We calculate positron annihilation parameters, namely the S and W parameters from the Doppler broadening spectroscopy and the positron lifetime , for defect-free states as well as cation ...mono-vacancies and vacancy complexes in nitride semiconductor alloys Al0.5Ga0.5N, In0.5Ga0.5N and Al0.5In0.5N. The obtained distributions of these parameters differ from compound to compound. Especially, the S-W relation for In0.5Ga0.5N is very different from that for Al0.5Ga0.5N. For the cation mono-vacancies, introducing local structural parameters, their correlations with S, W and are investigated. The S and variations are well described with the size distributions of the vacancies while the W variation is related to the presence of localized d electrons. For the vacancy complexes as well as the cation mono-vacancies, multiple-linear-regression models to describe S, W and are successfully constructed using the local structural parameters as descriptors. The S-W and S- relations are also compared with those for AlN, GaN and InN.
Abstract
A process for activating Mg and its relationship with vacancy-type defects in Mg-implanted GaN were studied by positron annihilation spectroscopy. Mg
+
ions were implanted with an energy of ...10 keV, and the Mg concentration in the subsurface region (≤ 50 nm) was on the order of 10
19
cm
−3
. After the Mg-implantation, N
+
ions were implanted to provide a 300-nm-deep box profile with a N concentration of 6 × 10
18
cm
−3
. From capacitance–voltage measurements, the sequential implantation of N was found to enhance the activation of Mg. For N-implanted GaN before annealing, the major defect species were determined to Ga-vacancy related defects such as divacancy. After annealing below 1000 °C, the clustering of vacancies was observed. Above 1200 °C annealing, however, the size of the vacancies started to decrease, which was due to recombinations of vacancy clusters and excess N atoms in the damaged region. The suppression of vacancy clustering by sequential N-implantation in Mg-implanted GaN was attributed to the origin of the enhancement of the Mg activation.
A void formation at nanoscale and a crack initiation at micrometer scale in CFRP during a tensile test were investigated by means of positron annihilation and X-ray computed tomography (X-CT). Free ...volumes in bisphenol-A epoxy resin in CFRP were probed, and a mean diameter of free volumes at room temperature was determined to be 0.49 nm. The crack initiation and propagation in the CFRP sample was observed by in-situ measurements of X-CT during a tensile test. For the sample after the tensile test, free volumes with a mean diameter of 0.70 nm were introduced near fractures locations. In the same locations, the density of free volumes which intrinsically existed in the polymer resins decreased. Thus, the introduction of the free volumes was attributed to breaks in molecular chains upon fracture and a resultant agglomeration of intrinsic free volumes in the polymer matrix.
Planar vacuum‐fluorescent‐display devices emitting polarized UV‐C, blue, and green light are demonstrated using immiscible Al1−xInxN nanostructures grown in nonpolar m‐directions. Despite the ...presence of high concentration of nonradiative recombination centers, the Al1−xInxN nanostructures emit polarized light with the luminescence lifetimes of 22–32 ps at 300 K. This defect‐resistant radiative performance suggests supernormal localized characteristics of electron–hole pairs.
We investigated epoxy resin consisting of diglycidyl ether of bisphenol A (DGEBA) and bis-(p-aminocyclohexyl)methane (PACM) and found that the density increased and decreased in the low- and ...high-conversion regions, respectively, by using experiments and all-atom (AA) molecular dynamics (MD) simulations. To understand this feature qualitatively, we conducted course-grained (CG) MD simulations. For the flexible and rigid CG models, the calculated density increased and decreased monotonically, respectively, in contrast to the experimental density. To develop a more realistic CG model, which is denoted as CG-EP, we derived angular parameters based on AA-MD simulations. It was found that the CG-EP successfully reproduced the trend of the experimental density, suggesting the importance of molecular flexibility. In addition, the progress of the conversion monotonically increased the free volume hole size, which is consistent with the result of positron annihilation lifetime spectroscopy. Furthermore, we experimentally observed that the Young's modulus suddenly decreased at 50%, as the conversion progressed. The CG analysis indicated that this trend was also attributed to the molecular flexibility.
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•Density of the present epoxy resin is maximum near the 50% conversion.•A developed coarse-grained model can reproduce the experimental density.•Molecular flexibility of epoxy resin significantly affects the density.•Molecular flexibility of epoxy resin significantly affects the Young's modulus.
•Vacancies in GaN grown by OVPE method were studied using positron annihilation.•Major defect species was identified as Ga-vacancy coupled with oxygen atoms.•Relationship between vacancies and sample ...transparency was discussed.•Epitaxial layer grown on OVPE-GaN by using MOVPE was also characterized.•Electron traps in MOVPE-GaN layer were discussed.
Defects in bulk GaN grown by the oxide vapor phase epitaxy (OVPE) method were probed by using positron annihilation spectroscopy. Measurements of the Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons revealed that the major defect species in OVPE-GaN was Ga vacancy-type defects coupled with oxygen atoms such as VGa(ON)n and VGaVN(ON)n. The formation of vacancy-oxygen complexes was attributed to the high concentration of oxygen incorporated in the samples during their growth process. A correlation between such defect complexes and sample transparencies was observed. An 8-μm-thick epitaxial film grown on OVPE-GaN by using metalorganic vapor phase epitaxy was also characterized. The vacancy concentration in the GaN film was found to be under or close to the detection limit of positron annihilation (1015 cm−3), suggesting that an epitaxial layer can be grown on without being influenced by vacancies in the substrates.
Mg ions were implanted into Si-doped (5 × 1017 cm-3) n-GaN at a dose of 1.5 × 1011 or 1.5 × 1012 cm-2. MOS diodes were used to characterize the implanted GaN after 300 °C annealing for 3 h and after ...additional 500 °C annealing for 3 min. Although capacitance-voltage (C-V) characteristics varied with the dosage, the effects of acceptor-like defects induced by ion implantation were observed in the C-V characteristics independently of dosage and annealing temperature. A defect level at approximately 0.25 eV below the conduction band edge was detected electrically. By positron annihilation spectroscopy, its origin was identified as a divacancy consisting of Ga and N vacancies. It was found that its density compared with that of as-implanted GaN decreased with 300 °C annealing, and further increased with 500 °C annealing. This phenomenon was explained on the basis of the difference between the diffusion barriers of possible point defects.
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