Thermal Conductivity Of Diamond Research Articles

Adhesion, bonding and mechanical properties of Mo doped diamond/Al (Cu) interfaces: A first principles study

Diamond reinforced metal matrix composites are considered as a promising thermal management material. The interface between diamond and metal matrix determines the properties of the material to some extent. In this paper, the atomic structure, work of adhesion and binding characteristics of diamond (1 1 1)/Al (Cu) (1 1 1) clean interface are studied using first principles calculations. The influence of Mo doping on the interface interaction, tensile properties and thermal conductivity of diamond (1 1 1)/Al (Cu) (1 1 1) interface is discussed. It is revealed that the work of adhesion of diamond/Al interface is much higher than that of diamond/Cu interface and the magnetic moments also prove this point of view. Mo doping increases the binding of diamond/Al interface and diamond/Cu interface, with an increase rate of 7.2% and 28.4%, respectively. The tensile strength is decreased due to the Mo doping, but the breaking elongation of diamond/Al (Cu) composites is effectively improved, which is intimately related to the charge density distribution and the localization of density states at the Fermi level. Furthermore, Mo is a good additive element to improve the thermal conductivity of diamond/Cu interface, which gives an interpretation of the reported experimental results through the view of the atomic and electronic structures.

Integration of polycrystalline Ga2O3 on diamond for thermal management

Gallium oxide (Ga2O3) has attracted great attention for electronic device applications due to its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is significantly lower than that of other wide bandgap semiconductors such as SiC, AlN, and GaN, which will impact its ability to be used in high power density applications. Thermal management in Ga2O3 electronics will be the key for device reliability, especially for high power and high frequency devices. Similar to the method of cooling GaN-based high electron mobility transistors by integrating it with high thermal conductivity diamond substrates, this work studies the possibility of heterogeneous integration of Ga2O3 with diamond for the thermal management of Ga2O3 devices. In this work, Ga2O3 was deposited onto single crystal diamond substrates by atomic layer deposition (ALD), and the thermal properties of ALD-Ga2O3 thin films and Ga2O3–diamond interfaces with different interface pretreatments were measured by Time-domain Thermoreflectance. We observed a very low thermal conductivity of these Ga2O3 thin films (about 1.5 W/m K) due to the extensive phonon grain boundary scattering resulting from the nanocrystalline nature of the Ga2O3 film. However, the measured thermal boundary conductance (TBC) of the Ga2O3–diamond interfaces is about ten times larger than that of the van der Waals bonded Ga2O3–diamond interfaces, which indicates the significant impact of interface bonding on TBC. Furthermore, the TBC of the Ga-rich and O-rich Ga2O3–diamond interfaces is about 20% smaller than that of the clean interface, indicating that interface chemistry affects the interfacial thermal transport. Overall, this study shows that a high TBC can be obtained from strong interfacial bonds across Ga2O3–diamond interfaces, providing a promising route to improving the heat dissipation from Ga2O3 devices with lateral architectures.

Orders of magnitude reduction in the thermal conductivity of polycrystalline diamond through carbon, nitrogen, and oxygen ion implantation

Despite the exceptional thermal and mechanical functionalities of diamond, its superlative properties are highly subject to the presence of point defects, dislocations, and interfaces. In this study, polycrystalline diamond is ion implanted with C3+, N3+, and O3+ ions at an energy of 16.5 MeV, producing an amorphous layer at the projected range and a damaged crystalline region between the surface and amorphous layer. Using time-domain thermoreflectance in combination with thermal penetration depth calculations based upon the multilayer heat diffusion equation, it is determined that reductions in the thermal conductivity can span nearly two orders of magnitude while still maintaining a polycrystalline structure within the regions thermally probed. Dynamical diffraction simulations of high-resolution x-ray diffraction measurements demonstrate the formation of a strained layer localized at the end of range, with much lower levels of strain near the surface. Furthermore, within the polycrystalline region above the amorphous layer, the average number of displacements-per-atom from the ion irradiation is found to be <1%, with mass impurity concentrations much less than 1%. These low defect concentrations within the thermally probed region demonstrate the remarkably large impact that dilute levels of defects from the ion implantation can have on the thermal conductivity of diamond.

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

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 the 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 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, nonequilibrium molecular dynamics 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.

(Invited) Raman Thermography of β-Ga2­O3 Nanomembrane FETs on Diamond

Despite the extremely low thermal conductivity of Ga2O3, remarkable progress in device technology has been achieved in a very short time since commercial HVPE epilayers were first introduced by Novel Crystal Technology, Japan. For example, Schottky barrier diodes with a forward current of 2 A were recently demonstrated, and thicker, low-doped epi has pushed breakdown voltage beyond 2 kV [1, 2]. Recent demonstrations of Raman thermography in Ga2O3 transistors, however, have proven that a heat spreading solution must be developed in order to realize the potential of Ga2O3 for high power operation [3]. As all devices eventually become thermally limited, effective heat dissipation will also assist in improving reliability and eventual commercialization of Ga2O3 devices. A number of approaches have been devised for GaN HEMTs in the past, where heat generated by the 2DEG under high electric field near the gate can push device temperatures above GaN reliability limits [4, 5]. In this work, we show for the first time that a reasonable operating temperature can be achieved in a Ga2O3 device at a practical power density. While wafer-scale thermal management solutions for Ga2O3 are still being developed, we have exploited the natural (100) cleave plane of Ga2O3 to mechanically exfoliate a highly-doped Ga2O3 membrane onto an insulating single-crystalline CVD diamond substrate and subsequently fabricate a Ga2O3-channel FET using standard electron-beam lithography techniques. Thermal characterization in the transistor channel was performed using a Raman thermography setup by first calibrating the Raman shift of the 169 cm-1 line as a function of temperature on a thermal chuck. This phonon line was chosen as its temperature dependent shift is relatively linear [6]. Independent measurements of the Au contact temperature using thermoreflectance imaging and modeling of the device produced a good agreement with the data [7, 8]. The combination of a thin, highly Sn-doped (2.7x1018 cm-3) Ga2O3 channel, high quality ALD Al2O3 gate dielectric, high thermal conductivity diamond substrate, and a very high critical field for both the channel and substrate (8 MV/cm for Ga2O3, 10 MV/cm for diamond) allowed for a high current density to be maintained under high drain bias. As a result, a maximum DC power density of about 60 W/mm and channel temperature of about 160 °C were simultaneously obtained before destructive failure occurred in the Ga2O3 FET channel. To the authors’ best knowledge, this is the highest power density demonstrated by any semiconductor device. [1] J. Yang et al., IEEE Trans. Electr. Dev., vol. 65, no. 7, pp. 2790, 2018. [2] J. Yang et al., ECS Jour. Solid State Sci. and Technol., vol. 7, no. 5, pp. Q92-Q96, 2018. [3] J.W. Pomeroy et al., IEEE Electr. Dev. Lett., 2018. DOI: 10.1109/LED.2018.2887278. [4] M.J. Tadjer et al., IEEE Electron Dev. Lett., vol. 33, no. 1, pp. 23-25, 2012. [5] K.D. Chabak et al., IEEE Electr. Dev. Lett., vol. 31, no. 2, pp. 99-101, 2010. [6] D. Dohy, G. Lucazeau, A. Revcolevschi, J. Solid State Chem. 45, 180-192 (1982). [7] J. Noh et al., Proc. Dev. Res. Conf., 2018. DOI: 10.1109/DRC.2018.8442276. [8] J. Noh et al., “High Performance β-Ga2O3 Nano-membrane Field Effect Transistors on a High Thermal Conductivity Diamond Substrate,” IEEE Jour. Electr. Dev. Soc., under review.

Thermal characteristics of GaN-on-diamond HEMTs: Impact of anisotropic and inhomogeneous thermal conductivity of polycrystalline diamond

The CVD growth of polycrystalline diamond (PCD) on GaN is a novel and effective approach to improve heat dissipation for high power GaN-based transistors. However, the growth-induced spatial variation in PCD's thermal conductivity makes it difficult to assess the thermal characteristics of GaN-on-diamond devices using bulk diamond properties. By including the depth-dependent anisotropic thermal conductivity of PCD, a finite element thermal simulation is used to study the heat spreading process within a device. The effective thermal conductivity (κeff) of the PCD substrate as “seen” by the device is found to be lower than the average thermal conductivity (κavg) of PCD, suggesting that the low thermal conductivity diamond film during the initial growth has a significant impact on the device thermal resistance. Moreover, κeff is investigated as a function of the thermal boundary resistance between GaN and diamond, the gate pitch of the device as well as the grain boundary conductance and the grain evolution rate of diamond polycrystals. The results provide reliable thermal assessment for the PCD substrate as used in a GaN-on-diamond device and have important implications for thermally optimizing the device layout and engineering the thermal conductivity of PCD heat spreaders for devices.

Understanding the thermal conductivity of Diamond/Copper composites by first-principles calculations

SThe present paper investigates the nanoscale thermal transport at diamond/copper (dCu) interfaces using the density functional theory (DFT) and the atomistic Green's function method. The DFT calculations show the boundary scattering become negligible for diamond particles larger than 10 μm. The temperature-dependent thermal conductivity of the dCu composite is predicted according to the thermal boundary conductance and thermal conductivity from DFT calculations. The results show a low thermal boundary conductance (18.5–26.9 MW/m2K at 300 K) at dCu interfaces causes the reduction in the thermal conductivity of dCu composites with small diamond particles or at low temperatures. Due to the dominant effects of interfacial resistance, adding small diamond particles (<16 μm) in a Cu matrix can only cause a reduction in the effective thermal conductivity of composites regardless of temperature. For diamond particles larger than 16 μm, the enhancement of effective thermal conductivity is most noticeable at relatively low temperatures (120 K ∼ 170 K), and thereafter the enhancement becomes smaller due to the decrease of the diamond thermal conductivity. The results provide insights to understand the mechanism of thermal transport at dCu interfaces and guidelines to engineer the thermal properties of dCu composites.

High Performance ${\beta}$ -Ga2O3 Nano-Membrane Field Effect Transistors on a High Thermal Conductivity Diamond Substrate

To suppress severe self-heating under high power density, we herein demonstrate top-gate nano-membrane ${\beta }$ -gallium oxide ( ${\beta }$ -Ga2O3) field effect transistors on a high thermal conductivity diamond substrate. The devices exhibit enhanced performance, with a record high maximum drain current of 980 mA/mm for top-gate ${\beta }$ -Ga2O3 field effect transistors and 60% less temperature increase from reduced self-heating, compared to the device on a sapphire substrate operating under identical power density. With improved heat dissipation, ${\beta }$ -Ga2O3 field effect transistors on a diamond substrate are validated using an ultrafast high-resolution thermoreflectance imaging technique, Raman thermography, and thermal simulations.

Heat Exchange Evaluation at Thermo-Adhezion Method of Extraction of Diamond Raw from Kimberlite Ore

In the work thermo-adhesion a method of selecting and extracting the grains of diamond from a mixture of related minerals is developed. The study presents estimates of the heat transfer of the proposed scheme extraction based on the difference between the thermal conductivity of diamond and related minerals. The high selectivity thermo-adhesion selection and extraction of diamonds with the division concentrate on useful component and waste rock is shown.

Investigation and Comparison of the DIBL Parameter and Thermal Effects of SOD Transistors and SOI Transistors and Improving Them with the Change of Their BOX Thicknesses

In this paper, thermal effects and Drain Induced barrier lowering (DIBL) of silicon-on-insulator (SOI) and silicon-on-diamond (SOD) transistors with 22 nm channel lengths using hydrodynamic simulations have been investigated. Thermal conductivity of diamond in contrast to thermal conductivity of silicon dioxide is significantly higher. Hence, the heat transfers faster in silicon-on-diamond transistors. Lattice temperature of SODs is lower than that of similar SOIs. By using SODs in Integrated circuits with the first transistor turning on and active, neighboring transistors will have the same level of heat as the active transistor. As a result, the DIBL factor will be increased; this is an undesired phenomenon in CMOS applications. To resolve this issue, we propose a new method which is the thickness reduction of buried diamond layers inside of transistors. Due to this change, DIBL of active transistor will be improved, the exceeding lattice heat of side transistors will be evacuated through the devices and their temperatures will be deduced in large scale.

Nanocrystalline Diamond Integration with III-Nitride HEMTs

Reduced performance in Gallium Nitride (GaN)-based high electron mobility transistors (HEMTs) as a result of self-heating has been well-documented. To mitigate this effect, the incorporation of high thermal conductivity diamond heat spreading films or substrates has been proposed. A mid-process integration scheme, termed “gate-after-diamond,” is shown to improve the thermal budget for NCD deposition and enables scalable, large-area diamond coating without degrading the Schottky gate metal. The optimization of this process step is presented in this work. Nanocrystalline (NCD)-capped devices had a 20% lower channel temperature at equivalent power dissipation. Improved electrical characteristics were also observed, notably improved on-resistance and breakdown voltage, and reduced gate leakage.

(Invited) Challenges and Opportunities for GaN on Diamond Devices

The integration of CVD diamond is seen as an important technology for the passive thermal management of high power GaN devices. Diamond films are often attached to GaN devices either through direct growth on the GaN or through a plasma activated attachment process. Regardless of the method used, a thermal resistance between the GaN and diamond will exist at the interface either through the nucleation and growth of low thermal conductivity diamond at the interface or through a large phonon mismatch at the GaN diamond interace for the bonded approach. While recent demonstrations have shown the benefits of using CVD diamond in lowering the temperature of GaN HEMTs, the theoretical benefits of CVD diamond have not been achieved due to the need to control thermal boundary resistance and the elimination of low conductivity diamond near the interace with GaN. In this work, we will present data on the thermal performance of GaN on Diamond HEMTs fabricated using bonding as well as CVD growth on the GaN. To better understand the thermal response of these devices, the thermal conductivity of the diamond and quality of the diamond measured using Raman spectroscopy and microscopy approaches will be presented. In addition, the thermal boundary resistance between the GaN and diamond will be presented. A link between the thermal conductivity, thermal boundary resistance, and the bonding or growth approach will be used to elucidate their impact on the total thermal resistance of the GaN on Diamond system. Furthermore, the mechanical integrity of the GaN on diamond interface which can lead to a degradation in TBR measured using a transient thermoreflectance method will be shown. Finally, the temperature distribution in GaN on Diamond HEMTs will be measured and correlated to models of the devices employind properties measured in our experiments. Finally, the propspects for GaN on Diamond based on CVD growth and direct bonding approaches will be discussed.

Thermal Management of Hotspots Using Diamond Heat Spreader on Si Microcooler for GaN Devices

A diamond heat spreader has been applied on the hybrid Si microcooler for the improvement of the hotspots cooling capability for GaN devices. The microwave chemical vapor deposition diamond heat spreader under tests is of thickness 400 μm and thermal conductivity as high as 1500 ~ 2000 W/mK, and is bonded through the thermal compression bonding process at chip level. Eight hotspots, each of size 450 × 300 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , were fabricated on a Si thermal test chip to mimic the heating areas of eight GaN units. Heat dissipation capabilities were studied and compared through experimental tests and thermal/fluid simulations, and consistent results have been obtained. Using the diamond heat spreader, to dissipate 70-W heating power, the maximum chip temperature can be reduced by 40.4% and 27.3%, compared with the structure without a heat spreader and the one with a copper heat spreader, respectively. While maintaining the maximum hotspot temperature under 160°C, 10-kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> hotspot heat flux can be dissipated. The thermal effects of the heat spreader thickness, the diamond thermal conductivity, and the bonding layer are investigated. Based on the simulation results, the higher power density of the GaN device can be dissipated, while maintaining the peak gate temperature under 200°C. The concentrated heat flux has been effectively reduced using a diamond heat spreader, and much better cooling capability of the Si microcooler has been achieved for high-power GaN devices.

High thermal conductivity diamond coated graphite by microwave plasma chemical vapour deposition

Diamond film was grown on high thermal conductivity graphite substrate using microwave plasma chemical vapour deposition method. Nanodiamond particles were uniformly seeded on the substrate to generate high nucleation density by a spray gun. The continuous and high purity diamond film was obtained, and growth rate was up to 2.7 μm h− 1. The thickness, surface morphology, quality and composite phase of the film were analysed by SEM, Raman and X-ray diffraction. It was shown that graphite coated with diamond presented a higher thermal conductivity (520 W m− 1 k− 1) than copper. Furthermore, this coated material with high thermal conductivity, good strength and non-conductive surface will make it possible to be widely used in thermal management field.

Effect of nitrogen and vacancy defects on the thermal conductivity of diamond: Anab initioGreen's function approach

We show that impurities and vacancies affect the thermal conductivity much more strongly than what is predicted by widely accepted models. When local distortions around point defects are strong, standard perturbative approaches fail, and phonon scattering can only be accounted for by an exact Green's function calculation. We apply the theory to the study, from first-principles, of nitrogen and vacancy defects in diamond. The thermal conductivity is computed by solving the linearized Boltzmann transport equation. The Born approximation underestimates the phonon scattering cross sections of nitrogen and vacancies by factors of 3 and 10, respectively. Thermal conductivity calculations are in good agreement with experiment.

Estimation of Cutting Conditions in Precision Micromachining of CuNi Alloys of Varying Composition

A non-dimensional model is developed that relates cutting edge temperatures to process inputs (speed and feed) and outputs (cutting and thrust forces) as well as tool and work thermal properties in micromachining with diamond tools. The temperature estimates are used to determine activation energy for thermally activated chemical wear of a PCD tool milling Monel 400 (Cu 32, Ni 65 wt%). Activation energy estimates are from 15 to 60kJ/ mole, depending on systematic uncertainties, such as diamond thermal conductivity, that still need to be addressed.

Thermal conductivity of diamond under extreme pressure: A first-principles study

Using a first-principles approach based on density functional perturbation theory and an exact numerical solution to the phonon Boltzmann equation, we show that application of high compressive hydrostatic pressure dramatically increases the thermal conductivity of diamond. We connect this enhancement to the overall increased frequency scale with pressure, which makes acoustic velocities higher and reduces phonon-phonon scattering rates. Of particular importance is the often-neglected fact that heat-carrying acoustic phonons are coupled through lattice anharmonicity to higher frequency optic modes. An increase in optic mode frequencies with pressure weakens this coupling and contributes to driving the diamond thermal conductivities to far larger values than in any material at ambient pressure and temperature.

Second sound waves in diamond

The possibility of propagation of second sound waves in diamond single crystals depending on their dimensions, concentrations of isotopes and temperature is studied. At this correct account of phonon scattering on boundaries is important. The calculation of phonon collision frequencies is carried out in the reduced isotropic crystal model using second and third modules of elasticity and in Callaway model on the basis of experimental data on diamond thermal conductivity. Both models give us the consistent values of parameters under which the propagation of SSW is possible. It is discovered in concentrations of isotopes 13C < 10 − 6 , temperatures T < 80 K. Such a good agreement provides the reliability of received results and shows the efficiency of reduced isotropic crystal model in the description of diamond properties in low temperature range.

Impact of keV-energy argon clusters on diamond and graphite

Impact of keV-energy size-selected Arn (n=16, 27, 41) cluster ions on diamond and graphite is studied both experimentally and by molecular dynamics simulations. For the case of diamond, relatively high cluster kinetic energies (above certain threshold) are required to produce severe radiation damage and originate crater formation on the surface. This is related to very strong chemical bonds and both the melting (or sublimation) point and thermal conductivity of diamond being the highest among the solids. For the case of graphite, which is layered material with weak van der Waals bonds between the graphene planes, significant radiation damage is already introduced by impact of clusters with low kinetic energies (a few tens of eV/atom). However, collisions of the argon clusters cause very elastic response of the graphene planes that leads to efficient closure of the craters which could be formed at the initial stage of impact.

Effect of coating on the microstructure and thermal conductivities of diamond–Cu composites prepared by powder metallurgy

Diamond–Cu composites from the direct combination of diamond and Cu show low thermal conductivities due to weak interface and high thermal resistance as a result of chemical incompatibility. In this paper, a new method is proposed to strengthen interfacial binding between diamond and Cu by coating strong carbide-forming elements, e.g., Ti or Cr on the surface of the diamond through vacuum micro-deposition. Interfacial thermal resistance of diamond–Cu composites is greatly decreased when diamond particles are coated by a Cr or Ti layer of a certain thickness before combining with Cu. Thermal conductivity is also increased several times. Cr coating can reduce more effectively interface thermal resistance between diamond and Cu than Ti coating. Moreover, it has a smaller negative impact on the thermal conductivity of the Cu matrix, resulting in higher thermal conductivity of Cr-coated diamond–Cu composites. Through the vacuum micro-deposition technology, Cr on the diamond particle surface is present in the form Cr 7C 3 near diamond and a pure Cr outer layer at 2:1. The optimum thickness is within 0.6–0.9 μm; at this depth, the thermal conductivities of 70 vol% diamond–Cu composites can be increased four times and reach as high as 657 W/m K. In this work, an original theoretical model is proposed to estimate the thermal conductivities of composite materials with an interlayer of a certain thickness. The predicted values from this model are in good agreement with the experimental values.