Showing papers by "Samuel Graham published in 2017"

Solution-based electrical doping of semiconducting polymer films over a limited depth

Abstract: A solution process for the diffusion of dopants in organic semiconducting films over a limited depth has been developed. The method is applied to single polymers and donor–acceptor mixtures, and for the realization of single-layer solar cells.

Rethinking phonons: The issue of disorder

Abstract: Current understanding of phonons treats them as plane waves/quasi-particles of atomic vibration that propagate and scatter. The problem is that conceptually, when any level of disorder is introduced, whether compositional or structural, the character of vibrational modes in solids changes, yet nearly all theoretical treatments continue to assume phonons are still waves. For example, the phonon contributions to alloy thermal conductivity (TC) rely on this assumption and are most often computed from the virtual crystal approximation (VCA). Good agreement is obtained in some cases, but there are many instances where it fails—both quantitatively and qualitatively. Here, we show that the conventional theory and understanding of phonons requires revision, because the critical assumption that all phonons/normal modes resemble plane waves with well-defined velocities is no longer valid when disorder is introduced. Here we show, surprisingly, that the character of phonons changes dramatically within the first few percent of impurity concentration, beyond which phonons more closely resemble the modes found in amorphous materials. We then utilize a different theory that can treat modes with any character and experimentally confirm its new insights. In solids, atoms continuously vibrate in collective motions with quantized amplitudes that can be described in terms of quasiparticles known as phonons—which are responsible for heat transfer and sound. Phonons are usually treated as waves that propagate and scatter, but this approach can sometimes fail when dealing with materials with disorder. A team of researchers from the Georgia Institute of Technology, led by Asegun Henry, show that by focussing on phonon correlation rather than scattering, it is possible to more accurately capture the changes in vibrational mode behavior as a function of disorder.

Characterization of AlGaN/GaN HEMTs Using Gate Resistance Thermometry

Abstract: In this paper, gate resistance thermometry (GRT) was used to determine the channel temperature of AlGaN/GaN high electron-mobility transistors. Raman thermometry has been used to verify GRT by comparing the channel temperatures measured by both techniques under various bias conditions. To further validate this technique, a thermal finite-element model has been developed to model the heat dissipation throughout the devices. Comparisons show that the GRT method averages the temperature over the gate width, yielding a slightly lower peak temperature than Raman thermography. Overall, this method provides a fast and simple technique to determine the average temperature under both steady-state and pulsed bias conditions.

Near-isothermal-isobaric compressed gas energy storage

Abstract: In this paper, the effectiveness of storing energy by compressing and expanding a condensable gas is evaluated A high efficiency energy storage system, which stores energy by compressing/expanding gas (air) using a liquid (water) piston has been recently introduced and extensively studied With the use of the liquid piston, the inefficient gas turbomachines used in conventional gas compression/expansion systems are replaced with high efficiency hydraulic machines Utilizing heat transfer techniques and replacing the non-condensable air with a condensable gas (ie CO2, synthetic refrigerants, hydrocarbon refrigerants, etc) have been proposed as methods to improve energy density and roundtrip efficiency of such systems, leading to near isothermal and near isobaric charge/discharge processes In order to investigate the effectiveness of the proposed concept, a miniature lab-scale experimental setup was designed and built to investigate the compression/expansion characteristics and energy storage efficiency of a system utilizing R134a as the energy storage (primary) working fluid, and mineral refrigerant oil as the liquid piston (secondary) working fluid Several tests are carried out to quantify energy storage efficiency and energy density It is found that improving heat transfer rates from/into the storage fluid results in increased efficiency and energy density A heat-transfer enhancement strategy to achieve near isothermal, isobaric expansion and compression is proposed and investigated experimentally Some results are generalized and presented in non-dimensional form which can be applied to describe and/or design scaled-up systems Lastly, a potential working fluid for a scaled-up system is discussed

Pool boiling characteristics and critical heat flux mechanisms of microporous surfaces and enhancement through structural modification

Abstract: Experimental studies have shown that microporous surfaces induce one of the highest enhancements in critical heat flux (CHF) during pool boiling. However, microporous surfaces may also induce a very large surface superheat (>100 °C) which is not desirable for applications such as microelectronics cooling. While the understanding of the CHF mechanism is the key to enhancing boiling heat transfer, a comprehensive understanding is not yet available. So far, three different theories for the CHF of microporous surfaces have been suggested: viscous-capillary model, hydrodynamic instability model, and dryout of the porous coatings. In general, all three theories account for some aspects of boiling phenomena. In this study, the theories are examined through their correlations with experimental data on microporous surfaces during pool boiling using deionized (DI) water. It was found that the modulation of the vapor-jet through the pore network enables a higher CHF than that of a flat surface based on the hydrodyna...

Solution-Processed Doping of Trilayer WSe2 with Redox-Active Molecules

Abstract: The development of processes to controllably dope two-dimensional semiconductors is critical to achieving next-generation electronic and optoelectronic devices. In this study, n- and p-doping of highly uniform large-area trilayer WSe2 is achieved by treatment with solutions of molecular reductants and oxidants. The sign and extent of doping can be conveniently controlled by the redox potential of the (metal−)organic molecules, the concentration of dopant solutions, and the treatment time. Threshold voltage shifts, the direction of which depends on whether a p- or n-dopant is used, and tunable channel current are observed in doped WSe2 field-effect transistors. Detailed physical characterization including photoemission (ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy) and Raman spectroscopy provides fundamental understanding of the underlying mechanism. The origin of the doping is the electron-transfer reactions between molecular dopants and 2D semiconductors and results in a sh...

Thermal conductivity measurements on suspended diamond membranes using picosecond and femtosecond time-domain thermoreflectance

Abstract: We report on the room temperature in-plane thermal conductivity measurements on a 1-micrometer thick suspended diamond membrane grown by chemical vapor deposition using two different time domain thermoreflectance (TDTR) setups. The first setup is at Stanford University and the second is at Georgia Institute of Technology. Despite the differences between the two setups and the difficulty associated with diamond membranes thermal measurements, we demonstrate excellent repeatability from each setup and a very good agreement between the two setups. The paper outlines steps considered by both groups to minimize the measurement uncertainty and achieve such agreement. The measurement results show that the thermal conductivity displays a large variability across the membrane. The sensitivity and uncertainty analyses suggest that this variability could be a result of the nonuniformity in the diamond and aluminum coating thicknesses across the sample.

Near room-temperature direct encapsulation of organic photovoltaics by plasma-based deposition techniques

Abstract: Plasma-assisted atomic layer deposition (ALD) is used for the deposition of environmental barriers directly onto organic photovoltaic devices (OPVs) at near room temperature (30 °C). To study the effect of the ALD process on the organic materials forming the device, the precursor diffusion and intermixing at the interface during the growth of different plasma-assisted ALD inorganic barriers (i.e. Al2O3 and TiO2) onto the organic photoactive layer (P3HT:ICBA) was investigated. Depth profile x-ray photoelectron spectroscopy was used to analyze the composition of the organic/inorganic interface to investigate the infiltration of the plasma-assisted ALD precursors into the photoactive layer as a function of the precursor dimension, the process temperature, and organic layer morphology. The free volume in the photoactive layer accessible to the ALD precursor was characterized by means of ellipsometric porosimetry (EP) and spectroscopic ellipsometry as a function of temperature. The organic layer is shown to exhibit free volume broadening at high temperatures, increasing the infiltration depth of the ALD precursor into the photoactive layer. Furthermore, based on previous investigations, the intrinsic permeation properties of the inorganic layers deposited by plasma-assisted ALD were predicted from the nano-porosity content as measured by EP and found to be in the 10−6 gm−2 d−1 range. Insight from our studies was used to design and fabricate multilayer barriers synthesized at near-room temperature by plasma-assisted ALD in combination with plasma-enhanced CVD onto organic photovoltaic (OPVs) devices. Encapsulated OPVs displayed shelf-lifetimes up to 1400 h at ambient conditions.

Note: A single specimen channel crack growth technique applied to brittle thin films on polymer substrates

Abstract: We introduce an external-load-assisted thin film channel crack growth technique to measure the subcritical crack growth properties of thin films (i.e., crack velocity, v, versus the strain energy release rate, G), and demonstrate it using 250-nm-thick SiNx films on poly(ethylene terephthalate) substrates. The main particularity of this technique is that it requires a polymer substrate to allow loading to large strains (in order to induce channel cracking) without substrate fracture. Its main advantages are to provide a full v-G curve with a single specimen while relying on a simple specimen preparation and straightforward crack growth characterization. Importantly, the technique can be employed for a much larger range of thin films compared to the residual-stress-driven, thin film channel crack growth tests, including ultrathin films and thin film with residual compressive stresses. The restrictions to a proper use of this technique, related to the (visco)plastic deformation of the substrate, are discussed.

Thermal Rectification in CVD Diamond Membranes Driven by Gradient Grain Structure

Abstract: As one of the basic components of phononics, thermal diodes transmit heat current asymmetrically similar to electronic rectifiers and diodes in microelectronics. Heat can be conducted through them easily in one direction while being blocked in the other direction. In this work, we report an easily-fabricated mesoscale chemical vapor deposited (CVD) diamond thermal diode without sharp temperature change driven by the gradient grain structure of CVD diamond membranes. We build a spectral model of diamond thermal conductivity with complete phonon dispersion relation to show significant thermal rectification in CVD diamond membranes. To explain the observed thermal rectification, the temperature and thermal conductivity distribution in the CVD diamond membrane are studied. Additionally, the effects of temperature bias and diamond membrane thickness are discussed, which shed light on tuning the thermal rectification in CVD diamond membranes. The conical grain structure makes CVD diamond membranes, and potentially other CVD film structures with gradient grain structure, excellent candidates for easily-fabricated mesoscale thermal diodes without a sharp temperature change.

Integrated Copper Heat Slugs and EMI Shields in Panel Laminate (LFO) and Glass Fanout (GFO) Packages for High Power RF ICs

Abstract: This paper demonstrates, for the first time, ultra-thin, panel laminate fan-out (LFO) and glass fan-out (GFO) packages with embedded copper heat spreaders and electromagnetic shields for packaging high-power RF ICs in much smaller form factors and at potentially much lower cost than current ceramic and metal flange packages. This unique double-sided package addresses the thermal dissipation requirements of 30-100W power amplifiers by bonding the IC directly to a large copper heat spreader embedded in the substrate, using high thermal conductivity die-attach paste. It also addresses the RF, microwave and mm-wave performance requirements by utilizing low-loss tangent glass and polymer dielectrics, as opposed to lossy epoxy dielectrics or mold compounds. The combination of glass and high temperature polymers also enables superior harsh environment reliability with built-in stress buffer layers to mitigate the CTE mismatch induced stresses from large copper thermal structures.

Experimental considerations of CVD diamond film measurements using time domain thermoreflectance

Abstract: Diamond has the highest known thermal conductivity of any known bulk material, but the properties of synthetic diamond films often fall far short of this high level. The DARPA program Thermal Transport in Diamond Films for Electronics Thermal Management brings together researchers from five universities to comprehensively characterize the thermal transport and material properties of CVD diamond thin films in an effort to better how to further improve the thermal transport properties and understand how accurately these properties can be measured using time domain thermoreflectance and Raman spectroscopy. Here we summarize the results of the thermal measurements of diamond conducted via time domain thermoreflectance (TDTR) using two different systems and discuss some difficulties of accurately measuring the thermal conductivity of micron-thick anisotropic films that often have high surface roughness. We also report that in certain cases the thermal conductivity and thermal boundary conductance of CVD diamond films has been improved to the point of making them highly attractive for thermal management of high power electronic devices.

Ultraviolet and visible micro-Raman and micro-photoluminescence spectroscopy investigations of stress on a 75-mm GaN-on-diamond wafer

Abstract: Investigations of stress distributions and material quality across a 75-mm wafer consisting of device-quality GaN integrated with a diamond substrate are presented. Stress in the GaN are mapped both over the full wafer and across the layer along the growth direction. Ultraviolet (UV) and visible micro-Raman and UV photoluminescence (PL) spectroscopy from both sides of the wafer reveal an unexpected gradient between the tensile stress at the free GaN surface (∼0.86–0.90 GPa) and the GaN/diamond interface (∼0.05–0.23 GPa). The stresses obtained exhibit good cross-wafer uniformity. The stress gradient is understood through variations in the material along the growth direction of the layers due to the presence of threading dislocations which result in local stress relaxation. Transmission electron microscopy confirms the presence of extended defects to be greater near the interface with diamond, corresponding to the initial GaN growth regime, and diminished toward the surface where transistors would be fabricated in a full device technology. Finite element (FE) simulations describing the observed stress dependence along with TEM imaging of the GaN cross-section support the relaxation interpretation.

Preliminary Performance Evaluation of a Ground-Level Integrated Diverse Energy Storage (GLIDES) Prototype System

Abstract: 1 This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). ABSTRACT Increased generation capacity from intermittent renewable electricity sources being brought on-line combined with an electrical grid ill-equipped to handle the mismatch between electricity generation and use, necessitates advanced bulk energy storage technologies. This paper introduces once such technology, GLIDES (Ground-Level Integrated Diverse Energy Storage), which stores energy by compressing/expanding a gas (air) using a liquid (water) piston. A Pelton turbine is utilized as the energy extraction machine through which high head water is passed. This paper is the first to report on experimental system performance of the GLIDES technology.

Probing Growth-Induced Anisotropic Thermal Transport in CVD Diamond Membranes by Multi-frequency and Multi-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. With generated heat fluxes within these devices reaching magnitude close to ten times of that at the sun surface, chemical vapor deposition (CVD) diamond is an attractive candidate to aid in the extraction of this heat in order to keep the operating temperatures of these high power electronics as low as possible. Due to the observed inhomogeneous structure, CVD diamond membranes exhibit a 3D anisotropic thermal conductivity which may result in significantly different cooling performance from expected in a given application. In this work, time domain thermoreflectance (TDTR) is used to measure the thermal properties of an 11.8-{\mu}m 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 MHz to 11.6 MHz to tune the heat penetration depth, and subsequently the part of diamond sampled by TDTR. We divide the membrane into ten sublayers and assume isotropic thermal conductivity in each sublayer. From this, we observe a 2D gradient of the depth-dependent thermal conductivity for this membrane. By measuring the same region with a smaller spot size at multiple frequencies, the in-plane and cross-plane thermal conductivity are extracted respectively. 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 insight toward an improved understanding of heat conduction inhomogeneity in CVD polycrystalline diamond membrane that is important for applications of thermal management of high power electronics.

Cooling of power electronics by integrating sintered Cu particle wick onto a direct-bond copper substrate

Abstract: Power electronics devices can be limited in operation, and lifetime by the ability to remove heat and maintain low junction temperatures. A common architecture for power electronics devices is a circuit soldered onto a direct-bonded-copper (DBC) substrate, which is attached to a heat spreader or and heat sink. The removal of some of these interfaces and materials, however, can reduce the overall thermal resistance and allow for the devices to operate at much higher power levels. One approach to do this is to directly cool the backside of the DBC power substrate. By sintering copper particles to the DBC, evaporative cooling can be integrated into the architecture without the need for another bond. In this study heat fluxes up to 75 W/cm2 were applied through a test chip attached to a 15 mm × 17 mm DBC substrate with varying thicknesses of sintered copper particles on the backside. The copper porous layer was partially submerged into deionized (DI) water in order to allow for capillary wicking and evaporative cooling from the backside. Heat was dissipated through the thermal test chip, while the junction temperature was measured. The surface temperature of the porous copper layer was measured with a FLIR thermal camera. From this set-up the evaporative heat transfer coefficient (HTC) was computed and the thermal resistance calculated. The thermal resistance was found to be in good agreement with numerical models. Overall, the thermal resistance was reduced by about half when compared to the conventional packaging design.

Thermal boundary conductance across epitaxial ZnO/GaN interfaces: Assessment of phonon gas models and atomistic Green's function approaches for predicting interfacial phonon transport

Abstract: We present experimental measurements of the thermal boundary conductance (TBC) from $77 - 500$ K across isolated heteroepitaxially grown ZnO films on GaN substrates. These data provide an assessment of the assumptions that drive the phonon gas model-based diffuse mismatch models (DMM) and atomistic Green's function (AGF) formalisms for predicting TBC. Our measurements, when compared to previous experimental data, suggest that the 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 $490\lbrack +150, -110\rbrack$ MW m$^{-2}$ K$^{-1}$. The disagreement among the DMM and AGF and the experimental data these elevated temperatures suggests a non-negligible contribution from additional modes contributing to TBC that not accounted for in the fundamental assumptions of these harmonic formalisms, such as inelastic scattering. Given the high quality of these ZnO/GaN interface, these results provide an invaluable critical and quantitive assessment of the accuracy of assumptions in the current state of the art of computational approaches for predicting the phonon TBC across interfaces.

Thermal characterization of GaN vertical p-i-n diodes

Abstract: In this study, pioneering research was performed on GaN p-i-n diodes for the first ever assessment of surface temperature distribution by incorporating the use of infrared (IR) thermography, thermoreflectance thermal imaging, Raman thermometry, and thermal simulations. Each technique was advanced in order to obtain self-consistent results with higher accuracy. A two-temperature emissivity calibration procedure was utilized for IR measurements to acquire a temperature map of the p-contact metallization. Higher spatial resolution thermoreflectance thermal imaging was performed with a diverse range of illumination wavelengths including 470 nm and 530 nm. To confirm the results of thermoreflectance, TiO2 thermal nanoprobes were deposited on the device surface which enabled Raman thermometry to be performed on the p-contact metallization. Coherence of the techniques was then validated through thermal modeling. The results suggest that IR thermography, when using the two-temperature emissivity correction procedure, gives reasonable results at high power dissipating conditions. Thermoreflectance and nanopowder assisted Raman thermometry are viable options for GaN vertical device temperature assessment. However, results from Raman thermometry possess relatively large uncertainties and thermoreflectance measurements require multiple illumination wavelengths to ensure the validity of the measured temperatures that are derived from the thermoreflectance calibration coefficient.

Efficient single-phase cooling techniques for durable power electronics module

Abstract: This study explores the feasibility of single phase liquid channel cooling, pin fin cooling and spray cooling techniques for heat removal from a power electronic substrate. The substrate is formed using an AlN layer directly bonded to an AlSiC heat sink with a copper circuit layer. With a comparative assessment of the three cooling techniques using analytical modeling, the heat transfer coefficient and pressure drop in the package are optimized for a fixed coolant pumping power. Computational simulations of the three optimized geometries are performed to estimate the device temperatures and the maximum heat rates that can be dissipated with this pumping power. It is concluded that channel cooling yields the best thermal management performance and its use is recommended in the development of a packaged assembly.