Xiangjin Wu

All Publications

• Probing the Melting Transitions in Phase-Change Superlattices via Thin Film Nanocalorimetry. Nano letters Zhao, J., Khan, A. I., Efremov, M. Y., Ye, Z., Wu, X., Kim, K., Lee, Z., Wong, H. P., Pop, E., Allen, L. H. 2023

  Abstract

  Phase-change superlattices with nanometer thin sublayers are promising for low-power phase-change memory (PCM) on rigid and flexible platforms. However, the thermodynamics of the phase transition in such nanoscale superlattices remain unexplored, especially at ultrafast scanning rates, which is crucial for our fundamental understanding of superlattice-based PCM. Here, we probe the phase transition of Sb2Te3 (ST)/Ge2Sb2Te5 (GST) superlattices using nanocalorimetry with a monolayer sensitivity (∼1 Å) and a fast scanning rate (105 K/s). For a 2/1.8 nm/nm Sb2Te3/GST superlattice, we observe an endothermic melting transition with an ∼240 °C decrease in temperature and an ∼8-fold decrease in enthalpy compared to those for the melting of GST, providing key thermodynamic insights into the low-power switching of superlattice-based PCM. Nanocalorimetry measurements for Sb2Te3 alone demonstrate an intrinsic premelting similar to the unique phase transition of superlattices, thus revealing a critical role of the Sb2Te3 sublayer within our superlattices. These results advance our understanding of superlattices for energy-efficient data storage and computing.

  View details for DOI 10.1021/acs.nanolett.3c01049

  View details for PubMedID 37171275

• Energy Efficient Neuro-inspired Phase Change Memory Based on Ge4 Sb6 Te7 as a Novel Epitaxial Nanocomposite. Advanced materials (Deerfield Beach, Fla.) Khan, A. I., Yu, H., Zhang, H., Goggin, J. R., Kwon, H., Wu, X., Perez, C., Neilson, K. M., Asheghi, M., Goodson, K. E., Vora, P. M., Davydov, A., Takeuchi, I., Pop, E. 2023: e2300107

  Abstract

  Phase change memory (PCM) is a promising candidate for neuro-inspired, data-intensive artificial intelligence applications, which relies on the physical attributes of PCM materials including gradual change of resistance states and multilevel operation with low resistance drift. However, achieving these attributes simultaneously remains a fundamental challenge for PCM materials such as Ge2 Sb2 Te5 , the most commonly used material. Here we demonstrate bi-directional gradual resistance changes with ∼10x resistance window using low energy pulses in nanoscale PCM devices based on Ge4 Sb6 Te7 , a new phase change nanocomposite material. These devices show 13 resistance levels with low resistance drift for the first 8 levels, resistance on/off ratio of ∼1000, and low variability. These attributes are enabled by the unique microstructural and electrothermal properties of Ge4 Sb6 Te7 , a nanocomposite consisting of epitaxial SbTe nanoclusters within the Ge-Sb-Te matrix, and a higher crystallization but lower melting temperature than Ge2 Sb2 Te5 . These results advance the pathway towards energy-efficient analog computing using PCM. This article is protected by copyright. All rights reserved.

  View details for DOI 10.1002/adma.202300107

  View details for PubMedID 36720651

• Understanding Interface-Controlled Resistance Drift in Superlattice Phase Change Memory IEEE ELECTRON DEVICE LETTERS Wu, X., Khan, A., Ramesh, P., Perez, C., Kim, K., Lee, Z., Saraswat, K., Goodson, K. E., Wong, H., Pop, E. 2022; 43 (10): 1669-1672

  View details for DOI 10.1109/LED.2022.3203971

  View details for Web of Science ID 000861441600023

• Unveiling the Effect of Superlattice Interfaces and Intermixing on Phase Change Memory Performance. Nano letters Khan, A. I., Wu, X., Perez, C., Won, B., Kim, K., Ramesh, P., Kwon, H., Tung, M. C., Lee, Z., Oh, I., Saraswat, K., Asheghi, M., Goodson, K. E., Wong, H. P., Pop, E. 2022

  Abstract

  Superlattice (SL) phase change materials have shown promise to reduce the switching current and resistance drift of phase change memory (PCM). However, the effects of internal SL interfaces and intermixing on PCM performance remain unexplored, although these are essential to understand and ensure reliable memory operation. Here, using nanometer-thin layers of Ge2Sb2Te5 and Sb2Te3 in SL-PCM, we uncover that both switching current density (Jreset) and resistance drift coefficient (v) decrease as the SL period thickness is reduced (i.e., higher interface density); however, interface intermixing within the SL increases both. The signatures of distinct versus intermixed interfaces also show up in transmission electron microscopy, X-ray diffraction, and thermal conductivity measurements of our SL films. Combining the lessons learned, we simultaneously achieve low Jreset 3-4 MA/cm2 and ultralow v 0.002 in mushroom-cell SL-PCM with 110 nm bottom contact diameter, thus advancing SL-PCM technology for high-density storage and neuromorphic applications.

  View details for DOI 10.1021/acs.nanolett.2c01869

  View details for PubMedID 35876819

• First Demonstration of Ge2Sb2Te5-Based Superlattice Phase Change Memory with Low Reset Current Density (~3 MA/cm2) and Low Resistance Drift (~0.002 at 105°C) IEEE Symposium on VLSI Technology and Circuits (VLSI Technology and Circuits) Khan, A., Perez, C., Wu, X., Won, B., Kim, K., Kwon, H., Ramesh, P., Neilson, K. M., Asheghi, M., Saraswat, K., Lee, Z., Oh, I., Wong, H., Pop, E. 2022

  View details for DOI 10.1109/VLSITechnologyandCir46769.2022.9830348

• Multiple Tunable Hyperbolic Resonances in Broadband Infrared Carbon-Nanotube Metamaterials PHYSICAL REVIEW APPLIED Roberts, J., Ho, P., Yu, S., Wu, X., Luo, Y., Wilson, W. L., Falk, A. L., Fan, J. A. 2020; 14 (4)

  View details for DOI 10.1103/PhysRevApplied.14.044006

  View details for Web of Science ID 000576948600002

• Uniform and ultrathin high-kappa gate dielectrics for two-dimensional electronic devices NATURE ELECTRONICS Li, W., Zhou, J., Cai, S., Yu, Z., Zhang, J., Fang, N., Li, T., Wu, Y., Chen, T., Xie, X., Ma, H., Yan, K., Dai, N., Wu, X., Zhao, H., Wang, Z., He, D., Pan, L., Shi, Y., Wang, P., Chen, W., Nagashio, K., Duan, X., Wang, X. 2019; 2 (12): 563-571

  View details for DOI 10.1038/s41928-019-0334-y

  View details for Web of Science ID 000512908500010