Enhanced Thermal Boundary Conductance across GaN/SiC Interfaces with AlN Transition Layers.

Heat dissipation plays a crucial role in the performance and reliability of high-power GaN-based electronics. While AlN transition layers are commonly employed in the heteroepitaxial growth of GaN-on-SiC substrates, concerns have been raised about their impact on thermal transport across GaN/SiC interfaces. In this study, we present experimental measurements of the thermal boundary conductance (TBC) across GaN/SiC interfaces with varying thicknesses of the AlN transition layer (ranging from 0 to 73 nm) at different temperatures. Our findings reveal that the addition of an AlN transition layer leads to a notable increase in the TBC of the GaN/SiC interface, particularly at elevated temperatures. Structural characterization techniques are employed to understand the influence of the AlN transition layer on the crystalline quality of the GaN layer and its potential effects on interfacial thermal transport. To gain further insights into the trend of TBC, we conduct molecular dynamics simulations using high-fidelity deep learning-based interatomic potentials, which reproduce the experimentally observed enhancement in TBC even for atomically perfect interfaces. These results suggest that the enhanced TBC facilitated by the AlN intermediate layer could result from a combination of improved crystalline quality at the interface and the "phonon bridge" effect provided by AlN that enhances the overlap between the vibrational spectra of GaN and SiC.

Thermal Transport across Ion-Cut Monocrystalline ß-Ga2O3 Thin Films and Bonded ß-Ga2O3-SiC Interfaces.

The ultrawide band gap, high breakdown electric field, and large-area affordable substrates make ß-Ga2O3 promising for applications of next-generation power electronics, while its thermal conductivity is at least 1 order of magnitude lower than other wide/ultrawide band gap semiconductors. To avoid the degradation of device performance and reliability induced by the localized Joule-heating, proper thermal management strategies are essential, especially for high-power high-frequency applications. This work reports a scalable thermal management strategy to heterogeneously integrate wafer-scale monocrystalline ß-Ga2O3 thin films on high thermal conductivity SiC substrates by the ion-cutting technique and room-temperature surface-activated bonding technique. The thermal boundary conductance (TBC) of the ß-Ga2O3-SiC interfaces and thermal conductivity of the ß-Ga2O3 thin films were measured by time-domain thermoreflectance to evaluate the effects of interlayer thickness and thermal annealing. Materials characterizations were performed to understand the mechanisms of thermal transport in these structures. The results show that the ß-Ga2O3-SiC TBC values are reasonably high and increase with decreasing interlayer thickness. The ß-Ga2O3 thermal conductivity increases more than twice after annealing at 800 °C because of the removal of implantation-induced strain in the films. A Callaway model is built to understand the measured thermal conductivity. Small spot-to-spot variations of both TBC and Ga2O3 thermal conductivity confirm the uniformity and high quality of the bonding and exfoliation. Our work paves the way for thermal management of power electronics and provides a platform for ß-Ga2O3-related semiconductor devices with excellent thermal dissipation.

Probing photoelectrical transport in lead halide perovskites with van der Waals contacts.

Lead halide perovskites have attracted increasing interest for their exciting potential in diverse optoelectronic devices. However, their charge transport properties remain elusive, plagued by the issues of excessive contact resistance and large hysteresis in ambient conditions. Here we report a van der Waals integration approach for creating high-performance contacts on monocrystalline halide perovskite thin films with minimum interfacial damage and an atomically clean interface. Compared to the deposited contacts, our van der Waals contacts exhibit two to three orders of magnitude lower contact resistance, enabling systematic transport studies in a wide temperature range. We report a Hall mobility exceeding 2,000 cm2 V-1 s-1 at around 80 K, an ultralow bimolecular recombination coefficient of 3.5 × 10-15 cm3 s-1 and a photocurrent gain >106 in the perovskite thin films. Furthermore, magnetotransport studies reveal a quantum-interference-induced weak localization behaviour with a phase coherence length up to 49 nm at 3.5 K. Our results lay the foundation for exploring new physics in this class of 'soft-lattice' materials.

One-Pot Self-Templated Growth of Gold Nanoframes for Enhanced Surface-Enhanced Raman Scattering Performance.

As one of the representative metallic hollow nanostructures, Au nanoframes have shown fascinating properties such as strong localized surface plasmon resonance associated with emerging applications such as surface-enhanced Raman scattering (SERS) sensors. In this study, for the first time, a facile one-pot synthetic approach for hollow Au nanoframes is demonstrated by directly etching Au nanoplates, that is, the so-called self-templates. A novel growth mechanism has been revealed that involves a synergistic function of Ag and Br ions. The presence of Ag+ leads to the observation of self-limiting Au film thickness, whereas Au{111} facets are preferentially attacked by the presence of Br- in the reaction ambient. More importantly, graphene is introduced to prevent/minimize aggregation during the formation of Au nanoframes. The combined simulation and experimental studies show that the hybrid platform made of graphene/Au nanoframes is capable of detecting analytes at concentration levels down to 10-9 M by using the SERS technique.

Ultra-high vacuum dc magnetron sputter-deposition of 0001-textured trigonal α-Ta2C/Al2O3(0001) thin films.

We report on the effects of substrate temperature (1073 K ≤ T s ≤ 1373 K) and deposition time t (= 3 ~ 30 min.) on the crystallinity of Ta2C/Al2O3(0001) thin films grown via ultra-high vacuum direct current magnetron sputtering of TaC target in 20 mTorr (2.7 Pa) pure Ar atmospheres. Using X-ray diffraction and transmission electron microscopy, we determine that the layers are 0001-oriented, trigonal-structured α-Ta2C at all T s. With increasing T s, we obtain smoother and thinner layers with enhanced out-of-plane coherency and decreasing unit cell volume. Interestingly, the Ta2C 0001 texture improves with increasing T s up to 1273 K above which the layers are relatively more polycrystalline. At T s = 1373 K, during early stages of deposition, the Ta2C layers grow heteroepitaxially on Al2O3(0001) with ( 0001 ) Ta 2 C ‖ ( 0001 ) Al 2 O 3 and [ 10 1 ¯ 0 ] Ta 2 C ‖ [ 11 2 ¯ 0 ] Al 2 O 3 . With increasing t, we observe the formation of anti-phase domains and misoriented grains resulting in polycrystalline layers. We attribute the observed enhancement in 0001 texture to increased surface adatom mobilities and the development of polycrystallinity to reduced incorporation of C in the lattice with increasing T s. We expect that our results help develop methods for the synthesis of high-quality Ta2C thin films.

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

The development of electronic devices, especially those that involve heterogeneous integration of materials, has led to increased challenges in addressing their thermal operational temperature demands. The heat flow in these systems is significantly influenced or even dominated by thermal boundary resistance at the interface between dissimilar materials. However, controlling and tuning heat transport across an interface and in the adjacent materials has so far drawn limited attention. In this work, we grow chemical vapor-deposited diamond on silicon substrates by graphoepitaxy and experimentally demonstrate tunable thermal transport across diamond membranes and diamond-silicon interfaces. We observed the highest diamond-silicon thermal boundary conductance (TBC) measured to date and increased diamond thermal conductivity due to strong grain texturing in the diamond near the interface. Additionally, nonequilibrium molecular dynamics simulations and a Landauer approach are used to understand the diamond-silicon TBC. These findings pave the way for tuning or increasing thermal conductance in heterogeneously integrated electronics that involve polycrystalline materials and will impact applications including electronics thermal management and diamond growth.

Thermal Boundary Conductance Across Heteroepitaxial ZnO/GaN Interfaces: Assessment of the Phonon Gas Model.

We present experimental measurements of the thermal boundary conductance (TBC) from 78-500 K across isolated heteroepitaxially grown ZnO films on GaN substrates. This data provides an assessment of the underlying assumptions driving phonon gas-based models, such as the diffuse mismatch model (DMM), and atomistic Green's function (AGF) formalisms used to predict TBC. Our measurements, when compared to previous experimental data, suggest that TBC can be influenced by long wavelength, zone center modes in a material on one side of the interface as opposed to the '"vibrational mismatch"' concept assumed in the DMM; this disagreement is pronounced at high temperatures. At room temperature, we measure the ZnO/GaN TBC as 490[+150,-110] MW m-2 K-1. The disagreement among the DMM and AGF, and the experimental data at elevated temperatures, suggests a non-negligible contribution from other types of modes that are not accounted for in the fundamental assumptions of these harmonic based formalisms, which may rely on anharmonicity. Given the high quality of these ZnO/GaN interfaces, these results provide an invaluable, critical, and quantitative assessment of the accuracy of assumptions in the current state of the art computational approaches used to predict phonon TBC across interfaces.

Direct Visualization of Thermal Conductivity Suppression Due to Enhanced Phonon Scattering Near Individual Grain Boundaries.

Understanding the impact of lattice imperfections on nanoscale thermal transport is crucial for diverse applications ranging from thermal management to energy conversion. Grain boundaries (GBs) are ubiquitous defects in polycrystalline materials, which scatter phonons and reduce thermal conductivity (κ). Historically, their impact on heat conduction has been studied indirectly through spatially averaged measurements, that provide little information about phonon transport near a single GB. Here, using spatially resolved time-domain thermoreflectance (TDTR) measurements in combination with electron backscatter diffraction (EBSD), we make localized measurements of κ within few µm of individual GBs in boron-doped polycrystalline diamond. We observe strongly suppressed thermal transport near GBs, a reduction in κ from ∼1000 W m-1 K-1 at the center of large grains to ∼400 W m-1 K-1 in the immediate vicinity of GBs. Furthermore, we show that this reduction in κ is measured up to ∼10 µm away from a GB. A theoretical model is proposed that captures the local reduction in phonon mean-free-paths due to strongly diffuse phonon scattering at the disordered grain boundaries. Our results provide a new framework for understanding phonon-defect interactions in nanomaterials, with implications for the use of high-κ polycrystalline materials as heat sinks in electronics thermal management.