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Now showing items 1 - 16 of 18

  • Phonon scattering in strained transition layers for GaN heteroepitaxy

    Cho, Jungwan   Li, Yiyang   Hoke, William E.   Altman, David H.   Asheghi, Mehdi   Goodson, Kenneth E.  

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  • Fundamental limits for near-junction conduction cooling of high power GaN-on-diamond devices

    Song, Changhwan   Kim, Jihyun   Lee, Hyoungsoon   Cho, Jungwan  

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  • Phonon conduction in GaN-diamond composite substrates

    Cho, Jungwan   Francis, Daniel   Altman, David H.   Asheghi, Mehdi   Goodson, Kenneth E.  

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  • Low cross-plane thermal conductivity of sub-1 µm polycrystalline silicon thin films for thermoelectric applications

    Kim, Jihyun   Cho, Jungwan  

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  • Thermal transport: Cool electronics

    Cho, Jungwan   Goodson, Kenneth E.  

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  • Low thermal conductivity of atomic layer deposition yttria-stabilized zirconia (YSZ) thin films for thermal insulation applications

    Cho, Jungwan   Park, Joonsuk   An, Jihwan  

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  • [IEEE 2014 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS) - La Jolla, CA, USA (2014.10.19-2014.10.22)] 2014 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS) - Thermal Interface Resistance Measurements for GaN-on-Diamond Composite Substrates

    Cho, Jungwan   Won, Yoonjin   Francis, Daniel   Asheghi, Mehdi   Goodson, Kenneth E.  

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  • [IEEE 2012 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS) - La Jolla, CA, USA (2012.10.14-2012.10.17)] 2012 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS) - Temperature Dependent Thermal Resistances at GaN-Substrate Interfaces in GaN Composite Substrates

    Cho, Jungwan   Li, Yiyang   Altman, David H.   Hoke, William E.   Asheghi, Mehdi   Goodson, Kenneth E.  

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  • [IEEE 2012 13th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm) - San Diego, CA, USA (2012.05.30-2012.06.1)] 13th InterSociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems - Thermal characterization of GaN-on-diamond substrates for HEMT applications

    Cho, Jungwan   Zijian Li,    Bozorg-Grayeli, Elah   Kodama, Takashi   Francis, Daniel   Ejeckam, Felix   Faili, Firooz   Asheghi, Mehdi   Goodson, Kenneth E.  

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  • Fundamental limits for near-junction conduction cooling of high power GaN-on-diamond devices

    Song, Changhwan   Kim, Jihyun   Lee, Hyoungsoon   Cho, Jungwan  

    The integration of differing materials can enable breakthrough performance for semiconductor devices. One example is the integration of gallium nitride (GaN) and diamond to form GaN-on-diamond, which enables high-power GaN devices to achieve extreme power densities and, arguably, approaches fundamental limits for conduction cooling. Here, we examine the fundamental limits for near-junction phonon conduction cooling of GaN-on-diamond devices via finite element calculations of their lowest possible thermal resistance. A semi-classical transport theory for phonons interacting with interfaces and defects is used to calculate the in-plane thermal conductivity of a GaN epilayer and thereby accurately account for the thermal spreading resistance of the GaN layer. The device thermal resistance of a state-of-the-art GaN-on-diamond structure is predicted to be similar to 13.0 Kmm W-1 for a 12 finger device with 30 mu m gate-to-gate spacing and a power dissipation of 5 W mm(-1). For the same multifinger cell geometry and dissipated power, device thermal resistances as low as similar to 10.0 K mm W-1 may be possible with assuming anisotropic but homogeneous diamond, as well as the absence of phonon scattering by external defects in the GaN layer and interface.
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  • Phonon Conduction in Silicon Nanobeam Labyrinths

    Park, Woosung   Romano, Giuseppe   Ahn, Ethan C.   Kodama, Takashi   Park, Joonsuk   Barako, Michael T.   Sohn, Joon   Kim, Jin   Cho, Jungwan   Marconnet, Amy M.   Asheghi, Mehdi   Kolpak, Alexie M.   Goodson, Kenneth E.  

    Here we study single-crystalline silicon nanobeams having 470 nm width and 80 nm thickness cross section, where we produce tortuous thermal paths (i.e. labyrinths) by introducing slits to control the impact of the unobstructed "line-of-sight" (LOS) between the heat source and heat sink. The labyrinths range from straight nanobeams with a complete LOS along the entire length to nanobeams in which the LOS ranges from partially to entirely blocked by introducing slits, s =3D 95, 195, 245, 295 and 395 nm. The measured thermal conductivity of the samples decreases monotonically from similar to 47 W m(-1) K-1 for straight beam to similar to 31 W m(-1) K-1 for slit width of 395 nm. A model prediction through a combination of the Boltzmann transport equation and ab initio calculations shows an excellent agreement with the experimental data to within similar to 8%. The model prediction for the most tortuous path (s =3D 395 nm) is reduced by similar to 14% compared to a straight beam of equivalent cross section. This study suggests that LOS is an important metric for characterizing and interpreting phonon propagation in nanostructures.
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  • Low Thermal Resistances at GaN-SiC Interfaces for HEMT Technology

    Cho, Jungwan   Bozorg-Grayeli, Elah   Altman, David H.   Asheghi, Mehdi   Goodson, Kenneth E.  

    The temperature rise in AlGaN/GaN high-electron-mobility transistors depends strongly on the GaN-substrate thermal interface resistance (TIR). We apply picosecond time-domain thermoreflectance measurements to GaN-SiC composite substrates with varying GaN thickness to extract both the TIR and the intrinsic GaN thermal conductivity at room temperature. Two complementary data extraction methodologies yield 4-5 m(2) . K/GW for the GaN-SiC TIR and 157-182 W/m . K for the GaN conductivity. The GaN-SiC interface resistance values reported here, as well as the TIR experimental uncertainties documented in this letter, are substantially lower than those reported previously for this material combination.
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  • Thermal Conduction across Metal-Dielectric Sidewall Interfaces

    Park, Woosung   Kodama, Takashi   Park, Joonsuk   Cho, Jungwan   Sood, Aditya   Barako, Michael T.   Asheghi, Mehdi   Goodson, Kenneth E.  

    The heat flow at the interfaces of complex nanostructures is three-dimensional in part due to the nonplanarity of interfaces. One example common in nano systems is the situation when a significant fraction of the interfacial area is composed of sidewalls that are perpendicular to the principal plane, for example, in metallization structures for complementary metal-oxide semiconductor transistors. It is often observed that such sidewall interfaces contain significantly higher levels of microstructural disorder, which impedes energy carrier transport and leads to effective increases in interfacial resistance. The impact of these sidewall interfaces needs to be explored in greater depth for practical device engineering, and a related problem is that appropriate characterization techniques are not available. Here, we develop a novel electrothermal method and an intricate microfabricated structure to extract the thermal resistance of a sidewall interface between aluminum and silicon dioxide using suspended nanograting structures. The thermal resistance of the sidewall interface is measured to be similar to 16 +/- 5 m(2) K GW(-1), which is twice as large as the equivalent horizontal planar interface comprising the same materials in the experimental sample. The rough sidewall interfaces are observed using transmission electron micrographs, which may be more extensive than at interfaces in the substrate plan in the same nanostructure. A model based on a two-dimensional sinusoidal surface estimates the impact of the roughness on thermal resistance to be similar to 2 m(2) K GW(-1). The large disparity between the model predictions and the experiments is attributed to the incomplete contact at the Al-SiO2 sidewall interfaces, inferred by observation of underetching of the silicon substrate below the sidewall opening. This study suggests that sidewall interfaces must be considered separately from planar interfaces in thermal analysis for nanostructured systems.
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  • Thickness-dependent thermal conductivity of ultrathin (< 100 nm) barium titanate films

    Kim, Yechan   Park, Joonsuk   Shin, Jeongwoo   An, Jihwan   Cho, Jungwan  

    Thin film barium titanate (BaTiO3) is a promising material in the electronics and ceramics industry owing to its compelling dielectric properties. A number of works have investigated its dielectric and structural properties, but less studied are its thermal properties particularly at sub-100 nm thicknesses. Here, we measure the room-temperature thermal conductivity of ultrathin (< 100 nm), pulsed laser deposited BaTiO3 films. The measured thermal conductivities are thickness-dependent, and this trend is consistent with the thickness-dependent crystallinity of the films. Transmission electron microscopy analysis of the films reveals the presence of an initial amorphous layer similar to 60 nm thick from the growth interface and the subsequent formation of columnar grains of width similar to 12 nm that are embedded within an amorphous matrix. For a region that incorporates grains with columnar morphology, we find that cross-plane heat conduction may be favored by 30-40% over in-plane heat conduction due to the columnar morphology of grains.
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  • Thermal Properties of Anisotropic and/or Inhomogeneous Suspended Thin Films Assessed via Dual-Side Time-Domain Thermoreflectance:A Numerical Study

    Cho, Jungwan  

    Time-domain thermoreflectance (TDTR) is a powerful method for measuring thermal properties, such as thermal conductivity and thermal boundary resistance, of a broad variety of thin-film materials and interfaces. Dual-side TDTR, in which measurements are performed on the top and bottom sides of a suspended region of a thin film of interest, has recently emerged as an effective way to investigate the thermal properties of a film that is thermally anisotropic and/or inhomogeneous. Despite its experimental versatility, dual-side TDTR has yet to be fully interrogated. In this work, we examine the thermal conductivity and boundary resistance of anisotropic and/or inhomogeneous suspended thin films, extracted by dual-side TDTR on these films via numerical simulation. We start from a simple case of an anisotropic or inhomogeneous suspended membrane and then consider the combined case where the suspended membrane is both anisotropic and inhomogeneous. Taken together with analysis of measurement sensitivity, we aim to provide a general guideline for data extraction methodologies for dual-side TDTR on anisotropic and/or inhomogeneous suspended thin films.
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  • Interference-free mid-IR laser absorption detection of methane

    Pyun, Sung Hyun   Cho, Jungwan   Davidson, David F.   Hanson, Ronald K.  

    A novel, mid-IR scanned-wavelength laser absorption diagnostic was developed for time-resolved, interference-free, absorption measurement of methane concentration. A differential absorption (peak minus valley) scheme was used that takes advantage of the structural differences of the absorption spectrum of methane and other hydrocarbons. A peak and valley wavelength pair was selected to maximize the differential cross-section (sigma(peak) (minus) (valley)) of methane for the maximum signal-to-noise ratio, and to minimize that of the interfering absorbers. Methane cross-sections at the peak and valley wavelengths were measured over a range of temperatures, 1000 to 2000 K, and pressures 1.3 to 5.4 atm. The cross-sections of the interfering absorbers were assumed constant over the small wavelength interval between the methane peak and valley features. Using this diagnostic, methane concentration time histories during n-heptane pyrolysis were measured behind reflected shock waves in a shock tube. The differential absorption scheme efficiently rejected the absorption interference and successfully recovered the vapor-phase methane concentration. These measurements allowed the comparison with methane concentration time-history simulations derived from a current n-heptane reaction mechanism (Sirjean et al 2009 A high-temperature chemical kinetic model of n-alkane oxidation JetSurF version 1.0).
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