Interface Phonons Research Articles

Localized interfacial phonon modes at the electronic axion domain wall

Localized interfacial phonon modes at the electronic axion domain wall

Thermoelectric properties of InGaN/GaN superlattices structure with high indium composition quantum dots

High indium composition InGaN is a promising material for thermoelectric harvesting application, which can work at high temperature and extreme environments. Due to the strong composition segregation, high indium composition InGaN material usually forms localized quantum dots, which advantageously enhances the thermoelectric (TE) properties. In this research, the two-dimensional InGaN/GaN superlattices (SLs) structured TE material with high In composition of 35% quantum dots is first grown and characterized. Using open-circuit voltage measurement, the Seebeck coefficient (S) exhibits a high value of −571 μV/K. Analysis indicates this relatively high S value is related to the increased density of electron states near the Fermi level induced by the reduced dimensionality, resulting in a power factor of 11.83 × 10−4W/m·K2. The dense boundary between InGaN quantum dots also increases the interface phonon scattering, thereby suppressing the heat transportation and leading to a low thermal conductivity (k) value of 19.9 W/m·K. As a result, a TE figure of merit (ZT) value of 0.025 is demonstrated in the sample. This work first clarifies the impact of embedded quantum dots in InGaN/GaN SLs structure on TE properties. It is very conductive for the design and fabrication of low-dimensional GaN based TE device.

Mediating coherent acoustic phonon oscillation of a 2D semiconductor/3D dielectric heterostructure by interfacial engineering

Abstract Coherent acoustic phonon (CAP) oscillation of 2D layered semiconductor/3D dielectric heterostructure generated by femtosecond laser pulse excitation can realize ultrafast photoacoustic conversion, while the photoacoustic conversion efficiency suffers from interfacial phonon scatterings of simultaneously laser-induced lattice heat. Here, taking advantage of graphene’s high thermal conductivity and large acoustic impedance, we demonstrate that phonon scatterings can be markedly mediated in a MoS$_2$/graphene/glass heterostructure via femtosecond laser pump-probe measurements. Equilibrium temperatures of MoS$_2$ lattice have been cooled down by about 45 %. As a benefit, both lifetime of CAP oscillations and pump pulse-picosecond acoustic pulse energy conversion efficiency have been enhanced by a factor of about 2. Our results constitute insights into CAP manipulations via interfacial engineering, that are fundamentally important for ultrafast photoacoustics based on 2D layered semiconductors.

Experimental study of thermal transport across metal/h-BN interfaces: Comparison with metal/graphite interfaces

The continuing miniaturization of layered material electronics highlights the increasingly significant role of interfacial thermal conductance (G) in the efficient nanoscale heat dissipation in these electronics. Despite its importance, the mechanisms of thermal transport across the interfaces of layered materials and the underlying substrate remain unclear. In this work, we comprehensively investigate the interfacial thermal and phonon transport across layered-material interfaces through our ultrafast pump-probe measurements and detailed calculations on G for the interfaces of hexagonal boron nitride (h-BN), one of the most promising layered materials. We first experimentally measure Gc for interfaces between four different metals (Al, Pd, Au and Ti) and h-BN along c-axis using time-domain thermoreflectance (TDTR) from 80 to 300 K. We successfully achieve high accuracy of our measurements with uncertainty <10 % through using the carefully chosen high modulation frequency (15.6 MHz) and relatively large laser spot size (22.5 μm). We find that the measured Gc for interfaces between metals (except Al) and (100) plane of h-BN is lower than that of metal/graphite interfaces, while Gc of Al/h-BN is comparable to that of Al/graphite interfaces. Furthermore, together with our proposed anisotropic model and the diffuse mismatch model (DMM), we determine the transmission probability for the various metal/h-BN interfaces and find that the strong bonding strength enhances the phonon transmission across Ti/h-BN or Ti/graphite interfaces. Finally, we predict the orientation-dependent thermal conductance (i.e., the different interfacial thermal transport along c-axis (Gc) and in-plane (Gab) crystallographic orientations) for both metal/h-BN and metal/graphite. We find Gab between metals and (001) plane is up to 2–3 times higher than Gc. This can be attributed to the highly anisotropic elastic properties in the h-BN and graphite. This work provides important insight into the phonon transmission mechanisms across metal/h-BN interfaces, and thus contributes significantly to the thermal management of layered material electronics.

Generalization of interfacial thermal conductance based on interfacial phonon localization

Interfacial energy transport is of great engineering and scientific importance. Traditional theoretical treatment based on phonon reflection and transmission only provides qualitative understanding of the interfacial thermal conductance (G). In the interface region, the material has gradual (covalent) or abrupt (van de Waals) physical structure transition, each transition features interface-region atomic interactions that are different from those of both adjoining sides. This difference makes the interface-region phonons extremely localized. Here, by constructing an “equivalent interfacial medium” (EIM) that accounts for the extremely localized phonon region, G can be described by a universal physical model that is characterized by an “interface characteristic temperature” (Θint) and energy carrier transfer time. The EIM model fits widely reported G ∼ T (T: temperature) data with high accuracy and provides remarkable prediction of G at different temperatures based on 2–3 experimental data points. Under normalized temperature (T/Θint) and interfacial thermal conductance (G/Gmax), all literature data of G can be universally grouped to a single curve. The EIM model provides a solid correlation between G and interfacial structure and is expected to significantly advance the physical understanding and design of interfacial energy transport toward high-efficiency energy conversion, transport, and micro/nanoelectronics.

Wafer-scale bonded GaN–AlN with high interface thermal conductance

Wide and ultrawide bandgap semiconductors, such as GaN, play a crucial role in high-power applications, yet their performance is often constrained by thermal management challenges. In this work, we introduce a high-quality interface between GaN and AlN, prepared through wafer-scale bonding and verified via high-resolution transmission electron microscopy and transport experiments. We experimentally measured the thermal boundary conductance of the GaN–AlN interface, achieving up to 320 MW/m2K at room temperature using an ultrafast optical technique and sensitivity examinations. Non-equilibrium atomistic Green's functions and density functional theory simulations were conducted to model the interface phonon modes and their contributions to thermal transport, demonstrating good agreement with the experimental results from 80 to 300 K. Additionally, we observed a size-dependent effect on the thermal boundary conductance related to the GaN film thickness from 180 to 450 nm, which we attributed to quasi-ballistic thermal transport through molecular dynamics simulations. Our study has demonstrated a scalable processing route for wafer-sized chip packaging and provides fundamental insights to mitigate near-junction thermal resistance. Further exploration of interface engineering could facilitate co-design strategies to advanced thermal management technologies.

Unraveling interfacial thermal transport in β-Ga2O3/h-BN van der Waals heterostructures

As global power consumption rapidly increases with generation of significant amount of heat, efficient thermal management in electronic equipments becomes an urgent task, which requires a comprehensive understanding on thermal transport in heterostructures within devices. Here, we present detailed examination on the interfacial thermal transport of van der Waals (vdW) heterostructures, composed of β-Ga2O3 and hexagonal boron nitride (h-BN) multilayers. Through extensive molecular dynamics simulations, we show that the interfacial thermal conductance (ITC) of β-Ga2O3/h-BN system becomes as high as 136.8MWm−2K−1, and the high ITC value results from substantial phonon interaction across the interface. In addition to the pristine interface, the effect of structural modulation including strain, vacancies and substitutional defects in h-BN multilayers on the ITC is also analyzed, and it is demonstrated that there exists ranges of strain values and defect concentrations which increase the ITC, and that the enhanced ITC is the result of the interplay among the interfacial distance, the overlap in the phonon density of states and elastic mismatch between β-Ga2O3 and h-BN multilayers. These results not only provide insights into understanding interfacial phonon transport in vdW systems but also offer guiding principles for designing efficient heat dissipators in device applications.

Utilization of doping and compositing strategy for enhancing the thermoelectric performance of CaMnO3 perovskite

The performance of high-temperature thermoelectric material CaMnO3 is primarily limited by its high electrical resistivity and thermal conductivity. In this study, we synergistically optimized its electrical and thermal transport properties by employing a combination of doping and compositing strategies. The results suggest that Yb doping promotes the transfer of electrons to the Mn d orbital and facilitates the double exchange interactions between neighbor Mn3+ and Mn4+, leading to decreased electrical resistivity and a transition of electron transport mechanism from semiconductor to metal. The further addition of CoAl2O4 nanoparticles improves the Seebeck coefficient by reducing carrier concentration and increasing electron scattering, ultimately enhancing the power factor. Meanwhile, the fluctuations in atomic mass and increased interface density strengthen the interface phonon scattering, which results in a significant reduction in lattice thermal conductivity. As a result, the Ca0.95Yb0.05MnO3 with 2 wt% CoAl2O4 nanoparticles exhibits a maximum ZT value of 0.12 at 850 K, representing a 180 % improvement compared to the pristine material. This study highlights the effectiveness of the combined doping and compositing strategy in enhancing the thermoelectric performance of CaMnO3 materials and offers a promising approach for optimizing other oxide thermoelectric materials.

Core-shell engineering of graphite nanosheets reinforced PVDF toward synchronously enhanced dielectric properties and thermal conductivity

Polymer composites integrating high dielectric permittivity (ε′) but low loss, large breakdown strength (Eb) and thermal conductivity (TC), have attracted widespread attention in electronic devices and power systems. To simultaneously achieve these properties in graphite nanosheet (GNS)/poly(vinylidene fluoride, PVDF), the GNS were first encapsulated by a layer of aluminum oxide (Al2O3), and then incorporated into PVDF, and the resulting PVDF nanocomposites’ dielectric properties and TC were explored in terms of the Al2O3 shell thickness and filler loading. The insulating Al2O3 shell serves as a barrier layer for the formation of leakage current and long-distance electron migration thus resulting in much lower dielectric loss and conductivity of the GNS@Al2O3/PVDF. It effectively mitigates the dielectric mismatch between the filler and host matrix, further introduces more traps that can capture charge carriers, and increases the barrier height for electron detrapping subsequently preventing the growth of electric trees and elevating the Eb. Moreover, the Al2O3 interlayer alleviates both the phonon density state and phonon impedance mismatches between the filler and matrix and facilitates the interfacial phonon transport thus leading to improved TC. The dielectric parameters and TC of the GNS@Al2O3/PVDF can be simultaneously modulated by optimizing the Al2O3′s thickness. This work offers an effective approach for designing and fabricating the polymeric nanodielectrics concurrently integrating high ε′ but low loss, enhanced Eb, and TC for prospective applications in power equipment and microelectronic devices.

Nonequivalent Atomic Vibrations at Interfaces in a Polar Superlattice.

In heterostructures made from polar materials, e.g., AlN-GaN-AlN, the nonequivalence of the two interfaces is long recognized as a critical aspect of their electronic properties; in that, they host different 2D carrier gases. Interfaces play an important role in the vibrational properties of materials, where interface states enhance thermal conductivity and can generate unique infrared-optical activity. The nonequivalence of the corresponding interface atomic vibrations, however, is not investigated so far due to a lack of experimental techniques with both high spatial and high spectral resolution. Herein, the nonequivalence of AlN-(Al0.65Ga0.35)N and (Al0.65Ga0.35)N-AlN interface vibrations is experimentally demonstrated using monochromated electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) and density-functional-theory (DFT) calculations are employed to gain insights in the physical origins of observations. It is demonstrated that STEM-EELS possesses sensitivity to the displacement vector of the vibrational modes as well as the frequency, which is as critical to understanding vibrations as polarization in optical spectroscopies. The combination enables direct mapping of the nonequivalent interface phonons between materials with different phonon polarizations. The results demonstrate the capacity to carefully assess the vibrational properties of complex heterostructures where interface states dominate the functional properties.

Localized Phonon Transport Study of GaN/SiO2 Core/shell Nanowires under Thermal-Stress Coupling.

In order to comprehensively explore the intricate mechanisms of thermo-mechanical interactions, this study employs the molecular dynamics method to investigate the influence of heat flux density, shell thickness and length, as well as stress on the radial interface phonon transport in GaN/SiO2 core/shell nanowire. Additionally, the surface eigenmode decomposition method is employed to analyze the interface phonon dispersion curves. The investigation reveals that with increasing heat flux density, internal thermal stresses intensify, leading to a complex distribution of thermal stresses within the system. Under the influence of thermal stress, the nonlinear acoustic properties interact with phonon scattering, resulting in the pronounced localization of interface phonons. Compressive stress causes an upshift in low-frequency phonons, while tensile stress induces a downward shift in the high-frequency optical branches at the interface. The localized phonon vibrations at the SiO2/GaN interface under nonuniform stress are identified as the primary cause for the abundant presence of nondispersive phonon modes at the radial interface. By elucidating the subtle interplay between lattice vibrations and stress fields, this study offers a novel and profound understanding of thermo-mechanical coupling effects, thereby providing innovative theoretical foundations for the design and performance management of thermoelectric devices.

Interlayer coupling modulated for thermoelectric transport characterization of black arsenic nanoscale devices

Modulating interlayer coupling modes can effectively enhance the thermoelectric properties of nanomaterials or nanoscale devices. By using density functional theory combined with non-equilibrium Green’s function method, we investigate the thermoelectric properties of zigzag-type black arsenic nanoscale devices with varying interlayer coupling modes. Our results show that altering the interlayer coupling mode significantly modulates the thermoelectric properties of the system. Specifically, we consider four coupling modes with different strengths, by modulating different interlayer overlap patterns. Notably, in the weaker interlayer coupling mode, the system exhibits enhanced thermoelectric properties due to increased interface phonon scattering, for example, the M4 reaching a peak value of 2.23 at μ = −0.73 eV. Furthermore, we explore the temperature-dependent behavior of each coupling model. The results suggest that the thermoelectric characteristics are more sensitive to temperature variations in the weaker coupling modes. These insights provide valuable guidance for enhancing the thermoelectric performance of nanoscale devices through precise interlayer coupling modulation.

Atomic-scale observation of localized phonons at FeSe/SrTiO3 interface

In single unit-cell FeSe grown on SrTiO3, the superconductivity transition temperature features a significant enhancement. Local phonon modes at the interface associated with electron-phonon coupling may play an important role in the interface-induced enhancement. However, such phonon modes have eluded direct experimental observations. The complicated atomic structure of the interface brings challenges to obtain the accurate structure-phonon relation knowledge. Here, we achieve direct characterizations of atomic structure and phonon modes at the FeSe/SrTiO3 interface with atomically resolved imaging and electron energy loss spectroscopy in an electron microscope. We find several phonon modes highly localized (~1.3 nm) at the unique double layer Ti-O terminated interface, one of which (~ 83 meV) engages in strong interactions with the electrons in FeSe based on ab initio calculations. This finding of the localized interfacial phonon associated with strong electron-phonon coupling provides new insights into understanding the origin of superconductivity enhancement at the FeSe/SrTiO3 interface.

Ion irradiation induced crystalline disorder accelerates interfacial phonon conversion and reduces thermal boundary resistance

Ion irradiation induced crystalline disorder accelerates interfacial phonon conversion and reduces thermal boundary resistance

Robust Thermal Transport across the Surface-Active Bonding SiC-on-SiC.

Surface-active bonding (SAB) is a promising technique for semiconductors directly bonding. However, the interlayer of the bonding interface and the reduced layer thickness may affect thermal transport. In this study, the temperature-dependent cross-plane thermal conductivity of 4H-SiC thin films and the effective thermal boundary resistance (TBReff) of the bonding SiC-on-SiC are measured by the multiple-probe wavelength nanosecond transient thermoreflectance (MW-TTR). The measured temperature-dependent cross-plane thermal conductivity of the 4H-SiC thin film exhibits good quantitative agreement with calculation by density functional theory (DFT) including higher-order four-phonon (4ph) scattering, especially at high temperatures (>400 K). The theoretical calculations indicate the non-negligible importance of 4ph scattering in 4H-SiC high-temperature applications, due to the significantly increasing 4ph scattering rate at increasing temperature and strong temperature dependence of 4ph scattering. The measured nonzero but small TBReff (2.33 + 0.43/-1.15 m2 K/GW) at the SiC-SiC interface is analyzed with molecular dynamics (MD) simulation, indicating that a strong bonding interface with an extremely thin interlayer is formed by the SAB process. Two-dimensional finite element simulations of the experimental equivalent structures are further investigated, and the significant effects (at least 19 °C) of TBReff on the maximum temperature (Tmax) are confirmed. This study provides insight into the fundamental phonon transport and interface thermal transport mechanism in SAB SiC-on-SiC and paves the way for improved 4H-SiC efficient device manufacturing and thermal management.

Synergetic improvement of dielectric properties and thermal conductivity in Zn@ZnO/carbon fiber reinforced silicone rubber dielectric elastomers

Dielectric elastomers (DEs) with large dielectric permittivity (ε') but low loss, high thermal conductivity (TC), are highly pursued in artificial muscles. To elevate the ε' and TC of raw zinc (Zn)/silicone rubber (SR), raw Zn particles were oxidized to Zn@ZnO (zinc oxide), and then incorporated into SR. Compared to raw Zn/SR, the Zn@ZnO/SR showcases the enhanced ε' and TC because the ZnO interlayer induces intraparticle polarization and promotes interfacial phonon transport. Further, synergetic improvement of ε' and TC is found when modicums carbon fibers (CF) was added into Zn@ZnO/SR. The ε' and TC of Zn@ZnO/CF/SR are 23.83 and 1.53 W/(m·K), corresponding to 3.41 and 0.48 W/(m·K) for raw Zn/SR at 60 phr fillers. However, dielectric loss and electric conductivity of Zn@ZnO/CF/SR are remained at low levels. This work provides a strategy for design and preparation of robust SR DEs with high ε' and TC but low loss for prospective applications.

Influence of temperature on bandgap shifts, optical properties and photovoltaic parameters of GaAs/AlAs and GaAs/AlSb p–n heterojunctions: insights from ab-initio DFT + NEGF studies

The III–V group semiconductors are highly promising absorbers for heterojunctions based solar cell devices due to their high conversion efficiency. In this work, we explore the solar cell properties and the role of electron–phonon coupling (EPC) on the solar cell parameters of GaAs/AlSb and GaAs/AlAs p–n heterojunctions using non-equilibrium Green function method (NEGF) in combination of ab-initio density functional theory (DFT). In addition, the band offsets at the heterointerfaces, optical absorption and bandgap shifts (BGSs) due to temperature are estimated using DFT + NEGF approach. The interface band gaps in heterostructures are found to be lower than bulk band gaps leading to a shift in optical absorption coefficient towards lower energy side that results in stronger photocurrent. The temperature dependent electronic BGS is significantly influenced by the phonon density and phonon energy via EPC. The phonon influenced BGS is found to change the optical absorption, photocurrent density and open-circuit voltage. In case of GaAs/AlSb junction, the interface phonons are found to have significantly higher energies as compared to the bulk phonons and thereby may have important implications for photovoltaic (PV) properties. Overall, the present study reveals the influence of EPC on the optical absorption and PV properties of GaAs/AlSb and GaAs/AlSb p–n heterojunctions. Furthermore, the study shows that the DFT + NEGF method can be successfully used to obtain the reasonable quantitative estimates of temperature dependent BGSs, optical absorption and PV properties of p–n heterojunctions.

2D MXene Interface Engineered Bismuth Telluride Thermoelectric Module with Improved Efficiency for Waste Heat Recovery

AbstractGraphene analog MXenes are the best options for interface engineering traditional thermoelectric materials. For the first time, a composite‐engineered TEG device composed of heavily doped bismuth and antimony telluride with incorporated Ti3C2Tx (MXene) nanoflakes is developed. Incorporated MXenes improved the electrical conductivity by carrier injection and reduces thermal conductivity by interfacial phonon scattering in both composites. The fabricated composite TEG device resulted in a maximum power of 1.14 mW and a power density of 6.1 mWcm−2. The fabricated composite TEG also demonstrates strong power generation stability and durability. Added MXenes improve the mechanical stability by employing a dispersion‐strengthening mechanism. Conclusively, the developed composite‐engineered TEG device is a facile and efficiency‐improving option for next‐generation bismuth telluride‐based commercial TEG devices.

Boosting phonon transport across AlN/SiC interface by fast annealing amorphous layers

The high increase in interface density has become the main bottleneck for heat dissipation in gallium nitride/aluminum nitride (AlN)/silicon carbide (SiC) based nanodevices. In this paper, the interfacial thermal conductance (ITC) of AlN/SiC interface is investigated by non-equilibrium molecular dynamics simulation. It is found that introducing amorphous layers at AlN/SiC interface will result in an enhancement of its ITC by 2.32 times. Three different amorphous layers are investigated and can be achieved by fast thermal annealing. Among them, the amorphous SiC layers work best, and the amorphous AlN layers work worst. Further spectral analysis reveals that the enhancement of ITC comes from the strengthening of interfacial inelastic phonon processes, which boosts the transport of modes at a wide frequency range. What is more, as the thickness of amorphous layers increases, the enhancement of ITC weakens. This research provides a highly operational strategy to enhance ITC and enriches our understanding of inelastic phonon process at interface.

Thermal transport mechanism of 4H–SiC/SiO2 heterostructures: a molecular dynamics study

Silicon carbide (SiC) is widely used in high-frequency, high-speed, and high-power applications such as power electronics, rail transportation, new energy vehicles, and aerospace. However, the thermal properties of the SiC/SiO2 interface, which is commonly found in SiC-based devices, are not yet fully understood. This study aims to investigate the influence of temperature and interface coupling strength on the interface thermal resistance (ITR) of 4H-SiC/SiO2 using non-equilibrium molecular dynamics simulations. Both crystalline and amorphous SiO2, as well as two interface contact modes (Si-terminated and C-terminated), have also been considered. The results reveal that the ITR of 4H-SiC/SiO2 is significantly affected by the interface coupling strength and contact modes. Under strong interface coupling conditions, the ITR for Si-terminated and C-terminated contacts modes of 4H-SiC/SiO2 interfaces are 8.077 × 10−10 m2KW−1 and 6.835 × 10−10 m2KW−1, respectively. However, under weak interface coupling conditions, these values increase to 10.142 × 10−10 m2KW−1 and 7.785 × 10−10 m2KW−1, respectively. Regardless of whether SiO2 is crystalline or amorphous, the ITR of the 4H-SiC/SiO2 interface exhibits a similar trend with increasing temperature (from 300 to 700 K). Additionally, the ITR of the amorphous SiO2 interface is smaller than that of the crystalline SiO2 interface under both strong and weak coupling conditions. To gain insights into the heat transport mechanism, the phonon density of states was analyzed to examine the phonon spectral characteristics under varying coupling strengths. These findings have implications for enhancing the thermal management and heat dissipation of SiC devices, providing a framework for controlling interface phonon scattering, and informing the thermal design of nanodevices.