Thermoreflectance Method Research Articles

Structure and thermophysical properties of GaN films deposited by reactive sputtering using a metal Ga target

The influence of dislocation density and impurities on the thermal conductivity of epitaxial GaN thin films on c-plane sapphire substrates was studied. GaN thin films with nominal thicknesses of 100, 300, and 1500 nm were fabricated by reactive direct current magnetron sputtering using a Ga metal target and a mixture gas of Ar and N2. A 300-nm-thick GaN film was also fabricated using a mixture gas of Ar–N2–H2. For all the 300- and 1500-nm-thick films, epitaxial growth was confirmed from the sixfold symmetry spots in the pole figure and selected area electron diffraction patterns. Rocking curves of GaN(0002) of these films showed highly oriented growth along the c-axis. The dislocation density deduced from the rocking curves of GaN(101¯0) ranged from 1011 to 1012 cm−2. In the cases where a mixture gas of Ar–N2 was used, films included O and H impurities on the order of 1022 atoms cm−3 in a layer of approximately 50–100 nm thickness near the substrate, and a low-impurity region with impurities on the order of 1021 atoms cm−3 existed above the high-impurity region. The addition of H2 to the sputtering gas led to a reduction in the impurity concentration to a level on the order of 1021 atoms cm−3; it also prevented the formation of the high-impurity region near the substrate. The thermal conductivity of GaN thin films on c-plane sapphire substrates was measured by the pulsed-light-heating thermoreflectance method. The thermal conductivity of the low-impurity region in the 300- and 1500-nm-thick films ranged from 14 to 18 W m−1 K−1. The dislocation density and the oxygen impurities in our films were attributed to inhibitory factors of the thermal conductivity. The breakdown of the bonding network caused by the formations of Ga-NH2 and Ga-OH was not a negligible inhibitory factor of heat conduction.

Composition dependent phase transition and its induced hysteretic effect in the thermal conductivity of WxMo1−xTe2

Recently, transition metal dichalcogenide (TMD) materials have shown promise in electronics and optoelectronics applications. Most of their properties are closely related to their abundant structural phases and phase transitions. For more practical applications in the future, it is necessary to tune the phase transitions in this material system. Here, we demonstrate the modulation of phase transitions in miscible WxMo1−xTe2 samples by appropriate alloying. The temperature dependent thermal conductivity along the c-axis, which strongly relates to the phase structures and the defect level, has been measured using the time-domain thermoreflectance method. In addition, a tunable hysteretic effect, induced by phase transitions, is observed in both thermal and electrical transport properties and confirmed by the consistent hysteresis in the Raman spectroscopic study. This hysteretic effect can be applied to realize phase-change storage devices. Furthermore, we provide a phase diagram to illustrate the composition dependent phase transition in WxMo1−xTe2. This work demonstrates an approach to modulate phase transition and thermal hysteresis in such a TMD material system by alloying engineering.

Thermal Conductivity Measurement of Thermoelectric Thin Films by a Versatility-Enhanced 2ω Method

The 2ω method is a technique to measure the cross-plane thermal conductivity, κ, of a film sample based on sinusoidal Joule-heating of a metal film deposited on the sample surface and its thermoreflectance (TR) measurement of surface temperature cooling due to the heat dissipation. The 2ω method is being paid attention because it is more cost-effective and easier to use than the conventional time domain thermoreflectance (TDTR) method or the 3ω method. In some cases, however, it is difficult to apply the conventional 2ω method to the κ measurement of high thermal resistance films such as general thermoelectric films due to its non-linear TR signal response to (2ω)−0.5. Here, we present a 2ω method based on a versatile TR signal analysis which enables the κ measurement of high thermal resistance film more explicitly than the conventional analysis based on a linear TR signal response. This method determines explicitly the thermal conductivities of PbTe films and PbTe/GeS superlattices grown on BaF2(111) substrates by hot wall epitaxy: κ = 2.1 ± 0.13 Wm−1 K−1 and κ = 0.71 ± 0.05 Wm−1 K−1, respectively. Furthermore, a significant impact of PbTe film crystallinity on thermal conductivity is demonstrated by comparative measurements between polycrystalline PbTe film and epitaxial PbTe film grown on the BaF2(111) substrates. These results demonstrate that our method can be a powerful tool to measure the thermal conductivity of thermoelectric films.

Thermal conductivity measurement of the He-ion implanted layer of W using transient thermoreflectance technique

Transient thermoreflectance method was applied on the thermal conductivity measurement of the surface damaged layer of He-implanted tungsten. Uniform damages tungsten surface layer was produced by multi-energy He-ion implantation with thickness of 450 nm. Result shows that the thermal conductivity is reduced by 90%. This technique was further applied on sample with holes on the surface, which was produced by the He-implanted at 2953 K. The thermal conductivity decreases to 3% from the bulk value.

Bimodal Control of Heat Transport at Graphene-Metal Interfaces Using Disorder in Graphene.

Thermal energy transport across the interfaces of physically and chemically modified graphene with two metals, Al and Cu, was investigated by measuring thermal conductance using the time-domain thermoreflectance method. Graphene was processed using a He2+ ion-beam with a Gaussian distribution or by exposure to ultraviolet/O3, which generates structural or chemical disorder, respectively. Hereby, we could monitor changes in the thermal conductance in response to varying degrees of disorder. We find that the measured conductance increases as the density of the physical disorder increases, but undergoes an abrupt modulation with increasing degrees of chemical modification, which decreases at first and then increases considerably. Moreover, we find that the conductance varies inverse proportionally to the average distance between the structural defects in the graphene, implying a strong in-plane influence of phonon kinetics on interfacial heat flow. We attribute the bimodal results to an interplay between the distinct effects on graphene’s vibrational modes exerted by graphene modification and by the scattering of modes.

Thermal conductivity of amorphous Al2O3/TiO2 nanolaminates deposited by atomic layer deposition

The thermophysical properties of Al2O3/TiO2 nanolaminates deposited by atomic layer deposition (ALD) are studied as a function of bilayer thickness and relative TiO2 content (0%–100%) while the total nominal thickness of the nanolaminates was kept at 100 nm. Cross-plane thermal conductivity of the nanolaminates is measured at room temperature using the nanosecond transient thermoreflectance method. Based on the measurements, the nanolaminates have reduced thermal conductivity as compared to the pure amorphous thin films, suggesting that interfaces have a non-negligible effect on thermal transport in amorphous nanolaminates. For a fixed number of interfaces, we find that approximately equal material content of Al2O3 and TiO2 produces the lowest value of thermal conductivity. The thermal conductivity reduces with increasing interface density up to 0.4 nm−1, above which the thermal conductivity is found to be constant. The value of thermal interface resistance approximated by the use of diffuse mismatch model was found to be 0.45 m2 K GW−1, and a comparative study employing this value supports the interpretation of non-negligible interface resistance affecting the overall thermal conductivity also in the amorphous limit. Finally, no clear trend in thermal conductivity values was found for nanolaminates grown at different deposition temperatures, suggesting that the temperature in the ALD process has a non-trivial while modest effect on the overall thermal conductivity in amorphous nanolaminates.

Enhancing the Thermal Conductance of Polymer and Sapphire Interface via Self-Assembled Monolayer.

Interfacial thermal conductance (ITC) receives enormous consideration because of its significance in determining thermal performance of hybrid materials, such as polymer based nanocomposites. In this study, the ITC between sapphire and polystyrene (PS) was systematically investigated by time domain thermoreflectance (TDTR) method. Silane based self-assembled monolayers (SAMs) with varying end groups, -NH2, -Cl, -SH and -H, were introduced into sapphire/PS interface, and their effects on ITC were investigated. The ITC was found to be enhanced up by a factor of 7 through functionalizing the sapphire surface with SAM, which ends with a chloride group (-Cl). The results show that the enhancement of the thermal transport across the SAM-functionalized interface comes from both strong covalent bonding between sapphire and silane-based SAM, and the high compatibility between the SAM and PS. Among the SAMs studied in this work, we found that the ITC almost linearly depends on solubility parameters, which could be the dominant factor influencing on the ITC compared with wettability and adhesion. The SAMs serve as an intermediate layer that bridges the sapphire and PS. Such a feature can be applied to ceramic-polymer immiscible interfaces by functionalizing the ceramic surface with molecules that are miscible with the polymer materials. This research provides guidance on the design of critical-heat transfer materials such as composites and nanofluids for thermal management.

Structure-thermal property correlation of aligned silicon dioxide nanorod arrays

Quantitative characterization of thermal properties of nanorod (NR) arrays appears to be challenging due to the complex combination of high volume of air voids, anisotropy, and structural non-uniformity. This work investigates the structure-thermal property correlation of arrays consisting of either vertically aligned or slanted silicon dioxide (SiO2) NRs, fabricated by the dynamic shadowing growth technique. We apply the frequency-dependent time-domain thermoreflectance method to quantify the thermal properties of SiO2 NR arrays that may possess inhomogeneity along the depth direction. The effective thermal conductivities of four SiO2 NR array films and one reference capping layer for the SiO2 NR array are obtained. The impact of the structure on the effective thermal conductivities of the SiO2 NR array is discussed. The lowest effective thermal conductivity among all samples in this work is found to be 0.13 W m–1 K−1 for the slanted NR array. We attribute the reduction in the effective thermal conductivity of the NR array to the discontinuous nature of SiO2 NRs, which reduces the density of the thermal transport channels and thus prevents heat flux from propagating downwards along the through-plane direction. The results from this work facilitate the potential applications of NR-array-based thermal insulators for micro-thermal devices.

Thermoreflectance-based in-depth stress distribution measurement technique for single-crystal silicon structures

In this paper, we suggest a new stress measurement technique based on the thermoreflectance method for the estimation of the in-depth stress distribution of fabricated silicon devices. Changing the modulated intensity of a heating laser beam of the frequency-domain thermoreflectance method (FD-TRM) can vary the estimation depth optionally. We developed a measurement system on the basis of the FD-TRM and demonstrated in-depth stress measurement for a single-crystal silicon (SCS) sample. The result measured at a modulation frequency of 3 MHz showed the phase distribution of the TR signal corresponding to the stress distribution determined by 632-nm-excited Raman spectroscopy. In addition, it was found that the phase distribution changed depending on the modulation frequency. The FD-TRM can be a powerful technique for estimating the in-depth stress distribution of silicon materials.

(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.

Filler-depletion layer adjacent to interface impacts performance of thermal interface material

When installing thermal interface material (TIM) between heat source and sink to reduce contact thermal resistance, the interfacial thermal resistance (ITR) between the TIM and heat source/sink may become important, especially when the TIM thickness becomes smaller in the next-generation device integration. To this end, we have investigated ITR between TIM and aluminum surface by using the time-domain thermoreflectance method. The measurements reveal large ITR attributed to the depletion of filler particles in TIM adjacent to the aluminum surface. The thickness of the depletion layer is estimated to be about 100 nm. As a consequence, the fraction of ITR to the total contact thermal resistance becomes about 20% when the TIM thickness is about 50 μm (current thickness), and it exceeds 50% when the thickness is smaller than 10 μm (next-generation thickness).

Measurement of heat diffusion across fuzzy tungsten layer

Heat diffusion across the fuzzy nanostructured tungsten (W) layer formed by helium plasma irradiation was measured using a pulsed light heating thermoreflectance method. By observing the heat diffusion across the nanostructured tungsten layer with a short (1ns) laser pulse heating, the averaged thermal conductivity of the nanostructured layer was deduced to be ∼1.5W/mK, which is ∼1% or less of that of pure (ideal) W.

Tuning the Interfacial Thermal Conductance between Polystyrene and Sapphire by Controlling the Interfacial Adhesion.

In polymer-based electric microdevices, thermal transport across polymer/ceramic interface is essential for heat dissipation, which limits the improvement of the device performance and lifetime. In this work, four sets of polystyrene (PS) thin films/sapphire samples were prepared with different interface adhesion values, which was achieved by changing the rotation speeds in the spin-coating process. The interfacial thermal conductance (ITC) between the PS films and the sapphire were measured by time domain thermoreflectance method, and the interfacial adhesion between the PS films and the sapphire, as measured by a scratch tester, was found to increase with the rotation speed from 2000 to 8000 rpm. The ITC shows a similar dependence on the rotation speed, increasing up to a 3-fold from 7.0 ± 1.4 to 21.0 ± 4.2 MW/(m(2) K). This study demonstrates the role of spin-coating rotation speed in thermal transport across the polymer/ceramic interfaces, evoking a much simpler mechanical method for tuning this type of ITC. The findings of enhancement of the ITC of polymer/ceramic interface can shed some light on the thermal management and reliability of macro- and microelectronics, where polymeric and hybrid organic-inorganic nano films are employed.

Anisotropic thermal conductivity in single crystal β-gallium oxide

The thermal conductivities of β-Ga2O3 single crystals along four different crystal directions were measured in the temperature range of 80–495 K using the time domain thermoreflectance method. A large anisotropy was found. At room temperature, the [010] direction has the highest thermal conductivity of 27.0 ± 2.0 W/mK, while that along the [100] direction has the lowest value of 10.9 ± 1.0 W/mK. At high temperatures, the thermal conductivity follows a ∼1/T relationship characteristic of Umklapp phonon scattering, indicating phonon-dominated heat transport in the β-Ga2O3 crystal. The measured experimental thermal conductivity is supported by first-principles calculations, which suggest that the anisotropy in thermal conductivity is due to the differences of the speed of sound along different crystal directions.

Thermal Conductivity of MWNT–Epoxy Composites by Transient Thermoreflectance

Multiwall nanotube composites with epoxy matrix were synthesized by sonication. Thermal conductivity of the composite samples was determined by a transient thermoreflectance method using indium film as a transducer. The thermal conductivity normal to the surface followed percolation behavior. The presence of higher mass fraction of MWNTs near the surface, and the higher purity and the larger aspect ratio of MWNTs were found to be responsible for significant improvement in thermal conductivity of the composites. The barrier to conduction was found to be the width of the epoxy film separating the MWNTs. Modeling analysis showed that the interface thermal conductance between MWNTs is fairly large and is not a limiting factor for the improvement in the thermal conductivity.

Mechanism of the Reduced Thermal Conductivity of Fishbone-Type Si Phononic Crystal Nanostructures

The mechanism of the reduced thermal conductivity of fishbone-type phononic crystal (PnC) nanostructures, in which ballistic phonon transport is dominant, was investigated with consideration of both the wave and particle nature of phonons. Phononic band diagrams were calculated for an Si nanowire and a fishbone-type PnC structure with a period of 100 nm, and a clear reduction of the group velocity of phonons, because of a zone-folding effect, was shown. Air-suspended Si nanowires and fishbone-type PnC structures were fabricated by electron beam (EB) lithography, and their thermal conductivities were measured by use of the originally developed micro time-domain thermoreflectance method. The PnC structure had a much lower thermal conductivity. We measured the thermal conductivity of a variety of PnC structures with different fin widths to investigate the mechanism of the reduced thermal conductivity observed. The result indicates that the increase of the phonon traveling distance. as a result of the fins, also results in reduced thermal conductivity.

Thermal conductivity changes upon neutron transmutation of 10B doped diamond

10B doped p-type diamond samples were subjected to neutron transmutation reaction using thermal neutron flux of 0.9 × 1013 cm−2 s−1 and fast neutron flux of 0.09 × 1013 cm−2 s−1. Another sample of epilayer grown on type IIa (110) single crystal diamond substrate was subjected to equal thermal and fast neutron flux of 1014 cm−2 s−1. The defects in the diamond samples were previously characterized by different methods. In the present work, thermal conductivity of these diamond samples was determined at room temperature by transient thermoreflectance method. The thermal conductivity change in the samples as a function of neutron fluence is explained by the phonon scattering from the point defects and disordered regions. The thermal conductivity of the diamond samples decreased more rapidly initially and less rapidly for larger neutron fluence. In addition, the thermal conductivity in type IIb diamond decreased less rapidly with thermal neutron fluence compared to the decrease in type IIa diamond subjected to fast neutron fluence. It is concluded that the rate of production of defects during transmutation reaction is slower when thermal neutrons are used. The thermal conductivity of epilayer of diamond subjected to high thermal and fast neutron fluence is associated with the covalent carbon network in the composite structure consisting of disordered carbon and sp2 bonded nanocrystalline regions.

Interfacial thermal resistance between high-density polyethylene (HDPE) and sapphire

To improve the thermal conductivity of polymeric composites, the numerous interfacial thermal resistance (ITR) inside is usually considered as a bottle neck, but the direct measurement of the ITR is hardly reported. In this paper, a sandwich structure which consists of transducer/high density polyethylene (HDPE)/sapphire is prepared to study the interface characteristics. Then, the ITRs between HDPE and sapphire of two samples with different HDPE thickness values are measured by time-domain thermoreflectance (TDTR) method and the results are ∼ 2 × 10−7 m2·K·W−1. Furthermore, a model is used to evaluate the importance of ITR for the thermal conductivity of composites. The model's analysis indicates that reducing the ITR is an effective way of improving the thermal conductivity of composites. These results will provide valuable guidance for the design and manufacture of polymer-based thermally conductive materials.

Investigating the Uneven Current Injection in Perovskite-Based Thin Film Bipolar Resistance Switching Devices by Thermal Imaging

Bipolar resistive switching (RS) phenomenon in planar Al/Pr 0.7 Ca 0.3 MnO 3 (PCMO)/Ti devices was investigated by thermoreflectance method. Thermal images of devices undergoing switching were used to quantify the unevenness of injected current under different voltage bias. At low resistance state, the injected current at the current crowding area of the Al/PCMO interface was 1.6 times higher than other regions of the interface. The uneven distribution of injected current indicated the existence of localized resistance at the interface, which cannot be simply measured by electrical measurements. The thermoreflectance method demonstrates the potential applications for in situ current profiling of the RS devices.

Reduced thermal conductivities of Si one-dimensional periodic structure and nanowire

Air-suspended Si nanowires and one-dimensional (1D) phononic crystals (PnC) nanostructures were fabricated and the thermal conductivity for each structure was measured. The nanostructures were fabricated by a top-down approach from a 145-nm-thick silicon-on-insulator wafer. The thermal conductivities of these nanostructures were measured by an all-optical system based on the time domain thermoreflectance method that we adapted to measure the properties of air-bridge structures. The reduction in thermal conductivity was clearly observed as the width of the nanowire is decreased due to more frequent surface scattering. The 1D PnC structure showed an unexpectedly low thermal conductivity, which is much lower than that of a nanowire with the average width of 1D PnC nanostructure.