Measurements of electron-phonon coupling factor and interfacial thermal resistance of metallic nano-films using a transient thermoreflectance technique

Using a transient thermoreflectance （TTR） technique, several Au films with different thicknesses on glass and SiC substrates are measured for thermal characterization of metMlic nano-films, including the electron phonon coupling factor G, interfazial thermal resistance R, and thermal conductivity Ks of the substrate. The rear heating-front detecting （RF） method is used to ensure the femtosecond temporal resolution. An intense laser beam is focused on the rear surface to heat the film, and another weak laser beam is focused on the very spot of the front surface to detect the change in the electron temperature. By varying the optical path delay between the two beams, a complete electron temperature profile can be scanned. Different from the normally used single-layer model, the double-layer model involving interfaciM thermal resistance is studied here. The electron temperature cooling profile can be affected by the electron energy transfer into the substrate or the electron-phonon interactions in the metallic films. For multiple-target optimization, the genetic algorithm （GA） is used to obtain both G and R. The experimental result gives a deep understanding of the mechanism of ultra-fast heat transfer in metals.

Thermal characterization of GaN heteroepitaxies using ultraviolet transient thermoreflectance

Thermal transport properties of GaN heteroepitaxial structures are of critical importance for the thermal management of high-power GaN electronic and optoelectronic devices. Ultraviolet(UV) lasers are employed to directly heat and sense the GaN epilayers in the transient thermoreflectance(TTR) measurement, obtaining important thermal transport properties in different GaN heterostructures, which include a diamond thin film heat spreader grown on GaN. The UV TTR technique enables rapid and non-contact thermal characterization for GaN wafers.

Probing thermal properties of vanadium dioxide thin films by time-domain thermoreflectance without metal film

Vanadium dioxide(VO_(2))is a strongly correlated material,and it has become known due to its sharp metal-insulator transition(MIT)near room temperature.Understanding the thermal properties and their change across MIT of VO_(2)thin film is important for the applications of this material in various devices.Here,the changes in thermal conductivity of epitaxial and polycrystalline VO_(2)thin film across MIT are probed by the time-domain thermoreflectance(TDTR)method.The measurements are performed in a direct way devoid of deposition of any metal thermoreflectance layer on the VO_(2)film to attenuate the impact from extra thermal interfaces.It is demonstrated that the method is feasible for the VO_(2)films with thickness values larger than 100 nm and beyond the phase transition region.The observed reasonable thermal conductivity change rates across MIT of VO_(2)thin films with different crystal qualities are found to be correlated with the electrical conductivity change rate,which is different from the reported behavior of single crystal VO_(2)nanowires.The recovery of the relationship between thermal conductivity and electrical conductivity in VO_(2)film may be attributed to the increasing elastic electron scattering weight,caused by the defects in the film.This work demonstrates the possibility and limitation of investigating the thermal properties of VO_(2)thin films by the TDTR method without depositing any metal thermoreflectance layer.

Fast-Converging and Asymptotic-Preserving Simulation of Frequency Domain Thermoreflectance

The heat conduction under fast external excitation exists in many experiments measuring the thermal conductivity in solids,which is described by the phonon Boltzmann equation,i.e.,the Callaway’s model with dual relaxation times.Such a kinetic system has two spatial Knudsen numbers related to the resistive and normal scatterings,and one temporal Knudsen number determined by the external oscillation frequency.Thus,it is a challenge to develop an efficient numerical method.Here we first propose the general synthetic iterative scheme(GSIS)to solve the phonon Boltzmann equation,with the fast-converging and asymptotic-preserving properties:(i)the solution can be found within dozens of iterations for a wide range of Knudsen numbers and frequencies,and(ii)the solution is accurate when the spatial cell size in the bulk region is much larger than the phonon mean free path.Then,we investigate how the heating frequency affects the heat conduction in different transport regimes.

Effect of chromium interlayer thickness on interfacial thermal conductance across copper/diamond interface

The thermal conductivity of diamond particles reinforced copper matrix composite as an attractive thermal management material is significantly lowered by the non-wetting heterointerface.The paper investigates the heat transport behavior between a 200-nm Cu layer and a single-crystalline diamond substrate inserted by a chromium(Cr)interlayer having a series of thicknesses from 150 nm down to 5 nm.The purpose is to detect the impact of the modifying interlayer thickness on the interfacial thermal conductance(h)between Cu and diamond.The time-domain thermoreflectance measurements suggest that the introduction of Cr interlayer dramatically improves the h between Cu and diamond owing to the enhanced interfacial adhesion and bridged dissimilar phonon states between Cu and diamond.The h value exhibits a decreasing trend as the Cr interlayer becomes thicker because of the increase in thermal resistance of Cr interlayer.The high h values are observed for the Cr interlayer thicknesses below 21 nm since phononic transport channel dominates the thermal conduction in the ultrathin Cr layer.The findings provide a way to tune the thermal conduction across the metal/nonmetal heterogeneous interface,which plays a pivotal role in designing materials and devices for thermal management applications.

Pressure-driven anomalous thermal transport behaviors in gallium arsenide

High-pressure has been widely utilized to improve material performances such as thermal conductiv-ityκand interfacial thermal conductance G.Gallium arsenide(GaAs)as a functional semiconductor has attracted extensive attention in high-pressure studies for its technological importance and complex structure transitions.Thermal properties of GaAs under high pressure are urgent needs in physics but remain elusive.Herein,we systematically investigateκGaAs and G Al/GaAs of multi-structure up to -23 GPa.We conclude that:(1)in pressurization,phonon group velocity,lattice defects,and electrons play a central role inκGaAs in elastic,plastic,and metallization regions,respectively.The increased phonon density of states(PDOS)overlap,group velocity,and interfacial bonding enhances G Al/GaAs.(2)In depressurization,electrons remain the dominant factor on κ GaAs from 23 to 13.5 GPa.G Al/GaAs increases dramatically at -12 GPa due to the larger PDOS overlap.With decompressing to ambient,lattice defects including grain size reduction,arsenic vacancies,and partial amorphization reduce κ GaAs to a glass-like value.Remarkably,the released G Al/GaAs is 2.6 times higher than that of the initial.Thus our findings open a new dimension in synergistically realizing glass-like κ and enhancing G,which can facilitate thermoelectric performance and its potential engineering applications.

Reduced thermal boundary conductance in GaN-based electronic devices introduced by metal bonding layer

Achieving high interface thermal conductance is one of the biggest challenges in the nanoscale heat transport of GaN-based devices such as light emitting diodes(LEDs),and high electron mobility transistors(HEMTs).In this work,we experimentally measured thermal boundary conductance(TBC)at interfaces between GaN and the substrates with AuSn alloy as a commonly-used adhesive layer by time-domain thermoreflectance(TDTR).We find that the TBCs of GaN/Ti/AuSn/Ti/Si,GaN/Ti/AuSn/Ti/SiC,and GaN/Ti/AuSn/Ti/diamond,are 16.5,14.8,and 13.2 MW·m^(-2)·K^(-1)at room temperature,respectively.Our measured results show that the TBC of GaN/Ti/AuSn/Ti/SiC interface is inferior to the TBC of pristine GaN/SiC interface,due to the large mismatch of phonon modes between AuSn/Ti and substrates,shown as the difference of Debye temperature of two materials.Overall,we measured the TBC at interface between GaN and thermal conductive substrates,and provided a guideline for designing the interface between GaN and substrate at HEMT from a thermal management point of view.

Enhanced thermal transport across monolayer MoS2

Characterizing the thermal properties of MoS2 is important for the design of electronic devices based on this material. We used frequency domain thermoreflectance to study the cross-plane thermal transport in mechanically exfoliated MoS2 samples supported on SiO2 and muscovite mica substrates. The thickness of MoS2 ranged between one and five layers, and the MoS2 layers were sandwiched between a metal layer and the substrate. In the case of mica, heat transport into the substrate remained the same whether or not a monolayer of MoS2 was present, whereas, for SiO2, heat transport was reduced by surface roughness. We observed a significant improvement in heat transport across monolayer MoS2 as compared to few layer MoS2. For MoS2 on SiO2, the effective thermal interface conductance was improved by more than three times if a monolayer was used. For MoS2 on mica, the thermal interface conductance was approximately two times better for monolayer MoS2. This implies that monolayer MoS2 has superior thermal properties and can be used in electronic devices as an intermediate layer between two materials. Additionally, we also report on the measurement of anisotropic thermal conductivity in bulk MoS2 and mica.

Efficient thermal dissipation in wafer-scale heterogeneous integration of single-crystalline𝛽β-Ga_(2)O_(3)thin film on SiC

The semiconductor,β-Ga_(2)O_(3)is attractive for applications in high power electronic devices with low conduction loss due to its ultra-wide bandgap(∼4.9 eV)and large Baliga’s figure of merit.However,the thermal conductivity of𝛽β-Ga_(2)O_(3)is much lower than that of other wide/ultra-wide bandgap semiconductors,such as SiC and GaN,which results in the deterioration of𝛽β-Ga_(2)O_(3)-based device performance and reliability due to self-heating.To overcome this problem,a scalable thermal management strategy was proposed by heterogeneously integrating wafer-scale single-crystalline𝛽β-Ga_(2)O_(3)thin films on a highly thermally conductive SiC substrate.Characterization of the transferred𝛽β-Ga_(2)O_(3)thin film indicated a uniform thickness to within±2.01%,a smooth surface with a roughness of 0.2 nm,and good crystalline quality with an X-ray rocking curves(XRC)full width at half maximum of 80 arcsec.Transient thermoreflectance measurements were employed to investigate the thermal properties.The thermal performance of the fabricated𝛽β-Ga_(2)O_(3)/SiC heterostructure was effectively improved in comparison with that of the𝛽β-Ga_(2)O_(3)bulk wafer,and the effective thermal boundary resistance could be further reduced to 7.5 m 2 K/GW by a post-annealing process.Schottky barrier diodes(SBDs)were fabricated on both a𝛽β-Ga_(2)O_(3)/SiC heterostructured material and a𝛽β-Ga_(2)O_(3)bulk wafer.Infrared thermal imaging revealed the temperature increase of the SBDs on𝛽β-Ga_(2)O_(3)/SiC to be one quarter that on the𝛽β-Ga_(2)O_(3)bulk wafer with the same applied power,which suggests that the combination of the𝛽-Ga_(2)O_(3)thin film and SiC substrate with high thermal conductivity promotes heat dissipation in𝛽β-Ga_(2)O_(3)-based devices.