Electrical Properties of Partial-RESET States in Ge-Rich GeSbTe PCM Cells

Phase change memories (PCMs) are a promising candidate for the next generation of electronic devices (e.g. neuromorphic and in-memory computing, artificial intelligence, …). Nowadays, the Ge₂Sb₂Te₅ (GST-225) alloy is one of the most common material for PCM, notably explained by its properties: high resistive contrast, fast switching speed, cycling performance…<br/>Nevertheless, the low crystallization temperature of GST-225 (around 150°C) makes it not suitable for embedded and automotive applications. Thus, recent work has proposed the Ge-rich GST alloy (GGST) as a good candidate to address high-temperature applications. Indeed, the crystallization temperature of such alloys increases with the Ge content enabling to reach crystallization temperatures above 300°C.<br/>During thermal annealing, GGST alloys undergo chemical phase separation and form grains of different phases during crystallization, such as GST and Ge [1]. In PCM cells, the GGST material experiences high-temperature heating when applying write pulses which significantly change the microstructure and chemical composition within a dome in the active region. This results in unique electrical properties even for the basic logic states, which show better resistance stability than offered by the conventional GST-225 cells [2]. Beyond binary data storage, GGST cells are also attracting attention for applications in multilevel storage. Given the complex phase change mechanism in GGST-based PCMs, it is essential to develop appropriate programming methods to access intermediate resistance states (IRS) in such cells and understand the origin of their electrical properties.<br/>This work combines electrical & physical characterizations to compare cells programmed to IRS using different methods. For this, we make use of “pRESET” pulses (partial RESET pulse) of low amplitudes which amorphize only a fraction of the dome. First, we compare the effect of using different pulse sequences to access IRS: RESET-SET-pRESET or direct SET-pRESET. We study the accuracy and reliability of these methods to target a specific resistance value possibly stable over cycling. (S)TEM techniques have been used to analyse the microstructure and composition of these cells. It is shown that the programming method has an impact on the shape of the amorphous dome. Finally, we complement these findings by showing results obtained by impedance spectroscopy which, beyond the single resistance of the cells, provides valuable quantitative information regarding their capacitance.<br/><br/><br/>REFERENCES:<br/>[1] Rahier, E., Ran, S., Ratel Ramond, N., Ma, S., Calmels, L., Saha, S., Mocuta, C., Benoit, D., Le Friec, Y., Luong, M., & Claverie, A. (2022). Crystallization of Ge-Rich GeSbTe Alloys: The Riddle Is Solved<i>. ACS Applied Electronic Materials, 4(6), 2682–2688.</i><br/><br/>[2] Andrea Redaelli, Elisa Petroni, & Roberto Annunziata (2022). Material and process engineering challenges in Ge-rich GST for embedded PCM<i>. Materials Science in Semiconductor Processing, 137, 106184.</i>