Composition Segregation of Ge-Rich GST and Its Effect on Reliability

Lee, Yung-Huei; Liao, Po‐Yung; Hou, Vincent; Heh, D.; Nien, Chih-Hung; Kuo, Wen-Hsien; Chen, Gary T.; Yu, S. M.; Chen, Yu‐Sheng; Wu, Jin-Shang; Bao, Xinyu; Diaz, C.H.

doi:10.1109/irps46558.2021.9405168

Cited by 11 publications

(12 citation statements)

References 14 publications

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“…However, the low crystallization temperature of GST-225 (between 120 and 180 °C) is at the origin of the low thermal stability of the different electrical states offered by the corresponding devices and is thus an obstacle to many possible applications, especially in the field of embedded technologies where operating temperatures are well above 200 °C. To improve performances and offer stable characteristics at higher temperatures, at least for digital devices, the enrichment of canonical GST-225 with Germanium (Ge-rich GST or GGST) is one of the solutions promoted by the industry. ,− Indeed, GGST has a much higher crystallization temperature (>300 °C), which guarantees the integrity of the encoded information at high temperatures.…”

Section: Introductionmentioning

confidence: 99%

Crystallization of Ge-Rich GeSbTe Alloys: The Riddle Is Solved

Rahier

Ran

Ramond

et al. 2022

ACS Appl. Electron. Mater.

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Among the phase-change materials, Ge-rich GeSbTe (GST) alloys are of considerable interest as they offer a much higher thermal stability than their congruent contenders, a desirable characteristic for embedded digital memories and neuromorphic devices. Up to now, the mechanisms by which such alloys crystallize and progressively switch from one resistivity state to the other remain unclear and very controversial. Using in situ synchrotron X-ray diffraction during isothermal annealing and advanced transmission electron microscopy techniques, we solve this riddle and unveil the mechanisms leading to the overall crystallization of such alloys. During annealing at 310 °C, the initially homogeneous and amorphous material undergoes a progressive phase separation, leading to the formation of Ge-rich regions of different compositions. During this decomposition, the first formed GeTe embryos crystallize and trigger the heterogeneous crystallization of the Ge cubic phase. As the phase separation proceeds, these embryos dissolve and the Ge phase gradually builds up through the nucleation of small grains. Only when this Ge cubic phase is largely formed, the remaining amorphous matrix may locally reach the Ge 2 Sb 2 Te 5 composition at which it can crystallize as large grains. Our density functional theory calculations confirm that the quite exotic Pnma GeTe structure we have experimentally identified is more stable than the regular R3m structure at nanometric sizes.

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Section: Introductionmentioning

confidence: 99%

Crystallization of Ge-Rich GeSbTe Alloys: The Riddle Is Solved

Rahier

Ran

Ramond

et al. 2022

ACS Appl. Electron. Mater.

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“…This issue poses a significant problem for programming operations in an active device. The local composition variation and phase separation into Ge-rich and Sb-rich regions compromise the device’s functionality [ 10 , 22 , 28 , 29 ]. Moreover, the excess Ge content in Ge-rich alloys increases oxidation susceptibility with lowered crystallization temperature and Te enrichment [ 9 , 30 ].…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2022

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Integration of the prototypical GeSbTe (GST) ternary alloys, especially on the GeTe-Sb2Te3 tie-line, into non-volatile memory and nanophotonic devices is a relatively mature field of study. Nevertheless, the search for the next best active material with outstanding properties is still ongoing. This search is relatively crucial for embedded memory applications where the crystallization temperature of the active material has to be higher to surpass the soldering threshold. Increasing the Ge content in the GST alloys seems promising due to the associated higher crystallization temperatures. However, homogeneous Ge-rich GST in the as-deposited condition is thermodynamically unstable, and phase separation upon annealing is unavoidable. This phase separation reduces endurance and is detrimental in fully integrating the alloys into active memory devices. This work investigated the phase separation of Ge-rich GST alloys, specifically Ge5Sb2Te3 or GST523, into multiple (meta)stable phases at different length scales in melt-quenched bulk and annealed thin film. Electron microscopy-based techniques were used in our work for chemical mapping and elemental composition analysis to show the formation of multiple phases. Our results show the formation of alloys such as GST213 and GST324 in all length scales. Furthermore, the alloy compositions and the observed phase separation pathways agree to a large extent with theoretical results from density functional theory calculations.

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“…One of the alternatives promoted by the industry is the enrichment of standard GST-225 with germanium (Ge-rich GST or GGST), which offers good stability at higher temperatures along with enhanced device performances. [8][9][10][11][12][13] Indeed, the significantly higher crystallization temperature of GGST alloys (>300 °C) ensures that the encoded information will remain intact when exposed to high temperatures during the soldering process and the lifespan of the device. [8][9][10][11][12][13] Initially, it was thought that the high crystallization temperature of GGST was resulting from the existence of a single but unknown "golden," Ge-rich, crystalline phase.…”

Section: Introductionmentioning

confidence: 99%

“…[8][9][10][11][12][13] Indeed, the significantly higher crystallization temperature of GGST alloys (>300 °C) ensures that the encoded information will remain intact when exposed to high temperatures during the soldering process and the lifespan of the device. [8][9][10][11][12][13] Initially, it was thought that the high crystallization temperature of GGST was resulting from the existence of a single but unknown "golden," Ge-rich, crystalline phase. [14,15] Based on the same belief, recent theoretical papers have investigated possible decomposition paths from "very Ge-rich" to "less Ge-rich" GST alloys during annealing.…”

Section: Introductionmentioning

confidence: 99%

Multistep Crystallization of Ge‐Rich GST Unveiled by In Situ synchrotron X‐ray diffraction and (scanning) transmission electron microscopy

Rahier

Luong

Ran

et al. 2023

Physica Rapid Research Ltrs

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Among many phase change materials, Ge‐rich GeSbTe (GST) alloys are of considerable interest due to their high thermal stability, a specification required for the next generation of embedded digital memories. This stability results from the fact that these alloys do not crystallize congruently but experience phase separation forming Ge and GST‐225 nanocrystals upon crystallization. However, the details of the crystallization process remain unclear. Combining in situ X‐ray diffraction studies during isothermal annealing and ex situ (scanning) transmission electron microscopy ((S)TEM) observations, the successive phases through which these alloys crystallize are identified. At low temperature (310 °C), the homogeneous amorphous material undergoes phase separation during wherein small regions of different Ge contents are formed. After a long incubation time, Pnma GeTe embryos first crystallize and trigger the heterogeneous crystallization of the Ge cubic phase. While the Ge phase progressively builds up through the addition of new small Ge crystals, cubic GeTe forms. At this point, the microstructure ceases to evolve, and Sb is still dispersed and contained within some remaining amorphous matrix surrounding Ge and GeTe crystals. Higher annealing temperatures (typically 400 °C) are needed to force Sb to diffuse and get incorporated into the GeTe grains to form cubic Ge2Sb2Te5.

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Composition Segregation of Ge-Rich GST and Its Effect on Reliability

Cited by 11 publications

References 14 publications

Crystallization of Ge-Rich GeSbTe Alloys: The Riddle Is Solved

Crystallization of Ge-Rich GeSbTe Alloys: The Riddle Is Solved

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

Multistep Crystallization of Ge‐Rich GST Unveiled by In Situ synchrotron X‐ray diffraction and (scanning) transmission electron microscopy

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