Cross-sectional Transmission Electron Microscopy Research Articles

Dynamic Electrode–Electrolyte Intermixing in Solid-State Sodium Nano-Batteries

Nanostructured solid-state batteries (SSBs) are poised to meet the demands of next-generation energy storage technologies by realizing performance competitive to their liquid-based counterparts while simultaneously offering improved safety and expanded form factors. Atomic layer deposition (ALD) is among the tools essential to fabricate nanostructured devices with challenging aspect ratios. Here, we report the fabrication and electrochemical testing of the first nanoscale sodium all-solid-state battery (SSB) using ALD to deposit both the V2O5 cathode and NaPON solid electrolyte followed by evaporation of a thin-film Na metal anode. NaPON exhibits remarkable stability against evaporated Na metal, showing no electrolyte breakdown or significant interphase formation in the voltage range of 0.05-6.0 V vs Na/Na+. Electrochemical analysis of the SSB suggests intermixing of the NaPON/V2O5 layers during fabrication, which we investigate in three ways: in situ spectroscopic ellipsometry, time-resolved X-ray photoelectron spectroscopy (XPS) depth profiling, and cross-sectional cryo-scanning transmission electron microscopy (cryo-STEM) coupled with electron energy loss spectroscopy (EELS). We characterize the interfacial reaction during the ALD NaPON deposition on V2O5 to be twofold: (1) reduction of V2O5 to VO2 and (2) Na+ insertion into VO2 to form NaxVO2. Despite the intermixing of NaPON-V2O5, we demonstrate that NaPON-coated V2O5 electrodes display enhanced electrochemical cycling stability in liquid-electrolyte coin cells through the formation of a stable electrolyte interphase. In all-SSBs, the Na metal evaporation process is found to intensify the intermixing reaction, resulting in the irreversible formation of mixed interphases between discrete battery layers. Despite this graded composition, the SSB can operate for over 100 charge-discharge cycles at room temperature and represents the first demonstration of a functional thin-film solid-state sodium-ion battery.

Passivation capping of InAs surface quantum dots by TMA/Al2O3: PL enhancement and blueshift suppression

Passivation capping that enhances the photoluminescence (PL) of molecular beam epitaxy (MBE)-grown InAs surface quantum dots (SQDs) is realized by ex situ low-temperature atomic layer deposition (ALD)-grown Al2O3. As the Al2O3 cap thickness increased from 2 to 30 nm, the PL intensity was enhanced by 2.7-fold and the blue shift was suppressed. This is in strong contrast to wet chemistry passivation and in situ GaAs capping by MBE, both of which resulted in significant PL blueshift, due to etching in the former, and In/Ga intermixing and strain in the latter. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) reveal that the Al2O3 cap layer mimics the shape of the underlying SQDs. The cross-sectional transmission electron microscopy (TEM) further reveals that the SQD size and shape remained unchanged after Al2O3 capping, which is in strong contrast to MBE-capping. The passivation mechanisms and native oxide reduction by trimethylaluminum (TMA), including self-clean-up reaction through ligand exchange, are discussed based on the results by x-ray photoelectron spectroscopy (XPS). A detailed comparison between Al2O3 and ZnO cap layers reveals the significance of the cap properties on the SQD size/shape and PL blueshift. While the size/shape of SQDs is preserved by Al2O3 capping, the apex is slightly removed by ZnO capping, resulting in a stronger PL blueshift compared to Al2O3.

Ohmic contact structures on β-Ga2O3 with n+ β-Ga2O3 pulsed laser deposition layers

Thin (40–150 nm), highly doped n+ (1019–1020 cm−3) Ga2O3 layers deposited using pulsed laser deposition (PLD) were incorporated into Ti/Au ohmic contacts on (001) and (010) β-Ga2O3 substrates with carrier concentrations between 2.5 and 5.1 × 1018 cm−3. Specific contact resistivity values were calculated for contact structures both without and with a PLD layer having different thicknesses up to 150 nm. With the exception of a 40 nm PLD layer on the (001) substrate, the specific contact resistivity values decreased with increasing PLD layer thickness: up to 8× on (001) Ga2O3 and up to 16× on (010) Ga2O3 compared with samples without a PLD layer. The lowest average specific contact resistivities were achieved with 150 nm PLD layers: 3.48 × 10−5 Ω cm2 on (001) Ga2O3 and 4.79 × 10−5 Ω cm2 on (010) Ga2O3. Cross-sectional transmission electron microscopy images revealed differences in the microstructure and morphology of the PLD layers on the different substrate orientations. This study describes a low-temperature process that could be used to reduce the contact resistance in Ga2O3 devices.

Single crystal-single crystal bonding of langasite exhibited high strength of 23.28 MPa

Langasite (LGS, La3Ga5SiO14) is a promising material in high-temperature piezoelectric devices due to its excellent thermal stability, piezoelectricity, and electrical property. A major challenge in the development of LGS-based devices is to form high-strength bonding of the brittle LGS. Here, we report that the single crystal-single crystal (dual-SC) bonding of LGS is realized through thermal activation under a low compression of 50 kPa for the first time. A record bonding strength of 23.28 MPa is achieved within the dual-SC bonded LGS/LGS structure (with a high bonding ratio exceeding 93%), which is 5 times higher than that of the recent reported LGS/LGS bonding structure (in which a relatively fragile amorphous interface layer is formed between the two LGS samples). The smooth and void-free dual-SC bonding interface of LGS is verified via the cross-sectional transmission electron microscopy (TEM) observations.

Fabrication of sub-5 nm uniform zirconium oxide films on corrugated copper substrates by a scalable polymer brush assisted deposition method

We demonstrate a polymer brush assisted approach for the fabrication of continuous zirconium oxide (ZrO2) films over large areas with high uniformity (pin-hole free) on copper (Cu) substrates. This approach involves the use of a thiol-terminated polymethyl methacrylate brush (PMMA-SH) as the template layer for the selective infiltration of zirconium oxynitrate (ZrN2O7). The preparation of a highly uniform covalently grafted polymer monolayer on the Cu substrate is the critical factor in fabricating a metal oxide film of uniform thickness across the surface. Infiltration is reliant on the chemical interactions between the polymer functional group and the metal precursor. A following reductive H2 plasma treatment process results in ZrO2 film formation whilst the surface Cu2O passive oxide layer was reduced to a Cu/Cu2O interface. Fundamental analysis of the infiltration process and the resulting ZrO2 film was determined by XPS, and GA-FTIR. Results derived from these techniques confirm the inclusion of the ZrN2O7 into the polymer films. Cross-sectional transmission electron microscopy and energy dispersive X-ray mapping analysis corroborate the formation of ZrO2 layer at Cu substrate. We believe that this quick and facile methodology to prepare ZrO2 films is potentially scalable to other high-κ dielectric materials of high interest in microelectronic applications.

Effects of GaN channel downscaling in AlGaN–GaN high electron mobility transistor structures grown on AlN bulk substrate

In this work, two series of AlGaN/GaN/AlN high electron mobility transistor (HEMT) heterostructures have been grown by molecular beam epitaxy on AlN bulk substrates. The effects of reduction in the GaN channel thickness as well as the AlGaN barrier thickness and composition on structural and electrical properties of the heterostructures have been studied. The material analysis involved high-resolution x-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. In a first series of HEMT structures grown with an aluminum content of 30% in the AlGaN barrier, the channel downscaling results in a reduction in the GaN strain relaxation rate but at the expense of degradation in the mean crystal quality and in the electron mobility with a noticeable increase in the sheet resistance. An opposite trend is noticed for the three-terminal breakdown voltage of transistors, so that a trade-off is obtained for a 50 nm width GaN channel HEMT, which exhibits a sheet resistance of 1700 Ω/sq. with transistors demonstrating three-terminal breakdown voltage up to 1400 V for 40 μm gate to drain spacing with static on resistance Ron = 32 mΩ cm2. On the other hand, a second series of HEMT structures with high aluminum content AlGaN barriers and sub-10 nm GaN channels have been grown perfectly strained with high sheet carrier densities allowing to preserve sheet resistances in the range of 880–1050 Ω/sq.

Opto-electronic properties of Zr[formula omitted]Hf[formula omitted]O[formula omitted]/Al/Zr[formula omitted]Hf[formula omitted]O[formula omitted] thin-films: The influence of substrate temperature

The influence of substrate temperature on ZrxHf1−xO2/Al/ZrxHf1−xO2 trilayer thin-films deposited on n-type Si(100) and commercial glass substrates using magnetron sputtering technique is systematically investigated with the substrate temperature ranging from 25 to 300°C. The deposited films are characterized by x-ray reflectivity (XRR), grazing incidence x-ray diffraction (GI-XRD), x-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FE-SEM), atomic force microscopy (AFM), cross-sectional transmission electron microscope (TEM), energy-dispersive x-ray spectroscopy (EDX) and UV–VIS spectroscopy to study the microstructural, morphological and optical properties. Capacitance–voltage (C–V) and current–voltage (I–V) measurements are performed to investigate the electrical properties of the trilayer thin-films. Physical characterization reveals that as the substrate temperature rises, the crystalline quality of the trilayer thin-films improves. Further, a rise in substrate temperature causes an increase in surface roughness and grain size. The trilayer thin-film deposited at 300°C shows improved optical transmittance over wavelengths 400–1100 nm. In addition this trilayer thin-film demonstrates highest figure of merit, lower leakage current as well as improved breakdown performance. Hence, ZrxHf1−xO2/Al/ZrxHf1−xO2 thin-film prepared at 300°C could be used in opto-electronic devices.

Study on the Structure of GaN films deposited on MoS2/Sapphire via Plasma-Assisted Molecular Beam Epitaxy

The gallium nitride (GaN) films were grown on molybdenum disulfide (MoS2) layers via plasma-assisted molecular beam epitaxy (PA-MBE). The heterostructures of the GaN film were studied using reflection high-energy electron diffraction (RHEED) and HR-XRD. The heterostructures of GaN/MoS2/sapphire were revealed through cross-sectional transmission electron microscopy (TEM). The surface texture of the GaN films was analyzed using FE-SEM. Single-crystal heterostructure GaN films can be obtained on 2D MoS2/c-sapphire. The RHEED demonstrated spot patterns with high intensity showing the single crystal structure constructed in the GaN films. The GaN films on the surface exhibited a hexagonal structure. TEM images taken perpendicular to the surface revealed that, even after 60 minutes of epitaxial growth, the thickness of the GaN films remained consistent at approximately 4 nm. However, the 2D MoS2 layer was not observable in the images due to harm incurred during heteroepitaxial growth. Based on the surface structure, it was found that GaN films were successfully grown on the MoS2 layers using the PA-MBE system.

Wide-gamut structural colours on oakblue butterflies by naturally tuned photonic nanoarchitectures.

The iridescent structural colours of butterflies, generated by photonic nanoarchitectures, often function as species-specific sexual signals; therefore, they are reproduced precisely from generation to generation. The wing scales of oakblue hairstreak butterflies (genus Arhopala, Theclinae, Lycaenidae, Lepidoptera) contain multi-layer photonic nanoarchitectures, which can generate a wide range of structural colours, from violet to green. By scanning (SEM) and cross-sectional transmission electron microscopy (TEM) investigation, the colour tuning mechanism of the cover scales was explored. We revealed that the characteristic size change of structural elements in similar photonic nanoarchitectures led to different structural colours that were examined by various reflectance spectrophotometry techniques. The measured structural properties of the naturally tuned photonic nanoarchitectures were used to calculate wing reflectances, which were compared with the measurement results. We found that the simulated structural colours were systematically redshifted by 95-126 nm as compared with the measured normal-incidence reflectance results. This is attributed to the swelling of the chitinous multi-layer structures during the standard TEM sample preparation and the tilt of the cover scales, which both affect the apparent layer thicknesses in the TEM cross-sections. We proposed a simulation correction and compared the results with the layer thicknesses measured on cryogenically prepared non-embedded SEM cross-sections.

Shedding new light on the dislocation-mediated plasticity in wurtzite ZnO single crystals by photoindentation

Dislocation-mediated plasticity in inorganic semiconductors and oxides has attracted increasing research interest because of the promising mechanical and functional properties tuned by dislocations. In this study, we investigated the effects of light illumination on the dislocation-mediated plasticity in hexagonal wurtzite ZnO, a representative third-generation semiconductor material. A (0001) 45o off sample was specially designed to preferentially activate the basal slip on (0001) plane. Three types of nanoindentation tests were performed under four different light conditions (550 nm, 334 nm, 405 nm, and darkness), including low-load (60 μN) pop-in tests, high-load (500 μN) nanoindentation tests, and nanoindentation creep tests. The maximum shear stresses at pop-in were found to approximate the theoretical shear strength regardless of the light conditions. The activation volume at pop-ins was calculated to be larger in light than in darkness. Cross-sectional transmission electron microscope images taken from beneath the indentation imprints showed that all indentation-induced dislocations were located beneath the indentation imprint in a thin-plate shape along one basal slip plane. These indentation-induced dislocations could spread much deeper in darkness than in light, revealing the suppressive effect of light on dislocation behavior. An analytical model was adopted to estimate the elastoplastic stress field beneath the indenter. It was found that dislocation glide ceased at a higher stress level in light, indicating the increase in the Peierls barrier under light illumination. Furthermore, nanoindentation creep tests showed the suppression of both indentation depth and creep rate by light. Nanoindentation creep also yielded a larger activation volume in light than in darkness.

Sliding wear and rolling contact fatigue behavior of M50 steel coated with atomic layer deposited lubricious oxide nanolaminates

Atomic layer deposition (ALD) was employed to deposit highly conformal and uniform nanolaminate coatings of ZnO/Al2O3/ZrO2/Al2O3 on M50 bearing steel substrates for sliding wear testing, as well as on CrN coated M50 bearing steel cylindrical rods and cups for three-ball on rod rolling contact fatigue (RCF) testing. The nanocrystalline ZnO top layer was structurally-engineered to achieve low surface energy, textured (0002)-oriented grains. To achieve this preferred orientation, amorphous Al2O3 was deposited as a seed layer, while crystalline ZrO2 acted as a high toughness/load bearing layer. The ALD nanolaminates reduced both the sliding wear rate (2 × 10-7 mm3/N·m) and enhanced the RCF resistance of M50 steel out to 6 million rolling contact fatigue cycles with no signs of spall. Based on cross-sectional transmission electron microscopy studies inside the wear surfaces, it was determined that sliding and rolling-induced plastic deformation was observed in ZnO layer where basal plane stacking faults were sheared parallel to the sliding and rolling directions. This intrafilm shear velocity accommodation mode mitigates the friction, reduces wear, and inhibits brittle fracture. Therefore, ALD ZnO/Al2O3/ZrO2/Al2O3 nanolaminates are candidate coatings that provide both sliding and RCF resistance in moving mechanical assemblies that require thin (∼10-300 nm), uniform and conformal solid lubricants.

Revealing the structure-property relationships of amorphous carbon tribofilms on platinum-gold surfaces

Nanocrystalline metal alloys have shown great promise as electrical contact materials, given their mechanical and tribological properties. In particular, platinum-gold (Pt–Au) nanocrystalline alloys have demonstrated coefficients of friction as low as 0.01 and specific wear rates on the order of 10−9 mm3 N−1 m−1, largely due to the formation of carbon-based tribofilms at the sliding interfaces. In this study, we advance our understanding of the Pt–Au tribofilm structure-property relations and growth mechanisms via high-throughput and high-resolution measurements as a function of Pt–Au composition. As the solute content increased from 0 at. % to 10 at. % Au, cross-sectional and plan-view transmission electron microscopy demonstrated a decrease in average grain size d and an accompanied increase in grain boundary (GB) segregation. The decrease in d and increase in GB solute segregation translated to a decrease in modulus Er and an increase in hardness H as determined via nanoindentation; the Er trend was mainly described using a rule-of-mixtures approximation, whereas the H trend was ascribed to solid solution strengthening and GB stabilization. The steady state-friction μ and wear rate decreased with the addition of Au; low Au-content films showed substrate wear, while high Au-content films showed stable tribofilm growth in both macroscale and nanoscale friction tests. The carbon bonding configuration of the tribofilms was investigated by near-edge X-ray absorption fine structure spectroscopic analyses and found to be similar to that of hydrogenated amorphous carbon films. Altogether, the study provided insight into the mechanistic origins of the tribofilms, thus opening the door to tunable properties ranging from mitigation for electrical contacts to the creation of self-healing films for solid lubricants.

Multilayer In-Plane Heterostructures Based on Transition Metal Dichalcogenides for Advanced Electronics.

In-plane heterostructures of transition metal dichalcogenides (TMDCs) have attracted much attention for high-performance electronic and optoelectronic devices. To date, mainly monolayer-based in-plane heterostructures have been prepared by chemical vapor deposition (CVD), and their optical and electrical properties have been investigated. However, the low dielectric properties of monolayers prevent the generation of high concentrations of thermally excited carriers from doped impurities. To solve this issue, multilayer TMDCs are a promising component for various electronic devices due to the availability of degenerate semiconductors. Here, we report the fabrication and transport properties of multilayer TMDC-based in-plane heterostructures. The multilayer in-plane heterostructures are formed through CVD growth of multilayer MoS2 from the edges of mechanically exfoliated multilayer flakes of WSe2 or NbxMo1-xS2. In addition to the in-plane heterostructures, we also confirmed the vertical growth of MoS2 on the exfoliated flakes. For the WSe2/MoS2 sample, an abrupt composition change is confirmed by cross-sectional high-angle annular dark-field scanning transmission electron microscopy. Electrical transport measurements reveal that a tunneling current flows at the NbxMo1-xS2/MoS2 in-plane heterointerface, and the band alignment is changed from a staggered gap to a broken gap by electrostatic electron doping of MoS2. The formation of a staggered gap band alignment of NbxMo1-xS2/MoS2 is also supported by first-principles calculations.

Low-Temperature Direct Growth of Nanocrystalline Multilayer Graphene on Silver with Long-Term Surface Passivation.

A wide variety of transition metals, including copper and gold, have been successfully used as substrates for graphene growth. On the other hand, it has been challenging to grow graphene on silver, so realistic applications by combining graphene and silver for improved electrode stability and enhanced surface plasmon resonance in organic light-emitting diodes and biosensing have not been realized to date. Here, we demonstrate the surface passivation of silver through the single-step rapid growth of nanocrystalline multilayer graphene on silver via low-temperature plasma-enhanced chemical vapor deposition (PECVD). The effect of the growth time on the graphene quality and the underlying silver characteristics is investigated by Raman spectroscopy, X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and cross-sectional annular dark-field scanning transmission electron microscopy (ADF-STEM). These results reveal nanocrystalline graphene structures with turbostratic layer stacking. Based on the XPS and ADF-STEM results, a PECVD growth mechanism of graphene on silver is proposed. The multilayer graphene also provides excellent long-term protection of the underlying silver surface from oxidation after 5 months of air exposure. This development thus paves the way toward realizing technological applications based on graphene-protected silver surfaces and electrodes as well as hybrid graphene-silver plasmonics.

Molecular beam epitaxy of KTaO3

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high-quality KTaO3 thin films by molecular-beam epitaxy. Tantalum was provided by either a suboxide source emanating a TaO2 flux from Ta2O5 contained in a conventional effusion cell or an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10% O3 + 90% O2) were simultaneously supplied with the TaO2 (or tantalum) molecular beams to grow the KTaO3 films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. Atomic force microscopy reveals atomic steps at the surface of the grown films. Reciprocal space mapping demonstrates that the films, when sufficiently thin, are coherently strained to the SrTiO3 (001) and GdScO3 (110) substrates.

Scratch adhesion evaluation of diamond like carbon coatings with alternate hard and soft multilayers

A series of four diamond like carbon coatings comprising alternating soft and hard layers was prepared by closed field unbalanced magnetron sputtering. A constant bias voltage of −40 V was used for soft layer deposition, while the hard layers were deposited at different bias voltages for different coatings in series. A uniform hard-to-soft layer thickness ratio of approximately 1:1.33 was maintained for all four coatings. Cross-sectional transmission electron microscopy of the coatings confirmed that these diamond like carbon coatings indeed exhibited distinct soft and hard layers. Moreover, increasing bias voltage during hard layer deposition also increased the residual stress in these coatings. The coatings were also subjected to scratch testing to determine the effect of increasing bias voltage on the scratch toughness. Scratch cracking mechanism changed from chevron tensile cracking at low bias voltages to buckle spallation at higher bias voltages. All the coatings exhibited substrate exposure at high normal loads beyond the upper critical load. The scratch crack propagation resistance parameter was reduced by ∼1.5 times when the bias voltage was increased to −140 V. It was demonstrated that coatings deposited at a hard layer bias voltage of −100 V demonstrated the best combination of mechanical properties, including the plastic deformation resistance (H3/E2 ratio) and elastic strain to failure (H/E ratio).

Oxygen ion migration induced polarity switchable SrFeOx memristor for high-precision handwriting recognition

This paper reports a repeatable polarity reversal behavior in Au/SrFeOx (SFO)/SrRuO3 (SRO) memristor. The set and reset voltage direction can be inverted by applying a suitable reverse bias voltage, and the SFO device always exhibits fine cycling stability and retention performance regardless of operation polarity. The cross-sectional transmission electron microscope (TEM) images of the SFO memristor on positive and negative set statuses display apparent perovskite SrFeO3-δ (PV-SFO) conductive filaments and brownmillerite SrFeO2.5 (BM-SFO) barrier layer, respectively. The surface oxygen atomic ratio characterized by X-ray photoelectron spectroscopy (XPS) demonstrates the oxygen ions migration behaviors during the polarity reversal processes. Moreover, the energy band structure evaluated by first-principles calculation suggests that insufficient oxygen injection into the BM-SFO barrier layer could reduce the band gap of BM-SFO and form a PV-SFO/BM-SFO+ interface observed by TEM with the lower Schottky barrier height, facilitating Schottky emission and conduction. Furthermore, the SFO device on negative set status shows preferable handwritten digit recognition accuracy of 98.81 % and faster convergence speed than positive set status due to the better synaptic weight update linearity. Hence, the alternated polarity reversal features of the SFO memristor may provide a new reference for resistive memory and synaptic memristors.

Formation of a-type dislocations near the InGaN/GaN interface during post-growth processing of epitaxial structures

Cross-sectional transmission electron microscopy studies often reveal a-type dislocations located either below or above the interfaces in InGaN/GaN structures deposited along the [0001] direction. We show that these dislocations do not emerge during growth but rather are a consequence of the stress state on lateral surfaces and mechanical processing, making them a post-growth effect. In cathodoluminescence mapping, these defects are visible in the vicinity of the edges of InGaN/GaN structures exposed by cleaving or polishing. Finite element calculations show the residual stress distribution in the vicinity of the InGaN/GaN interface at the free edge. The stress distribution is discussed in terms of dislocation formation and propagation. The presence of such defects at free edges of processed devices based on InGaN layers may have a significant negative impact on the device performance.

Characterization of dislocation of halide vapor phase epitaxial (001) β-Ga2O3 by ultrahigh sensitive emission microscopy and synchrotron X-ray topography and its influence on Schottky barrier diodes

This study elucidates the dislocation responsible for the leakage current in a halide vapor phase epitaxial (001) β-Ga2O3 Schottky barrier diode. A high reverse leakage current of −0.98 μA was observed at −100 V as an emission pattern via ultrahigh-sensitive emission microscopy. This was attributed to the presence of dislocation. Moreover, the Burgers vector of this dislocation was determined to be ⫽ 〈12〉 from the g·b invisibility criteria via synchrotron X-ray topography observation. This dislocation was speculated to be induced by the relaxation of the strain field surrounding the void and subsequently propagated to the surface. Cross-sectional scanning transmission electron microscopy revealed multiple threading dislocations along the (100) plane.

Nb–Nb direct bonding at room temperature for superconducting interconnects

Superconducting devices with high-density integration are required for applications, such as high-performance detectors and quantum computing. Here, the direct bonding of Nb electrodes at room temperature using a surface activated bonding method was investigated. We directly bonded Nb bonding pads and then performed current–voltage measurements in a 3He cryostat. A superconducting current of 1–5 mA was confirmed for the bonded interconnect. The transition temperature was 7.3–9.3 K. These values are affected by the quality of the bonding interface. According to cross-sectional transmission electron microscopy observations and energy-dispersive x-ray spectroscopy analysis of the bonding interface, the Nb layers were bonded with a 3-nm-thick intermediate layer made of Si and Nb between them. The Si sources were the bonding substrate and the bonding jig in the bonding apparatus.