Xiangjin Wu

All Publications

• Nonvolatile Phase-Only Transmissive Spatial Light Modulator with Electrical Addressability of Individual Pixels. ACS nano Fang, Z., Chen, R., Froch, J. E., Tanguy, Q. A., Khan, A. I., Wu, X., Tara, V., Manna, A., Sharp, D., Munley, C., Miller, F., Zhao, Y., Geiger, S., Bohringer, K. F., Reynolds, M. S., Pop, E., Majumdar, A. 2024

  Abstract

  Active metasurfaces with tunable subwavelength-scale nanoscatterers are promising platforms for high-performance spatial light modulators (SLMs). Among the tuning methods, phase-change materials (PCMs) are attractive because of their nonvolatile, threshold-driven, and drastic optical modulation, rendering zero-static power, crosstalk immunity, and compact pixels. However, current electrically controlled PCM-based metasurfaces are limited to global amplitude modulation, which is insufficient for SLMs. Here, an individual-pixel addressable, transmissive metasurface is experimentally demonstrated using the low-loss PCM Sb2Se3 and doped silicon nanowire heaters. The nanowires simultaneously form a diatomic metasurface, supporting a high-quality-factor (406) quasi-bound-state-in-the-continuum mode. A global phase-only modulation of 0.25pi (0.2pi) in simulation (experiment) is achieved, showing ten times enhancement. A 2pi phase shift is further obtained using a guided-mode resonance with enhanced light-Sb2Se3 interaction. Finally, individual-pixel addressability and SLM functionality are demonstrated through deterministic multilevel switching (ten levels) and tunable far-field beam shaping. Our work presents zero-static power transmissive phase-only SLMs, enabled by electrically controlled low-loss PCMs and individual meta-molecule addressable metasurfaces.

  View details for DOI 10.1021/acsnano.4c00340

  View details for PubMedID 38639708

• Novel nanocomposite-superlattices for low energy and high stability nanoscale phase-change memory. Nature communications Wu, X., Khan, A. I., Lee, H., Hsu, C. F., Zhang, H., Yu, H., Roy, N., Davydov, A. V., Takeuchi, I., Bao, X., Wong, H. P., Pop, E. 2024; 15 (1): 13

  Abstract

  Data-centric applications are pushing the limits of energy-efficiency in today's computing systems, including those based on phase-change memory (PCM). This technology must achieve low-power and stable operation at nanoscale dimensions to succeed in high-density memory arrays. Here we use a novel combination of phase-change material superlattices and nanocomposites (based on Ge4Sb6Te7), to achieve record-low power density ≈ 5 MW/cm2 and ≈ 0.7 V switching voltage (compatible with modern logic processors) in PCM devices with the smallest dimensions to date (≈ 40 nm) for a superlattice technology on a CMOS-compatible substrate. These devices also simultaneously exhibit low resistance drift with 8 resistance states, good endurance (≈ 2 × 108 cycles), and fast switching (≈ 40 ns). The efficient switching is enabled by strong heat confinement within the superlattice materials and the nanoscale device dimensions. The microstructural properties of the Ge4Sb6Te7 nanocomposite and its high crystallization temperature ensure the fast-switching speed and stability in our superlattice PCM devices. These results re-establish PCM technology as one of the frontrunners for energy-efficient data storage and computing.

  View details for DOI 10.1038/s41467-023-42792-4

  View details for PubMedID 38253559

  View details for PubMedCentralID PMC10803317

• 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