Thermal Conductivity Of Diamond Research Articles

Nanocrystalline Diamond Lateral Overgrowth for High Thermal Conductivity Contact to Unseeded Diamond Surface

We demonstrate diamond lateral overgrowth as a way of increasing the thermal conductivity of thin layers of diamond. The technique can be used as a way of growing diamond on top of semiconductors, creating a thin layer of high thermal conductivity diamond in direct contact with semiconductors and allowing for the encasement of GaN in high thermal conductivity diamond.As we move to higher and higher power densities, the requirements for heat removal become extreme. The best passive way to remove heat from a semiconductor is to have a high thermal conductivity heat spreader near the heat source. Having diamond under the substrate is good. Having diamond under the epi is better and having diamond on top and bottom of the epi is better still.As-grown thin layers of diamond on non-diamond substrates have nano-diamond seeds as the starting material. The diamond thermal conductivity is a function of the grain size (Especially for small grain material) [1]. If we can increase the grain of the diamond near the junction, we can increase the thermal conductivity near the junction and more efficiently spread the heat away from the source before rejecting it into the surrounding material.In the experiment, we start with a bare silicon wafer seeded with nano-diamond seeds then grow about one micron of diamond. We coat it with a few nanometers of SiN. Then pattern the SiN layer with 2μm wide features to facilitate the coalescence of LOG-NCD over the SiN. Then we grow a 2μm thick microwave-plasma CVD NCD film (800W, 15Torr, 750°C, 0.3% CH4/H2) over the patterned SiN film initiating directly from the exposed areas of the host NCD. The diamond growth does not initiate on the patterned silicon nitride without seeds. The exposed diamond acts as a nucleating center initiating the diamond regrowth. The regrown diamond spreads from the diamond crystals exposed from the first growth. The edge crystals closest to the SiN patterned area expand over the silicon nitride forming diamond conformal to the SiN surface. This creates large crystals over the unseeded area. Figure 1 is a transmission electron microscope (TEM) image of the sample. The TEM, the image can be split into two regions. The left has no patterning, the called-out grain is 100nm wide. The right is patterned, and the diamond grains are between 1,000 to 2,000nm wide.We measured the thermal conductivity of the diamond on this samples using the TDTR method. This method uses a laser many times larger in diameter than the thickness of the film. As such, we probe the out-of-plane thermal conductivity. The initial diamond growth near the silicon had an average thermal conductivity of 100W/mK. We measured the second micron of growth without patterns to have Tc of 130W/mK. The diamond directly above the patterns had Tc of 260W/mK with grains 10X larger than the un-patterned diamond.Since the diamond grains are columnar, the difference in thermal conductivity between the patterned and un-patterned diamonds was 2X. Lateral measurements would likely show even larger changes in thermal conductivity. With grain sizes of 100 nm to 250 nm (as is the case with seeded diamond), the in-plane thermal conductivity will be dominated by the quality of the grain/grain interfaces and the lateral grain size dimensions rather than for the quality of the lattice. [1] We used lateral overgrowth to increase the grains to microns rather than 100s of nm. This means that we move into a regime where thermal conductivity is dominated by the quality of the diamond in the grains rather than the grain boundaries. As such the thermal conductivity is increased and can be further improved by improving the crystal quality - a parameter which is controlled by the diamond growth conditions. Typically, samples grown with more than 1% methane / hydrogen ratio show lower thermal conductivity [2]. However higher concentrations are important to establish the initial diamond film without damage to the underlying semiconductor. Here, the lateral overgrowth and methane concentration can be balanced to achieve both high thermal conductivity and low damage to the underlying substrate.

Nanocrystalline Diamond Lateral Overgrowth for High Thermal Conductivity Contact to Unseeded Diamond Surface

We demonstrate unseeded lateral overgrowth of nanocrystalline diamond as a way of increasing the thermal conductivity of thin layers of diamond. The technique can be used as a way of growing diamond on top of semiconductors, creating a thin layer of high thermal conductivity diamond in direct contact with semiconductors and allowing for the encasement of GaN in high thermal conductivity diamond.

Enhancement of interfacial thermal conductance by introducing carbon vacancy at the Cu/diamond interface

Effective heat dissipation of semiconductor devices is crucial for their extended lifespan and operational stability, and the interface of semiconductors provides an effective window for thermal design and management. In this work, we have systematically investigated the effect of the carbon vacancy on the thermal conductivity of diamond and the interface thermal conductance (ITC) of Cu/diamond by using both first-principles calculation and molecular dynamics methods. Although the carbon vacancy leads to a decrease in the thermal conductivity of diamond, a marked increase in ITC from 37.98 MWm−2K−1 to about 177 MWm−2K−1 for diamond (1 1 1) plane and from 78.8 MWm−2K−1 to about 241 MWm−2K−1 for diamond (0 0 1) plane is observed between Cu and diamond with carbon vacancy. The increase of the ITC is mainly due to the anharmonic phonon scattering, revealed by the phonon density of states and phonon participation ratio.

High-quality fabrication of diamond straight microchannels heat sink with large aspect ratio microchannels using UV nanosecond laser based on multi-feed method

The high heat flux density of modern chips presents a major challenge to their development. Single-phase or two-phase flow heat dissipation techniques using high thermal conductivity materials are seen as a potential solution. However, creating microfluidic channels from ultra-high thermal conductivity diamond materials is a significant challenge. In this paper, we present an experimental study on the fabrication of diamond heat sinks with high aspect ratio microchannels using a UV-nanosecond laser processing system, along with two multi-feed processing techniques for the collecting grooves and microchannels. We used a 1 μm spacing grid path combined with a polished multi-feed process to fabricate a 1600 μm deep collecting groove with a smooth bottom morphology. We also proposed a multi-area multi-feed fabrication technique for microchannels with an aspect ratio of 8:1 and a depth of 1600 μm. Finally, we have successfully fabricated a diamond straight microchannels heat sink, featuring a high aspect ratio and low phase change damage.

Thermal conductivity of type-Ib HPHT synthetic diamond irradiated with electrons

The effect of irradiation with 3 MeV electrons and subsequent high-temperature annealing on the thermal conductivity κT of synthetic HPHT diamond containing impurity nitrogen at a concentration of about 100 ppm was studied at temperatures from 5 to 410 K. At each stage of the experiment, the optical absorption and photoluminescence spectra, magnetization and thermal conductivity were measured on two plates from the same diamond single crystal. Color centers and their concentration in different locations in the samples were determined from the optical spectra. The concentration of paramagnetic impurities in the samples was determined from the temperature and field dependencies of the paramagnetic magnetization. Thermal conductivity is suppressed by two orders of magnitude at temperatures from 35 to 50 K after irradiation to a fluence of 5×1018 cm−2. Annealing at a temperature of 1800° C restored the thermal conductivity of diamond at moderate and high temperatures, but not at temperatures below 120 K. The measured data on thermal conductivity were analyzed within the framework of the phenomenological Callaway model in order to understand the role of various phonon scattering processes and how it changes during irradiation and annealing.

Synergistic competition mechanism of phonon scattering in uniaxial (100) strain diamond: A first-principles study

The miniaturization of high-frequency electronic devices has led to heat accumulation in their components. The ultrahigh thermal conductivity of diamond makes it an ideal material for heat dissipation, but factors such as temperature changes during the working process, special structures, thermal stress, and lattice mismatch can affect performance. This study investigated the effects of stress, temperature, isotope purity, and feature size on the thermal conductivity of diamond using a first-principles method. The sensitivity to size effects of the thermal conductivity of uniaxially strained diamond differed according to the scale. Under a low temperature (100 K), and with a feature size of 1 µm, the thermal conductivity of diamond with large-scale tensile strain exceeded that of a diamond with small-scale strain, which was an anomalous phenomenon. Isotope purification did not significantly affect the variation rule of the thermal conductivity of strained diamond.

First-principles predictions of thermal conductivity of bulk diamond under isotropic and uniaxial (100) strains

The miniaturization of high-frequency electronic devices is associated with significant heat accumulation by individual components. Diamond exhibits ultrahigh thermal conductivity and is thus an ideal material for heat dissipation. Considering the sensitive stress-thermal responses of diamond, this study uses first-principles calculations and the phonon Boltzmann transport equation to compare and analyze the effects of uniaxial strain along the <100> crystal direction, as well as isotropic strain, on the thermal conductivity of bulk diamond. Under isotropic strain, the thermal conductivity is principally influenced by bond length; the relationship between thermal conductivity and the bond length deformation index follows a power law. During uniaxial strain, the bond angle and bond length change simultaneously, superimposing their effects on thermal conductivity. This study provides essential insights into the mechanisms of diamond thermal conductivity under different strains; the findings can guide prediction of the strain-thermal conductivity relationships of other crystalline materials.

Diamond rotors

The resolution of magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra remains bounded by the spinning frequency, which is limited by the material strength of MAS rotors. Since diamond is capable of withstanding 1.5–2.5x greater MAS frequencies, compared to state-of-the art zirconia, we fabricated rotors from single crystal diamond. When combined with bearings optimized for spinning with helium gas, diamond rotors could achieve the highest MAS frequencies to date. Furthermore, the excellent microwave transmission properties and thermal conductivity of diamond could improve sensitivity enhancements in dynamic nuclear polarization (DNP) experiments. The fabrication protocol we report involves novel laser micromachining and produced rotors that presently spin at ωr/2π = 111.000 ± 0.004 kHz, with stable spinning up to 124 kHz, using N2 gas as the driving fluid. We present the first proton-detected 13C/15N MAS spectra recorded using diamond rotors, a critical step towards studying currently inaccessible ex-vivo protein samples with MAS NMR. Previously, the high aspect ratio of MAS rotors (∼10:1) precluded fabrication of MAS rotors from diamond.

Orientated growth the 3D diamond/graphene hybrid arrays and the application in thermal interface materials

Diamond and graphene are considered to be one of the most promising thermal interface materials (TIMs) for electronic devices benefited from their highest thermal conductivity in the natural world. However, orientated fabrication of high thermal conductivity diamond and graphene hybrid arrays with three dimensions (3 D) thermal conductive networks are still problematic. Here, we used a unique one-step microwave plasma chemical vapor deposition, n-butylamine, as the liquid source to prepare a novel high thermal conductivity 3 D vertical diamond/graphene (VDG) hybrid arrays films. The orientated 3 D thermal conduction path of the VDG is regulated by the growth temperature, and the through-plane thermal conductivity value of the VDG700 films up to 97 W m−1 K−1. In the actual TIM performance measurement, the system cooling efficiency with our VDG as TIM is higher than the state-of-the-art commercial TIM, demonstrating the superior ability to solve the inter-facial heat transfer issues in electronic systems.

Integrating Diamond for Cooling Electronics

III-Nitrides promise to offer high-frequency power amplifiers at higher power densities. This is attractive for realizing device platforms for 5G and beyond. It also enables integrating Si-based electronics to complement functionalities. However, to obtain the full potential of III-Nitrides one must operate these transistors at higher power densities, for which thermal management is crucial.Due to the continuous need for higher performance in various applications ranging from DC to W-band frequencies, semiconductor devices are pushed to higher power densities. This results in a higher junction temperature owing to the Joule heating in the channel resulting in device performance degradation and premature failure. Using devices based on wide-bandgap (WBG) semiconductors such as Gallium Nitride (GaN) and Gallium Oxide (GaOx) with a larger critical electric field and high-temperature tolerance, one could increase the power density. However, in order to increase the output power to the levels that is promised by these wide bandgap materials, junction temperature reduction through device-level cooling is critical. We are actively researching thermal management at device, circuit and packaging level using synthetic polycrystalline (PC) diamond. PC-diamond, at an appropriate grain size offers a TC is higher than any competing thermal vias with metals without interfering the electrical performance. Over the past 5 years we have demonstrated the potential of integrating diamond with GaN and recently achieving a record low diamond/GaN thermal boundary resistance (TBR) along with a relatively high diamond thermal conductivity (TC). This remarkable thermal performance was be achieved by maintaining the electrical performance, as demonstrated by the unharmed channel charge and mobility results.There are two key requirements for thermal management using heat spreading from the channel: first, a low thermal boundary resistance (TBR) between the hot spot in the channel and the heat spreader is required (in this study between GaN channel and diamond). Second, a low thermal resistance path to the heat-sink is crucial from the diamond layer which is directly proportional to diamond’s thermal conductivity (TC). In collaboration with Univ of Bristol, we have reported a TBR of ~3 m2K/GW, to date, known to be the lowest reported TBR for GaN HEMT technology. Diamond-GaN TBR depends strongly on the interface smoothness and the thickness of interfacial Si3N4 layer. A thinner Si3N4 resulted in lower TBR and allowed superior phonon coupling from the channel to the spreader. We have also measured a remarkably high TC for a 2 μm-thick diamond layer yielding over 650 W/m.K. The diamond grains are near isotropic in shape that allows excellent in-plane and cross-plane thermal conductivity. Achieving high TC within a thin film of diamond, grown heterogeneously, underscores the importance of our technique, since it reduces the residual stress when integrating with different materials that involves many thin layers like GaN/AlGaN HEMT epi-layers.β-Ga2O3 is an emerging ultra-wide bandgap material showing promises for both power and RF devices. However, the material’s low thermal conductivity poses to be a challenge for efficient power delivery (at any frequencies). PC diamond was successfully grown on (‾¯201)β-Ga2O3 using proper nucleation technique and thermal characterizations were conducted. A thermal conductivity (diamond + nucleation) and thermal boundary resistance at the diamond/β-Ga2O3 interface of 110 ± 33 W/mK and 30.2 ± 1.8 m2K/GW, respectively were measured.Our current results demonstrates a very promising roadmap for wide bandgap semiconductors via diamond integration.

Research on controllable ozone oxidation on diamond surface

In recent years, there have been more and more researches on the surface modification of diamonds, however, the exact types and quantities of oxygen-related species on diamond surfaces and the method to control the condition parameters to obtain as many oxygen-containing groups as possible have been rarely studied so far. Therefore, in this work, we focused on these questions. And we find out that ozone oxidation would not affect the overall crystal structure and morphology of diamonds. Besides, changing oxidation time and ozone concentration would significantly influence the density of hydroxyl groups, which is manifested as a change of oxygen content. In order to make the hydroxyl density on diamond surface reach a high level (3.12 × 1014 units/cm2), so that diamonds can be better combined with the resin matrix, the ozone oxidation time should be 15 min, and the ozone concentration should be 115 g•m−3. And under these conditions, the thermal conductivity of diamond and polysiloxane composites can reach 8.02 W/mK.

Structural and thermal analysis of polycrystalline diamond thin film grown on GaN-on-SiC with an interlayer of 20 nm PECVD-SiN

A 1.5-μm polycrystalline diamond was deposited on the AlGaN/GaN heterojunction on the SiC substrate with a 20-nm SiN dielectric. A 4.9% increase in 2DEG density after the diamond growth due to the increase in tensile strain of the GaN layer is confirmed by micro-Raman measurements. The interfacial analysis through the transmission electron microscopy and electron energy loss spectroscopy shows a thickness reduction in the SiN layer of ∼1.7 nm, which converts to a thin SiC layer at the diamond/SiN interface, and no carbon diffusion is found in the SiN layer after the diamond growth. Device simulation using the thermal properties extracted by time domain thermoreflectance predicts a temperature drop of 17.1 °C when the diamond only covers the device access region and reveals that the improvement of thermal boundary resistance is much more effective than that of the diamond thermal conductivity for the top-side heat spreading, which is mainly due to the limited thickness of the top diamond film.

Polycrystalline diamond growth on β-Ga2O3 for thermal management

We report polycrystalline diamond epitaxial growth on β-Ga2O3 for device-level thermal management. We focused on establishing diamond growth conditions on β-Ga2O3 accompanying the study of various nucleation strategies. A growth window was identified, yielding uniform-coalesced films while maintaining interface smoothness. In this first demonstration of diamond growth on β-Ga2O3, a diamond thermal conductivity of 110 ± 33 W m−1 K−1 and a diamond/β-Ga2O3 thermal boundary resistance of 30.2 ± 1.8 m2K G−1 W−1 were measured. The film stress was managed by growth optimization techniques preventing delamination of the diamond film. This work marks the first significant step towards device-level thermal management of β-Ga2O3 electronic devices.

3-D Printed Microjet Impingement Cooling for Thermal Management of Ultrahigh-Power GaN Transistors

Future GaN-based radio frequency (RF) high-electron-mobility-transistors (HEMTs) can enable increased areal power dissipation by, for example, integrating GaN device layers with high thermal conductivity diamond substrates. To maximize the benefit of the ultrahigh-power-density electronic devices, improved package-level cooling methods are needed to prevent the package and heatsink becoming a thermal bottleneck. We demonstrate that 3-D printed polymeric microjet liquid impingement cooling can reduce the thermal resistance at the package level by ~60% with respect to GaN RF HEMTs mounted on conventional packaging.

Thickness Dependence and Anisotropy of Capped Diamond Thermal Conductivity on Cooling of Pulse-Operated GaN HEMTs

A capped thin-film diamond heat spreader is modeled for the reinforcement of hotspot cooling in the application of pulse-operated GaN-based high-electron mobility transistors (HEMTs). To precisely evaluate the cooling performance, thickness dependence and anisotropy of diamond film thermal conductivity are considered. A 3-D transient thermal model based on the finite element method is developed to simulate the pulse behavior. The sharp temperature oscillation occurring in the periodic pulse mode from the microsecond to millisecond time scales introduces substantial difficulty and challenge for accurate modeling. To this end, a scale-down model mimic of a recently reported GaN-based amplifier with kilowatts peak power level is built to significantly reduce the computational time and increase the accuracy while maintaining crucial thermal characteristics. The thermal characteristics under periodic pulse mode are revealed and the dynamic steady-state peak junction temperature is reliably obtained. The impacts of thermal boundary resistance (TBR), substrate material, duty cycle, and pulse period on the performance of the diamond heat spreader have been scrutinized. For the GaN-on-SiC HEMTs working at 2.5% duty cycle, 200-μs pulse period, and 19.58 W/mm peak power density, a 2-μm-thick diamond heat spreader can reduce the dynamic steady-state peak junction temperature by 14.7%.

Origin of abnormal thermal conductivity in group III-V boron compound semiconductors

Over the past half-century, according to Moore’s law, the sizes of transistors continue shrinking, and the integrated circuits have approached to their physical limits, which puts forward higher requirements for the thermal dissipation capacity of material. Revealing the physical mechanisms of heat conduction in semiconductors is important for thermal managements of devices. Experimentally, it was found that boron arsenide has a very high thermal conductivity compared with diamond, and boron arsenide has lattice constant close to silicon’s lattice constant, which can be heterogeneously integrated into silicon to solve the thermal management problem. However, group III-V boron compounds show abnormal thermal conductivities: the thermal conductivity of boron arsenide is significantly higher than that of boron phosphide and boron antimonide. Here, we use the first-principles calculation and the Boltzmann transport equation to study the thermal conductivity properties of the group III-V boron compounds. Comparison between the IV and III-V semiconductors shows that the high thermal conductivity of boron arsenide is due mainly to the existence of a large frequency gap between the acoustic and the optical branches. The energy sum of two acoustic phonons is less than energy of one optical phonon, which cannot meet the energy conservation requirements of three-phonon scattering, and then seriously restrict the probability of scattering of three phonons. The high thermal conductivity of diamond is due mainly to its great acoustic phonon group velocity. Although the boron phosphide also has a relatively large acoustic phonon group velocity, the frequency gap is relatively small, which cannot effectively suppress the three-phonon scattering, so the thermal conductivity of boron phosphide is less than that of boron arsenide. Although the frequency gap of boron antimonide is similar to that of boron arsenide, the thermal conductivity of boron antimonide is lower than that of boron arsenide due to its smaller acoustic phonon group velocity and larger coupling matrix element. The research provides a new insight into the design of semiconductor materials with high thermal conductivities.

Significance of viscous dissipation on the dynamics of ethylene glycol conveying diamond and silica nanoparticles through a diverging and converging channel

The enhancement of heat transfer in a new kind of hybrid nanofluid is more than that of a single nanoparticle. Also, the thermal conductivity of diamond, silver and gold is more than that of copper, titanium oxide and alumina. Hence, an attempt is made to study the influence of hybrid nanoparticles (diamond and silica) with ethylene glycol as a base fluid in a diverging/converging channel. Using similarity analysis, the governing system of equations is transformed to a set of nonlinear ODEs. The resulting system is evaluated computationally by using the most powerful shooting procedure with the support of MATLAB software. The influence of (diamond and silica-ethylene glycol) hybrid nanomaterial on various thermofluidic parameters in a Jeffery–Hamel channel is investigated. The outcomes of this work conclude that (1) the fluid flow velocity decreases in divergent channels with increasing Reynolds number and nanoparticle volume fraction. (2) The skin friction coefficient is found to be a rising function of diamond nanoparticles in the presence of the ethylene glycol-based fluid for divergent channel.

Microwave diamond devices technology: Field‐effect transistors and modeling

AbstractThis paper provides an overview of the developments in microwave diamond field‐effect transistor (D‐FET) technologies. Due to the ultrawide‐bandgap and high carrier velocity and thermal conductivity of diamond, it is a potential candidate for microwave power devices with high output power and operating frequency. Here, the properties of semiconductor materials for microwave applications are described. Then, the mechanisms of various diamond FETs are detailed. In recent years, hydrogen‐terminated diamond metal‐oxide‐semiconductor field‐effect transistors (HD MOSFETs) have been widely studied for their potential use in microwave power devices. Therefore, the structures and developments of HD MOSFETs are mainly discussed. Finally, in the prospective application of the HD FET microwave circuit, the state‐of‐the‐art large‐signal modeling of HD MOSFETs is presented.

Constructing efficient heat transfer channels at the interface of Diamond/Cu composites

ABSTRACT Understanding the mechanism of interfacial heat flux transfer is the basis of improving the thermal conductivity of diamond reinforced metal matrix composites. In this paper, the diamond/Cu(Ti) (38 vol.% diamond) composites with roughened surfaces are prepared by vacuum hot pressing sintering, and the effect of interfacial contact area on the thermal conductivity of composites is investigated by experiments and finite element simulations. A 48% (from 347 to 512 W·m−1·K−1, which is close to 90% of the theoretical value) enhancement in thermal conductivity is obtained. The roughened interfaces increase the contact areas, which provide more heat transfer channels and concentrated thermal flux routes and then optimize the efficiency of thermal flux transfer and then improve the thermal conductivity of the composites. These results provide the experimental and theoretical basis for improving the thermal conductivity of composites by the design of interface structure.

The effect of GaN epilayer thickness on the near-junction thermal resistance of GaN-on-diamond devices

Gallium nitride (GaN) heteroepitaxially integrated with diamond (GaN-on-diamond) is promising for high-power electronics due to the excellent heat spreading capability of diamond. A number of past works have examined the thermal properties of GaN-on-diamond devices, particularly the diamond thermal conductivity and the thermal boundary resistance (TBR) between the GaN and diamond, as well as the impact of these two properties on the thermal resistance of GaN-on-diamond devices. Much less investigated, however, is the effect of the thickness of the GaN epilayer on the thermal resistance of GaN-on-diamond devices. Here, we examine this effect through combining finite element simulations with calculations using a semi-classical phonon transport model. The latter considers phonon scattering on defects and interfaces and is utilized here to predict the in-plane thermal conductivity of the GaN epilayer versus layer thickness. This aims at considering the thermal spreading resistance within the GaN in a more accurate manner, which also depends on the layer thickness. Our simulation results indicate that with increasing GaN layer thickness the device thermal resistance monotonically decreases until it reaches the minimum at GaN thicknesses of ∼3.6 and ∼5.8 μm for GaN/diamond TBRs of 6.5 and 30 m2 K GW–1, respectively. A typical GaN thickness of 1 µm can provide a fairly low device thermal resistance, approximately 6 and 20% higher than the minimum thermal resistances for GaN/diamond TBRs of 6.5 and 30 m2 K GW–1, respectively. Reducing the GaN thickness below 1 µm can substantially increase the device thermal resistance, particularly when the GaN/diamond TBR is high, predominantly due to the increasing contribution of the interface. Increasing the GaN thickness above 1 µm can decrease the device thermal resistance but this change is not significant. Our findings presented here can offer a more in-depth understanding of near-junction thermal transport in GaN-on-diamond transistors.