Jie Zhao1,Asir Intisar Khan2,Mikhail Efremov3,Zichao Ye1,Xiangjin Wu2,H.S. Philip Wong2,Eric Pop2,Leslie Allen1
University of Illinois at Urbana-Champaign1,Stanford University2,University of Wisconsin—Madison3
Jie Zhao1,Asir Intisar Khan2,Mikhail Efremov3,Zichao Ye1,Xiangjin Wu2,H.S. Philip Wong2,Eric Pop2,Leslie Allen1
University of Illinois at Urbana-Champaign1,Stanford University2,University of Wisconsin—Madison3
Phase transitions involving both stable and metastable phases in superlattices (SLs) are the underlying driving forces for the operation of SL-based phase-change memory (PCM) devices. These SL stacks with nanometer-thin sublayers have been proven promising for low-power PCM. Measuring bulk thermodynamic properties is typically done with calorimetry techniques. However, PCM samples are extremely thin (~65 nm), making them inaccessible for most calorimetry methods that require large sample size (>1,000 nm).<br/><br/>Therefore, we use thin-film Nanocalorimetry [1, 2] to probe thermal effects in the as-deposited SLs directly (TiN-capped, all deposited by the same process as functional PCM devices). This unique method allows us to investigate the sample in its natural form of thin-film SL stacks (strain, grain size, and stoichiometry) as in SL-based PCM device. Nanocalorimetry is capable of scanning rates up to 3,000,000 K/s for 1−60 nm thick samples and has quantitatively revealed the Arrhenius crystallization kinetics of 20 nm thick Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5 </sub>in our recent work [3].<br/><br/>Here, we probe the phase transition of 65 nm thick Sb<sub>2</sub>Te<sub>3</sub>/Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> superlattices [4-6] using Nanocalorimetry. Our preliminary measurements quantitatively characterize the heat capacity of the SL samples in a broad temperature range (room temperature - 650 <sup>o</sup>C) and reveal several thermal effects that could be classified as glass-to-liquid and melting phase transitions. These thermodynamic properties could provide insights into the low-power switching of the SL-based PCM and thus advancing our understanding of SLs for use in the next generation of energy-efficient data storage devices on rigid and flexible platforms.<br/><br/>[1] 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/>[2] 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/>[3] Zhao, J.<i> et al.</i> Exploring “No Man's Land”—Arrhenius Crystallization of Thin-Film Phase Change Material at 1 000 000 K s− 1 via Nanocalorimetry. <i>Advanced Materials Interfaces</i> <b>9</b>, 2200429 (2022).<br/>[4] Khan, A. I.<i> et al.</i> Ultralow–switching current density multilevel phase-change memory on a flexible substrate. <i>Science</i> <b>373</b>, 1243-1247 (2021).<br/>[5] Khan, A. I.<i> et al.</i> Electro-Thermal Confinement Enables Improved Superlattice Phase Change Memory. <i>IEEE Electron Device Letters</i> <b>43</b>, 204-207 (2021).<br/>[6] Khan, A. I.<i> et al.</i> Unveiling the effect of superlattice interfaces and intermixing on phase change memory performance. <i>Nano Letters</i> <b>22</b>, 6285-6291 (2022).