Thermal Conductivity Of Substrate Research Articles

Channel Properties of Ga₂O₃-on-SiC MOSFETs

We report the characterization of the channel mobility properties of metal-oxide-semiconductor field-effect transistors (MOSFETs) on the heterogeneous $\beta $ -Ga2O3-on-SiC (GaOSiC) substrate fabricated by an ion-cutting process. The mobility of GaOSiC MOSFETs is significantly improved as the postannealing temperature of Ga2O3 channel increases from 900 °C to 1200 °C. The GaOSiC transistor annealed at 1200 °C exhibits mobility consistent with the $\beta $ -Ga2O3 donor wafer, which suggests that the defects in Ga2O3 channel induced by the H+ implantation for the ion-cutting step can be eliminated by the high-temperature annealing. As the ambient temperature ( ${T} _{{\mathrm {amb}}}$ ) increases from 0 °C to 150 °C, the mobility within the accumulation regime of GaOSiC MOSFETs decreases with the temperature following a ${T}_{{\mathrm {amb}}}^{-1}$ law, which is limited by the phonon scattering. The results of this work will be critically important for designing the transport properties of the GaOSiC channel, significantly advancing the development of Ga2O3 power devices on high thermal conductivity substrate.

Bifurcation analysis of oxidative coupling of methane in monolith, gauze or wire-mesh reactors

We use a multi-scale multi-mode reduced order model for coupled homogeneous-catalytic reaction systems to present a comprehensive bifurcation (ignition-extinction) analysis of the oxidative coupling of methane (OCM) in monolith, gauze or wire-mesh type reactors. We determine the impact of methane to oxygen ratio in the feed, space time, channel hydraulic radius (or gas phase to catalyst volume ratio), washcoat diffusional limitations, operating pressure and substrate thermal conductivity on the ignition and extinction behavior of the system as a function of the feed temperature. The computations show that for typical operating conditions, the methane conversion and C2 product selectivity are non-monotonic on the ignited branch and there exists an optimum point of operation away from the extinction point. The predicted methane, oxygen conversions and C2 selectivity are compared to reported experimental results in the literature. We also present the various species and temperature profiles along the length of the reactor and examine how these profiles are impacted by the substrate conductivity, space time and heat loss. The results obtained for monolith, gauze or wire-mesh reactors are compared to those in packed-bed reactors and suggestions are provided for scale-up and optimization of these reactors for carrying out OCM as well as other catalytic partial oxidations.

Droplet evaporation residue indicating SARS-COV-2 survivability on surfaces.

We conducted a systematic investigation of droplet evaporation on different surfaces. We found that droplets formed even with distilled water do not disappear with evaporation but instead shrink to a residue of a few micrometers lasting over 24 h. The residue formation process differs across surfaces and humidity levels. Specifically, under 40% relative humidity, 80% of droplets form residues on plastic and uncoated and coated glass, while less than 20% form on stainless steel and none on copper. The formation of residues and their variability are explained by modeling the evaporation process considering the presence of nonvolatile solutes on substrates and substrate thermal conductivity. Such variability is consistent with the survivability of SARS-CoV-2 measured on these surfaces. We hypothesize that these long-lasting microscale residues can potentially insulate the virus against environmental changes, allowing them to survive and remain infectious for extended durations.

Analysis of thickness and interfacial thermal resistance of Au microheater on glass or polyimide substrate

The 3ω method is widely used for measuring the thermal properties of a substrate or thin film on which a microheater is deposited. Recently, samples with non-formal shapes and has a thermal conductivity of 1 W/mK or below have been analyzed using the bidirectional 3ω method. In measuring the thermal conductivity of such samples, the measurement sensitivity and accuracy can be increased using a 3ω sensor fabricated on a low thermal conductivity substrate such as glass or polyimide. In this study, thermal characterization is conducted on a microheater deposited on glass and polyimide substrates for a 3ω sensor. The effects of the microheater thickness and the interfacial thermal resistance between the substrate and microheater, on the temperature amplitude and phase lag are analyzed. Results from analytic solutions considering the microheater thickness and the interfacial thermal resistance are compared with results from numerical analysis of two-dimensional conduction heat transfer and experiment.

(Invited) Heterogeneous Integration of GaN and Ga2O3 with High Thermal Conductivity Substrates Via Surface Activated Bonding

The high-power high-frequency devices of both GaN and Ga2O3 needs careful thermal management to migrate the self-heating effect during operation. One promising strategy is to integrate GaN and Ga2O3 with a high thermal conductivity substrate such as SiC and diamond. Epitaxial growth, usually at high temperature, met many challenges because of the lattice mismatch and thermal expansion mismatch. Recently, we realized room-temperature bonding of GaN and Ga2O3 with high thermal conductivity substrates via surface activated bonding methods, which can avoid the transition layer necessary for growth method and make the GaN or Ga2O3 device close to the high thermal conductivity substrates as much as possible.

Heat and mass transfer inside of a monolith honeycomb: From channel to full size reactor scale

This paper is devoted to the multi-scale model of the heat and mass transfer inside of a honeycomb monolith substrate. Due to computational limitations, monoliths are modelled as a continuum in full-scale catalytic reactors models. That makes it necessary to use correlations or sub-models derived from channel scale results to account for a physically consistent heat and mass transfer inside of the substrate. In this paper detailed computational models at a channel and reactor scales are analyzed. Catalytic oxidation of CO is used as a reaction and the fluid properties are considered to be temperature-dependent. First, a channel scale model is used to analyze Nusselt, Sherwood, Lewis, and Damköhler numbers inside of the monolith channels. Secondly, sub-models obtained at a channel level are implemented in a full-scale reactor model using the continuum approach, to evaluate the impact of using detailed vs. highly simplified correlations for heat and mass transfer. The reactor scale model accounts for the transitions of the flow regime, entrance length effects, an-isotropic substrate thermal conductivity and temperature-dependent fluid properties. According to the results, the Lewis number can deviate significantly from one in the entrance length, however, it approaches asymptotically to unity as the flow develops. Regarding Nusselt and Sherwood, current interpolating methodologies are not able to predict the correct value in the entrance region when Damköhler is low, nonetheless, are reasonably accurate for the asymptotic one.

Heteroepitaxial growth of β-Ga2O3 films on SiC via molecular beam epitaxy

β-Ga2O3 is a promising ultrawide bandgap semiconductor for next generation radio frequency electronics. However, its low thermal conductivity and inherent thermal resistance provide additional challenges in managing the thermal response of β-Ga2O3 electronics, limiting its power performance. In this paper, we report the heteroepitaxial growth of β-Ga2O3 films on high thermal conductivity 4H-SiC substrates by molecular beam epitaxy (MBE) at 650 °C. Optimized MBE growth conditions were first determined on sapphire substrates and then used to grow β-Ga2O3 on 4H-SiC. X-ray diffraction measurements showed single phase (2¯01) β-Ga2O3 on (0001) SiC substrates, which was also confirmed by TEM measurements. These thin films are electrically insulating with a (4¯02) peak rocking curve full-width-at-half-maximum of 694 arc sec and root mean square surface roughness of ∼2.5 nm. Broad emission bands observed in the luminescence spectra, acquired in the spectral region between near infrared and deep ultraviolet, have been attributed to donor-acceptor pair transitions possibly related to Ga vacancies and its complex with O vacancies. The thermal conductivity of an 81 nm thick Ga2O3 layer on 4H-SiC was determined to be 3.1 ± 0.5 W/m K, while the measured thermal boundary conductance (TBC) of the Ga2O3/SiC interface is 140 ± 60 MW/m2 K. This high TBC value enables the integration of thin β-Ga2O3 layers with high thermal conductivity substrates to meliorate thermal dissipation and improve device thermal management.

The effect of the substrate on the damage threshold of gold nano-antennas by a femtosecond laser

Gold nano-antennas with silica substrate may not be suitable for high power applications such as heat resisted magnetic recording, solar thermophotovoltaics, and nano-scale heat transfer systems. When a laser beam reaches to these nano-antennas, part of the light is absorbed by the metallic regions, leading to a temperature rise of the device. If these devices reach a temperature beyond its Tamman temperature (the temperature at which sintering of atoms or molecules start to occur), the antenna can be damaged. One strategy to allow the antenna to work at higher fluences (energy density) is to employ substrates that can quickly carry the heat away from the antennas. In this paper, we show that high thermal conductivity substrates, such as diamond, can allow the antenna to withstand 20 times higher fluence than a low thermal conductivity silica substrate.

Current‐Induced Planar Resistive Switching Mediated by Oxygen Migration in NiO/Pt Bilayer

AbstractNew materials and mechanisms to retain electrically tunable resistance states are highly pursed to advance information technology. In this study, a novel scenario of current‐induced planar resistive switching mediated by oxygen migration is unveiled in the NiO/Pt bilayers. Sawtooth‐like switching is successfully realized in various crystalline NiO, and the switching amplitude monotonically increases with the Pt thickness. On the other hand, the critical switching current is found to be strongly correlated with the substrate thermal conductivity hence suggesting a thermal origin. By directly probing the Ni2+ and O2− distribution before and after switching via electron energy loss spectrum, the switching behavior in NiO/Pt is assuredly associated with the planar redistribution of O2−, thereby setting forth an oxygen migration scenario underlying the switching process. Under this scenario, the planar resistive switching is extended to the SrTiO3/Pt, GdOx/Pt, and TbOx/Pt, and the manipulation of the switching characteristics is successfully achieved by controlling the oxygen content in the GdOx/Pt and TbOx/Pt bilayers. The results offer novel strategy to design new emergent memory devices through oxygen mediated planar resistive switching.

Three-omega thermal-conductivity measurements with curved heater geometries

The three-omega method, a powerful technique to measure the thermal conductivity of nanometer-thick films and the interfaces between them, has historically employed straight conductive wires to act as both heaters and thermometers. When investigating stochastically prepared samples such as two-dimensional materials and nanomembranes, residue and excess material can make it difficult to fit the required millimeter-long straight wire on the sample surface. There are currently no available criteria for how diverting three-omega heater wires around obstacles affects the validity of the thermal measurement. In this Letter, we quantify the effect of the wire curvature by performing three-omega experiments with a wide range of frequencies using both curved and straight heater geometries on SiO2/Si samples. When the heating wire is curved, we find that the measured Si substrate thermal conductivity changes by only 0.2%. Similarly, we find that wire curvature has no significant effect on the determination of the thermal resistance of an ∼65 nm SiO2 layer, even for the sharpest corners considered here, for which the largest measured ratio of the thermal penetration depth of the applied thermal wave to radius of curvature of the heating wire is 4.3. This result provides useful design criteria for three-omega experiments by setting a lower bound for the maximum ratio of the thermal penetration depth to wire radius of curvature.

Laser-induced anharmonicity vs thermally induced biaxial compressive strain in mono- and bilayer MoS2 grown via CVD

We studied the role of biaxial compressive strain in laser-induced anharmonicity effects of mono- and bilayer MoS2 grown by chemical vapor deposition. With the increased laser powers, the A1g phonon mode was strongly affected by the anharmonicity, whereas the E12g mode was negligibly affected by the anharmonic effects, which is attributed to the dominant nature of biaxial compressive strain over the effects of anharmonicity. In the case of the A1g phonon mode, anharmonicity effects dominated over the biaxial compressive strain owing to its large out-of-plane thermal expansion coefficient. The origin of biaxial compressive strain was explained by invoking the difference in linear thermal expansion coefficients of MoS2 and the substrate. Moreover, thermally induced biaxial compressive strain also influenced the exciton emission energy. The role of the substrate thermal conductivity was discussed in the light of anharmonicity of the MoS2 monolayer. In addition, the polarizability of Raman modes was found to be dependent on the second layer arrangement over the monolayer.

Role of substrate thermal conductivity and vapor pressure in dropwise condensation

The role of constriction resistance is examined experimentally using hydrophobic gold-plated copper, aluminum, carbon steel, and stainless steel condenser surfaces over a range of surface subcooling and vapor pressure. It is confirmed that the condensation heat transfer coefficient depends on the substrate thermal conductivity (five-folds lower for stainless steel compared to copper) and the tendency of a hydrophobic surface to maintain a small liquid-film resistance is controlled by the droplet growth and coalescence rate. These are accompanied by an increase in the departing droplet radius and the surface coverage with decreasing substrate thermal conductivity. Numerical thermal simulations isolate the role of constriction resistance from the role of the droplet distribution. Additionally, based on recently available molecular dynamics results, the liquid-vapor interfacial resistance is modeled as a vapor temperature jump using the vapor mean free path and the predicted pressure dependence is in good agreement with the experimental results.

Temperature dependence of the anomalous Nernst coefficient for Ni80Fe20 determined with metallic nonlocal spin valves

The anomalous Nernst effect, which generates an out-of-plane charge voltage in response to a thermal gradient perpendicular to the magnetization of a ferromagnet, can play a significant role in many spintronic devices where large thermal gradients exist. Since they typically include features deep within the submicron regime, nonlocal spin valves can be made very sensitive to this effect by lowering the substrate thermal conductance. Here, we use nonlocal spin valves suspended on thin silicon nitride membranes to determine the temperature dependence of the anomalous Nernst coefficient of 35 nm thick permalloy (Ni80Fe20) from 78 K to 300 K. In a device with a simple ferromagnet geometry, the transverse Seebeck coefficient shows a weak temperature dependence, with values at all T near 2.5 μV/K. Assuming previously measured values of the Seebeck coefficient for permalloy, which has a near-linear dependence on T, leads to a low temperature upturn in the anomalous Nernst coefficient RN. We also show that the temperature dependence of this coefficient is different when a constricted nanowire is used as the ferromagnetic detector element.

(Invited) Characterization of Thermal Effects in Wide Bandgap Semiconductor Materials and Devices

Wide bandgap electronics made from nitrides (e.g., Gallium Nitride (GaN)) and oxides (e.g., Gallium Oxide (Ga2O3)) are currently under development due to their potential to create some of the most advanced RF and power electronic devices in the world. However, the thermal response of these devices under applied electric fields can create large power densities (RF and power electronics) that must be understood. For many of these devices, the electrothermal response plays a strong role in both the acceptable operation or long-term failure and reliability of the devices. Thus, tools that can help elucidate these responses and provide a method to help design better devices is of critical need for this field.In this talk we will discuss advancements in thermal characterization techniques that have allowed new insights into electrothermal behavior in GaN and Ga2O3 materials and devices. Specifically, a focus on the role of device architecture and material processing will be addressed. For GaN and Ga2O3 RF and power electronics, the role of thermal interfaces in the heat dissipation of both lateral and vertical device architectures will be discussed. We will cover aspects of semiconductor-semiconductor as well as metal-semiconductor interfaces and their roles in device thermal management. A highlight will be the use of plasma activated bonding to provide ultralow thermal resistance interfaces that allow for heterogenous integration of nitrides and oxides with high thermal conductivity substrates. Finally, we will discuss the implications of these effects on device behavior through experiments and modeling.

Impact of the thermal environment on the analog temporal response of HfOx-based neuromorphic devices

Filamentary adaptive oxide devices based on HfOx are a promising technology for neuromorphic computing applications. The resistance of these devices depends on the concentration of oxygen vacancies in the filament region. A local temperature rise from joule heating plays a significant role in the movement of oxygen ions, making thermal management crucial to reliable performance. In this work, the role of the substrate thermal conductivity on the analog performance was investigated at biologically realistic pulse widths. Au/Ti/HfOx/Au adaptive oxide devices were fabricated on substrates with two orders of magnitude difference in thermal conductivity. A lower thermal conductivity substrate dissipates heat more slowly, resulting in a large initial change in resistance from a single operation pulse, which is detrimental to the desired analog behavior. The results were validated by a COMSOL Multiphysics® model that models the flow of heat in both samples.

Interfacial Thermal Conductance across Room-Temperature-Bonded GaN/Diamond Interfaces for GaN-on-Diamond Devices.

The wide bandgap, high-breakdown electric field, and high carrier mobility makes GaN an ideal material for high-power and high-frequency electronics applications, such as wireless communication and radar systems. However, the performance and reliability of GaN-based high-electron-mobility transistors (HEMTs) are limited by the high channel temperature induced by Joule heating in the device channel. Integration of GaN with high thermal conductivity substrates can improve the heat extraction from GaN-based HEMTs and lower the operating temperature of the device. However, heterogeneous integration of GaN with diamond substrates presents technical challenges to maximize the heat dissipation potential brought by the ultrahigh thermal conductivity of diamond substrates. In this work, two modified room-temperature surface-activated bonding (SAB) techniques are used to bond GaN and single-crystal diamond. Time-domain thermoreflectance (TDTR) is used to measure the thermal properties from room temperature to 480 K. A relatively large thermal boundary conductance (TBC) of the GaN/diamond interfaces with a ∼4 nm interlayer (∼90 MW/(m2 K)) was observed and material characterization was performed to link the interfacial structure with the TBC. Device modeling shows that the measured TBC of the bonded GaN/diamond interfaces can enable high-power GaN devices by taking full advantage of the ultrahigh thermal conductivity of single-crystal diamond. For the modeled devices, the power density of GaN-on-diamond can reach values ∼2.5 times higher than that of GaN-on-SiC and ∼5.4 times higher than that of GaN-on-Si with a maximum device temperature of 250 °C. Our work sheds light on the potential for room-temperature heterogeneous integration of semiconductors with diamond for applications of electronics cooling, especially for GaN-on-diamond devices.

氧化铝基板导热性能数值分析

氧化铝基板导热性能数值分析

Near-junction heat spreaders for hot spot thermal management of high power density electronic devices

Many high power (opto-) electronic devices such as transistors, diodes, and lasers suffer from significant hot spot temperature rises due to the high heat fluxes generated in their active area, which limits their performance, reliability, and lifetime. Employing high thermal conductivity materials near the heat source, known as near-junction heat spreaders, offers a low-cost and effective thermal management approach. Here, we present analytical heat spreader models and a methodology to evaluate their performance. Experimental demonstration of near-junction diamond heat spreaders on vertical GaN PiN diodes revealed significantly reduced spreading resistances, along with very low temperature gradients across the device. The findings in this work provide design guidelines and demonstrate excellent prospects, especially for the devices on low thermal conductivity substrates. The theoretical analysis of optimized diamond heat spreaders shows an 86% reduction of spreading resistance for GaN devices and 98% for Ga2O3 devices. In addition, our results show that a 3 μm-thick layer of high-quality CVD-deposited diamond heat spreaders on GaN-on-Si devices can provide better heat spreading than GaN-on-SiC devices and perform similar to GaN-on-diamond devices, highlighting the significant potential of heat spreaders as an effective and low-cost thermal management approach.

Implementation of phosphor sedimentation to reduce thermal instability issue affecting white LED luminescence

Purpose In general, lighting application, white light emitting diode (LED) usually exposed to an extreme operating temperature of above 90°C. It is well-known that luminous efficacy and spectral characteristic of white LED are dependent on the temperature, causing thermal effects on luminous efficacy and color shift of white LED become a critical application checkpoint to be addressed by white LED manufactures. Thus, the purpose of this paper is to minimize the thermal stability issue affecting white LED luminescence during operation by introducing phosphor sedimentation process. Design/methodology/approach The LED samples were assembled and sent for centrifugation with 0, 5 and 10 revolutions per second (rps), respectively, during phosphor sedimentation process. Luminescence properties of these LED samples were then characterized at a varying temperature to investigate the effect of phosphor sedimentation on the luminescence stability of LED samples. The LED samples were also cross-sectioned and analyzed to understand the phosphor sedimentation mechanism. Computational fluid dynamics (CFD) was applied to study the temperature distribution of the non-phosphor sediment (NPS) and phosphor sediment (PS) LED during operation to validate the hypotheses based on experimental data. Findings Experimental results show that the luminous intensity of PS LED samples degrades less significant at high temperature. The experimental results also show that the color coordinate for PS LED samples is more stable and is less blue-shifted than NPS LED samples as the temperature increased. These are because the heat generated by phosphor particles during operation can be dissipated effectively throughout a high thermal conductivity substrate after phosphor sedimentation. Thus, the phosphor temperature of PS LED is lower than NPS LED during operation as validated with the thermal simulation. Practical implications The study of this paper is applicable as a reference for industries who intend to resolve the thermal stability of white LED during operation. The luminescence properties changes as a function of the temperature study in this paper can be used to predict the application performances of white LED accurately. Apart from that, the analysis method of temperature distribution using CFD simulations can be extended by other CFD users in the future. Originality/value Implementation of phosphor sedimentation to reduce thermal instability issue of white LED has yet to be reported on previous studies. Most literature just studied the thermal instability issue of either assembled LED or raw material, without suggesting any solution to tackle the issue.

Plasma-sprayed nickel splats on chromium substrates: The role of substrate preheating and thermal conductivity

The effect of chromium substrate preheating on the spreading behavior of plasma-sprayed nickel splats was characterized. The interfacial features along the splat-substrate interface were analyzed by focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). In addition, the transient droplet spreading process was modelled in a simulation study. The results showed that the presence of a large fraction of surface moisture on the substrates induced splat fragmentation. However, splat fragmentation was completely constrained on chromium preheated to a low temperature (373 K) where a certain amount of surface moisture was still present. The high thermal conductivity of chromium substrates increased droplet solidification rates, decreasing the interaction time between the spreading droplet and the vaporized gas layer. Accordingly, splat fragmentation was suppressed and disk-shaped splats were formed. Fast solidification not only refined the final splat microstructure, but also promoted the formation of finger-splashed disk splats. In addition, the extent of elemental diffusion across the splat-substrate interface was decreased due to the low interfacial temperature.