High Thermal Stability and Low Thermal Resistance of Large Area GaN/3C-SiC/Diamond Junctions for Practical Device Processes.

Thermal management is critical in contemporary electronic systems, and integrating diamond with semiconductors offers the most promising solution to improve heat dissipation. However, developing a technique that can fully exploit the high thermal conductivity of diamond, withstand high-temperature annealing processes, and enable mass production is a significant challenge. In this study, the successful transfer of AlGaN/GaN/3C-SiC layers grown on Si to a large-size diamond substrate is demonstrated, followed by the fabrication of GaN high electron mobility transistors (HEMTs) on the diamond. Notably, no exfoliation of 3C-SiC/diamond bonding interfaces is observed even after annealing at 1100 °C, which is essential for high-quality GaN crystal growth on the diamond. The thermal boundary conductance of the 3C-SiC-diamond interface reaches ≈55 MW m-2 K-1, which is efficient for device cooling. GaN HEMTs fabricated on the diamond substrate exhibit the highest maximum drain current and the lowest surface temperature compared to those on Si and SiC substrates. Furthermore, the device thermal resistance of GaN HEMTs on the diamond substrate is significantly reduced compared to those on SiC substrates. These results indicate that the GaN/3C-SiC on diamond technique has the potential to revolutionize the development of power and radio-frequency electronics with improved thermal management capabilities.

Influence of gas environment and heating on atomic structures of platinum nanoparticle catalysts for proton-exchange membrane fuel cells.

Atomic-scale relaxations of platinum nanoparticles (Pt NPs) for fuel-cell catalysts are evaluated by spherical-aberration corrected environmental transmission electron microscopy (ETEM) under reference high-vacuum and N2 atmospheres, and then under reactive H2, CO and O2 atmospheres, combined with ex situ durability test using an electrochemical half-cell. In high-vacuum, increasing roughness due to continuous relaxation of surface-adsorbed Pt atoms is quantified in real-space. Under H2 and N2 atmospheres at a critical partial pressure of 1 × 10-2 Pa the stability of the surface facets is for the first time found to be improved. The adsorption behaviour of CO molecules is investigated using experimentally measured Pt-Pt bond lengths on the topmost surface layer of Pt NPs. The deactivation of Pt NPs in the anode environment of a proton-exchange-membrane fuel-cell is demonstrated at the atomic-scale in the ETEM, and the transformation of NPs into disordered nanoclusters is systematically quantified using the partial size distribution of Pt atomic clusters under controlled heating experiments at 423, 573 and 723 K.

Atom Probe Tomography Interlaboratory Study on Clustering Analysis in Experimental Data Using the Maximum Separation Distance Approach.

We summarize the findings from an interlaboratory study conducted between ten international research groups and investigate the use of the commonly used maximum separation distance and local concentration thresholding methods for solute clustering quantification. The study objectives are: to bring clarity to the range of applicability of the methods; identify existing and/or needed modifications; and interpretation of past published data. Participants collected experimental data from a proton-irradiated 304 stainless steel and analyzed Cu-rich and Ni-Si rich clusters. The datasets were also analyzed by one researcher to clarify variability originating from different operators. The Cu distribution fulfills the ideal requirements of the maximum separation method (MSM), namely a dilute matrix Cu concentration and concentrated Cu clusters. This enabled a relatively tight distribution of the cluster number density among the participants. By contrast, the group analysis of the Ni-Si rich clusters by the MSM was complicated by a high Ni matrix concentration and by the presence of Si-decorated dislocations, leading to larger variability among researchers. While local concentration filtering could, in principle, tighten the results, the cluster identification step inevitably maintained a high scatter. Recommendations regarding reporting, selection of analysis method, and expected variability when interpreting published data are discussed.

1.54 µm photoluminescence from Er:Ox centers at extremely low concentration in silicon at 300 K.

The demand for single photon emitters at λ=1.54 µm, which follows from the consistent development of quantum networks based on optical fiber technologies, makes Er:Ox centers in Si a viable resource, thanks to the I13/24→I415/2 optical transition of Er3+. While its implementation in high-power applications is hindered by the extremely low emission rate, the study of such systems in the low concentration regime remains relevant for quantum technologies. In this Letter, we explore the room-temperature photoluminescence at the telecomm wavelength from very low implantation doses of Er:Ox in Si. The lower-bound number of optically active Er atoms detected is of the order of 102, corresponding to a higher-bound value for the emission rate per individual ion of about 104 s-1.

Elemental Distribution in Multilayer Systems by Laser-Assisted Atom Probe Tomography with Various Analysis Directions.

Elemental distributions in a magnetic multilayer system with the structure Si substrate/Ta/NiFe/Ru/CoFeB/Ru/NiFe were studied using atom probe tomography (APT) along different analysis directions. The distributions of Ru and B atoms, which require a high evaporation field, were strongly influenced by the APT analysis direction. In particular, B in the CoFeB layer appeared near the interface with the lower Ru layer when the analysis was anti-parallel to the film growth direction, while B atoms were observed at the other side of the CoFeB layer when the analysis was parallel to the film growth direction. Moreover, when the analysis was perpendicular to the film growth direction, a homogenous distribution of B atoms was found within the CoFeB layer. Owing to this B behavior, the underlying Ru layer was affected in both of these analysis directions. In APT measurements of such a multilayer system composed of a stack of different evaporation field materials, evaluation of the elemental distribution around interfaces should be performed from more than one analysis direction.

High thermal conductivity in wafer-scale cubic silicon carbide crystals.

High thermal conductivity electronic materials are critical components for high-performance electronic and photonic devices as both active functional materials and thermal management materials. We report an isotropic high thermal conductivity exceeding 500 W m-1K-1 at room temperature in high-quality wafer-scale cubic silicon carbide (3C-SiC) crystals, which is the second highest among large crystals (only surpassed by diamond). Furthermore, the corresponding 3C-SiC thin films are found to have record-high in-plane and cross-plane thermal conductivity, even higher than diamond thin films with equivalent thicknesses. Our results resolve a long-standing puzzle that the literature values of thermal conductivity for 3C-SiC are lower than the structurally more complex 6H-SiC. We show that the observed high thermal conductivity in this work arises from the high purity and high crystal quality of 3C-SiC crystals which avoids the exceptionally strong defect-phonon scatterings. Moreover, 3C-SiC is a SiC polytype which can be epitaxially grown on Si. We show that the measured 3C-SiC-Si thermal boundary conductance is among the highest for semiconductor interfaces. These findings provide insights for fundamental phonon transport mechanisms, and suggest that 3C-SiC is an excellent wide-bandgap semiconductor for applications of next-generation power electronics as both active components and substrates.

Correlative atom probe tomography and scanning transmission electron microscopy reveal growth sequence of LPSO phase in Mg alloy containing Al and Gd.

Atom probe tomography (APT) and transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) have been used correlatively to explore atomic-scale local structure and chemistry of the exactly same area in the vicinity of growth front of a long-period stacking ordered (LPSO) phase in a ternary Mg-Al-Gd alloy. It is proved for the first time that enrichment of Gd atoms in four consecutive (0001) atomic layers precedes enrichment of Al atoms so that the formation of Al6Gd8 clusters occurs only after sufficient Al atoms to form Al6Gd8 clusters diffuse into the relevant portions. Lateral growth of the LPSO phase is found to occur by 'ledge' mechanism with the growth habit plane either {1[Formula: see text]00} or {11[Formula: see text]0} planes. The motion of ledges that give rise to lateral growth of the LPSO phase is considered to be controlled by diffusion of Al atoms.

Fabrication of GaN/Diamond Heterointerface and Interfacial Chemical Bonding State for Highly Efficient Device Design.

The direct integration of gallium nitride (GaN) and diamond holds much promise for high-power devices. However, it is a big challenge to grow GaN on diamond due to the large lattice and thermal-expansion coefficient mismatch between GaN and diamond. In this work, the fabrication of a GaN/diamond heterointerface is successfully achieved by a surface activated bonding (SAB) method at room temperature. A small compressive stress exists in the GaN/diamond heterointerface, which is significantly smaller than that of the GaN-on-diamond structure with a transition layer formed by crystal growth. A 5.3 nm-thick intermediate layer composed of amorphous carbon and diamond is formed at the as-bonded heterointerface. Ga and N atoms are distributed in the intermediate layer by diffusion during the bonding process. Both the thickness and the sp2 -bonded carbon ratio of the intermediate layer decrease as the annealing temperature increases, which indicates that the amorphous carbon is directly converted into diamond after annealing. The diamond of the intermediate layer acts as a seed crystal. After annealing at 1000 °C, the thickness of the intermediate layer is decreased to approximately 1.5 nm, where lattice fringes of the diamond (220) plane are observed.

Evaluation of spatial and temporal resolution on in situ annealing aberration-corrected transmission electron microscopy with proportional-integral-differential controller.

The in situ annealing observation in transmission electron microscope (TEM) is one of the effective methods for imaging thermally induced microstructural changes. For applying this dynamical characterization to bulk samples fabricated by ion-milling, electro-polishing or focused ion beam (FIB) mill, it is generally needed to use a heating-pot type system. We here report an initial trial to improve the spatial and temporal resolution during the in-situ annealing observation of bulk samples using a spherical aberration corrected (AC) TEM with a new thermal control unit. The information limit of 1.5 Å and the point resolution of 2.0 Å are achieved under isothermal annealing at 350°C, which is the same resolution at room temperature, and it is affected strongly of sample drift by the temperature variation. The sample is heated at a heating rate of +1.0°C/s, the drift distance observed by a TV readout speed CCD camera is less than 2.0 Å/s.

Weak-beam scanning transmission electron microscopy for quantitative dislocation density measurement in steels.

To evaluate dislocations induced by neutron irradiation, we developed a weak-beam scanning transmission electron microscopy (WB-STEM) system by installing a novel beam selector, an annular detector, a high-speed CCD camera and an imaging filter in the camera chamber of a spherical aberration-corrected transmission electron microscope. The capabilities of the WB-STEM with respect to wide-view imaging, real-time diffraction monitoring and multi-contrast imaging are demonstrated using typical reactor pressure vessel steel that had been used in an European nuclear reactor for 30 years as a surveillance test piece with a fluence of 1.09 × 1020 neutrons cm-2. The quantitatively measured size distribution (average loop size = 3.6 ± 2.1 nm), number density of the dislocation loops (3.6 × 1022 m-3) and dislocation density (7.8 × 1013 m m-3) were carefully compared with the values obtained via conventional weak-beam transmission electron microscopy studies. In addition, cluster analysis using atom probe tomography (APT) further demonstrated the potential of the WB-STEM for correlative electron tomography/APT experiments.

Boron distributions in individual core-shell Ge/Si and Si/Ge heterostructured nanowires.

Ge/Si and Si/Ge core-shell nanowires (NWs) have substantial potential for application in many kinds of devices. Because impurity distributions in Ge/Si and Si/Ge core-shell NWs strongly affect their electrical properties, which in turn affect device performance, this issue needs urgent attention. Here we report an atom probe tomographic study of the distribution of boron (B), one of the most important impurities, in two kinds of NWs. B atoms were doped into the Si regions of Ge/Si and Si/Ge core-shell NWs. It was found that the B atoms were randomly distributed in the Si shell of the Ge/Si core-shell NWs. In the Si/Ge core-shell NWs, on the other hand, the B distributions depended on the growth temperature and the B2H6 flux. With a higher growth temperature and an increased B2H6 flux, the B atoms piled up in the outer region of the Si core. However, the B atoms were observed to be randomly distributed in the Si core after decreasing both the growth temperature and the B2H6 flux.

Behavior of phosphorous and contaminants from molecular doping combined with a conventional spike annealing method.

The fabrication of future nanoscale semiconductor devices calls for precise placement of dopant atoms into their crystal lattice. Monolayer doping combined with a conventional spike annealing method provides a bottom-up approach potentially viable for large scale production. While the diffusion of the dopant was demonstrated at the start of the method, more sophisticated techniques are required in order to understand the diffusion, at the near surface, of P and contaminants such as C and O carried by the precursor, not readily accessible to direct time-of-flight secondary ion mass spectrometry measurements. By employing atom probe tomography, we report on the behavior of dopant and contaminants introduced by the molecular monolayer doping method into the first nanometers. The unwanted diffusion of C and O-related molecules is revealed and it is shown that for C and O it is limited to the first monolayers, where Si-C bonding formation is also observed, irrespective of the spike annealing temperature. From the perspective of large scale employment, our results suggest the benefits of adding a further process to the monolayer doping combined with spike annealing method, which consists of removing a sacrificial Si layer to eliminate contaminants.