Elimination Of Vacancies Research Articles

Unveiling the NIR modulation performance enhancement of VO2 endowed by oxygen vacancy elimination

Vanadium dioxide has emerged as a promising material for smart windows owing to the temperature-responsive variable near-infrared (NIR) transmittance. Yet, the poor NIR modulation ability challenges its efficiency in thermal management. In this study, by meticulously controlling the oxygen vacancy content at a low level, VO2 nanoparticles with excellent NIR modulation performance are achieved. Oxygen vacancy (VO) defects elimination leads to a remarkable decrease of reflectance in the monoclinic (M) phase, dramatically enhancing the near-infrared contrast of VO2 by 154 %. Density functional theory (DFT) calculations reveal that VO elimination favors low refractive index in the NIR region. The optimized experiment is carried out to prepare VO2 nanoparticles with low defects and high crystallinity. It shows the best NIR transmittance contrast at 1500 nm (ΔT1500 nm) of 24.4 %, simultaneously keeping a high luminous transmittance (Tlum) of 79.7 %. This study is believed to provide valuable guidance for the current defect and thermochromic performance study of VO2.

Potassium Iodide Doping for Vacancy Substitution and Dangling Bond Repair in InP Core-Shell Quantum Dots.

This work highlights the novel approach of incorporating potassium iodide (KI) doping during the synthesis of In0.53P0.47 core quantum dots (QDs) to significantly reduce the concentration of vacancies (i.e., In vacancies; VIn-) within the bulk of the core QD and inhibit the formation of InPOx at the core QD-Zn0.6Se0.4 shell interfaces. The photoluminescence quantum yield (PLQY) of ~97% and full width at half maximum (FWHM) of ~40 nm were achieved for In0.53P0.47/Zn0.6Se0.4/Zn0.6Se0.1S0.3/Zn0.5S0.5 core/multi-shell QDs emitting red light, which is essential for a quantum-dot organic light-emitting diode (QD-OLED) without red, green, and blue crosstalk. KI doping eliminated VIn- in the core QD bulk by forming K+-VIn- substitutes and effectively inhibited the formation of InPO4(H2O)2 at the core QD-Zn0.6Se0.4 shell interface through the passivation of phosphorus (P)-dangling bonds by P-I bonds. The elimination of vacancies in the core QD bulk was evidenced by the decreased relative intensity of non-radiative unpaired electrons, measured by electron spin resonance (ESR). Additionally, the inhibition of InPO4(H2O)2 formation at the core QD and shell interface was confirmed by the absence of the {210} X-ray diffraction (XRD) peak intensity for the core/multi-shell QDs. By finely tuning the doping concentration, the optimal level was achieved, ensuring maximum K-VIn- substitution, minimal K+ and I- interstitials, and maximum P-dangling bond passivation. This resulted in the smallest core QD diameter distribution and maximized optical properties. Consequently, the maximum PLQY (~97%) and minimum FWHM (~40 nm) were observed at 3% KI doping. Furthermore, the color gamut of a QD-OLED display using R-, G-, and B-QD functional color filters (i.e., ~131.1%@NTSC and ~98.2@Rec.2020) provided a nearly perfect color representation, where red-light-emitting KI-doped QDs were applied.

Structure analysis of the defects generated by a thermal spike in single crystal CeO2: A molecular dynamics study

In the present paper, we have extensively analyzed the atomic structures generated by supplying a thermal spike to the single crystal CeO2 that we previously analyzed for specimens by the molecular dynamics method [Y. Sasajima et al., Nucl. Instr. Meth. B 314 (2013) 202]. Our analysis results were compared with the atomic structures obtained by the actual experiments using HAADF-STEM and ABF-STEM [S. Takaki et al., Nucl. Instr. Meth. B 326 (2014) 140]. Our simulation results and the experimental observations agreed well on the following point that the lattice of cerium is maintained while oxygen atoms migrate from their equilibrium cites. In particular, our simulation reproduced the distribution of the numbers of oxygen atoms obtained from the analysis of TEM images. The detailed process of generation and elimination of vacancies has also been analyzed as a function of time. We found that the number of vacancies was increased abruptly immediately after the thermal spike, and the number subsequently dropped through a relaxation process within 3 ps.

Depth-Resolved Carrier Lifetime Measurements in 4H-SiC Epilayers Monitoring Carbon Vacancy Elimination

We address the key factors limiting charge carrier lifetime in 4H-SiC epilayers by demonstrating a viable method for eliminating carbon vacancy (VC) related Z1/2 lifetime killer sites and by introducing a novel approach in depth-resolved characterization of the carrier lifetimes across the epitaxial layer, which allows monitoring the efficacy of the proposed defect reduction scheme also exposing surface and interface recombination effects. We show that a moderate-temperature annealing conducted at 1500 °C for 6 hours under C-rich thermodynamic equilibrium conditions in effect eliminates carbon vacancies in epilayers to the levels below the detection limit (1011 cm-3) of DLTS measurements. The efficient reduction of VC-related Z1/2 sites upon thermal treatment is further proven by a significant increase of the minority carrier lifetime from 0.3µs to 1 µs, the upper limit apparently set by epilayer thickness dependent lifetime. Equally important is the extensive range of defect elimination as evidenced by consistently enhanced lifetime throughout entire 40 μm-thick epilayer, thus suggesting immediate practical implication as a lifetime control method suitable for variable thickness 4H-SiC epilayers.

P-Type transition-metal doping of large-area MoS2 thin films grown by chemical vapor deposition.

Two-dimensional transition metal dichalcogenides (e.g. MoS2) have recently emerged as a promising material system for electronic and optoelectronic applications. A major challenge for these materials, however, is to realize bipolar electrical transport properties (i.e. both p-type and n-type conduction), which is critical for enhancing device performance and functionalities. Here, we demonstrate the transition metal zinc as a p-type dopant in the otherwise n-type MoS2, through systematic characterizations of large area Zn-doped MoS2 thin films grown by a one-step chemical vapor deposition (CVD) approach. Raman characterization and X-ray photoelectron spectroscopy studies identified millimeter-scale, monolayer films with 1-2% Zn as dopants. Zinc doping suppresses n-type conductivity in MoS2 and shifts its Fermi level downwards. The stability and p-type nature of Zn dopants were further confirmed by density-functional-theory calculations of formation energies and electronic band structures. The electrical transport properties of Zn-MoS2 films can be influenced by stoichiometry, and p-type gate transfer characteristics were realized by thermal treatment under a sulfur atmosphere. Our work highlights transition-metal doping followed by sulfur vacancy elimination in CVD grown films as a promising route for achieving large area p-type transition metal dichalcogenide films that are essential for practical applications in electronics and optoelectronics.

Formation and Annihilation of Carbon Vacancies in 4H-SiC

The carbon vacancy (VC) is a major point defect in high-purity 4H-SiC epitaxial layers limiting the minority charge carrier lifetime. In layers grown by chemical vapor deposition techniques, the VC concentration is typically in the range of 1012 cm-3 and after device processing at temperatures approaching 2000 °C, it can be enhanced by several orders of magnitude. In the present contribution, we show that the cooling rate after high-temperature processing has a profound influence on the resulting VC concentration where a slow rate promotes elimination of VC. Further, isochronal annealing of as-grown and as-oxidized epi-layers protected by a carbon-cap was undertaken between 800 °C and 1600 °C. The results reveal that thermodynamic equilibrium of VC is established rather rapidly at moderate temperatures, reaching a VC concentration of only a few times 1011 cm-3 after 40 min at 1500 °C. Hence, the concept of eliminating VC’s by annealing at moderate temperatures under C-rich equilibrium conditions shows great promise and enables re-annealing of high-temperature processed wafers, in contrast to the procedures commonly used today to eliminate VC. In-diffusion of carbon interstitials and out-diffusion of VC’s are discussed as the kinetics processes establishing the thermodynamic equilibrium

Radiation-accelerated precipitation in Fe–Cr alloys

The kinetics of phase separation in Fe–Cr alloys under irradiation is modeled by Atomistic kinetic Monte Carlo simulations that include the formation, migration and elimination of vacancies and self-interstitials at point defects sinks. The evolution of the sink density is modeled by cluster dynamics, and taken into account in the Monte Carlo simulations by a rescaling of the time. The results are in good agreement with available experimental observations of neutron irradiation at 290 °C. The irradiation is found to accelerate the kinetics of phase separation by orders of magnitude, with an acceleration factor given by the increase in point defect concentrations. The microstructure evolution is qualitatively the same as during isothermal annealing, except in the vicinity of point defect sinks. The effects of equilibrium and radiation induced segregations at grain boundaries are considered. The ballistic mixing occurring in displacement cascades is modeled, and is found to be insufficient to produce the dissolution of chromium rich precipitates at 290 °C, even at high dose rates. Therefore, it cannot explain the absence of precipitation observed during ion irradiations.

Elimination of vacancies in titanium monoxide under high pressure in combination with high temperature

The material design with controlled structure was performed on titanium monoxide with structural vacancies on both sublattices. The polycrystalline powder of disordered titanium monoxide TiO0.98 with B1 structure was subjected to high pressures up to 60 kbar and high temperatures up to 2273 K. The X-ray powder diffraction of as-prepared and treated powders revealed cubic single-phase crystals. The lattice constant of this phase increased from 417.6(3) to 418.5(3) pm due to decreasing vacancy concentration on the titanium sublattice from 13.3 to 8.5 at.% after a highest pressure-highest temperature treatment for 1 h.

Enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epitaxial layers by combination of thermal oxidation and hydrogen annealing

We investigated the enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epilayers (NA ≃ 2 × 1014 cm−3) by postgrowth processing. A carrier lifetime of 2.8 µs in an as-grown epilayer is increased to 5.1 µs by carbon vacancy elimination, i.e., thermal oxidation at 1400 °C for 48 h. It reaches 10 µs by subsequent hydrogen annealing at 1000 °C for 10 min. The carrier lifetime in the as-grown epilayer is also increased to 4.0 µs by only hydrogen annealing. These results suggest that, in addition to carbon vacancy, there is another lifetime killer in p-type SiC, which cannot be eliminated by thermal oxidation but can be passivated by hydrogen annealing.

Characterization of intrinsic hysteresis of pentacene-based organic thin-film transistor through in-situ real-time electrical measurement

The intrinsic hysteresis of a pentacene-based organic thin-film transistor was characterized through home-designed in-situ real-time electrical measurement. The device exhibited intrinsic hysteresis after the device fabrication without breaking the vacuum, which has not been observed previously. Similar behavior was observed when introducing the nitrogen gas. Compared with the measurement condition of vacuum or nitrogen gas, exposure to the ambient air resulted in a severe hysteresis. It was attributed to both the acceptor-like traps at the organic/dielectric interface and the donor-like traps in the transport channel. When the chamber was vacuumed out again, a significantly reduced hysteresis was obtained almost the same as that measured just after device fabrication, indicating the reversibility of the extrinsic hysteresis. We also related the hysteresis to the morphological change under different deposition rates of pentacene. The smoother surface at higher deposition rate caused reduced hysteresis because of the elimination of vacancies near the pentacene/dielectric interface.

Self-healing mechanism of irradiation defects near [formula omitted] grain boundary in copper

Exploring the mechanism for elimination of vacancies and interstitial defects around grain boundaries is of special significance for nuclear materials. Formation energies and diffusion barriers of point defects at different sites near Σ=11(113) grain boundary in copper have been calculated by first principle in order to investigate the interactions between grain boundaries and defects. The results show that grain boundaries (GBs) facilitate the diffusion of both vacancy and interstitial defects to GBs, and the interstitials are more active than the vacancies in this kind of diffusions. Furthermore, with low energy barrier, interstitials could be emitted from the grain boundary and annihilate bulk vacancies, indicating that grain boundaries can play a role of the sink for both radiation-induced defects and significantly improve the radiation resistance of the material.

Effect of amorphous FePO4 coating on structure and electrochemical performance of Li1.2Ni0.13Co0.13Mn0.54O2 as cathode material for Li-ion batteries

Lithium-rich layered cathode Li1.2Mn0.54Ni0.13Co0.13O2 is synthesized by a co-precipitation method followed by high-temperature treatment and surface coated with different amount of amorphous FePO4. The microstructure and electrochemical performance of the as-prepared cathode materials are investigated systematically. It is demonstrated that the Li1.2Mn0.54Ni0.13Co0.13O2 particles are uniformly coated with amorphous FePO4. With proper amount of amorphous FePO4 coating layer, significant improvements in discharge capacity, initial Coulombic efficiency, rate capability, cycle performance, and thermal stability are achieved at room temperature. Specifically, the 3 wt.% FePO4-coated cathode exhibits the highest discharge specific capacities (271.7 mAh g−1 at C/20), improved initial Coulombic efficiency (85.1%), and best cyclability (discharge capacity of 202.6 mAh g−1 at C/2 after 100 cycles), while the 1 wt.% FePO4-coated cathode displays the best rate capability (194.3 mAh g−1 at 1 C rate and 167.9 mAh g−1 at 2 C rate). The charge–discharge curves and electrochemical impedance spectra reveal that the improved electrochemical performances are due to the suppression of both the oxygen vacancy elimination at the end of the first charge and side reactions of the cathode with the electrolyte, as well as the decrease in charge transfer polarization by the FePO4 coating layer.

Effect of the amido Ti precursors on the atomic layer deposition of TiN with NH3

The effect of the amide Ti precursors, tetrakis dimethyl amido titanium (TDMAT), tetrakis ethylmethyl amido titanium (TEMAT), and tetrakis diethyl amido titanium (TDEAT) on the atomic layer deposition of TiN film with ammonia was studied. Surface decomposition mechanism of each precursor was studied with in-situ Fourier transform infrared spectroscopy. It was confirmed that ethyl ligand in the precursor was more stable than methyl and the surface decomposition temperature of TDMAT, TEMAT, and TDEAT was 175, 200, and 250 °C on the SiO2 surface, respectively. The resistivity of the film was decreased with the increase in the substrate temperature due to the film crystallization. The TiN film deposited with TDMAT gave the lowest resistivity even though the atomic layer deposition temperature window was lowest due to the largest amount of carbon incorporation. It was confirmed that carbon incorporation leads to TiC formation and suppressed the postdeposition oxygen uptake possibly due to the elimination of vacancy in the film.

Centric and Non-centric Ca3Au∼7.5Ge∼3.5: Electron-Poor Derivatives of La3Al11. Syntheses, Structures, and Bonding Analyses

Two La(3)Al(11) type derivatives have been discovered in the Ca-Au-Ge system and structurally characterized by single-crystal X-ray diffraction. Compositions Ca(3)Au(7.16(6))Ge(3.84(6)) (1) and Ca(3)Au(7.43(9))Ge(3.57(9)) (2) lie within a non-centric Imm2 phase region with a approximately 4.40 A, b approximately 13.06 A, c approximately 9.60 A. The Au-richer and electron-poorer Ca(3)Au(7.50(1))Ge(3.50(1)) (3) and Ca(3)Au(8.01(1))Ge(2.99(1)) (4) occur within a centric Pnnm phase region with a approximately 9.50 A, b approximately 13.20 A, c approximately 4.43 A. Both phases contain complex [Au,Ge](11)(6-) polyanionic networks made up of hexagonal and pentagonal prisms that are filled with the electropositive Ca atoms. Both 3:11 phases represent opposed 1 x 3 x 1 superstructure distortions of CaAu(2)Ge(2) (ThCr(2)Si(2) type, I4/mmm), the structure of which has also been re-determined in this work. Linear muffin-tin-orbital (LMTO) calculations reveal that the symmetry variations induced by changes of the Au contents in the present 3:11 phases are consequences of bonding and structural optimizations. The hypothetical "CaAu(2.33-2.67)Ge(1.33-1.00)cube(0.33)" compositions, which are close to those of 1-4, follow through creation and elimination of vacancies within the electronegative networks of CaAu(2)Ge(2).

Theoretical models for work function control (Invited Paper)

We have studied Fermi level pinning (FLP) of Hf-based high- k gate stacks based on thermodynamics based on an O vacancy model. Our study shows that FLP cannot be avoided when the system is under thermal equilibrium. O exposure to aim O vacancy elimination is not effective, since O vacancy elimination condition is equivalent to the Si substrate oxidation which leads to the increase in Equivalent oxide thickness (EOT). We also studied the mechanism of FLP induced by the reduction with H 2 anneal. FLP with H 2 anneal is governed by the O vacancy annihilation reaction by reducing SiO 2 interface layer. Based on these considerations, we propose some recipes for obtaining band-edge-work-function metals.

Theoretical Studies on Fermi Level Pining of Hf-Based High-k Gate Stacks Based on Thermodynamics

We have studied Fermi level pinning of Hf-based high-k gate stacks based on thermodynamics by our O vacancy model. Our study shows that FLP cannot be avoided when the system is under thermal equilibrium. O injection to aim O vacancy elimination is hopeless, since O vacancy elimination condition is equivalent to the Si substrate oxidation which leads to the increase in EOT. We also studied the mechanism of FLP induced by H2 anneal. FLP by H2 anneal is governed by the O vacancy annihilation reaction by reducing SiO2 interface layer. Moreover, we briefly discuss the recipe for obtaining band-edge-work-function metals.

Numerical Model for Oxide Scale Growth with Explicit Treatment of Vacancy Fluxes

In the framework of research on behaviour of nuclear waste containers, to evaluate the effects of possible evolution of experimental conditions, as well as evolution of parameters controlling oxidation rate during long-term interim storage, a numerical model has been developed in order to take into account non-stationary states. To anticipate effects like cold working of the metal on the scale growth kinetics and risks of scale detachment by over saturation of vacancies at the metal/oxide interface in the course of scale growth, the model is based on the calculation of chemical species, but also vacancies profiles evolution in the oxide and the metal following a simple time integration. An original numerical treatment is proposed to easily describe elimination of vacancies by introducing sink strength in the metal. The first calculations are presented and discussed.

First stage of the structural evolution of austenite in Cu-Al-Ni shape memory alloys

Two shape memory Cu-Al-Ni alloys, a polycrystal and a single crystal, exhibiting a martensitic transformation close to 130 °C (in the as-quenched state) have been studied. Specimens have been quenched after heat treatment at 850 °C. The structural evolutions of the high temperature phase (austenite) have been studied for thermal treatments performed below 200 °C. Investigations have been carried out using electrical resistivity measurements, TEM (Transmission Electron Microscopy) observations and X-ray diffraction analysis. The main structural modifications are observed in the polycrystalline alloy and concern first, the reordering process of the austenite structure (B2 $\to$ L2 1 ), and second, the precipitation of the (Cu 9 Al 4 ) γ 2 phase. In the single crystal alloy, the evolutions are very slight and localized on the structural defects. Particular attention is paid to the role of the quenched-in vacancy elimination on the observed mechanisms. In addition, the incidence of the structural evolution on the transformation temperatures is also discussed.

Interstitial cluster motion in displacement cascades

Displacement cascades of energies up to 30 keV were investigated by molecular dynamics simulations in α-iron, nickel and in ordered intermetallic compounds Ni 3Al. Different models of embedded-atom potentials were used in conjunction with a modification of the short-range repulsive interaction. Interstitial clusters were found to be very mobile at any size. The direction of the dumbbells in a cluster configuration was different as compared to that in an isolated configuration. A glide mechanism and a one-dimensional motion of these clusters were observed in different systems under investigation. Several types of cluster–cluster interactions have been observed: coalescence of two clusters with distinct glide cylinders assisted by the flip of the Burgers vector of one of the clusters, growth of a cluster by the absorption of isolated interstitials and elimination of vacancies swept by a large interstitial cluster. The migration trajectories and the diffusion coefficients of some clusters were investigated.

A positron investigation of electron-irradiation induced defects in γ-TiAl intermetallic compounds

Abstract The isochronal recovery of irradiation induced defects was investigated in γ-TiAl intermetallic compounds (with 50 and 54 at% Al) by positron lifetime measurements after 2.5 MeV electron irradiation at 21 K. In the as-irradiated condition, the analysis of the results and the comparison with published data led to a value τd=230±5 ps for the lifetime of vacancy-trapped positrons. The lifetime variations observed during isochronal anneals at increasing temperatures are consistent with vacancy migration around 450 K. The observation of a progressive decrease in the lifetime of trapped positron, during the migration and elimination of vacancies, suggests that they do not form unrelaxed three-dimensional clusters, and that another type of positron traps is simultaneously present.