Vacancy-type Defects Research Articles

Modulation of the Dielectric Properties of CaCu<sub>3</sub>Ti<sub>4</sub>O<sub>12</sub> System with Gd-Substitution

To specify the effects of modulating mechanism of Gd substitution on the dielectric properties for Ca1-xGdxCu3Ti4O12 system, the XRD, SEM and positron positron annihilation technique have been implemented. The results display that both grain size and vacancy-type defect concentration play important roles in controlling the dielectric properties of CaCu3Ti4O12. The dielectric properties are improved by adding an appropriate amount of Gd additives (x=0.01) but weakened by higher Gd doping content since this is closely related to the grain size and concentration of vacancy-type defect in the samples.

First-Principles Calculation of Defect Formation and Positron Annihilation States in Bi<sub>2</sub>Te<sub>3</sub>

Defect formation energy in Bi2Te3 thermoelectric material was calculated using a first principles approach based on the Density Functional Theory (DFT). For vacancy-type defect, the Te1 vacancy (VTe1) is the most stable defect with low formation energy in both Bi-rich and Te-rich conditions, which indicates that the Te1 vacancies have higher probability to be formed. For antisite defects, the formation energy of BiTe1 is much lower than that of BiTe2 in Bi-rich condition, while in Te-rich condition it is beneficial for TeBi with lower formation energy. Positron wave function distribution and positron lifetimes of different annihilation states in Bi2Te3 were also calculated using the atomic superposition (ATSUP) method. The positron bulk lifetime in Bi2Te3 is about 231 ps, and for the neutral vacancy-type defects without relaxation, the positron lifetimes of VBi, VTe1 and VTe2 are 275 ps, 295 ps and 269 ps, respectively.

Investigation of Hydrogen Sorption-Desorption Processes at Gas-Phase Hydrogenation and Defects Formation in Titanium by Means of Electron-Positron Annihilation Techniques

The results of hydrogen sorption and desorption processes investigation at commercially pure titanium alloy during hydrogenation at gas atmosphere are shown in this article. Titanium alloy hydrogenation at temperatures 350, 450 и 550 °С leads to δ-hydrides formation in samples’ volume. Hydrogen sorption rates were calculated on the linear parts of sorption curves and equal to 0.15·10-4 wt%/s at 350 °C, 0.86·10-4 wt%/s at 450 °C, 1.55·10-4 wt%/s at 550 °C. Phase transition in titanium-hydrogen system during thermally stimulated hydrogen desorption investigation by the means of short-wave diffraction of synchrotron radiation shows hydrides dissociation until 520-530 °C. Then α→β transition takes place until 690-720 °C and at this temperatures the phase transformation ends and additional hydrogen desorption peak appears in thermally stimulated hydrogen desorption curve. Defect structure at different hydrogen concentration investigation by the means of electron-positron annihilation techniques shows that hydrogen penetration into titanium leads to crystal lattice expansion. It initiates vacancy-type defects formation which reacts with hydrogen and forms defect-hydrogen complexes.

Radiation-Induced Defects in Si after High Dose Proton Irradiation

Experiments on investigation of the radiation defects produced as a result of high energy proton irradiation of single crystal Si wafers are carried out. Parameters of the proton irradiation facility are presented. It is shown that the most efficient radiation defect formation correlates with the position of the Bragg peak of ionization losses. LT spectra were measured just after irradiation and then after keeping Si samples during 3 months of at room T. We did not observe any variation of the number density of the defects, except for the 7th wafer, where most part of protons was stopped. An efficient annealing of the vacancy-type defects starts at temperatures slightly lower than 100 °C (during 10 min). Annealing at about 700 °C leads to recovering of the monoexponrntial shape of the LT spectra.

Positron Annihilation Spectroscopic Studies of Cu<sub>2</sub>Te Thermoelectric Material

Positron annihilation lifetime spectroscopy (PALS) and coincident Doppler-broadening spectroscopy (CDBS) have been used for investigating the evolution of vacancy-type defects in the thermoelectric material Cu2Te which annealed at different temperatures. The results of PALS show that a fraction of positrons has got annihilated at the surfaces and the sample which annealed at 450 °C has the highest concentration of surface defects. The average positron lifetime and the S parameter have the same trends which gradually increase with the increase of the annealing temperature. This change implies that the total concentration of the defects has been changed with the change of the annealed temperatures. The results of the CDBS ratio spectrum and S-W plot indicate that the defect species have no change after annealing at different temperatures.

The Evolution Behavior of Defects in the Nanofilms of W/Cu and W Probed by Doppler Broadening Positron Annihilation Spectroscopy

W/Cu multilayer nanofilms and pure W nanofilms were prepared in pure Ar and He/Ar mixing atmosphere by radio frequency magnetron sputtering method. The defect evolution of the samples was characterized by Doppler broadening positron annihilation spectroscopy (DB-PAS).The results show that plenty of defects can be produced by introducing helium (He) into W/Cu multilayer nanofilms. With the natural storage time increasing, the helium located in the near surface of W/Cu multilayer nanofilm would be released gradually and induce the coalescence of the helium related defects due to the diffusion of the helium and defects. In addition, the pure W nanofilms were irradiated by 30 keV helium ions and 40 keV hydrogen (H) ions in sequence at room temperature. From the DB-PAS analysis, it can be shown that a large number of vacancy-type defects are produced due to the He and/or H irradiation. H ions would be trapped by He related defects and produced He-H-V complexes.

Oxide Microstructural Changes Accompanying Pore Formation During Anodic Oxidation of Aluminum

Porous anodic oxide formation results from a morphological instability of uniform barrier oxide growth leading to the establishment of pores. To gain insight into this process, evidence for microstructural changes in the oxide accompanying pore initiation in anodic alumina was sought through two types of experiments: potentiodynamic anodizing and stress measurements during dissolution of the anodic films. Cyclic voltammetry during anodizing in sulfuric acid shows that pore formation coincides with the appearance of localized ohmic- conducting regions close to the oxide-solution interface of individual pores. These defects induce large current increases which are apparently responsible for growth of concave scalloped features on the metal-oxide interface at the pore base. It is argued that interface defects are generated by surface forces in the oxide that accompany flow contributing to pore initiation. Stress measurements reveal that pore initiation introduces tensile residual stress into the oxide at the pore base, suggesting the formation of vacancy-type defects in the film. Such defects are explained by increases of local conduction current density due to the concentration of anodizing current in pores. Both sets of experiments reveal that pore formation generates locally defective oxide, which may help explain the phenomenon of burning during high-rate anodizing.

Formation mechanism of luminescence spectra of carbon nitride films doped by europium chloride CNx: EuCl3

Luminescence and absorption spectra of nanostructured carbon nitride (CNx)-films doped by EuCl3 were studied. It was found that the films with europium concentration equal to 4.5at% and 10at% placed on the glass substrate exhibit a luminescence spectrum specific to the EuCl3 compound but with a frequency shift depending on the rare-earth dopant primary concentration. Comparison of the luminescence spectra with theoretical calculations showed a considerable part of the coordination complexes [EuCl9]6- entering into chemical bonding with the carbon nitride film matrix that directly impacts the covalency degree of the《Eu3+–Сl1-》bond. It was found that some of the complexes have vacancy-type defects but the number of the vacancies is small and could be well identified only in films with a higher europium concentration.

The Role of Low-Coordinated Sites on the Adsorption of Glycerol on Defected Ptn/Pt(111) Substrates: A Density Functional Investigation within the D3 van der Waals Correction

Several experimental studies have been reported for the conversion of glycerol on transition metal surfaces; however, only few studies have addressed the role of surface defects in the glycerol–substrate interactions. Here, we report ab initio calculations based on density functional theory within the D3 van der Waals (vdW) correction to investigate the adsorption properties of glycerol on flat and defected Pt(111) substrates, namely, (i) flat surface, (ii) dispersed adatoms, (iii) linear-type defect, (iv) island-type defect, and (v) vacancy-type defect. In the lowest and higher energy configurations, glycerol binds to the flat and defected Pt(111) substrates via one or two hydroxyl groups, in which the anionic O atom binds to cationic Pt site with the O–H bond nearly parallel to the Pt–Pt bonds in several cases, which indicates a contribution of the HO–Pt interaction to the adsorption energy. The PBE adsorption energy is stronger on the low-coordinated sites of single Pt adatoms and six-adatom triangular...

Positron annihilation study of the vacancy clusters in ODS Fe–14Cr alloys

Oxide dispersion strengthened Fe14Cr and Fe14CrWTi alloys produced by mechanical alloying and hot isostatic pressing were subjected to isochronal annealing up to 1400 °C, and the evolution and thermal stability of the vacancy-type defects were investigated by positron annihilation spectroscopy (PAS). The results were compared to those from a non-oxide dispersion strengthened Fe14Cr alloy produced by following the same powder metallurgy route. The long lifetime component of the PAS revealed the existence of tridimensional vacancy clusters, or nanovoids, in all these alloys. Two recovery stages are found in the oxide dispersion strengthened alloys irrespective of the starting conditions of the samples. The first one starting at T > 750 °C is attributed to thermal shrinkage of large vacancy clusters, or voids. A strong increase in the intensity of the long lifetime after annealing at temperatures in the 800–1050 °C range indicates the development of new vacancy clusters. These defects appear to be unstable above 1050 °C, but some of them remain at temperatures as high as 1400 °C, at least for 90 min.

Positron annihilation study of defects in copper irradiated with swift Xe26+ ions

Variable energy positron beam and conventional positron lifetime spectroscopy were used to study pure copper exposed to irradiation with 167 MeV Xe26+ heavy ions with different doses of 1012, 1013, 5 × 1013, 1014 ions/cm2. The presence of vacancy-type defects induced by implantation was confirmed in Doppler spectroscopy characteristics. Decreasing of the positron diffusion length from 135 nm for unirradiated sample, to 82 nm for the lowest and to 46 nm for the highest applied dose was noted. It points out increasing defect concentration with the increase of the fluence. Additionally, the existence of one kind of defects in all irradiated samples was noticed. The defected zone covered with the implanted range and the so called “long range effect” was not confirmed.

On positrons irradiation in the defects of vacancy type

It was predicted the radiation of low energy positrons at their interaction with the defects of the vacancy type. Models of positron traps have been proposed. The energy transitions of positrons in the defects were calculated.

Impact of vacancies on the thermal conductivity of graphene nanoribbons: A molecular dynamics simulation study

Equilibrium molecular dynamics simulation using 2nd generation Reactive Bond Order interatomic potential has been performed to model the thermal transport of nanometer sized zigzag defected graphene nanoribbons (GNRs) containing several types of vacancies. We have investigated the thermal conductivity of defected GNRs as a function of vacancy concentration within a range of 0.5% to 5% and temperature ranging from 300K to 600K, along with a comparative analysis of those for pristine GNRs. We find that, a vacancy concentration of 0.5% leads to over 90% reduction in the thermal conductivity of GNRs. At low defect concentration, the decay rate is faster but ceases gradually at higher defect concentration. With the increasing temperature, thermal conductivity of defected GNRs decreases but shows less variation in comparison with that of pristine GNRs at higher temperatures. Such comprehensive study on several vacancy type defects in GNRs can provide further insight to tune up the thermal transport characteristics of low dimensional carbon nanostructures. This eventually would encourage the characterization of more stable thermal properties in thermal devices at an elevated temperature as well as the potential applicability of GNRs as thermoelectrics.

Advances in applications of positron annihilation spectroscopy to investigating semiconductor microstructures

Positron annihilation spectroscopy has unique advantage for detecting the micro-defects and microstructures in materials,especially for investigating the negatively charged defects such as cation vacancies in semiconductors.It is a powerful tool to characterize the important features for vacancy-type defects localized electron states within the forbidden energy gap and cation vacancy which provides the key information about the type and distribution of microdefects. Positron annihilation lifetime and Doppler broadening spectroscopy are the major methods of analyzing the vacancy formation,evolution and distribution mechanism.Importantly,the slow positron beam technique can provide the dependences of surface,defect and interface microstructure information on depth distribution in semiconductor thin film.Vacancy and impurity elements can change the ambient electron density in material.They also induce the middle band,which will have dramatic effects on optical and electrical performance.And the variation of electron density will exert furtherinfluences on the positron-electron annihilation mechanism and process.For the fundamental experiments in semiconductors,fabrication technology,thermal treatment,ion implantation/doping,irradiation etc, positron annihilation spectroscopy technology has been extensively applied to detecting the detailed electron density and momentum distribution,and gained the information about microstructure and defects.It can guide the fundamental researches in experiment and give optimal design of the technology and properties about semiconductors.In principle, defect concentrations can be derived and an indication can be obtained about the nature of the defect.Results are presented showing that cation vacancies can be easily detected.Also charge states and defect levels in the band gap are accessible.By combining the positron annihilation spectroscopy with optical spectroscopies or other experimental methods,it is possible to give detailed identifications of the defects and their chemical surroundings.The positron annihilation spectroscopy technology is a very special and effective nuclear spectroscopy analysis method in studying semiconductor microstructure.In this review,the research progress in applications of positron annihilation spectroscopy technology to semiconductors is reported,which focuses on the experimental results from the Positron Research Platform located in Institute of High Energy Physics,Chinese Academy of Sciences.Under different growth modes and ways of treating semiconductors,the experimental results about the internal micro-defect formation mechanism of material, evolution mechanism,and defect feature research progress are reviewed Future challenges including the analysis of electropositivity vacancy (i.e.oxygen vacancy) and of multi-ion implantation phenomena are also presented new technologies such as digitization and new theory will make the positron annihilation spectroscopy portable and reliable.

Prediction of positron-annihilation parameters for vacancy-type defects in ternary alloy semiconductors by data-scientific approach

We calculated positron annihilation parameters for mono- and di-vacancies in ternary semiconductors Al0.5Ga0.5N and In0.5Ga0.5N. It has been found that the obtained annihilation parameters are well correlated with structural parameters. By constructing multiple linear regression models using selected (about 1/4 of the total) datasets as training sets in order to reduce computational cost, we could predict annihilation parameters for the rest.

Suppression of vacancy cluster growth in concentrated solid solution alloys

Large vacancy clusters, such as stacking-fault tetrahedra, are detrimental vacancy-type defects in ion-irradiated structural alloys. Suppression of vacancy cluster formation and growth is highly desirable to improve the irradiation tolerance of these materials. In this work, we demonstrate that vacancy cluster growth can be inhibited in concentrated solid solution alloys by modifying cluster migration pathways and diffusion kinetics. The alloying effects of Fe and Cr on the migration of vacancy clusters in Ni concentrated alloys are investigated by molecular dynamics simulations and ion irradiation experiment. While the diffusion coefficients of small vacancy clusters in Ni-based binary and ternary solid solution alloys are higher than in pure Ni, they become lower for large clusters. This observation suggests that large clusters can easily migrate and grow to very large sizes in pure Ni. In contrast, cluster growth is suppressed in solid solution alloys owing to the limited mobility of large vacancy clusters. The differences in cluster sizes and mobilities in Ni and in solid solution alloys are consistent with the results from ion irradiation experiments.

Probing the effect of point defects on the leakage blocking capability of Al0.1Ga0.9N/Si structures using a monoenergetic positron beam

Vacancy-type defects in Al0.1Ga0.9N were probed using a monoenergetic positron beam. Al0.1Ga0.9N layers with different carbon doping concentrations ([C] = 5 × 1017−8 × 1019 cm−3) were grown on Si substrates by metalorganic vapor phase epitaxy. The major defect species in Al0.1Ga0.9N was determined to be a cation vacancy (or cation vacancies) coupled with nitrogen vacancies and/or with carbon atoms at nitrogen sites (CNs). The charge state of the vacancies was positive because of the electron transfer from the defects to CN-related acceptors. The defect charge state was changed from positive to neutral when the sample was illuminated with photon energy above 1.8 eV, and this energy range agreed with the yellow and blue luminescence. For the sample with high [C], the charge transition of the vacancies under illumination was found to be suppressed, which was attributed to the trapping of emitted electrons by CN-related acceptors. With increasing [C], the breakdown voltage under the reverse bias condition increased. This was explained by the trapping of the injected electrons by the positively charged vacancies and CN-related acceptors.

Defects and Charging Processes in Li-Ion Battery Cathodes Studied by Operando Magnetometry and Positron Annihilation

A brief report is given on recent studies of the atomistic processes during charging of battery cathode material LixCoO2 by means of magnetometry and positron annihilation. A set-up for operando magnetometry is implemented which, for the first time, allowed to continuously monitor the distinct variation of the magnetic susceptibility χ of LixCoO2 which occurs during consecutive charging and discharging cycles. The variation of χ with Li+ content in the concentration range 1>x≥0.77 arises from a variation of the electronic density of states and from electronic correlation effects. The χ (x)-behaviour for x<0.77 shows that oxygen is involved in the charging process. Positron annihilation reveals vacancy-type defects on the Li-sublattice, the size of which increases with Li-extraction. Indication for Li-reordering at the reversibility limit of Li extraction is found which correlates with χ (x)-variations in this concentration regime. First measurements on LixCoO2 thin-films performed at the positron beam line NEPOMUC of FRM II at the Heinz Maier-Leibnitz neutron source will be presented.

First-cycle defect evolution of Li1−xNi1/3Mn1/3Co1/3O2 lithium ion battery electrodes investigated by positron annihilation spectroscopy

Abstract In this study the structure and evolution of vacancy type defects in lithium ion batteries are investigated in respect of crystallographic properties. The relation between positron annihilation and electronic structure is discussed in terms of structural dynamics during the lithiation process. Samples of Li1−xNi1/3Mn1/3Co1/3O2 (NMC-111) electrodes with decreasing lithium content (x = 0–0.7) covering the whole range of state of charge were electrochemically prepared for the non-destructive analysis using positron coincidence Doppler broadening spectroscopy (CDBS). The positron measurements allowed us to observe the evolution of the defect structure caused by the delithiation process in the NMC-111 electrodes. The combination of CDBS with X-ray diffraction for the characterization of the lattice structures enabled the analysis of the well-known kinetic-hindrance-effect in the first charge-discharge cycle and possible implications of vacancy ordering. In particular, CDBS revealed the highest degree of relithiation after discharge to 3.0 V at 55 °C. For the first time, we report on the successful application of CDBS on NMC-111 electrodes yielding new insights in the important role of defects caused by the delithiation process and the kinetic hindrance effect.

Behavior of copper contamination on backside damage for ultra-thin silicon three dimensional stacking structure

Bumpless interconnects and ultra-thinning of 300mm wafers for three-dimensional stacking technology have been studied. In our previous work, thinning effects using device wafers <10μm thick were reported. No degradation occurred in the retention time even in a 4-μm-thick DRAM wafer. In this study, the behavior of Cu contamination on a <3-μm-thick DRAM wafer was investigated. The wafer was thinned down by coarse (#320 grit size) grinding and fine (#2000 grit size) grinding. This thinning condition had 200-nm-thick ground damage remaining for the gettering effect. The DRAM wafer was intentionally contaminated with Cu on the damaged layer, and 250°C-60min of heating was carried out during adhesive bonding and de-bonding. Degradation in the device characteristics was found. However, the analytical results indicated that the Cu did not diffuse into the thin Si. Thus, a Cu contaminated blanket wafers having a damaged layer were prepared and annealed until 1000°C-30min. Secondary ion mass spectroscopy, transmission electron microscopy and positron annihilation spectroscopy were evaluated. The Cu was trapped in the vacancy-type defects of the 200nm damaged layer until a 700°C anneal. After an 800°C anneal, the Cu was eliminated on the damaged surface because of the Si recrystallization. The gettering ability of the damaged layer is suitable for 3D multi-level stacking regarding thermal stability.