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AbstractA GaN-on-diamond structure is the most promising candidate for improving the heat dissipation efficiency of GaN-based power devices. Room-temperature bonding of GaN and diamond is an efficient technique for fabricating this structure. However, it is extremely difficult to polish diamond to an average roughness (Ra) below 0.4 nm, especially for polycrystalline diamond. In this work, Room-temperature bonding of GaN and rough-surfaced diamond with a SiC layer was successfully achieved by a surface-activated bonding (SAB) method. The diamond surface’s initial Ra value was 0.768 nm, but after deposition of the SiC layer, the Ra decreased to 0.365 nm. The SiC layer formed at the as-bonded GaN/diamond interface was amorphous, with a thickness of about 7 nm. After annealing at 1000-°C, the amorphous SiC layer became polycrystalline, and its thickness increased to approximately 12 nm. These results indicate that the deposition of a SiC layer on diamond can efficiently lower the diamond surface’s roughness and thus facilitate room-temperature bonding.

Samples of ultra-high-purity tungsten prepared using chemical vapour deposition (CVD) technique were irradiated with neutrons at temperatures Tirr= 373–483 K (stage II of defect recovery) and Tirr= 573–673 K (stage III) up to 0.15 displacements per atom (dpa) in the Belgian reactor (BR2). The study of the microstructure of neutron-damaged samples using transmission electron microscopy (TEM) revealed visible defects with a predominance of dislocation loops. With an increase in the neutron irradiation temperature, the spatial distribution of the loops acquired pronounced inhomogeneity, and their average size moderately increased. Cavities and voids were not observed. Irradiation-induced hardening was found and a linear correlation was obtained between Vickers microhardness and nanohardness for undamaged and neutron-irradiated CVD-W samples. Irradiation of tungsten with neutrons led to a significant increase in the retention of deuterium, which accumulated mainly in vacancy-type traps. Furthermore, the influence of the columnar grain structure in low-dose neutron-irradiated tungsten seemed to be non-trivial upon deuterium retention.

AbstractA new technique of diamond and InGaP room temperature bonding in atmospheric air is reported. Diamond substrate cleaned with H2SO4/H2O2mixture solution is bonded to InGaP exposed after removing the GaAs layer by the H2SO4/H2O2/H2O mixture solution. The bonding interface is free from interfacial voids and mechanical cracks. An atomic intermixing layer with a thickness of about 8 nm is formed at the bonding interface, which is composed of C, In, Ga, P, and O atoms. After annealing at 400 °C, no exfoliation occurred along the bonding interface. An increase of about 2 nm in the thickness of the atomic intermixing layer is observed, which plays a role in alleviating the thermal stress caused by the difference of the thermal expansion coefficient between diamond and InGaP. The bonding interface demonstrates high thermal stability to device fabrication processes. This bonding method has a large potential for bonding large diameter diamond and semiconductor materials.

Electrical properties of heterojunctions of group-III nitrides are largely sensitive to interface charges due to the discontinuity of polarizations. By means of surface-activated bonding of double-side polished freestanding GaN (0001) wafers, we fabricate Ga-face/Ga-face and N-face/N-face interfaces with antiparallel spontaneous polarizations, i.e. interfaces with the greatest discontinuity of polarizations, to investigate their electrical and nanostructural properties. Built-in potential of N-face/N-face interface is smaller than that of Ga-face/Ga-face interface after a post-bonding annealing at 600 °C. The difference in built-in potentials between the two antiparallel polarized interfaces is analyzed in the framework of charge-neutrality-level model with effects of antiparallel polarizations incorporated, and the density of interface states is roughly estimated. The leak is enhanced in both Ga-face/Ga-face and N-face/N-face interfaces by annealing at higher temperatures. Contribution of defects observed in the vicinity of bonding interfaces is suggested.

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:O_x centers in Si a viable resource, thanks to the I_13/24→I4_15/2 optical transition of Er^3+. 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:O_x in Si. The lower-bound number of optically active Er atoms detected is of the order of 10^2, corresponding to a higher-bound value for the emission rate per individual ion of about 10^4  s^−1.

Chemical composition around diamond/silicon heterointerfaces fabricated by surface activated bonding (SAB) at room temperature is examined by energy-dispersive X-ray spectroscopy under scanning transmission electron microscopy. Iron impurities segregate just on the bonding interfaces, while oxygen impurities segregate off the bonding interfaces in the silicon side by 3–4 nm. Oxygen atoms would segregate so as to avoid the amorphous compound with silicon and carbon atoms, self-organized at the bonding interfaces in the SAB process. When the bonding interfaces are annealed at 1000 °C, the amorphous compound converts into cubic silicon carbide (c-SiC), and nano-voids 5–15 nm in size are formed at the region between silicon and c-SiC, at which the oxygen density is high before annealing. The nano-voids can act as the gettering sites in which metal impurities are preferentially agglomerated, and the impurity gettering would help to improve the electronic properties of the bonding interfaces by annealing.

In this work, we fabricated Ga2O3(001)/Si(100) and Ga2O3(010)/Si(100) heterointerfaces by surface activated bonding at room temperature and investigated the effect of Si thickness on the thermal stability of the heterointerfaces by heating the bonding samples at different temperatures. The heterointerface with a thin Si exhibited a good thermal stability at 1000 °C. A 4 nm thick intermediate layer with a uniform thickness was formed at the as-bonded Ga2O3(001)/Si(100) heterointerface, but for the as-bonded Ga2O3(010)/Si(100) heterointerface, an intermediate layer with a non-uniform thickness was formed. The thickness of both intermediate layers ranged from 3.6 to 5.4 nm and decreased after annealing at 500 °C, followed by an increase after annealing at 1000 °C. The component of the intermediate layer includes Ga, O, and Si atoms.

We fabricate AlGaN/GaN high electron mobility transistors (HEMTs) on diamond substrates by transferring 8 μm heterostructures grown on 3C-SiC/Si templates and subsequently applying the conventional device process steps. No exfoliation of 3C-SiC/diamond bonding interfaces is observed during 800 °C annealing, the essential step for forming ohmic contacts on nitrides. The thermal resistance of HEMTs on diamond is 35% of that of HEMTs on Si, which is assumed to be the origin of smaller negative drain conductance in on-diamond HEMTs. The results imply that the bonding-first process is applicable for fabricating low-thermal-resistance HEMTs with thick nitride layers.

Post-irradiation annealing (PIA) behavior of irradiation-induced microstructural changes and hardening of two kind of A533B (first generation (1stGENS: 0.16 wt.% Cu) and second generation (2ndGENS: 0.04 wt.% Cu)) steels after neutron-irradiation of 3.9 × 1019 n cm–2 at 290 °C has been studied by positron annihilation spectroscopy, atom probe tomography and Vickers microhardness measurements. In the 1stGENS, clear two recovery stages are observed: (i) as-irradiated state to 450 °C and (ii) 450 to 600 °C. The first stage is due to annealing out of the most of irradiation-induced vacancy-related defects (VRDs), and the second stage corresponds to dissolving irradiation-induced Cu-rich solute nano-clusters (CRSCs). The experimental hardening is almost twice of the hardening due to the CRSCs estimated by Russell-Brown model below 350 °C, but almost the same as the estimation from 400 to 550 °C. In the 2ndGENS, the VRDs and non-Cu-rich solute nano-clusters (NCRSCs) recover at 450 °C. No CRSC has been formed even in all the annealing process. The experimental hardening is almost twice of the hardening estimated due to the NCRSCs by Russell-Brown model below 400 °C.

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A new two-dimensional angular correlation of annihilation radiation apparatus is described. Position-sensitive photomultiplier tubes coupled with two-dimensional arrays of small BGO scintillator blocks make simple and compact position-sensitive γ-ray detectors. With a sample-detector distance of 5 m, an angular resolution of 1.1 mrad FWHM and a coincidence count rate of ∼2.4 cps per mCi are obtained. Its performance is demonstrated by the result of a test measurement for KI crystal in which non-localized positronium exists at low temperatures.

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Structural phase transitions between various kinds of superlattice structures formed on a Si(111) surface have been investigated by spot analysis of reflection high-energy electron diffraction (RHEED). Reversible transitions induced by temperature changes and irreversible ones induced by metal depositions were observed. Detailed discussions on the dynamics of the phase transitions are made by quantitative analyses of integrated spot intensity and profile. For a phase transition of 7'7 ↔ 1'1 structures on a clean Si(111) surface, a hysteresis with temperature difference of 5°C. between in heating and cooling processes was found in the spot intensity change, indicating a first-order transition. Hysteresis was hardly recognized, on the other hand, for transitions of Au-induced superstructures (5×2-Au or [image omitted]×[image omitted]-Au) ↔ 1×1-Au. The spot profiles were found to be broadened during the transition of Si(111)-[image omitted]×[image omitted]-Au ↔ 1×1-Au, which was a signature of a continuous transition, while the profiles remained unchanged during the transitions of the 7×7 ↔ 1×1 and 5×2-Au ↔ 1×1-Au phases. Structural conversions induced by In adsorption on the Si(111) surface kept at constant temperatures were also analyzed. The conversions at room temperature were totally dependent on the initial substrate surface structures; the 7×7 surface did not show any structural conversion with In adsorption, while the [image omitted]×[image omitted]-In surface successively converted to a 2×2 and a [image omitted]×[image omitted] phase with coverage increase. The structural transitions at elevated temperatures were sensitively dependent on the temperatures. Sequences of transitions among the 7×7, 4×1, [image omitted]×[image omitted], [image omitted]1×[image omitted], and [image omitted]×4 were quantitatively revealed as changes in RHEED spot intensity.

The three-dimensional distribution of oxygen atoms segregated at ?9{114} grain boundaries (GBs) in Czochralski-grown silicon ingots is analyzed within a high spatial resolution of less than 0.5 nm by atom probe tomography combined with a focused ion beam (FIB) operated at ?150 °C. The analysis reveals a segregation of oxygen atoms within a range of 2.5 nm across the GB plane, which is much narrower in comparison with the previous reports obtained using a conventional FIB. The oxygen concentration profile accurately reflects the distribution of the segregation sites, which exist at bond-centered sites under tensile stresses above 2 GPa, as calculated by ab initiolocal stress calculations.

Non-coherent ?3{111} grain boundaries (GBs) with a positive deviation in the tilt angle (??110?> 70.5°) exhibit a high recombination activity in high-performance multicrystalline silicon ingots. Most of the GB segments are composed of edge-type dislocations with the Burgers vector bof a/3?111?, unlike Lomer dislocations with b= a/2?110? observed for negative deviations, arranged on coherent ?3{111} GB segments. Stretched ?110? reconstructed bonds along the tilt axis are introduced so as not to form dangling bonds, and large strains are generated around the dislocation cores. Oxygen and carbon atoms segregating due to the strains would induce the recombination activity.

The three-dimensional (3D) distribution of nanosized silicon (Si) crystallites within a hydrogenated nanocrystalline Si (nc-Si:H) material is examined by laser-assisted atom probe tomography (APT). The amorphous and crystalline phases in nc-Si:H are distinguished by obtaining the 3D density distribution of H atoms, because the former contains a high H density. The H content in the amorphous phase is estimated to be approximately 15 at% by APT, which is consistent with that obtained by infrared spectroscopy. Thus, the 3D analysis of H distribution via APT is a powerful method to visualize the real shape of nanosized crystallites within nc-Si:H materials.

Laser-assisted atom probe tomography was used to identify the impurity distribution in Si heterojunction (SHJ) solar cells composed of thin doped/intrinsic amorphous Si layers on the textured surface of a crystalline Si wafer. A site-specific lift-out technique involving a focused ion beam enabled the selection of a ?2 × 2 ?m2area on an arbitrary pyramidal surface. The distributions of B, P and C in the amorphous Si layers introduced by p-type (trimethyl-borane or diborane) or n-type (phosphine) dopant gases were investigated. Standard guidelines for the assessment of the H content in amorphous Si of SHJ solar cells were provided.

An interlaboratory comparison for positron annihilation lifetime (PAL) measurements for pure nickel and fused silica was performed. Based on the reported positron (for nickel) and positronium (for fused silica) lifetimes, the uncertainties in the PAL measurements were estimated.

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The effect of P or B predoping on As diffusion in polycrystalline Si was investigated by atom probe tomography. In all samples, a high concentration of As was found at grain boundaries, indicating that such boundaries are the main diffusion path. However, As grain-boundary diffusion was suppressed in the B-doped sample and enhanced in the P-doped sample. In a sample codoped with both P and B, As diffusion was somewhat enhanced, indicating competition between the effects of the two dopants. The results suggest that As grain-boundary diffusion can be controlled by varying the local concentration of P or B.

The successful formation of abrupt phosphorus (P) d-doping profiles in germanium (Ge) is reported. When the P d-doping layers were grown by molecular beam epitaxy (MBE) directly on Ge wafers whose surfaces had residual carbon impurities, more than a half the phosphorus atoms were confined successfully within a few nm of the initial doping position even after the growth of Ge capping layers on the top. On the other hand, the same P layers grown on Ge buffer layers that had much less carbon showed significantly broadened P concentration profiles. Current-voltage characteristics of Au/Ti/Ge capping/P d-doping/n-Ge structures having the abrupt P d-doping layers with carbon assistance showed excellent ohmic behaviors when P doses were higher than 1 x 1014 cm[?]2 and the capping layer thickness was as thin as 5 nm. Therefore, the insertion of carbon around the P doping layer is a useful way of realizing ultrashallow junctions in Ge.

The impact of carbon (C) co-implantation on boron (B) activation in crystalline silicon was investigated. The detailed distribution of B and C atoms and B activation ratios dependent on the C ion-implantation energies were examined based on three-dimensional spatial mappings of B and C obtained by atom probe tomography and from depth profiles of their concentrations from secondary ion mass spectrometry and depth profiles of carrier concentrations with spreading resistance measurements. At all C implantation energies (8, 15, and 30 keV), B out-diffusion during activation annealing was reduced, so that more B atoms were observed in the C co-implanted samples. The carrier concentration was decreased throughout the entire implanted region for C implantation energies of 15 and 30 keV, although it was only increased at greater depths for C co-implantation at 8 keV. Two different effects of C co-implantation, (I) reduction of B out-diffusion and (II) influence of B activation, were confirmed.

Although characterization of hydrogen (H) in silicon (Si) is of interest from a device physics viewpoint, this is difficult using conventional analytical methods. In this study, deuterium (D) was used as a proxy for H, and the distribution of implanted D-ions in Si(100) was investigated using atom probe tomography (APT). D can be distinguished from the residual H gas in the APT chamber. The APT results indicated that D tended to form clusters, whose distribution was similar to that of 311 defects identified using transmission electron microscopy, suggesting that the D ions segregate in the vicinity of these defects.

Three-dimensional dopant distributions in actual n- and p-channel metal-oxide-semiconductor devices of 65 nm node in two kinds of commercially available products were investigated by atom probe tomography (APT). Detailed and quantitative dopant distributions in gate, gate oxide, and channel regions were successfully obtained by APT. In particular, similarities as well as differences in the dopant distributions of these two devices, which were made by different fabrication processes, were clarified in detail by estimating the dopant concentrations in the grain, at the grain boundary, and at the interface of the polycrystalline Si gate individually.

Laser-assisted atom probe tomography (APT) was successfully applied to analyze the elemental distributions in a high-k/metal gate stack of a commercially available 45 nm node real device. APT revealed the multilayer structure of the high-k/metal gate stack with nearly atomic-scale resolution, and successfully detected small amounts of Zr in the thin layer of high-kHfO2dielectrics and H in the Ti layer of the metal gate. The present results demonstrate the usefulness of APT as a tool of elemental analysis in nanoscale multilayer device structures.

The momentum distribution of positronium (Ps) in BaF2single crystal has been investigated in a temperature range from 10 to 297 K. The existence of the delocalized and localized Ps has been confirmed. The momentum distribution of the delocalized Ps, a central peak at zero momentum and satellite peaks at the momenta corresponding to the reciprocal lattice vectors, has a large width even at 10 K. The width is almost independent of temperature. The behavior is different from that of the delocalized Ps in other ionic crystals. This indicates that the delocalized Ps in BaF2is strongly coupled to the phonon field.

Randomness of channel dopant distribution in metal-oxide-semiconductor field-effect transistor (MOSFET) structures was analyzed by laser-assisted atom probe tomography. Three-dimensional dopant distributions of boron and arsenic atoms in MOSFET channels which define the threshold voltage were obtained with nearly atomic scale resolution. In order to achieve highly statistical precision, about 130 million atoms were detected in the channel region. We found that the distribution of boron atoms was consistent with a random solid solution in a matrix. Meanwhile, the arsenic atom distribution is slightly deviated from the random distribution, implying the possibility of arsenic cluster formation.