Stanford Advisors

All Publications

  • Heat Conductor-Insulator Transition in Electrochemically Controlled Hybrid Superlattices. Nano letters Zhou, J., Wu, Y., Kwon, H., Li, Y., Xiao, X., Ye, Y., Ma, Y., Goodson, K. E., Hwang, H. Y., Cui, Y. 2022


    Designing materials with ultralow thermal conductivity has broad technological impact, from thermal protection to energy harvesting. Low thermal conductivity is commonly observed in anharmonic and strongly disordered materials, yet a microscopic understanding of the correlation to atomic motion is often lacking. Here we report that molecular insertion into an existing two-dimensional layered lattice structure creates a hybrid superlattice with extremely low thermal conductivity. Vibrational characterization and ab initio molecular dynamics simulations reveal strong damping of transverse acoustic waves and significant softening of longitudinal vibrations. Together with spectral correlation analysis, we demonstrate that the molecular insertion creates liquid-like atomic motion in the existing lattice framework, causing a large suppression of heat conduction. The hybrid materials can be transformed into solution-processable coatings and used for thermal protection in wearable electronics. Our work provides a generic mechanism for the design of heat insulators and may further facilitate the engineering of heat conduction based on understanding atomic correlations.

    View details for DOI 10.1021/acs.nanolett.2c01407

    View details for PubMedID 35715219

  • Electro-Thermal Confinement Enables Improved Superlattice Phase Change Memory IEEE ELECTRON DEVICE LETTERS Khan, A., Kwon, H., Chen, M. E., Asheghi, M., Wong, H., Goodson, K. E., Pop, E. 2022; 43 (2): 204-207
  • Thermal Characterization of Metal-Oxide Interfaces Using Time-Domain Thermoreflectance with Nanograting Transducers. ACS applied materials & interfaces Kwon, H., Perez, C., Park, W., Asheghi, M., Goodson, K. E. 2021


    Metal-oxide thermal boundary conductance (TBC) strongly influences the temperature rise in nanostructured systems, such as dense interconnects, when its value is comparable to the thermal conductance of the amorphous dielectric oxide. However, the thermal characterization of metal-amorphous oxide TBC is often hampered by the measurement insensitivity of techniques such as time-domain thermoreflectance (TDTR). Here, we use metal nanograting structures as opto-thermal transducers in TDTR to measure the TBC of metal-oxide interfaces. Combined with an ultrafast pump-probe laser measurement approach, the nanopatterned structures amplify the contribution of the thermal boundary resistance (TBR), the inverse of TBC, over the thermal resistance of the adjacent material, thereby enhancing measurement sensitivity. For demonstration purposes, we report the TBC between Al and SiO2 films. We then compare the impact of Al grating dimensions on the measured TBC values, sensitivities, and uncertainties. The grating periods L used in this study range from 150 to 300 nm, and the bridge widths w range from 72 to 205 nm. With the narrowest grating transducers (72 nm), the TBC of Al-SiO2 interfaces is measured to be 159-48+61 MW m-2 K-1, with the experimental sensitivity being 5* higher than that of a blanket Al film. This improvement is attributed to the reduced contribution of the SiO2 film thermal resistance to the temperature signal from TDTR response. The nanograting measurement approach described here is promising for the thermal characterization of a variety of nanostructured metal-amorphous passivation systems and interfaces common in semiconductor technology.

    View details for DOI 10.1021/acsami.1c12422

    View details for PubMedID 34797056

  • Thermal Interface Enhancement via Inclusion of an Adhesive Layer Using Plasma-Enhanced Atomic Layer Deposition. ACS applied materials & interfaces Kwon, H., Perez, C., Kim, H. K., Asheghi, M., Park, W., Goodson, K. E. 2021


    Interfaces govern thermal transport in a variety of nanostructured systems such as FinFETs, interconnects, and vias. Thermal boundary resistances, however, critically depend on the choice of materials, nanomanufacturing processes and conditions, and the planarity of interfaces. In this work, we study the interfacial thermal transport between a nonreactive metal (Pt) and a dielectric by engineering two differing bonding characters: (i) the mechanical adhesion/van der Waals bonding offered by the physical vapor deposition (PVD) and (ii) the chemical bonding generated by plasma-enhanced atomic layer deposition (PEALD). We introduce 40-cycle (2 nm thick), nearly continuous PEALD Pt films between 98 nm PVD Pt and dielectric materials (8.0 nm TiO2/Si and 11.0 nm Al2O3/Si) treated with either O2 or O2 + H2 plasma to modulate their bonding strengths. By correlating the treatments through thermal transport measurements using time-domain thermoreflectance (TDTR), we find that the thermal boundary resistances are consistently reduced with the same increased treatment complexity that has been demonstrated in the literature to enhance mechanical adhesion. For samples on TiO2 (Al2O3), reductions in thermal resistance are at least 4% (10%) compared to those with no PEALD Pt at all, but could be as large as 34% (42%) given measurement uncertainties that could be improved with thinner nucleation layers. We suspect the O2 plasma generates stronger covalent bonds to the substrate, while the H2 plasma strips the PEALD Pt of contaminants such as carbon that gives rise to a less thermally resistive heat conduction pathway.

    View details for DOI 10.1021/acsami.0c19197

    View details for PubMedID 33914509

  • Uncovering Thermal and Electrical Properties of Sb2Te3/GeTe Superlattice Films. Nano letters Kwon, H., Khan, A. I., Perez, C., Asheghi, M., Pop, E., Goodson, K. E. 2021


    Superlattice-like phase change memory (SL-PCM) promises lower switching current than conventional PCM based on Ge2Sb2Te5 (GST); however, a fundamental understanding of SL-PCM requires detailed characterization of the interfaces within such an SL. Here we explore the electrical and thermal transport of SLs with deposited Sb2Te3 and GeTe alternating layers of various thicknesses. We find up to an approximately four-fold reduction of the effective cross-plane thermal conductivity of the SL stack (as-deposited polycrystalline) compared with polycrystalline GST (as-deposited amorphous and later annealed) due to the thermal interface resistances within the SL. Thermal measurements with varying periods of our SLs show a signature of phonon coherence with a transition from wave-like to particle-like phonon transport, further described by our modeling. Electrical resistivity measurements of such SLs reveal strong anisotropy (∼2000×) between the in-plane and cross-plane directions due to the weakly interacting van der Waals-like gaps. This work uncovers electrothermal transport in SLs based on Sb2Te3 and GeTe for the improved design of low-power PCM.

    View details for DOI 10.1021/acs.nanolett.1c00947

    View details for PubMedID 34270270

  • Two-Fold Reduction of Switching Current Density in Phase Change Memory Using Bi2Te3 Thermoelectric Interfacial Layer IEEE ELECTRON DEVICE LETTERS Khan, A., Kwon, H., Islam, R., Perez, C., Chen, M. E., Asheghi, M., Goodson, K. E., Wong, H., Pop, E. 2020; 41 (11): 1657–60
  • Thermal and Manufacturing Design Considerations for Silicon-Based Embedded Microchannel-Three-Dimensional Manifold Coolers-Part 2: Parametric Study of EMMCs for High Heat Flux (similar to 1kW/cm(2)) Power Electronics Cooling Jung, K., Hazra, S., Kwon, H., Piazza, A., Jih, E., Asheghi, M., Gupta, M., Degner, M., Goodson, K. E. ASME. 2020

    View details for DOI 10.1115/1.4047883

    View details for Web of Science ID 000576282500019

  • Tungsten-doped Ge2Sb2Te5 phase change material for high-speed optical switching devices APPLIED PHYSICS LETTERS Guo, P., Burrow, J. A., Sevison, G. A., Kwon, H., Perez, C., Hendrickson, J. R., Smith, E. M., Asheghi, M., Goodson, K. E., Agha, I., Sarangan, A. M. 2020; 116 (13)

    View details for DOI 10.1063/1.5142552

    View details for Web of Science ID 000524534800001

  • Phase Change Dynamics and Two-Dimensional 4-Bit Memory in Ge2Sb2Te5 via Telecom-Band Encoding ACS PHOTONICS Sevison, G. A., Farzinazar, S., Burrow, J. A., Perez, C., Kwon, H., Lee, J., Asheghi, M., Goodson, K. E., Sarangan, A., Hendrickson, J. R., Agha, I. 2020; 7 (2): 480–87
  • Tunable Dielectric and Thermal Properties of Oxide Dielectrics via Substrate Biasing in Plasma-Enhanced Atomic Layer Deposition. ACS applied materials & interfaces Kim, Y. n., Kwon, H. n., Han, H. S., Kim, H. J., Kim, B. S., Lee, B. C., Lee, J. n., Asheghi, M. n., Prinz, F. B., Goodson, K. E., Lim, J. n., Sim, U. n., Park, W. n. 2020


    The ability to control the properties of dielectric thin films on demand is of fundamental interest in nanoscale devices. Here, we modulate plasma characteristics at the surface of a substrate to tune both dielectric constant and thermal conductivity of amorphous thin films grown using plasma-enhanced atomic layer deposition. Specifically, we apply a substrate bias ranging from 0 to ∼117 V and demonstrate the systematic tunability of various material parameters of Al2O3. As a function of the substrate bias, we find a nonmonotonical evolution of intrinsic properties, including density, dielectric constant, and thermal conductivity. A key observation is that the maximum values in dielectric constant and effective thermal conductivity emerge at different substrate biases. The impact of density on both thermal conductivity and dielectric constant is further examined using a differential effective medium theory and the Clausius-Mossotti model, respectively. We find that the peak value in the dielectric constant deviates from the Clausius-Mossotti model, indicating the change of oxygen fraction in our thin films as a function of substrate bias. This finding suggests that the increased local strength of plasma sheath not only enhances material density but also controls the dynamics of microstructural defect formation beyond what is possible with conventional approaches. Based on our experimental observations and modeling, we further build a phenomenological relation between dielectric constant and thermal conductivity. Our results pave invaluable avenues for optimizing dielectric thin films at the atomic scale for a wide range of applications in nanoelectronics and energy devices.

    View details for DOI 10.1021/acsami.0c11086

    View details for PubMedID 32915545

  • Optical and electrical properties of phase change materials for high-speed optoelectronics Burrow, J. A., Guo, P., Sevison, G. A., Kwon, H., Perez, C., Asheghi, M., Hendrickson, J. R., Sarangan, A., Goodson, K. E., Agha, I., IEEE IEEE. 2019
  • Direct Visualization of Thermal Conductivity Suppression Due to Enhanced Phonon Scattering Near Individual Grain Boundaries. Nano letters Sood, A., Cheaito, R., Bai, T., Kwon, H., Wang, Y., Li, C., Yates, L., Bougher, T., Graham, S., Asheghi, M., Goorsky, M., Goodson, K. E. 2018


    Understanding the impact of lattice imperfections on nanoscale thermal transport is crucial for diverse applications ranging from thermal management to energy conversion. Grain boundaries (GBs) are ubiquitous defects in polycrystalline materials, which scatter phonons and reduce thermal conductivity (kappa). Historically, their impact on heat conduction has been studied indirectly through spatially averaged measurements, that provide little information about phonon transport near a single GB. Here, using spatially resolved time-domain thermoreflectance (TDTR) measurements in combination with electron backscatter diffraction (EBSD), we make localized measurements of kappa within few mum of individual GBs in boron-doped polycrystalline diamond. We observe strongly suppressed thermal transport near GBs, a reduction in kappa from 1000 W m-1 K-1 at the center of large grains to 400 W m-1 K-1 in the immediate vicinity of GBs. Furthermore, we show that this reduction in kappa is measured up to 10 mum away from a GB. A theoretical model is proposed that captures the local reduction in phonon mean-free-paths due to strongly diffuse phonon scattering at the disordered grain boundaries. Our results provide a new framework for understanding phonon-defect interactions in nanomaterials, with implications for the use of high-kappa polycrystalline materials as heat sinks in electronics thermal management.

    View details for PubMedID 29631399

  • HIGH STABILITY THERMAL ACCELEROMETER BASED ON ULTRATHIN PLATINUM ALD NANOSTRUCTURES Everhart, C. M., Kaplan, K. E., Winterkorn, M. M., Kwon, H., Provine, J., Asheghi, M., Goodson, K. E., Prinz, F. B., Kenny, T. W., IEEE IEEE. 2018: 976–79