Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping

Abstract

Ge-rich GeSbTe alloys allowed overcoming temperature limitations of Phase-Change Memory technology. In this paper, we present a thorough investigation of the structural evolution and crystallization process of these alloys as a function of increasing temperature of annealing. We highlight the progressive rearrangement of the structure towards the demixing of Ge and GeSbTe phases. In particular, we show the stability of Sb-Te units and the development of Ge-Te bonds around these features. We observe the formation of a transient GeSbTe phase, which is driven by crystallization phenomena, leading to a gradual diffusion and expulsion of Ge. Therefore, the system moves towards the complete separation of Ge and Ge2Sb2Te5 stable phases. Furthermore, we investigate the effect of N doping in Ge-rich GeSbTe, which induces the formation of Ge-N bonds. Such features are demonstrated to be responsible for a delayed structural reorganization to higher temperatures, thus affecting the entire process of crystallization and phase separation in the alloy.

The effect of Ge content on structural evolution of Ge-rich GeSbTe alloys at increasing temperature

“…In Ge35GST and Ge45GST, the first transition is separated into two steps, evidencing the growth of an intermediary phase in the system, which will be explained later in the text. A similar double step transition in Ge-rich GST alloys was already observed in Resistivity vs Temperature (R vs T) measurements [14]. A second sharp increase of ψ can be observed in Ge-rich GST at higher temperatures and may be attributed to the growth of the GST phase once Ge is completely segregated and starts to crystallize.…”

“…Based on the previous investigations on Ge45GST alloy [14], we initially focus on this composition in order to describe the evolution of the main structural features with increasing temperature. This description will further allow to highlight the main structural changes common to all the investigated Ge-rich GST samples.…”

“…Ge-Te, Sb-Te, a-Ge, etc.) [14]. The Ge enrichment of GST alloys is known to be beneficial to the thermal stability of the amorphous layer; however, a clear understanding of the impact of the Ge content on the structure of the alloys and its evolution in temperature is still missing.…”

Multilayer OTS Selectors Engineering for High Temperature Stability, Scalability and High Endurance

“…After 30 minutes of annealing, we observe a slight change in the main features highlighted in Raman spectra, and a shift of the Ge-N broad band, indicative of both an amorphous structural relaxation and GeN features reorganization. In particular, a shift of the Ge-N vibration modes towards higher wavenumbers, as observed in SSGN, could be due to a gradual change of Ge and N atoms arrangement, towards Ge-N features poorer in Ge content, driven by an already started phase segregation and following crystallization, as already observed in other chalcogenide based systems [12].…”

Phase Separation in Ge-Rich GeSbTe at Different Length Scales: Melt-Quenched Bulk versus Annealed Thin Films

“…Increasing the Ge content in the GST alloys shows an increase in the crystallization temperature, promoting high data retention and endurance [ 10 , 21 , 22 ]. Moreover, in addition to Ge enriching, nitrogen doping of Ge-rich GST alloys has also been studied for increased thermal stability and higher crystallization temperatures, which are attractive for future memory devices [ 9 , 23 , 24 ].…”