Vacancy-type Defects Research Articles

Increased p-type conductivity in GaNxSb1−x, experimental and theoretical aspects

The large increase in the p-type conductivity observed when nitrogen is added to GaSb has been studied using positron annihilation spectroscopy and ab initio calculations. Doppler broadening measurements have been conducted on samples of GaNxSb1−x layers grown by molecular beam epitaxy, and the results have been compared with calculated first-principle results corresponding to different defect structures. From the calculated data, binding energies for nitrogen-related defects have also been estimated. Based on the results, the increase in residual hole concentration is explained by an increase in the fraction of negative acceptor-type defects in the material. As the band gap decreases with increasing N concentration, the ionization levels of the defects move closer to the valence band. Ga vacancy-type defects are found to act as positron trapping defects in the material, and the ratio of Ga vacancy-type defects to Ga antisites is found to be higher than that of the p-type bulk GaSb substrate. Beside Ga vacancies, the calculated results imply that complexes of a Ga vacancy and nitrogen could be present in the material.

Rare sulfides enriched in K, Tl and Pb from the Noril'sk and Salmagorsky complexes, Russia: new data and implications

AbstractNew results (compositional data and reflectance values) are reported for some rare sulfides enriched in K, Tl and Pb, which are related to djerfisherite, thalfenisite, bartonite, a “Cl-bearing bartonite”, or chlorbartonite, and also for shadlunite, from the Noril'sk and Salmagorsky complexes, Russia. Our observations and comparisons with relevant data in the literature imply that: (1) bartonite is probably a S-dominant (or Cl-free) analogue of djerfisherite; and a “Cl-bearing bartonite” and chlorbartonite are probably compositional variants of the djerfisherite–bartonite series. (2) The most probable formulae of bartonite and djerfisherite are (K,Me2+)6(Fe,Cu,Ni)25–xS26(S,Cl) and (K,Me2+)6(Fe,Cu,Ni)25–xS26(Cl,S), where 0 ≤ x ≤ 5, respectively. (3) Two independent substitution mechanisms probably operate in the natural series. A coupled substitution [Me2+ + S2– ↔ K+ + Cl–] is reflected by an observed deficit in K, accompanied by the incorporation of Me2+(Pb, Fe, or Ni) in the K site. Another mechanism is inferred to be [2Fe3+ + 〈 ↔ 3Fe2+], which assumes the existence of vacancy-type defects at the Me site. Thus, the second mechanism could possibly control the existing variations of Σ(Fe, Cu, Ni) in the range of ∼21 to 25 a.p.f.u., documented in djerfisherite- and bartonite-type minerals. The minerals analysed from Noril'sk, which are free of Cl and related to bartonite and to a Tl-dominant analogue of bartonite (unnamed species), probably crystallized from microvolumes of late fluid rich in K and Tl, under conditions of relatively low oxygen fugacity in the environment. Uniform contentss of Fe and Cu, observed in coexisting phases of normal (Cl-bearing) djerfisherite and bartonite (or Cl-free analogue of djerfisherite) at Salmagorsky imply that they reached equilibrium with regard to the distribution of these elements during crystallization. These phases probably formed as a result of fluctuations in the ratios of sulfur and chlorine fugacity in a fluid at a postmagmatic hydrothermal stage.

Positron annihilation spectroscopic studies of multiferroic Bi1-xPrxFeO3 nanocrystalline compounds

Positron lifetime (PL) and coincidence Doppler broadening spectroscopic (CDBS) experiments were carried out on BiFeO3 samples doped with Pr(in place of Bi) in concentrations (x) = 0, 0.05, ..., 0.35. In the initial stages of doping, the existing Bi3+ vacancies are occupied by Pr3+ ions and positron trapping is reduced. From x = 0.15 to 0.35, more number of positrons diffused to the surfaces of the nanocrystallites. The CDBS ratio curves showed enhancement of the peak at pL = 10.2×10-3 m0c due to positron annihilation with the 2p electrons of oxygen. It suggested increased trapping of positrons in newly created cationic vacancy-type defects due to the mismatch of ionic radii of Pr3+ substituting Bi3+ ions. The lattice parameters decreased (3.9661-3.9248Å) while the nanocrystallitesizes reduced and increased (28.6-34.2nm) during the doping.

Vacancies in InxGa1−xN/GaN multiple quantum wells fabricated on m-plane GaN probed by a monoenergetic positron beam

Vacancy-type defects in InxGa1−xN/GaN multiple-quantum-well (MQW) structures fabricated on m-plane GaN by metal–organic chemical vapor deposition have been studied using a monoenergetic positron beam. Through measurements of Doppler broadening spectra of the annihilation radiation, the vacancy-type defects in MQW structures were probed. The positron trapping rate of defects decreased under photon illumination, which is attributed to the emission of electrons from those defects and/or the suppression of the positron diffusion by optically active defects. The energy level of the defects was close to the energy of photoluminescence emissions. The relationship between the energy width of the photoluminescence line and the defects is discussed.

Thermal evolution of vacancy-type defects in quenched FeCrNi alloys

The effect of isochronal annealing on vacancy-type defects in quenched FeCrNi alloys and SUS316 was investigated via positron annihilation technique. Vacancy-type defects clustered and grew with increasing annealing temperatures of up to 523 K and, in FeCrNi alloys, were gradually annihilated with increasing temperature. The annihilation temperature was relatively insensitive to the addition of Mo and nonmetal elements, and after annealing at 673 K, the vacancy-type defects were annihilated and dislocation-type defects were formed in all of the alloys. In addition, due to the formation of Mo-vacancy complexes, the density of defects in the Mo-diluted FeCrNi model alloy was lower than that in the FeCrNi model alloy. The long lifetime of vacancy-type defects in commercial stainless steel SUS316 was smaller than that in the FeCrNi model alloys due to the nonmetal-element-induced change in mobility of the defects. Moreover, the vacancy-type and dislocation-type defects contributed to the S and W parameters of positron annihilation during the entire annealing treatment.

Enhanced dielectric and piezoelectric properties in lead-free Bi0.5Na0.5TiO3–BaTiO3–SrTiO3 thin films with seed layer

0.755(Bi0.5Na0.5)TiO3–0.065BaTiO3–0.18SrTiO3 (BNT–BT–ST) thin films were deposited on the Pt(111)/Ti/SiO2/Si substrates with seed layer. Extremely enhanced electrical properties exhibited in the BNT–BT–ST film with seed layer, such as large dielectric constant (630) and piezoelectric coefficient d33 (140pm/V) and low dielectric loss (0.032). The enhancement of electrical properties can be attributed to the seed layer, which offers nucleation site to reduce crystallization activation energy and facilitates polarization response. The seed layer also acts as a capacitive interface layer, which reduces the vacancy-type defects effectively.

Effect of Xe26+ ion irradiation on the microstructural evolution and mechanical properties of Zr–1Nb at room and high temperature

Zr–1Nb samples were irradiated with 6.37MeV Xe26+ ions to 5dpa at room and high temperature and the microstructural evolution and mechanical properties were investigated. After irradiation, X-ray diffraction (XRD) and slow positron annihilation technique (SPAT) were used to study the microstructure and the results demonstrated that the microstrain, domain size, dislocation density and vacancy-type defects increased. The less increases in these microstructural parameters are due to the annihilation of defects at high temperature irradiation. The presence of the defects are responsible for the enhanced hardness of irradiation at room or high temperature and activation energy of irradiation at room temperature. The reason of the lowest activation energy of irradiation at high temperature still needs further clarification.

Enhanced damage buildup in C+-implanted GaN film studied by a monoenergetic positron beam

Wurtzite GaN films grown by hydride vapor phase epitaxy were implanted with 280 keV C+ ions to a dose of 6 × 1016 cm−2. Vacancy-type defects in C+-implanted GaN were probed using a slow positron beam. The increase of Doppler broadening S parameter to a high value of 1.08–1.09 after implantation indicates introduction of very large vacancy clusters. Post-implantation annealing at temperatures up to 800 °C makes these vacancy clusters to agglomerate into microvoids. The vacancy clusters or microvoids show high thermal stability, and they are only partially removed after annealing up to 1000 °C. The other measurements such as X-ray diffraction, Raman scattering and Photoluminescence all indicate severe damage and even disordered structure induced by C+-implantation. The disordered lattice shows a partial recovery after annealing above 800 °C. Amorphous regions are observed by high resolution transmission electron microscopy measurement, which directly confirms that amorphization is induced by C+-implantation. The disordered GaN lattice is possibly due to special feature of carbon impurities, which enhance the damage buildup during implantation.

Preferential Cu precipitation at extended defects in bcc Fe: An atomistic study

As a starting point to understand Cu precipitation in RPV alloys, molecular dynamics and Metropolis Monte-Carlo simulations are carried out to study the effect of lattice defects on Cu precipitation by taking Fe–Cu system as a model alloy. Molecular dynamics simulations show that owing to the high heat of mixing and positive size mismatch, Cu is attracted by vacancy type defects such as vacancies and voids, and tensile stress fields. In accordance, preferential precipitation of Cu is observed in Metropolis Monte-Carlo simulations at dislocations, prismatic loops and voids. The interaction of Cu with a stress field, e.g., that associated with a dislocation or a prismatic loop, is dominated by elastic effect and can be well described by the linear-elasticity theory. For prismatic loops, the attraction to Cu is found to be size-dependent with opposite trends displayed by vacancy and interstitial loops. The size-dependences can be explained by considering the stress fields produced by these loops. The current results will be useful for understanding the effect of neutron irradiation on Cu precipitation in reactor-pressure-vessel steels.

Thermal Behavior of Residual Defects in Low-Dose Arsenic- and Boron-Implanted Silicon After High-Temperature Rapid Thermal Annealing

We investigated the thermal behavior of defects remaining in low-dose (<;10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) arsenic- and boron-implanted Si after high-temperature (1100 °C) rapid thermal annealing (RTA). The defects remaining after RTA were characterized as vacancy-type defects, and confirmed to be created by nonequilibrium states that occur during the extremely rapid cooling step of the RTA sequence. They were gradually removed by applying additional furnace annealing (FA) (i.e., thermal equilibrium heating process) at 300-400 °C. At the range of 500-600 °C, however, carbon- and oxygen-related point defects were newly created. These defects were confirmed to be eliminated at 700 °C, and the crystal quality was significantly improved. When using a rapid thermal process for heat treatment after low-dose impurity implantation, it is necessary to apply an equilibrium thermal treatment at >700 °C to remove residual damage as well as to activate impurities.

Synergistic effect of oxygen and nitrogen functionalities for graphene-based quantum dots used in photocatalytic H2 production from water decomposition

Surface intact nitrogen-doped graphene oxide quantum dots (iNGO-QDs), which are based on the abundant elements C, H, O, and N, exhibit outstanding photocatalytic performance in H2 production through water decomposition. In this study, we synthesize the iNGO-QDs by thermally treating graphene oxide sheets in NH3 with subsequent ultrasonic exfoliation. The iNGO-QDs exhibit sizes of 2 to 6nm and consist of approximately 3 graphitic layers. Nitrogen doping in QD synthesis repairs the vacancy-type defects of GO and introduces n-type conductivity to compensate for the unbalanced charges on p-type GO, thereby suppressing leaks of photogenerated charges. Coexistence of p- and n-domains in the iNGO-QDs may form photochemical diodes to facilitate charge separation. Under visible light illumination, the iNGO-QDs steadily catalyze the production of H2 from an aqueous solution containing triethanolamine. When deposited with Pt as the cocatalyst, the iNGO-QDs exhibit high activity in H2 generation at an apparent quantum yield of 12.8% under monochromatic light (420nm) irradiation. The high activity of the iNGO-QDs can be attributed to the synergistic effect of the oxygen and nitrogen functionalities in facilitating charge separation and transfer.

Effect of incorporation of deuterium on vacancy-type defects of a-C:H films prepared by plasma CVD

Amorphous deuterated carbon (a-C:D) films were prepared using plasma-enhanced chemical vapor deposition (PECVD) from CH4/CD4 and CH4/D2 source gases. For CH4/CD4, the gas flow ratio of CD4/(CD4+CH4) was varied from 0 to 100%. For CH4/D2, the additional partial gas pressure of D2 was increased from 1 to 7Pa as the flow rate increased, and the partial gas pressure and the flow rate of CH4 were maintained at 10Pa and 10sccm, respectively. The concentrations of hydrogen (H) and deuterium (D) relative to carbon (C) in the films and the film densities were determined by elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy (RBS). Positron annihilation spectroscopy (PAS) was performed to measure the vacancy-type defect of the film. The S value obtained from PAS measurement correlates to the vacancy-type defect in the film. The film hardness was also measured using a nanoindenter.For CH4/CD4, the D concentration in the film increased when the CD4/(CH4+CD4) gas ratio increased, whereas the concentration of H decreased. For CH4/D2, the D concentration in the film increased with increasing D2 partial pressure, whereas the concentration of H decreased. From the PAS results, the S value increased with increasing CD4/(CH4+CD4) gas ratio for CH4/CD4, whereas the S value did not change with any D2 partial pressure for CH4/D2. The hardness and the mass density of the films decreased when the CD4/(CH4+CD4) gas ratio increased for CH4/CD4, whereas the hardness and the density did not change with any D2 partial pressure for CH4/D2. A correlation among the S value, the film hardness and the film density was observed, and the S value, the film hardness and the film density did not correlate to the D concentration in the film. These findings suggest that information about the vacancy-type defect of the hydrogenated amorphous carbon films is crucial for evaluation of their mechanical properties and density.

Enhanced electrochemical performance of ion-beam-treated 3D graphene aerogels for lithium ion batteries

High energy light-ion (3.8MeV He) bombardment is used to introduce lattice defects in a 3-dimensional (3D) interconnected network of graphene aerogels (GAs). When these materials are used as anodes for lithium ion batteries, we observe improved percentage reversible capacity and cycle stability compared to those without ion-beam treatment. Furthermore, all ion-beam treated 3D graphene samples exhibit substantially higher Coulombic efficiencies, suggesting at beneficial role of vacancy-type defects in stabilizing solid-electrolyte interphases. Although 3D graphene exhibits initial reversible capacities that are 2–3 times higher than that of graphite (∼372mAh/g), fast capacity fading is observed but becomes more stable after ion-beam treatment. Our experimental results demonstrate that ion-beam treatment is an effective route to tune and produce good-performance graphene electrodes, and that vacancy-type defects help to promote reversible lithium storage capacity in graphene. We further observe that 3D GAs irradiated to the highest dose studied (1016cm−2) fail rapidly upon electrochemical cycling, likely caused by the excessive ion-beam damage and graphene restacking. Raman I(D)/I(D′) signature is considered linked to defect type in graphene and thus is proposed, for the first time, as an indicator of the reversible capacity for GAs.

Solid-State Reaction and Vacancy-Type Defects in Bilayer Fe/Hf Studied by the Slow Positron Beam

The positron annihilation lifetimes and the Doppler broadening by slow positron beam are measured in thin Fe films with thickness 500 nm, a thin Hf film with thickness 100 nm, and the bilayer Fe (50 nm)/Hf (50 nm) on quartz glass substrate. We have analyzed the behavior in vacancy-type defects in each layer through some deposition temperatures and annealing. It is observed that the thin Fe film, the thin Hf film, and the bilayer Fe (50 nm)/Hf (50 nm) already contain many vacancy-type defects. We have investigated the change of densities of the vacancy-carbon complex and the small vacancy-cluster with carbons, through solid-state amorphization of Fe (50 nm)/Hf (50 nm) bilayer.

Positron annihilation studied defects and their influence on thermal conductivity of chemically synthesized Bi2Te3 nanocrystal

Bismuth telluride (Bi2Te3) and its alloys are regarded as the best thermoelectric materials available nowadays at room temperature and can be well prepared by using existing technology. In this paper, Bi2Te3 nanocrystals are prepared by hydrothermal method and then treated by a spark plasma sintering (SPS) process at five temperatures of 300, 350, 400, 450 and 500 ℃ each for 5 min under a pressure of 20 MPa. X-ray diffraction (XRD) and positron annihilation spectroscopy are used to study the microstructures of the samples after SPS treatment at different temperatures. According to the XRD patterns, the diffraction peaks of the as-grown powder are consistent with those indicated in the standard card for Bi2Te3, which confirms successful synthesis of Bi2Te3 powders. Scanning electron microscope images show that the particles of all the samples take on flake-like structures, and the particle sizes increase from about 100 nm to a few m with the sintering temperature increasing from 350 to 500 ℃. This suggests significant reorganization of nanograins in sintering process, and some grains are agglomerated into larger particles. However, the grain sizes estimated from the X-ray diffraction peaks show little change in all the samples sintered at temperatures between 300-500 ℃. And most of the grains have sizes around 30 nm. Positron lifetime spectra are measured for Bi2Te3 samples sintered at different temperatures. The measurements reveal vacancy defects existing in all the sintered samples. With the increase of sintering temperature, there appears no significant change in trapped positron lifetime (2). This suggests that the defect size has no change during sintering. However, intensity I2 decreases monotonically with increasing sintering temperature, which indicates the lowering of vacancy concentration. The average positron lifetime shows a monotonous decrease with increasing sintering temperature, which indicates the recovery of vacancy defects at higher sintering temperatures. The thermal conductivity of the sample increases from 0.3 Wm-1K-1 to about 2.4 Wm-1K-1 with the sintering temperature increasing from 300 to 500 ℃. Since the lattice thermal conductivity dominates the total thermal conductivity, it can be inferred that sintering at higher temperature leads to the increase of lattice thermal conductivity. According to the positron annihilation lifetime result, the vacancy defects in the interface region gradually recover after sintering at higher temperatures. This shows good correlation with the increase of lattice thermal conductivity, indicating that vacancy-type defects are effective phonon scattering centers for Bi2Te3.

Characterization of vacancy-type defects in semiconductor device materials by means of positron annihilation

Characterization of vacancy-type defects in semiconductor device materials by means of positron annihilation

Formation of vacancy-type defects in titanium nickelide

In this report we briefly review the current state-of-the-art and challenges in determining point defect properties from first-principles calculations as well as from experimental measurements in titanium nickelid. . Based on the vacancy formation energy and their activation energy for vacancy migration in TiNi, vacancy mediated diffusion mechanism was examined. The behavior of vacancy defects in the TiNi structural phase transition has been described.

Compensating vacancy defects in Sn- and Mg-dopedIn2O3

MBE-grown Sn- and Mg-doped epitaxial ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ thin-film samples with varying doping concentrations have been measured using positron Doppler spectroscopy and compared to a bulk crystal reference. Samples were subjected to oxygen or vacuum annealing and the effect on vacancy type defects was studied. Results indicate that after oxygen annealing the samples are dominated by cation vacancies, the concentration of which changes with the amount of doping. In highly Sn-doped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$, however, these vacancies are not the main compensating acceptor. Vacuum annealing increases the size of vacancies in all samples, possibly by clustering them with oxygen vacancies.

Investigation of vacancy-type defects in helium irradiated FeCrNi alloy by slow positron beam

The evolution of microstruture for Fe16.7Cr14.5Ni model alloy and 316 stainless steel irradiated with 140keV He ions were studied by Positron annihilation spectroscopy. The fluences were 1×1016 and 5×1016 Heions/cm2. The irradiation temperature was room temperature and 573K, respectively. The variation of S parameter-incident positron energy profile indicated that large amount of vacancy-type defects formed after He ion irradiation. Meanwhile, helium atoms deposited in bulk and certain amount of He-vacancy complexes were formed. The vacancy-type defects could be the major defects in track region and He-vacancy complexes would be the main defects in cascade region. The vacancy-type defects could migrate and aggregate to form vacancy clusters and even microvoids at elevated temperature irradiation. The diffusion mechanism of helium atoms might be changed at different irradiation temperature.

Positron annihilation Doppler broadening spectroscopy study on Fe-ion irradiated NHS steel

In order to study the evolution of irradiation-induced vacancy-type defects at different irradiation fluences and temperatures, a new type of ferritic/martensitic (F/M) steel named NHS (Novel High Silicon) was irradiated by 3.25MeV Fe-ion at room temperature and 723K to fluences of 4.3×1015 and 1.7×1016ions/cm2. After irradiation, vacancy-type defects were investigated with variable-energy positron beam Doppler broadening spectra. Energetic Fe-ions produced a large number of vacancy-type defects in the NHS steel, but one single main type of vacancy-type defect was observed in both unirradiated and irradiated samples. The concentration of vacancy-type defects decreased with increasing temperature. With the increase of irradiation fluence, the concentration of vacancy-type defects increased in the sample irradiated at RT, whereas for the sample irradiated at 723K, it decreased. The enhanced recombination between vacancies and excess interstitial Fe atoms from deeper layers, and high diffusion rate of self-interstitial atoms further improved by diffusion via grain boundary and dislocations at high temperature, are thought to be the main reasons for the reversed trend of vacancy-type defects between the samples irradiated at RT and 723K.