Thin-Film Nanocalorimetry Study of The Melting Transitions in 2D Phase-Change Superlattices

Today's nanoelectronics are reaching the limits of energy and latency for numerous data-intensive applications, including the Internet of Things (IoT) and artificial intelligence (AI). Improving the energy efficiency and speed of data storage remains at the core of tackling this grand challenge. Phase-change memory (PCM) is a storage technology combining the high speed of dynamic random-access memory and the non-volatility of flash memory. By simply stacking ~2 nm Sb₂Te₃and Ge₂Sb₂Te₅ together, superlattice (SL) phase change memory devices demonstrated an 8 times reduction in power consumption, compared to conventional PCM devices [1]. The unexpected device-level success lies in the unique thermodynamic properties of such two-dimensional (2D) heterostructures, which, unfortunately, are inaccessible to conventional characterization methods with limited sensitivity and speed. Thus, the underlying science responsible for success has remained a mystery.<br/>Nanocalorimetry (NanoDSC) [2, 3] is an advanced characterization method with monolayer sensitivity (~1 Å) and fast scanning rate (10⁶ K/s). This unique tool allows us to investigate the transitions of 2D heterostructure in its natural form of thin-film stacks as in SL-based PCM devices. In our recent work (2023 Nano Letters [4]), we applied NanoDSC to 65 nm Sb₂Te₃/Ge₂Sb₂Te₅ superlattices. An unexpected low-T metastable melting transition is uncovered along with an 8 times reduction in the enthalpy compared to its bulk counterparts. Such a transition provides a thermodynamic insight into the low-current switching of the superlattice PCM and advances the understanding of novel 2D heterostructures. The findings revealed here using the Nanocalorimetry technique will also inspire and enable a systematic material search for new 2D heterostructures for future energy-efficient nanoelectronics.<br/><br/>[1] Khan, A. I. et al. Unveiling the effect of superlattice interfaces and intermixing on phase change memory performance. <i>Nano Letters</i> 22, 6285-6291 (2022).<br/>[2] Lai, S., Guo, J., Petrova, V., Ramanath, G. & Allen, L. Size-dependent melting properties of small tin particles: nanocalorimetric measurements. <i>Physical review letters</i> <b>77</b>, 99 (1996).<br/>[3] Efremov, M. Y., Olson, E. A., Zhang, M., Zhang, Z. & Allen, L. H. Glass transition in ultrathin polymer films: calorimetric study. <i>Physical Review Letters</i> <b>91</b>, 085703 (2003).<br/>[4] Zhao, Jie, et al. "Probing the Melting Transitions in Phase-Change Superlattices via Thin Film Nanocalorimetry." Nano Letters 23.10 (2023): 4587-4594.