. We show that it then gradually inserts itself into existing cubic GeTe grains to form increasingly Sb-rich GST phases (with 0.1&lt;Sb/Te&lt;0.4) and to form new grains of hexagonal Sb-rich phases (Sb₂, Sb₂Te₃, GST-147 etc...).<br/>This study shows that the crystallization kinetics of GGST are limited by the phase separation between Ge-rich and Sb-rich regions. This crystallization does not generate exotic phases but grains of listed Sb-poor cubic phases and of Sb-rich hexagonal phases. The GST-225 phase is not formed directly (as often claimed without demonstration) but gradually, via the insertion of Sb into the cubic GeTe lattice. All these phases are prone to be found in the pristine and active areas of PCM devices based on GGST.