Seungha Shin

In-Plane Thermal Conductivity of Polycrystalline Chemical Vapor Deposition Graphene with Controlled Grain Sizes

Manipulation of the chemical vapor deposition graphene synthesis conditions, such as operating P, T, heating/cooling time intervals, and precursor gas concentration ratios (CH4/H2), allowed for synthesis of polycrystalline single-layered graphene with controlled grain sizes. The graphene samples were then suspended on 8 μm diameter patterned holes on a silicon-nitride (Si3N4) substrate, and the in-plane thermal conductivities k(T) for 320 K < T < 510 K were measured to be 2660-1230, 1890-1020, and 680-340 W/m·K for average grain sizes of 4.1, 2.2, and 0.5 μm, respectively, using an opto-thermal Raman technique. Fitting of these data by a simple linear chain model of polycrystalline thermal transport determined k = 5500-1980 W/m·K for single-crystal graphene for the same temperature range above; thus, significant reduction of k was achieved when the grain size was decreased from infinite down to 0.5 μm. Furthermore, detailed elaborations were performed to assess the measurement reliability of k by addressing the hole-edge boundary condition, and the air-convection/radiation losses from the graphene surface.

Anisotropic control of thermal transport in graphene/Si heterostructures

The cross-plane interaction across interface changes phonon kinetics and spectrum near the interface, and the interaction effects on both in-plane and cross-plane thermal transport are investigated in graphene/Si heterostructure. The interaction with substrates dramatically reduces the in-plane thermal conductivity of graphene by changing the behaviors of the out-of-plane phonons as well as adding phonon-substrate scatterings. Applying pressure up to 2.6 GPa to the sandwiched graphene reduces the cross-plane interfacial thermal resistance by 50% without altering the in-plane thermal conductivity in a significant way. The pressure increases the inter-layer coupling and creates a low-energy phonon transport channel between graphene and Si with minor effects on phonons propagating along the graphene. This study suggests the anisotropic control of thermal transport, and the physics and calculation results can be used to improve the thermal design and analysis in two-dimensional nano-electronic devices.

Die level thermal storage for improved cooling of pulsed devices

In many communications applications semiconductor devices operate in a pulsed mode, where rapid temperature transients are continuously experienced within the die. We proposed a novel junction-level cooling technology where a metallic phase change material (PCM) was embedded in close proximity to the active transistor channels without interfering with the devices electrical response. Here we present multiscale simulations that were performed to determine the thermal performance improvement and electrical performance impact under pulsed operating conditions. The modeling effort was focused on Gallium Nitride (GaN) on Silicon (Si) chips with Indium (In) as the PCM. To accurately capture the microscale transient melting process, a hierarchical multiscale model was developed that includes linking of atomistic-level molecular dynamics simulations and macroscale finite element analysis simulations. Macroscale physics, including the melting process, were captured with a transient two-dimensional finite element analysis (FEA) model. The FEA model also includes interfacial and contact resistances between the semiconductor materials and PCM. Non-equilibrium Molecular Dynamic (MD) simulations were performed to estimate the value of the interfacial resistances between the Si substrate and the In PCM, which included a new interatomic potential between In and Si that was developed from experimental scattering results available in the literature. The thermal modeling results indicate 26% more heat can be dissipated through the PCM enhanced transistor while maintain a safe operating temperature. A separate electrical modeling effort showed that the metallic PCM layer did not create appreciable parasitic capacitances as long as the PCM was farther than 1μm from the active channel. The lower, more constant temperatures achieved by this technology can help improve the reliability and performance of future communication devices.

Entropy production in hot-phonon energy conversion to electric potential

We apply phonon and electron nonequilibrium-population statistical entropy analysis to the recently introduced phonon energy to electric potential conversion heterobarrier with its height optimized for optical phonon absorption under steady electric current. The entropy production rates for phonon and electron subsystems depend on their interaction kinetics and occupancy distributions, indicating the direction of the processes. Under upstream thermal equilibrium among electrons and acoustic and optical phonons, we predict an upper limit of 42% energy conversion for GaAs heterobarrier at 300 K, while the reported Monte Carlo prediction of 19% efficiency is below this limit. We show that for upstream electrons in thermal equilibrium with the acoustic phonons, while under supply of hot optical phonons, the conversion efficiency increases significantly, making integration of the barrier into optical phonon emitting circuits and devices very attractive.

Sintering of multiple Cu–Ag core–shell nanoparticles and properties of nanoparticle-sintered structures

Cu–Ag core–shell (CS) nanoparticles (NP) have been synthesized to replace pure Ag NP paste in order to lower the cost while maintaining excellent thermal and electrical conductivities for electronic applications. In this study, a multiple-CS-NP sintering model with molecular dynamics is employed to investigate the NP size and temperature dependency of the sintering process, as well as mechanical and thermodynamic properties of the sintered structures. Porosity and multiple particle effects are included, which allow for more accurate analysis than the conventional two- or three-NP sintering model. We unravelled the sintering mechanism at room temperature, and the interplay of liquid and solid surface diffusion during sintering at higher temperatures. Interfacial atoms have a higher mobility than surface atoms and contribute to a higher densification in the multiple-CS-NP model. A more densified structure yields higher Youngs modulus, yield strength and Poissons ratio, while lowering isothermal compressibility. The coefficient of thermal expansion and specific heat capacity exhibit grain-size and porosity independence. This multiple-CS-NP model provides a theoretical basis for determining NP configuration and sintering conditions for desirable properties.

High-power flexible AlGaN/GaN heterostructure field-effect transistors with suppression of negative differential conductance

We investigate thermo-electronic behaviors of flexible AlGaN/GaN heterostructure field-effect transistors (HFETs) for high-power operation of the devices using Raman thermometry, infrared imaging, and current-voltage characteristics. A large negative differential conductance observed in HFETs on polymeric flexible substrates is confirmed to originate from the decreasing mobility of the two-dimensional electron gas channel caused by the self-heating effect. We develop high-power transistors by suppressing the negative differential conductance in the flexible HFETs using chemical lift-off and modified Ti/Au/In metal bonding processes with copper (Cu) tapes for high thermal conductivity and low thermal interfacial resistance in the flexible hybrid structures. Among different flexible HFETs, the ID of the HFETs on Cu with Ni/Au/In structures decreases only by 11.3% with increasing drain bias from the peak current to the current at VDS = 20 V, which is close to that of the HFETs on Si (9.6%), solving the probl...

Preliminary first principle based electro-thermal coupled solver for silicon carbide power devices

The electrical behavior of high power, high frequency power devices is dramatically affected by heat generation caused due to interactions between energetic electrons and the lattice. It is critical that these interactions are taken into account when attempting to predict the electrical behavior of these devices. Since the hydrodynamic model follows the mass, momentum and energy transfer between the electrons/holes and phonons, it is capable of simultaneously predicting the electrical and thermal performance of these devices. This model is accurate and computationally efficient when properly correlated material parameters are available. In this work, a preliminary hydrodynamic transport device solver capable of accurately predicting the thermal and electrical performance of SiC devices is presented.

Enhanced laser cooling of CO2–Xe gas using (0200) excitation

Laser irradiation with a 9.6 μm wavelength resonant with the (0200) level improves the anti-Stokes cooling of CO2 gas. Excitation of the (0200) level increases cooling by producing a larger population of (0001), despite the higher-energy photon absorption, compared to a (1000) level excitation. Further selection of macroconditions (temperature, pressure, Xe diluent atomic fraction, and geometric parameters) enhances cooling by reducing parasite gas conduction through slower thermal energy transport and increasing the nonequilibrium population of the excited (0001) level by fast species diffusion and small collisional relaxation. We include reabsorption, and then find the conditions for optimal cooling.