Metastable Vacancy Research Articles

Photoinduced Generation of Metastable Sulfur Vacancies Enhancing the Intrinsic Hydrogen Evolution Behavior of Semiconductors

Hydrogen evolution over pristine semiconductors is desirable but seldom realized in powdered photocatalysis. It requires the catalyst surface simultaneously possessing efficient electron transfer and rapid H2 production properties. The current semiconductor photocatalysts have to depend on additional cocatalysts to achieve the H2 evolution process. Herein, theoretical and experimental results demonstrate that metastable sulfur vacancy could significantly enhance the intrinsic H2 evolution behavior of semiconductors. The hydrogen adsorption free energy (ΔGH) of CdS could be optimized to ΔGH = 0.01 eV, much lower than that over thermodynamically stable vacancies (ΔGH = 0.31 eV). The experiment is conducted based on kinds of supported CdS nanoparticles prepared with the anion‐exchange method. A series of in situ characterizations disclose that a metastable sulfur vacancy forms under photoexcitation and is stable during the reaction. These metastable sulfur vacancies cause the formation of intermediate states between the valence band and the conduction band that increase transportation and utilization of photogenerated electrons. The conceptual finding of the critical role of the metastable vacancy in enhancing H2 evolution would bring new thinking on the design of semiconductor photocatalysts to be less dependent on cocatalysts.

First-principles calculations of solute transport in zirconium: Vacancy-mediated diffusion with metastable states and interstitial diffusion

Zirconium alloys are the most widely used nuclear fuel cladding materials for light water power reactors where irradiation damage causes solute redistribution, leading to degradation of alloy properties such as corrosion resistance. Designing radiation-tolerant zirconium alloys requires a thorough understanding of the atomic-scale transport behavior of the alloying elements in Zr. We perform density function theory calculations to investigate the diffusion of Sn, Cr, Fe, Be, Al, and Ni in the hexagonal close-packed (HCP) Zr matrix. We develop a methodology to accurately model the metastable vacancy states along the basal migration path, known to occur in group IV metals. We compute the vacancy-mediated solute diffusion coefficients and drag ratios using the kinetic Monte Carlo method and an analytic Green's function method---the agreement between the two validates our methodology. The computed diffusion coefficients of Sn and Al show good agreement with the experimental data and we expect these solutes to diffuse via the vacancy-mediated mechanism. We use a Green's function approach, parameterized with data from density functional theory calculations, to compute the interstitial diffusion coefficients of Cr, Fe, Be, and Ni in the HCP Zr lattice. The computed diffusion coefficients of Cr, Ni, and Be agree with the experimental measurements within one order of magnitude, while those of Fe are within two orders of magnitude of the experimental measurements. The drag ratios for Cr, Fe, Be, and Ni are positive up to 1235 K, which suggests that nonequilibrium vacancy fluxes could drag these solutes toward sinks such as dislocation loops and grain boundaries. We also compute the transport coefficients without including the metastable states, and using the eight- and thirteen-frequency model. Our results show significant differences in drag ratio for the eight- and thirteen-frequency model predictions compared with the Green's function methodology, but smaller errors in the solute diffusivity. Combining interstitial and vacancy-mediated diffusivities, we predict the unusual result that increased vacancy concentration slows down solute diffusivity, while a sufficiently high vacancy concentration can change the dominant mechanism to an accelerated vacancy-mediated diffusion.

Thermodynamics of interfacial interactions in composites based on nanodispersed NiO and methacrylic acid derivatives

Interfacial interactions in composites based on nanodispersed nickel oxide and methacrylate polymer matrices are studied via isothermal microcalorimetry. The thermochemical cycle is used to estimate the concentration dependences of the enthalpies of mixing of nickel oxide with, respectively, poly(methyl methacrylate), poly(butyl methacrylate), poly(acrylic acid), poly(methacrylic acid), and the butyl methacrylate copolymers containing 1 and 5 mol % methacrylic acid units. On the basis of these data, the maximum enthalpies of adhesion and the parameters of the glassy structure of the polymers are calculated. It is shown that the increase in the absolute values of the maximum enthalpy of adhesion is related to a gain in the fraction of carboxyl groups in the macromolecular chain. Strengthening of interfacial interactions for glassy polymers correlates with an increase in the fraction of metastable vacancies and in the thickness of the adhesion glassy layer.

Sub-Bandgap Light-Induced Carrier Generation at Room Temperature in Silicon Carbide MOS Capacitors

Carrier generation characteristics in n-type substrate SiC MOS capacitors induced by sub-bandgap energy light are reported. The generation rate is high enough to create an inversion layer in ~20 minutes with monochromatic light (front side illumination) of energy 2.1 eV in 4H-SiC for electric fields smaller than 1 MV/cm. Generation and recovery results strongly indicate involvement of a metastable defect whose efficiency as a generation center increases under hole-rich and decreases under electron-rich conditions. The generation dependence on bias history and light energy shows the defect to have properties consistent with the metastable silicon vacancy / carbon vacancy-antisite complex (VSi / Vc - CSi).

Positive Gate-Bias Temperature Stability of RF-Sputtered $\hbox{Mg}_{0.05}\hbox{Zn}_{0.95}\hbox{O}$ Active-Layer Thin-Film Transistors

This paper investigates the positive gate-bias temperature stability of RF-sputtered bottom-gate Mg0.05Zn0.95O active-layer thin-film transistors (TFTs) annealed at 200 °C for 5 h and 350 °C for 30 min. Although the TFT devices initially exhibited similar electrical characteristics, the TFTs annealed at 350 °C demonstrated stability characteristics superior to those annealed at 200 °C. This result is due to the improved crystallinity and more stable phase with greater proportion of Zn replaced by Mg in the ZnO crystals. The results also reveal a hump shape in the subthreshold region of the transfer characteristics, which is induced by the positive gate-bias stress at elevated temperatures. The hump phenomenon was suppressed in the TFT annealed at 350 °C. The hump disappeared shortly after removing the positive gate bias, suggesting that this phenomenon was meta-stable and resulted from gate-bias-induced electric field. One possible mechanism responsible for the hump formation in the transfer curve is the gate-field-induced back-channel parasitic transistor. Alternatively, this hump phenomenon might have been due to the creation of meta-stable vacancies in which the neutral defects were thermally excited and released electrons into the active layer to form a leakage path when the TFTs were subjected to gate-bias stress at elevated temperatures.

Investigation and Generation of Vacancies in Alpha Quartz by Soft X-Rays

with electrons and electric dipole of atoms transitions can generate metastable vacancies, point defects, and induce chemical reactions. Usually, for point defects generation doses of gamma rays are used. We measured values of the Bragg reflections of X-rays and calculated mean square deviations of atoms in crystal lattice for defining the dynamics of irradiated point defects. We accomplished infrared measurements for establishing of generated chemical reactions, and conductivity measurements were also done.

Superdiffusion in Si Crystal Lattice Irradiated by Soft X-Rays

We considered the reasons of superdiffusivity and measured profiles of boron and phosphorus in crystalline silicon at room temperature. The superdiffusivity or ultrafast diffusion of metastable vacancies at room temperature in Si crystal irradiated by soft X-rays was obtained experimentally. In this work, we presented experimentally obtained diffusion coefficients of singly and doubly negatively charged long-lived excited vacancies. These high concentration charged metastable vacancies (about 10 cm−3) at room temperature can very fast diffuse changing electrical conductivity and the Hall mobility of carriers. We measured the superdiffusivity of negatively charged vacancies, generated by the Auger effect in the regions of the sample, which were not affected by X-rays. In this paper, we presented the obtained superdiffusion profiles of boron and phosphorus in crystalline silicon measured with secondary-ion mass spectrometer.

Li Ion Diffusion Mechanisms in the Crystalline Electrolyte γ-Li3PO4

Solid state lithium ion electrolytes are becoming increasingly important in batteries and related technologies. We have used first-principles modeling techniques based on density functional theory and the nudged elastic band method to examine possible Li ion diffusion mechanisms in idealized crystals of the electrolyte material . In modeling the Li ion vacancy diffusion, we find direct hopping between neighboring metastable vacancy configurations to have a minimal migration barrier of . In modeling the Li ion interstitial diffusion, we find an interstitialcy mechanism, involving the concerted motion of an interstitial Li ion and a neighboring Li ion of the host lattice, that can result in a migration barrier as low as . The minimal formation energy of a Li ion vacancy-interstitial pair is determined to be . Assuming the activation energy for intrinsic defects to be given by , the calculations find for ionic diffusion in crystalline , in good agreement with reported experimental values of .

Crystal Lattice and Carriers Hall Mobility Relaxation Processes in Si Crystal Irradiated by Soft X-rays

We applied soft X-rays for investigation of dynamics of the Frenkel point defects in a Si crystal during its saturation with metastable vacancies with neighboring Si atoms in excited states produced in the lattice after ejection of the Auger electrons. The irradiated irregularities and defects of the lattice cause a change of the Bragg reflection maxima. Several resonance phenomena are related to metastable states introduced into Si crystal by soft X-rays irradiation. The resonance of mean square displacements of Si atoms in the lattice and the resonance of the Hall mobility after irradiation are obtained and considered.

Investigation of Compound Relaxation Processes in Crystal Lattice Dynamics of Si Irradiated by Soft X-rays

We applied soft X-rays for investigation of dynamics of Frenkel point defects in a Si crystal during its saturation with metastable vacancies with neighboring Si atoms in excited states or vacancies with neighboring Si atoms in interstitial states produced in the lattice after ejection of Auger electrons. The irradiated irregularities and defects of the lattice cause a change of Bragg reflection maxima. Several resonance phenomena related to the metastable states introduced into Si crystal by soft X-rays irradiation have been detected.

Nonlinear diffusion in excited Si crystals

We discuss the properties of the nonlinear diffusion equation for the case of diffusion in excited systems. The diffusion coefficient is directly proportional to the concentration of impurities and depends on the time in a special way. For the description of the excited systems, we used a special temperature function, which defined the time dependent diffusion coefficient and the Boltzmann's distribution of the excited vacancies or impurity atoms in solids. This model was used for approximation of very fast diffusion of metastable vacancies irradiated by soft X-rays in an excited Si crystal.

Nonlinear Diffusion in Excited HgCdTe and Si Crystals

We discuss the properties of the nonlinear diffusion equation for the case of diffusion in excited systems. The diffusion coefficient is directly proportional to the concentration of impurities and depends on time in a special way. For the description of the excited systems, we used a special temperature function, which defined the time dependent diffusion coefficient and the Boltzmann distribution of the excited vacancies or impurity atoms in solids. This model was used for the approximation of indium concentration profiles in HgCdTe of a rapid diffusion component and very fast diffusion of metastable vacancies irradiated by soft X-rays in an excited Si crystal.

Observation of Andreev-Pushkarov vacancy clusters in phase-separated solid solutions of 4He in 3He

New features are observed for the pressure in a phase-separated dilute solid solution of 4He in 3He subjected to multiple temperature cycling within the phase-separation region. The results are explained within the framework of the hypothesis of A.F. Andreev and D.I. Pushkarov that the vacancies in a crystal without ideal periodicity are surrounded by clusters with a periodic structure. The equation for determining the radius of a cluster of pure 4He in a solution of 4He in 3He is refined. This hypothesis is shown to provide quantitative agreement between the calculated and experimental data under the assumption that the homogenization of the phase-separated solution is accompanied by the formation of metastable vacancies with a concentration of ∼(4–5)×10−5.

Device-relevant point defects in GaAs and InP

Positron annihilation is used to determine the vacancy ionization levels and the atomic structure of acceptors, vacancy-type or ion-type, in GaAs and InP materials. Positron annihilation yields information on the size of the vacancy open volume via the annihilation lifetime in the vacancy τ V, on its charge state via the positron trapping coefficient μ V( T) and the concentration C V via the positron trapping rate K V = μ V C V. It is shown that V(As) and V(Ga) vacancies play a role as compensating centers in semi-insulating (SI) and n-type GaAs. V(As) vacancies absorb near-band edge light and interact with holes or electrons depending on their charge state V(As) +, V(As) 0 or V(As) −. The conversion EL2 to EL2 * generates metastable vacancies which are part of the EL2 * structure.

Infrared absorption related to the metastable state of arsenic antisite defects in electron-irradiated GaAs.

Positron-annihilation and infrared-absorption experiments have been performed to study the metastable defects in electron-irradiated semi-insulating GaAs. The positron results show that an irradiation-induced metastable vacancy can be generated by illumination with 1.1-eV light at 25 K. The vacancy can be optically recovered by either 0.85- or 1.3-eV photons at 25 K. These generation and recovery bands are identical to those detected earlier for irradiation-induced As antisite defects in magnetic circular dichroism of absorption experiments. We thus conclude that the metastable vacancy belongs to the atomic structure of the metastable state of the As antisite. The metastable As antisite absorbs infrared light, suggesting that its ionization level is in the energy gap and not in the conduction band as in as-grown GaAs. This difference can be explained by the presence of other defects complexed with the As antisite in electron-irradiated GaAs.

Introduction of Metastable Vacancy Defects in Electron Irradiated Semi-Insulating GaAs

Positron-lifetime experiments have been performed to investigate the metastability of the point defects produced in the electron irradiation of semi-insulating GaAs. The measurements in darkness indicate the presence of Ga vacancies and Ga antisite defects in a negative charge state. Illumination at 25 K reveals another type of a defect, which has a vacancy in its metastable state. The metastable vacancies can be observed most effectively after illumination with 1.1-eV photons and they are persistent up to the annealing temperature of 80--100 K. The introduction rate of the metastable defects is about 0.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which is close to the values reported earlier for the As antisite. The metastable properties of the defects resemble those of the well-known EL2 center in as-grown GaAs. We associate these defects to As antisites, which exhibit the metastability predicted by the theory: in the metastable configuration the As antisite atom relaxes away from the lattice position, leaving a Ga site vacant.

Introduction of metastable vacancy defects in electron-irradiated semi-insulating GaAs.

Positron-lifetime experiments have been performed to investigate the metastability of the point defects produced in the electron irradiation of semi-insulating GaAs. The measurements in darkness indicate the presence of Ga vacancies and Ga antisite defects in a negative charge state. Illumination at 25 K reveals another type of a defect, which has a vacancy in its metastable state. The metastable vacancies can be observed most effectively after illumination with 1.1-eV photons and they are persistent up to the annealing temperature of 80--100 K. The introduction rate of the metastable defects is about 0.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which is close to the values reported earlier for the As antisite. The metastable properties of the defects resemble those of the well-known EL2 center in as-grown GaAs. We associate these defects to As antisites, which exhibit the metastability predicted by the theory: in the metastable configuration the As antisite atom relaxes away from the lattice position, leaving a Ga site vacant.

Metastable vacancy in the EL2 defect in GaAs studied by positron-annihilation spectroscopies.

We have performed positron-lifetime and Doppler-broadening experiments before and after illumination of undoped semi-insulating GaAs with 0.7--1.5-eV photons. When the EL2 defect is transformed to the metastable state by 1.1--1.3-eV photons, we observe a metastable vacancy defect. Its concentration is proportional to the EL2 concentration. The metastable vacancy has smaller open volume than isolated Ga and As vacancies and it is connected to a negative charge. The vacancy is generated with an optical cross section of 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$ at the photon energy of 1.15 eV and it recovers at 120 K. As these properties are identical to those of the metastable state of EL2, we conclude that the observed vacancy is involved in its atomic configuration. The results are in good agreement with the isolated As antisite model for the structure of the EL2 defect.

Spin polarons in solid3He: Suggestions for further experiments

A proposal is made for a simple way to test experimentally whether anomalies in the low temperature thermodynamic properties of solid3He are due to metastable trapped vacancies in spin polarons. The idea is to perform susceptibility measurements before and after applying a strong magnetic field for some time, with the aid of a SQUID or NMR. If this magnetic annealing reduces the susceptibility, it provides evidence for the existence of metastable trapped vacancies in the original sample. With this procedure it may also be possible to clarify the issue of the existence of a “vacancy solid.” Finally, the effects of the tendency of vacancies to cluster are discussed.

Vacancy ordered phases and one-dimensional quasiperiodicity

The metastable vacancy ordered phases observed in aluminium transition metal alloys on rapid solidification or vapour deposition can be considered as a periodic arrangement of a truncated quasiperiodic string based on the Fibonacci sequence along the 〈111〉 stacking direction of the original CsCl cell. Using the projection formalism developed in the context of quasicrystals, the diffraction patterns of the vacancy ordered phases are calculated for both commensurate and incommensurate projection from a periodic cubic cell in four dimensions. These are compared with experimentally observed patterns. It is shown that at increasingly longer periodicity the patterns from commensurate crystals become indistinguishable from the truly quasiperiodic one. It is suggested that there is a strong link between vacancy ordered phases and quasicrystals.