Showing papers by "Samuel Graham published in 2018"

Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

Abstract: J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

Low Thermal Boundary Resistance Interfaces for GaN-on-Diamond Devices

Abstract: The development of GaN-on-diamond devices holds much promise for the creation of high-power density electronics. Inherent to the growth of these devices, a dielectric layer is placed between the GaN and diamond, which can contribute significantly to the overall thermal resistance of the structure. In this work, we explore the role of different interfaces in contributing to the thermal resistance of the interface of GaN/diamond layers, specifically using 5 nm layers of AlN, SiN, or no interlayer at all. Using time-domain thermoreflectance along with electron energy loss spectroscopy, we were able to determine that a SiN interfacial layer provided the lowest thermal boundary resistance (<10 m2K/GW) because of the formation of an Si–C–N layer at the interface. The AlN and no interlayer samples were observed to have TBRs greater than 20 m2K/GW as a result of a harsh growth environment that roughened the interface (enhancing phonon scattering) when the GaN was not properly protected.

Direct Visualization of Thermal Conductivity Suppression Due to Enhanced Phonon Scattering Near Individual Grain Boundaries.

Abstract: 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 (κ). 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 κ within few μm of individual GBs in boron-doped polycrystalline diamond. We observe strongly suppressed thermal transport near GBs, a reduction in κ 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 κ is measured up to ∼10 μm 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-κ polycrystalline materials as heat sinks in electronics thermal management.

Probing Growth-Induced Anisotropic Thermal Transport in High-Quality CVD Diamond Membranes by Multifrequency and Multiple-Spot-Size Time-Domain Thermoreflectance.

Abstract: The maximum output power of GaN-based high-electron mobility transistors is limited by high channel temperature induced by localized self-heating, which degrades device performance and reliability. Chemical vapor deposition (CVD) diamond is an attractive candidate to aid in the extraction of this heat and in minimizing the peak operating temperatures of high-power electronics. Owing to its inhomogeneous structure, the thermal conductivity of CVD diamond varies along the growth direction and can differ between the in-plane and out-of-plane directions, resulting in a complex three-dimensional (3D) distribution. Depending on the thickness of the diamond and size of the electronic device, this 3D distribution may impact the effectiveness of CVD diamond in device thermal management. In this work, time-domain thermoreflectance is used to measure the anisotropic thermal conductivity of an 11.8 μm-thick high-quality CVD diamond membrane from its nucleation side. Starting with a spot-size diameter larger than the thickness of the membrane, measurements are made at various modulation frequencies from 1.2 to 11.6 MHz to tune the heat penetration depth and sample the variation in thermal conductivity. We then analyze the data by creating a model with the membrane divided into ten sublayers and assume isotropic thermal conductivity in each sublayer. From this, we observe a two-dimensional gradient of the depth-dependent thermal conductivity for this membrane. The local thermal conductivity goes beyond 1000 W/(m K) when the distance from the nucleation interface only reaches 3 μm. Additionally, by measuring the same region with a smaller spot size at multiple frequencies, the in-plane and cross-plane thermal conductivities are extracted. Through this use of multiple spot sizes and modulation frequencies, the 3D anisotropic thermal conductivity of CVD diamond membrane is experimentally obtained by fitting the experimental data to a thermal model. This work provides an improved understanding of thermal conductivity inhomogeneity in high-quality CVD polycrystalline diamond that is important for applications in the thermal management of high-power electronics.

Thermal Boundary Conductance Across Heteroepitaxial ZnO/GaN Interfaces: Assessment of the Phonon Gas Model.

Abstract: We present experimental measurements of the thermal boundary conductance (TBC) from 78-500 K across isolated heteroepitaxially grown ZnO films on GaN substrates. This data provides an assessment of the underlying assumptions driving phonon gas-based models, such as the diffuse mismatch model (DMM), and atomistic Green's function (AGF) formalisms used to predict TBC. Our measurements, when compared to previous experimental data, suggest that TBC can be influenced by long wavelength, zone center modes in a material on one side of the interface as opposed to the '"vibrational mismatch"' concept assumed in the DMM; this disagreement is pronounced at high temperatures. At room temperature, we measure the ZnO/GaN TBC as 490[+150,-110] MW m-2 K-1. The disagreement among the DMM and AGF, and the experimental data at elevated temperatures, suggests a non-negligible contribution from other types of modes that are not accounted for in the fundamental assumptions of these harmonic based formalisms, which may rely on anharmonicity. Given the high quality of these ZnO/GaN interfaces, these results provide an invaluable, critical, and quantitative assessment of the accuracy of assumptions in the current state of the art computational approaches used to predict phonon TBC across interfaces.

Thermal characterization of gallium nitride p-i-n diodes

Abstract: In this study, various thermal characterization techniques and multi-physics modeling were applied to understand the thermal characteristics of GaN vertical and quasi-vertical power diodes Optical thermography techniques typically used for lateral GaN device temperature assessment including infrared thermography, thermoreflectance thermal imaging, and Raman thermometry were applied to GaN p-i-n diodes to determine if each technique is capable of providing insight into the thermal characteristics of vertical devices Of these techniques, thermoreflectance thermal imaging and nanoparticle assisted Raman thermometry proved to yield accurate results and are the preferred methods of thermal characterization of vertical GaN diodes Along with this, steady state and transient thermoreflectance measurements were performed on vertical and quasi-vertical GaN p-i-n diodes employing GaN and Sapphire substrates, respectively Electro-thermal modeling was performed to validate measurement results and to demonstrate the effect of current crowding on the thermal response of quasi-vertical diodes In terms of mitigating the self-heating effect, both the steady state and transient measurements demonstrated the superiority of the tested GaN-on-GaN vertical diode compared to the tested GaN-on-Sapphire quasi-vertical structure

Transient Thermal Characterization of AlGaN/GaN HEMTs Under Pulsed Biasing

Abstract: The development of steady-state thermal characterization techniques for AlGaN/GaN high-electron mobility transistors (HEMTs) has been used to measure the device’s peak temperature under DC conditions. Despite these methods enabling the accurate quantification of the device’s effective thermal resistance and power density dependence, transient thermometry techniques are necessary to understand the nanoscale thermal transport within the active GaN layer where the highly localized joule heating occurs. One technique that has shown the ability to achieve this is transient thermoreflectance imaging (TTI). The accuracy of TTI is based on using the correct thermoreflectance coefficient. In the past, alternative techniques have been used to adjust the thermoreflectance coefficient to match the correct temperature rise in the device. This paper provides a new method to accurately determine the thermoreflectance coefficient of a given surface and is validated via an electrical method: gate resistance thermometry (GRT). Close agreement is shown between the temperature rise of the passivated gate metal measured by TTI and the averaged gate temperature monitored by GRT. Overall, TTI can now be used to thermally map GaN HEMTs under pulsed conditions providing simultaneously a submicrosecond temporal resolution and a submicrometer spatial resolution.

Tunable Thermal Energy Transport across Diamond Membranes and Diamond-Si Interfaces by Nanoscale Graphoepitaxy

Abstract: The development of electronic devices, especially those that involve heterogeneous integration of materials, has led to increased challenges in addressing their thermal operational-temperature demands. The heat flow in these systems is significantly influenced or even dominated by thermal boundary resistance at interface between dissimilar materials. However, controlling and tuning heat transport across an interface and in the adjacent materials has so far drawn limited attention. In this work, we grow chemical-vapor-deposited (CVD) diamond on silicon substrates by graphoepitaxy and experimentally demonstrate tunable thermal transport across diamond membranes and diamond-silicon interfaces. We observed the highest diamond-silicon thermal boundary conductance (TBC) measured to date and increased diamond thermal conductivity due to strong grain texturing in the diamond near the interface. Additionally, non-equilibrium molecular-dynamics (NEMD) simulations and a Landauer approach are used to understand the diamond-silicon TBC. These findings pave the way for tuning or increasing thermal conductance in heterogeneously integrated electronics that involve polycrystalline materials and will impact applications including electronics thermal management and diamond growth.

Enhancing the thermosiphon-driven discharge of a latent heat thermal storage system used in a personal cooling device

Abstract: Personal cooling devices reduce energy loads by allowing buildings to operate with elevated setpoint temperatures, without compromising on the occupant comfort. One such novel technology called the Roving Comforter (RoCo) uses a compact R134a based vapor compression system for cooling. Following its cooling operation, during which waste heat from the condensing refrigerant is stored in a phase change material (PCM), a two-phase loop thermosiphon is used to discharge (solidify) the PCM to enable its next operation. The transient operation of this thermosiphon is the focus of the present article. Use of a PCM as the storage medium provides high energy density due to the ability to store thermal energy as latent heat during the phase transition; however, the discharge rate is limited by the low thermal conductivity of the PCM. Insertion of a graphite foam within the PCM can increase the rate of discharge and decrease the downtime of the cooling device. Since graphite enhancement involves a tradeoff between improving the discharge time at the expense of PCM volumetric latent heat, the impact of graphite foam density on the PCM discharge rate is investigated by using a Modelica-based transient model of the thermosiphon. The semi-empirical model, which uses relevant heat transfer coefficient and pressure drop correlations for both refrigerant and airside heat transfer, captures the complex phenomena involving simultaneous phase change of the refrigerant and the PCM. The graphite enhanced PCM selected from this analysis results in a 51% reduction in the discharge time with addition of only 5% to the thermal storage weight, without compromising the required cooling time.

A Comparative Study on the Junction Temperature Measurements of LEDs With Raman Spectroscopy, Microinfrared (IR) Imaging, and Forward Voltage Methods

Abstract: Energy efficiency, long life, exceptional color, and performance of solid-state light sources have resulted in a rapidly increasing trend in a number of practical applications especially for general lighting after a long history of incandescent lamps. Besides, light-emitting diodes (LEDs) have thermal limitations that are vital for device quality and lifetime. Specifically, to improve the heat dissipation, one major parameter used to evaluate the LED performance is thermal resistance ( $R$ ). Reducing the resistance can improve the heat flow from the p-n junction to ambient during operation. To quantify this parameter, the LED junction temperature ( $T_{J}$ ) must be determined. In this paper, the junction temperatures are first measured with forward voltage method (FVM), Raman spectroscopy, and infrared (IR) imaging for a 465-nm bare blue LED chip (without any phosphor coating). Then, the same samples have been coated with a phosphor-particles added epoxy mixture (%13, 4300 CCT) to convert blue to white light, and the junction temperatures were measured again experimentally with the previously mentioned three methods and compared to each other. While IR imaging shows better capability on capturing the possible hotspots over the surface, Raman method and FVM were in reasonably good agreement on measuring the junction temperature for 465-nm blue (uncoated) LED chip. However, the measurements performed after coating have shown slightly different results with IR imaging and Raman methods, while FVM has shown consistent results for coated chips.

Diffuson-driven Ultralow Thermal Conductivity in Amorphous Nb2O5 Thin Films

Abstract: Niobium pentoxide (Nb2O5) has been extensively reported for applications of electrochemical energy storage, memristors, solar cells, light emitting diodes (LEDs), and electrochromic devices. The thermal properties of Nb2O5 play a critical role in device performance of these applications. However, very few studies on the thermal properties of Nb2O5 have been reported and a fundamental understanding of heat transport in Nb2O5 is still lacking. The present work closes this gap and provides the first study of thermal conductivity of amorphous Nb2O5 thin films. Ultralow thermal conductivity is observed without any size effect in films as thin as 48 nm, which indicates that propagons contribute negligibly to the thermal conductivity and that the thermal transport is dominated by diffusons. Density-function-theory (DFT) simulations combined with a diffuson-mediated minimum-thermal-conductivity model confirms this finding. Additionally, the measured thermal conductivity is lower than the amorphous limit (Cahill model), which proves that the diffuson model works better than the Cahill model to describe the thermal conduction mechanism in the amorphous Nb2O5 thin films. Additionally, the thermal conductivity does not change significantly with oxygen vacancy concentration. This stable and low thermal conductivity facilitates excellent performance for applications such as memristors.

Review of Inorganic Salt Hydrates with Phase Change Temperature in Range of 5 to 60°C and Material Cost Comparison with Common Waxes

Abstract: Phase change materials (PCMs) with desirable phase change temperatures can be used to provide a constant temperature thermal source or sink for diverse applications. As such, incorporating PCMs into building materials, equipment, or appliances can shift and/or reduce the energy load. The motivation of this work is to identify low-cost inorganic salt hydrate PCMs that can complement current building systems and designs, and compare them with common paraffins. In this work, we analyzed inorganic salt hydrates with phase change temperatures in the range of 5-60°C, to target both space heating and cooling applications. The properties of the salt hydrates were compared with paraffins over the same temperature range. The results showed that PCMs with a melting temperature above 20°C, salt hydrates have advantages over paraffins including higher thermal energy density (45-120 kWh/m for salt hydrates; 45-60 kWh/m for paraffins) and generally lower material energy cost (1-20 $/kWh for salt hydrates; 20-30 $/kWh for comparable paraffins). For PCMs with a melting temperature less than 20°C, the material cost is higher for both salt hydrates and paraffins (30-110 $/kWh for both classes of materials) and salt hydrates retain their advantage of greater thermal energy density (50-120 kWh/m for salt hydrates; 45-60 kWh/m for paraffins). In all cases, factors including thermal cyclability, stability, congruency, corrosion, and supercooling must be considered when comparing paraffins and salt hydrates for a particular application. Finally, we give an overview of enhancement techniques for salt hydrate PCMs and find that limited efforts have been pursued to tune salt hydrate phase change temperatures, with a wider range of studies investigating stabilization and minimization of supercooling. This analysis shows the potential of developing salt hydrate PCMs for low-cost heating and cooling thermal energy storage systems for a range of applications.

Improving the Transient Thermal Characterization of GaN HEMTs

Abstract: Transient thermoreflectance imaging (TTI) is a thermometry technique employed to map the surface temperature distribution of GaN HEMTs. The accuracy of the technique is dependent on applying the correct thermoreflectance coefficient to the region of interest on the device surface. TTI has shown high accuracy when measuring the temperature rise of metals such as Schottky contacts but the technique has never been verified when applied to semiconductors. Using UV LED excitation sources, TTI of the GaN channel in GaN/Si HEMTs is presented and verified for the first time via the comparison of the gate metal temperature. A pixel by pixel calibration method is implemented to improve the spatial accuracy and account for any variability in the thermoreflectance coefficient across the device. The improvements of TTI discussed in this study make the technique an accurate and effective method to measure the temperature distribution of both the gate metal and GaN.

Thermal rectification in thin films driven by gradient grain microstructure

Abstract: As one of the basic components of phononics, thermal rectifiers transmit heat current asymmetrically similar to electronic rectifiers in microelectronics. Heat can be conducted through them easily in one direction while being blocked in the other direction. In this work, we report a thermal rectifier that is driven by the gradient grain structure and the inherent gradient in thermal properties as found in these materials. To demonstrate their thermal rectification properties, we build a spectral thermal conductivity model with complete phonon dispersion relationships using the thermophysical properties of chemical vapor deposited (CVD) diamond films which possess gradient grain microstructures. To explain the observed significant thermal rectification, the temperature and thermal conductivity distribution are studied. Additionally, the effects of temperature bias and film thickness are discussed, which shed light on tuning the thermal rectification based on the gradient microstructures. Our results show that the columnar grain microstructure makes CVD materials unique candidates for mesoscale thermal rectifiers without a sharp temperature change.

UV Thermal Imaging of RF GaN Devices with GaN Resistor Validation

Abstract: Shrinking features and growing device complexity in advanced microwave devices has increased the challenge of fully understanding device thermal behavior on the sub-micron scale. Predicting the device static and dynamic thermal behavior is essential for ensuring optimal tradeoffs between performance and device reliability. Thermal imaging based on the Thermoreflectance Principle can meet the challenges imposed by these compact, high power density RF devices by providing submicron spatial resolution and temporal resolution in the picosecond range. This technique overcomes the limitations of traditional thermal imaging techniques such as IR and Micro-Raman and some of the specific challenges in measuring GaN devices. In the past, Thermoreflectance Imaging has been shown to accurately estimate the temperature rise of metals using visible wavelength excitation sources. This paper presents a novel method to estimate the temperature directly on GaN surfaces using UV wavelengths. These UV thermoreflectance measurements were verified with measurement of an on-chip GaN mesa resistor. Temperature measurements on top of the field plate and inside the GaN channel were compared for a commercial GaN HEMT both on Si and SiC substrate. The advantages and disadvantages will be presented for the thermoreflectance technique for thermal imaging.

Influence of Polymer Substrate Damage on the Time Dependent Cracking of SiNx Barrier Films.

Abstract: This work is concerned with the long-term behavior of environmentally-assisted subcritical cracking of PECVD SiNx barrier films on polyethylene terephthalate (PET) and polyimide (PI) substrates. While environmentally-assisted channel cracking in SiNx has been previously demonstrated, with constant crack growth rates over short periods of time (<1 hour) during which no substrate damage was observed, the present experiments over longer periods reveal a regime where cracking also develops in the polymer substrate. This time-dependent local cracking of the polymer underneath the channel crack is expected based on creep rupture or static fatigue. Our combined in-situ microscopy and finite-element modeling results highlight the combined effects of neighboring cracks and substrate cracking on the crack growth rate evolution in the film. In most cases, the subcritical crack growth rates decrease over time by up to two orders of magnitude until steady-state rates are reached. For SiNx on PI, crack growth rates were found to be more stable over time due to the lack of crack growth in the substrate as compared to SiNx on PET. These results provide a guideline to effectively improving the long-term reliability of flexible barriers by a substrate possessing high strength which limits substrate damage.

Thermal and Thermomechanical Modeling to Design a Gallium Oxide Power Electronics Package

Abstract: There is significant interest in the power electronics industry in transitioning from silicon to wide-bandgap devices. Gallium oxide devices have the potential to offer comparable or even superior performance than other wide-bandgap devices, but at a much lower cost. Recent breakthroughs include demonstration of a laboratory-scale gallium oxide transistors and diodes; however, a functional power electronics package for these devices is yet to be developed. In this paper, the research methodology in designing an electronics package for gallium oxide devices is outlined. Finite element-based thermal and thermomechanical modeling simulations were conducted to realize a package design that meets the combined target of minimal thermal resistance and improved reliability. Different package designs that include various material combinations and cooling configurations were explored, and their thermal and thermomechanical performance are reported. Furthermore, the short-circuit withstanding capabilities of gallium oxide devices were studied and compared with silicon carbide.

Design and Surface Modification of PET Substrates Using UV/Ozone Treatment for Roll-to-Roll Processed Solar Photovoltaic (PV) Module Packaging

Abstract: For many flexible electronic and photonic devices, roll-to-roll processing technology has been a novel driving force for their development. This processing technology can also be applied to solar photovoltaic (PV) module packaging. However, to fit its processing needs, there are several challenges. This paper presents surface modification of PET carrier film using UV/Ozone treatment to improve adhesion with the adhesive material. Whereas previous studies also utilized treatment methods to increase the wettability characteristics, in this paper we present the results of a relatively weaker irradiation technique, UV/Ozone treatment at room temperature condition. We found that this treatment method increases the wettability or surface energy by up to 1.4-fold and demonstrates a 28.3% increase in average adhesion strength.

Direct Wafer Bonding of GaN for Power Devices Applications

Abstract: Over the past decade the interest in gallium nitride – based electronic applications increased significantly The economical advantages of GaN-based applications imposed the development of new manufacturing technologies Among them, wafer bonding process solutions could be further developed and optimized for the manufacturing of GaN engineered substrates The most significant benefit of using wafer bonding for GaN-based applications is the possibility to combine GaN with various substrate materials, offering a high flexibility by imposing strict quality requirements only to the surfaces of the substrates and not to their crystal structureThis work proposes two different process flows for "GaN-on-Something" substrates manufacturing The features and benefits of the two process flows are illustrated by experimental results

Molecular dynamics simulation of oxygen transport characteristics in the electrolyte membrane of PEMFC

Abstract: Purpose This paper aims to investigate the oxygen transport characteristics in the electrolyte membrane of proton exchange membrane fuel cell (PEMFC), in particular, the water content dependence and the microscopic view of the molecular transport. Design/methodology/approach Molecular dynamics simulation is used to examine the oxygen transport characteristics in the electrolyte membrane of PEMFC that we have experimentally observed in our previous study. Findings Molecular dynamics simulation well predicts the diffusion coefficient of oxygen in the membrane. It was found that the oxygen molecules have preference in their transport passage that governs the property. Originality/value First attempt is to theoretically examine the experimentally observed water uptake dependence of the oxygen diffusion coefficient in membrane and to explain the mechanism.

Low Temperature Direct Bonding Of Aluminum Nitride To AlSiC Substrates

Abstract: Disclosed herein are power electronic modules formed by directly bonding a heat sink to a dielectric substrate using transition liquid phase bonding.