Borazine Promoted Growth of Highly Oriented Thin Films.

We report on a phenomenon, where thin films sputter-deposited on single-crystalline Al2O3(0001) substrates exposed to borazine─a precursor commonly used for the synthesis of hexagonal boron nitride layers─are more highly oriented than those grown on bare Al2O3(0001) under the same conditions. We observed this phenomenon in face-centered cubic Pd, body-centered cubic Mo, and trigonal Ta2C thin films grown on Al2O3(0001). Interestingly, intermittent exposure to borazine during the growth of Ta2C thin films on Ta2C yields better crystallinity than direct deposition of monolithic Ta2C. We attribute these rather unusual results to a combination of both enhanced adatom mobilities on, and epitaxial registry with, surfaces exposed to borazine during the deposition. We expect that our approach can potentially help improve the crystalline quality of thin films deposited on a variety of substrates.

Self-Organized Growth of 111-Oriented (VNbTaMoW)N Nanorods on MgO(001).

Refractory high-entropy alloy nitride, (VNbTaMoW)N, layers are grown on single-crystalline MgO(001) via ultrahigh vacuum direct current magnetron sputtering of a VNbTaMoW target in Kr/N2 gas mixtures at 1073 K. X-ray diffraction, scanning and transmission electron microscopy, and energy dispersive X-ray spectroscopy characterizations revealed the formation of B1-structured, 111-textured (V0.21Nb0.18Ta0.19Mo0.21W0.21)N1.05 with lattice parameter a = 0.4249 nm. The alloy nitride film exhibits dense columnar microstructure near the substrate-film interface with coherent 001 grain growth limited to a few tens of nanometers, followed by an outgrowth of quasi one-dimensional nanorods with 3-fold symmetric facets. We attribute the self-organized growth of rather unusual 111-textured nanorods on isostructural MgO(001) to kinetic limitations of the sputter-deposition process exacerbated by the sluggish diffusion of the multicomponent adspecies and the preferential growth of {111} crystals.

Surface Reconstruction of Halide Perovskites During Post-treatment.

Postfabrication surface treatment strategies have been instrumental to the stability and performance improvements of halide perovskite photovoltaics in recent years. However, a consensus understanding of the complex reconstruction processes occurring at the surface is still lacking. Here, we combined complementary surface-sensitive and depth-resolved techniques to investigate the mechanistic reconstruction of the perovskite surface at the microscale level. We observed a reconstruction toward a more PbI2-rich top surface induced by the commonly used solvent isopropyl alcohol (IPA). We discuss several implications of this reconstruction on the surface thermodynamics and energetics. Particularly, our observations suggest that IPA assists in the adsorption process of organic ammonium salts to the surface to enhance their defect passivation effects.

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.

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.

Ultrahigh vacuum dc magnetron sputter-deposition of epitaxial Pd(111)/Al2O3(0001) thin films.

Pd(111) thin films, ∼245 nm thick, are deposited on Al2O3(0001) substrates at ≈0.5Tm, where Tm is the Pd melting point, by ultrahigh vacuum dc magnetron sputtering of Pd target in pure Ar discharges. Auger electron spectra and low-energy electron diffraction patterns acquired in situ from the as-deposited samples reveal that the surfaces are compositionally pure 111-oriented Pd. Double-axis x-ray diffraction (XRD) ω-2θ scans show only the set of Pd 111 peaks from the film. In triple-axis high-resolution XRD, the full width at half maximum intensity Γω of the Pd 111 ω-rocking curve is 630 arc sec. XRD 111 pole figure obtained from the sample revealed six peaks 60°-apart at a tilt angles corresponding to Pd 111 reflections. XRD ϕ scans show six 60°-rotated 111 peaks of Pd at the same ϕ angles for 11[Formula: see text]3 of Al2O3 based on which the epitaxial crystallographic relationships between the film and the substrate are determined as [Formula: see text]ǁ[Formula: see text] with two in-plane orientations of [Formula: see text]ǁ[Formula: see text] and [Formula: see text]ǁ[Formula: see text]. Using triple axis symmetric and asymmetric reciprocal space maps, interplanar spacings of out-of-plane (111) and in-plane (11[Formula: see text]) are found to be 0.2242 ± 0.0003 and 0.1591 ± 0.0003 nm, respectively. These values are 0.18% lower than 0.2246 nm for (111) and the same, within the measurement uncertainties, as 0.1588 nm for (11[Formula: see text]) calculated from the bulk Pd lattice parameter, suggesting a small out-of-plane compressive strain and an in-plane tensile strain related to the thermal strain upon cooling the sample from the deposition temperature to room temperature. High-resolution cross-sectional transmission electron microscopy coupled with energy dispersive x-ray spectra obtained from the Pd(111)/Al2O3(0001) samples indicate that the Pd-Al2O3 interfaces are essentially atomically abrupt and dislocation-free. These results demonstrate the growth of epitaxial Pd thin films with (111) out-of-plane orientation with low mosaicity on Al2O3(0001).

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.

Experimental evidence of auxeticity in ion implanted single crystal calcite.

We report initial experimental evidence of auxeticity in calcite by ion implanting (1010) oriented single crystalline calcite with Ar+ at room temperature using an ion energy of 400 keV and a dose of 1 × 1014 cm-2. Lattice compression normal to the substrate surface was observed, which is an atypical result for ion implanted materials. The auxetic behavior is consistent with predictions that indicate auxeticity had been predicted along two crystallographic directions including [1010]. Materials with a positive Poisson's ratio experience lattice expansion normal to the substrate surface when ion implanted, whereas lattice contraction normal to the surface is evidence of auxetic behavior. Triple-axis X-ray diffraction measurements confirmed the auxetic strain state of the implanted calcite substrates. Reciprocal space maps for the symmetric 3030 and asymmetric 1450 reflections revealed that the implanted region was fully strained (pseudomorphic) to the bulk of the substrate, as is typical with implanted single crystals. A symmetric (3030) ω:2θ line scan was used with X-ray dynamical diffraction simulations to model the strain profile and extract the variation of compressive strain as a function of depth normal to the substrate surface. SRIM calculations were performed to obtain a displacement-per-atom profile and implanted Ar+ concentration profile. It was found that the strain profile matches the displacement-per-atom profile. This study demonstrated the use of ion implantation and X-ray diffraction methods to probe mechanical properties of materials and to test predictions such as the auxeticity.

Steady-state methods for measuring in-plane thermal conductivity of thin films for heat spreading applications.

The development of high thermal conductivity thin film materials for the thermal management of electronics requires accurate and precise methods for characterizing heat spreading capability, namely, in-plane thermal conductivity. However, due to the complex nature of thin film thermal property measurements, resolving the in-plane thermal conductivity of high thermal conductivity anisotropic thin films with high accuracy is particularly challenging. Capable transient techniques exist; however, they usually measure thermal diffusivity and require heat capacity and density to deduce thermal conductivity. Here, we present an explicit uncertainty analysis framework for accurately resolving in-plane thermal conductivity via two independent steady-state thermometry techniques: particle-assisted Raman thermometry and electrical resistance thermometry. Additionally, we establish error-based criteria to determine the limiting experimental conditions that permit the simplifying assumption of one-dimensional thermal conduction to further reduce thermal analysis. We demonstrate the accuracy and precision (<5% uncertainty) of both steady-state techniques through in-plane thermal conductivity measurements of anisotropic nanocrystalline diamond thin films.

Experimental observation of localized interfacial phonon modes.

Interfaces impede heat flow in micro/nanostructured systems. Conventional theories for interfacial thermal transport were derived based on bulk phonon properties of the materials making up the interface without explicitly considering the atomistic interfacial details, which are found critical to correctly describing thermal boundary conductance. Recent theoretical studies predicted the existence of localized phonon modes at the interface which can play an important role in understanding interfacial thermal transport. However, experimental validation is still lacking. Through a combination of Raman spectroscopy and high-energy-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we report the experimental observation of localized interfacial phonon modes at ~12 THz at a high-quality epitaxial Si-Ge interface. These modes are further confirmed using molecular dynamics simulations with a high-fidelity neural network interatomic potential, which also yield thermal boundary conductance agreeing well with that measured in time-domain thermoreflectance experiments. Simulations find that the interfacial phonon modes have an obvious contribution to the total thermal boundary conductance. Our findings significantly contribute to the understanding of interfacial thermal transport physics and have impact on engineering thermal boundary conductance at interfaces in applications such as electronics thermal management and thermoelectric energy conversion.

High In-Plane Thermal Conductivity of Aluminum Nitride Thin Films.

High thermal conductivity materials show promise for thermal mitigation and heat removal in devices. However, shrinking the length scales of these materials often leads to significant reductions in thermal conductivities, thus invalidating their applicability to functional devices. In this work, we report on high in-plane thermal conductivities of 3.05, 3.75, and 6 µm thick aluminum nitride (AlN) films measured via steady-state thermoreflectance. At room temperature, the AlN films possess an in-plane thermal conductivity of ∼260 ± 40 W m-1 K-1, one of the highest reported to date for any thin film material of equivalent thickness. At low temperatures, the in-plane thermal conductivities of the AlN films surpass even those of diamond thin films. Phonon-phonon scattering drives the in-plane thermal transport of these AlN thin films, leading to an increase in thermal conductivity as temperature decreases. This is opposite of what is observed in traditional high thermal conductivity thin films, where boundaries and defects that arise from film growth cause a thermal conductivity reduction with decreasing temperature. This study provides insight into the interplay among boundary, defect, and phonon-phonon scattering that drives the high in-plane thermal conductivity of the AlN thin films and demonstrates that these AlN films are promising materials for heat spreaders in electronic devices.

Self-catalyzed epitaxial growth of vertical indium phosphide nanowires on silicon.

Vertical indium phosphide nanowires have been grown epitaxially on silicon (111) by metalorganic vapor-phase epitaxy. Liquid indium droplets were formed in situ and used to catalyze deposition. For growth at 350 degrees C, about 70% of the wires were vertical, while the remaining ones were distributed in the 3 other <111> directions. The vertical fraction, growth rate, and tapering of the wires increased with temperature and V/III ratio. At 370 degrees C and V/III equal to 200, 100% of the wires were vertical with a density of approximately 1.0 x 10(9) cm(-2) and average dimensions of 3.9 mum in length, 45 nm in base width, and 15 nm in tip width. X-ray diffraction and transmission electron microscopy revealed that the wires were single-crystal zinc blende, although they contained a high density of rotational twins perpendicular to the <111> growth direction. The room temperature photoluminescence spectrum exhibited one peak centered at 912 +/- 10 nm with a FWHM of approximately 60 nm.

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.

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.

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.

Approaching the Practical Conductivity Limits of Aerosol Jet Printed Silver.

Previous efforts to directly write conductive metals have been narrowly focused on nanoparticle ink suspensions that require aggressive sintering (>200 °C) and result in low-density, small-grained agglomerates with electrical conductivities <25% of bulk metal. Here, we demonstrate aerosol jet printing of a reactive ink solution and characterize high-density (93%) printed silver traces having near-bulk conductivity and grain sizes greater than the electron mean free path, while only requiring a low-temperature (80 °C) treatment. We have developed a predictive electronic transport model which correlates the microstructure to the measured conductivity and identifies a strategy to approach the practical conductivity limit for printed metals. Our analysis of how grain boundaries and tortuosity contribute to electrical resistivity provides insight into the basic materials science that governs how an ink formulator or process developer might approach improving the conductivity. Transmission line measurements validate that electrical properties are preserved up to 20 GHz, which demonstrates the utility of this technique for printed RF components. This work reveals a new method of producing robust printed electronics that retain the advantages of rapid prototyping and three-dimensional fabrication while achieving the performance necessary for success within the aerospace and communications industries.

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.

Author Correction: Solid-phase hetero epitaxial growth of α-phase formamidinium perovskite.

A Correction to this paper has been published: https://doi.org/10.1038/s41467-020-19846-y .

Solid-phase hetero epitaxial growth of α-phase formamidinium perovskite.

Conventional epitaxy of semiconductor films requires a compatible single crystalline substrate and precisely controlled growth conditions, which limit the price competitiveness and versatility of the process. We demonstrate substrate-tolerant nano-heteroepitaxy (NHE) of high-quality formamidinium-lead-tri-iodide (FAPbI3) perovskite films. The layered perovskite templates the solid-state phase conversion of FAPbI3 from its hexagonal non-perovskite phase to the cubic perovskite polymorph, where the growth kinetics are controlled by a synergistic effect between strain and entropy. The slow heteroepitaxial crystal growth enlarged the perovskite crystals by 10-fold with a reduced defect density and strong preferred orientation. This NHE is readily applicable to various substrates used for devices. The proof-of-concept solar cell and light-emitting diode devices based on the NHE-FAPbI3 showed efficiencies and stabilities superior to those of devices fabricated without NHE.