Decrease In Defect Concentration Research Articles

Unraveling the Deep‐Level Defects Induced Optical Losses in LilnSe2 Crystal toward Enhancement of Mid‐Infrared Laser Radiation

AbstractMid‐infrared (MIR) lasers generated by nonlinear optical (NLO) crystals have aroused extensive attention in various military and civilian applications. Among the MIR NLO crystal family, LiInSe2 (LISe) is a promising candidate in the generation of MIR lasers due to its wide transmission range, large bandgap, and high laser‐induced damage threshold. However, the intrinsic defects in LISe induce optical losses, which will induce laser damage and hinder the performance of MIR laser generation. In this work, the mechanism of optical absorption induced by defects is revealed in LISe through transient absorption (TA) spectroscopy. It is verified that the deep‐level defects in LISe give rise to the extra optical absorption, which is owing to the excited state absorption (ESA) and photo‐induced absorption (PIA). Accordingly, for improving the optical properties of LISe crystals, Li2Se vapor‐mediated annealing to reduce defects concentration is rationally designed. The results confirmed that the optical properties of annealed LISe are improved due to the decrease of defects concentration. Ultimately, the performance of MIR laser generation is successfully enhanced by using the annealed LISe crystal. This work provides deep insights into the defect‐induced absorption losses in LISe crystals and promotes the application of LISe crystals in MIR laser generation.

Vertical GaN PIN Structure with Intrinsic AlGaN Drift Layer Grown Using Metal‐Organic Chemical Vapor Deposition

In this study, it is aimed to examine the DC characteristics of a vertical PIN diode featuring AlGaN as the drift layers. In the investigation, observing the changes in DC characteristics with the variation of Al content (0%–4%) in the drift layers as well as its thickness is focused on. By applying AlGaN as the drift layer, the semi‐insulating properties are augmented and the detrimental background carbon impurity is reduced, leading to a reduction in the leakage current and an improvement in the breakdown voltage characteristics of the device. Room‐temperature and low‐temperature photoluminescence measurements confirm a reduction in the intensity of both yellow luminescence and blue luminescence, which can be attributed to the decrease in defect concentration. Furthermore, to minimize the occurrence of premature breakdown voltage, p‐type GaN is meticulously etched into a bevel shape, and subsequent confirmation through the utilization of a scanning electron microscope reveals an inclination of 13°. The outcome of the analysis on the DC characteristic indicates an improvement in reverse‐bias characteristics with the increase in the Al ratio in the drift layer for the 2 μm series. Furthermore, for the 20 μm Al0.04Ga0.96N, a breakdown voltage of 2540 V is recorded.

Electronic structure and magnetic properties of noble metal (Rh, Ru, Pd, Ag) adsorbed vacancy‐defective arsenene: A first‐principles study

AbstractThe effects of vacancy defects and noble metals (NM = Rh, Ru, Pd, Ag) on the structural stability, electronic structure, and magnetic properties of arsenene were investigated using a first‐principles study. Vacancy defects have a great influence on both the magnetic properties and the band gap of arsenene. With the decrease in defect concentration, the magnetic moment of the vacancy defect system can be increased, and the bandgap transition from metal‐direct–indirect can be realized. After NM adsorption of vacancy defect arsenene, we considered three defect concentrations. It was indicated that there was a large adsorption energy between NM and vacancy‐defective arsenene, indicating that the adsorption system was relatively stable while the change in defect concentration did not have a significant effect on charge transfer. Except for the Ag adsorption system, the defect concentration can effectively modulate the band gap of the adsorption system and make it exhibit various electronic structures, such as semiconductors and metals. In terms of magnetic properties, not all NM atoms can induce arsenene to appear magnetic. Except for the Ag adsorption system, the defect concentration can make the adsorption system exhibit a variety of magnetic properties. It can induce the magnetic state of vacancy‐defective arsenenes or lead to the disappearance of magnetism. The spin polarization of the adsorption system mainly originates from the NM atoms or As atoms around the vacancies.

Ultrafast, room temperature rejuvenation of SiC Schottky diodes from forward current-induced degradation

In this work, we demonstrate the rejuvenation of Ti/4H-SiC Schottky barrier diodes after forward current-induced degradation, at room temperature and in a few seconds, by exploiting the physics of high-energy electron interactions with defects. The diodes were intentionally degraded to a 42% decrease in forward current and a 9% increase in leakage current through accelerated electrical stressing. The key feature of our proposed rejuvenation process is very high current density electrical pulsing with low frequency and duty cycle to suppress any temperature rise. The primary stimulus is, therefore, the electron wind force, which is derived from the loss of the momentum of the high energy electrons upon collision with the defects. Such defect-specific or “just in location” mobilization of atoms allows a significant decrease in defect concentration, which is not possible with conventional thermal annealing that requires higher temperatures and longer times. We show evidence of rejuvenation with additional improvement in leakage current (16%) and forward current (38%) beyond the pristine condition. Transmission electron microscopy, geometric phase analysis, Raman spectroscopy, and energy dispersive x-ray-spectroscopy reveal the enhancement of defects and interfaces. The ultrafast and room temperature process has the potential for rejuvenating electronic devices operating in high power and harsh environmental conditions.

Direct Evidence on Effect of Oxygen Dissolution on Thermal and Electrical Conductivity of AlN Ceramics Using Al Solid-State NMR Analysis.

Aluminum nitride, with its high thermal conductivity and insulating properties, is a promising candidate as a thermal dissipation material in optoelectronics and high-power logic devices. In this work, we have shown that the thermal conductivity and electrical resistivity of AlN ceramics are primarily governed by ionic defects created by oxygen dissolved in AlN grains, which are directly probed using 27Al NMR spectroscopy. We find that a 4-coordinated AlN3O defect (ON) in the AlN lattice is changed to intermediate AlNO3, and further to 6-coordinated AlO6 with decreasing oxygen concentration. As the aluminum vacancy (VAl) defect, which is detrimental to thermal conductivity, is removed, the overall thermal conductivity is improved from 120 to 160 W/mK because of the relatively minor effect of the AlO6 defect on thermal conductivity. With the same total oxygen content, as the AlN3O defect concentration decreases, thermal conductivity increases. The electrical resistivity of our AlN ceramics also increases with the removal of oxygen because the major ionic carrier is VAl. Our results show that to enhance the thermal conductivity and electrical resistivity of AlN ceramics, the dissolved oxygen in AlN grains should be removed first. This understanding of the local structure of Al-related defects enables us to design new thermal dissipation materials.

Defect emission in Cs3Cu2I5 and CsCu2I3 halide films

Cs3Cu2I5 and CsCu2I3 have shown potential applications as optoelectronic materials, but the mechanism of their emission remains controversial. It is widely believed that the Stokes shifts of Cs3Cu2I5 (1.45 eV) and CsCu2I3 (1.63 eV) are caused by self-trapped excitons, but the apparent changes in emission intensity observed experimentally are difficult to explain by this theory. In this paper, the defect emission mechanism is investigated: Iodine vacancies (VI) are widely present in iodide materials, but the effect of such point defects on Cs3Cu2I5 and CsCu2I3 has been little studied. Theoretical calculations based on the first-principles suggest that the VI defect (3.05 eV) may be directly related to the Stokes shift of Cs3Cu2I5 (similar conclusions were obtained for CsCu2I3). In the experiments related to VI, the emission quenching in the iodination treatment is associated with the decrease of defect concentration, while the emission recovery in the annealing treatment stems from the increase of defect concentration caused by iodine decomposition. The iodination and annealing treatments for CuI shows that VI defects play an important role in the emission of iodide. In addition, experiments to control the defect concentration by RTP treatment also showed that VI defects have an effect on the emission intensity of Cs3Cu2I5 and CsCu2I3. Based on the experimental results and theoretical calculations, the emission mechanism of Cs3Cu2I5 and CsCu2I3 may be the semiconductor defect emission theory. We believe that our research on the defect emission theory will provide new insights into the application of Cs3Cu2I5 and CsCu2I3.

Zeta potential-tunable silica abrasives and fluorinated surfactants in chemical mechanical polishing slurries

Chemical mechanical planarization (CMP) is a vital process for smoothing and polishing the surfaces of various material layers in the semiconductor device fabrication. CMP slurries, chiefly composed of abrasives and various chemicals, play a key role in the planarization performance owing to the synergistic effects. Wafer quality must be investigated simultaneously with the planarization performance, because mechanical and chemical defects on wafer surfaces inevitably occur during the CMP process and should be avoided for yield enhancement. In this study, the planarization performance of dielectric layers (SiO2) was considerably improved while maintaining low wafer defect density, avoiding the trade-off between planarization performance and wafer defect density. The balanced performance was achieved via the combination of the tailoring of the zeta potential of colloidal silica nanoparticles (CSNs) and the decrease of defect concentration using fluorinated surfactant (FS)-induced intermolecular repulsion between the wafer surfaces and the additives. High-quality wafer surfaces with rare defect, low surface roughness, and no contamination (fungus and bacteria) were thus achieved. The combination of aminosilane treatment and FS effects investigated in this study can enhance the planarization performance while maintaining low defect/contaminant density not only for CMP slurries but also for various other polishing/abrasion applications.

Improvement of Colloidal Characteristics in a Precursor Solution by a PbI2-(DMSO)2 Complex for Efficient Nonstoichiometrically Prepared CsPbI2.8Br0.2 Perovskite Solar Cells

The optoelectronic properties of all-inorganic perovskite solar cells are greatly affected by the quality characteristics of films, such as the defect concentration, crystal growth orientation, crystallinity, and morphology. In this study, a PbI2-(DMSO)2 complex is adopted to partially replace PbI2 as the lead source in the preparation of perovskite precursor solutions. Due to the rapid dispersion of the PbI2-(DMSO)2 complex in a solvent, raw materials can rapidly react to form perovskite colloids with a narrow size distribution. Such uniform colloidal particles are found to be beneficial for achieving films with improved quality and highly orientated growth along the [001] direction. The optimized film exhibits a clearly improved crystallinity and a decrease in defect concentration from 4.29 × 1015 cm-3 to 3.20 × 1015 cm-3. The device based on the obtained all-inorganic CsPbI2.8Br0.2 perovskite finally achieves an increase in photovoltaic power conversion efficiency from 10.5 to 14.15%. In addition, the environmental stability of the device also benefits from the improved film quality. After 480 h of storage in air, the device can still maintain nearly 80% of its initial performance.

Effect of solution aging on properties of spin coated zinc oxide thin films

A series of multilayer zinc oxide (ZnO) films were deposited on glass substrate by solution processed spin coating method. The effect of aging time of ZnO sol on the structural and optical properties of the films was studied. The structural characteristics of the samples were analyzed by an X-ray diffractometer (XRD) and Scanning electron microscopy (SEM). The optical properties were studied by a UV–Vis spectrophotometer and a fluorescence spectrometer. The obtained ZnO thin films show very good transmittance with average transmittance of 85% with no fluctuations in the 300–600 nm range of visible spectrum. The band gap increases with the increase in the sol aging time. The enhanced crystalline quality of films and decrease of defects concentration contributed to improve the ultraviolet emission and weaken the visible emissions with prolonging sol aging time.

A study of the microstructural stability and defect evolution in an ODS Eurofer steel by means of Electron Microscopy and Positron Annihilation Spectroscopy

An approach to improve the performance of steels for fusion reactors is to reinforce them with oxide nanoparticles. These can hinder dislocation and grain boundary movement and trap radiation-induced defects, thus increasing creep and radiation damage resistance. The present work investigates the thermal stability of the microstructure and the evolution of defects in a 0.3% Y2O3 dispersed Eurofer steel. Samples were annealed for 1 h under vacuum, from 600 K to 1600 K, followed by cooling inside the furnace. Electron Microscopy techniques and Vickers Hardness were used to characterize the microstructure and evaluate its thermal stability. Positron Annihilation Spectroscopy Doppler Broadening (PASDB) was used to monitor the evolution of defects, such as dislocations and vacancies, and their interaction with Y-O based nanoparticles. Several types of events take place simultaneously in the material, due to its initial deformation caused by mechanical alloying, the presence of oxide particles and austenitic phase transformation. Annealing up to 1000 K shows that the Y-O based nanoparticles keep the microstructure refined. Upon cooling from 1200 K (above Ac3), martensite forms with an equiaxed morphology, instead of the conventional lath form, due to the pinning of prior-austenite grain boundaries by the oxide nanoparticles. Annealing at 1400 K and 1600 K results in the progressive coarsening of Y-O based nanoparticles and their loss of ability to pin grain boundaries. PASDB shows that annealing up to 1200 K leads to an overall decrease in defect concentration, mainly due to recovery of dislocations. After annealing at 1400 K and 1600 K, PASDB indicates the formation of a different type of positron trap. The hypothesis is that, at these temperatures, clusters of thermal vacancies are trapped by the oxide nanoparticles, accumulating at their interfaces with the matrix and being retained in the material upon cooling to room temperature.

Extending Carrier Lifetimes in Lead Halide Perovskites with Alkali Metals by Passivating and Eliminating Halide Interstitial Defects.

Defects, such as halide interstitials, act as charge recombination centers, induce degradation of halide perovskites, and create major obstacles to applications of these materials. Alkali metal dopants greatly improve perovskite performance. Using ab initio nonadiabatic molecular dynamics, it is demonstrated that alkalis bring favorable effects. The formation energy of halide interstitials increases by up to a factor of four in the presence of alkali dopants, and therefore, defect concentration decreases. When defects are present, alkali metals strongly bind to them. Halide interstitials introduce mid-gap states that rapidly trap charge carriers. Alkalis eliminate the trap states, helping to maintain high current density. Further to charge trapping, the interstitials accelerate charge recombination. By passivating the interstitials, alkalis make carrier lifetimes up to seven times longer than in defect-free perovskites and up to thirty times longer than in defective perovskites.

Extending Carrier Lifetimes in Lead Halide Perovskites with Alkali Metals by Passivating and Eliminating Halide Interstitial Defects

AbstractDefects, such as halide interstitials, act as charge recombination centers, induce degradation of halide perovskites, and create major obstacles to applications of these materials. Alkali metal dopants greatly improve perovskite performance. Using ab initio nonadiabatic molecular dynamics, it is demonstrated that alkalis bring favorable effects. The formation energy of halide interstitials increases by up to a factor of four in the presence of alkali dopants, and therefore, defect concentration decreases. When defects are present, alkali metals strongly bind to them. Halide interstitials introduce mid‐gap states that rapidly trap charge carriers. Alkalis eliminate the trap states, helping to maintain high current density. Further to charge trapping, the interstitials accelerate charge recombination. By passivating the interstitials, alkalis make carrier lifetimes up to seven times longer than in defect‐free perovskites and up to thirty times longer than in defective perovskites.

Free volume estimation in Au and Ag mixed sodium antimonate glass ceramics by means of positron annihilation

Earlier, we have reported the results of quantitative studies on structural and a variety of physical properties (that include non-linear optical, piezo-optical and electrical characteristics) of Na2O-Sb2O3:Ag2O/Au2O3 glass ceramics. In continuation of these studies, this part of the investigation is devoted to estimate the concentration of the defects and the free volume space (up to the nano scale) entrenched in the titled glass ceramics (as functions of the concentrations of dopant noble metal ions) by means of positron annihilation lifetime spectroscopy (PAS) technique. Such defects are predicted to have tremendous influence on above mentioned physical properties of these glass ceramics. Glasses of the composition (40-x) Na2O–60 Sb2O3: xAg2O/Au2O3 (x = 0.025–0.15 mol% for Ag2O glasses and x = 0.025–0.1 mol% for Au2O3 glasses) were prepared by melt quenching and subsequent heat-treatment for prolonged times. PAS studies were performed using 22Na positron source of strength 0.1 MBq. The results of these studies indicated significant influence of the concentrations Ag2O and Au2O3 on positron annihilation lifetime in the samples. In both the series of the samples, the third component of intensity, I3 (among the three components) is observed to be the longest-lived component. Further, for Ag2O doped glass ceramics, the value of I3 is found to be the maximal, whereas the life time component τ3, concentration of nano sized free volume defects and their volume fraction (fv), cavity radius (R) are found to be minimal for the samples doped with 0.1 mol% Ag2O and for the Au2O3 doped glass ceramics, these parameters have exhibited continuous decreasing trend with increase of Au2O3 concentration. Such decrease of defect concentration is attributed to the increasing degree of participation of antimony ions in the glass network forming with Sb(V)O4 structural units. Overall, information obtained regarding the dependence of free volume defect concentration on the contents of dopants is well in accordance with conclusions drawn from various macroscopic properties of these materials presented in our earlier reports.

Stabilities and electronic properties of vacancy-doped Ti2CO2

The stabilities and electronic properties of vacancy-doped Ti2CO2 have been investigated by a first-principles method. The formation energies of carbon (C) vacancies are relatively small. And they change from negative to positive as the defect concentration of C vacancies increases. Atomic-vacancy-induced defect states appear in the band structure of defective Ti2CO2. The Fermi energy of C vacancy (VC)-doped Ti2CO2 shifts upward compared with that of perfect Ti2CO2, making VC-doped Ti2CO2 exhibit metallic character. A localized defect state of oxygen vacancy (VO)-doped Ti2CO2 occurs in the midgap of pristine Ti2CO2. And as the defect concentration decreases, the VO-induced band becomes more localized with nearly flat band dispersion. The defective state of VO-doped Ti2CO2 acts as an electron donor state when Ti2CO2 is used as a semiconducting material. Further, the stabilities and electronic properties of different C and O divacancy configurations are explored. Investigation of the C divacancy (V2C) in a 4 × 4 × 1 Ti2CO2 supercell indicates that the C vacancy configuration distribution has little influence on the electronic conductivity of V2C-doped Ti2CO2. Hence it is possible to modulate the electrical conductivity of Ti2CO2 by adjusting the C vacancy concentration. Investigation of the O divacancy (V2O) suggests that the O vacancy configuration distribution changes the electronic band structure at the Fermi level of V2O-doped Ti2CO2.

Comparative study on sintering kinetics of as-milled and annealed CoCrFeNi high entropy alloy powders

This work attempts to understand the effect of initial conditions, i.e., as-milled powder and phase stabilized nearly single phase powder (through annealing), on sintering kinetics of CoCrFeNi high entropy alloy powders. Dilatometric study indicates, axial shrinkage reduced from about 5% to 3% when powder was annealed. Analysis of shrinkage through non-isothermal method shows activation energies of sintering increased significantly for a given model, in annealed powder compared to as-milled powder. Calculated activation energies indicate, in as-milled powder grain boundary diffusion seems to be the dominating mechanism, attributed to the presence of ultrafine grains and a large fraction of grain boundaries. On the other hand, annealing helped to form almost single phase HEA, decrease in defect concentration and reduced grain boundary fraction with increased crystallites; which led to increase in activation energies. Coefficients of thermal expansion have also been studied and it was found to vary with temperature range.

Phase evolution, mechanical and corrosion behavior of Fe(100-x) Ni(x) alloys synthesized by powder metallurgy

In the present investigation, Fe(100-x) Ni(x) alloys (x = 10, 20, 30, 40 and 50 wt%) were synthesized through the evolution of γ-taenite and α-kamacite phases by powder metallurgy route using commercially available Fe and Ni powders. Mechanically mixed powders of Fe and Ni were compacted at room temperature and sintered at three different temperatures 1000, 1200 and 1250 °C for 1 h. Both Ni concentration and sintering temperature have shown a strong impact on the phase formation, tribological and electrochemical behavior. Micro structural study has shown the formation of taenite (γ-Fe,Ni) and kamacite (α-Fe,Ni) phases in the sintered specimens. An increase in Ni fraction resulted in formation of more taenite which reduces hardness and wear resistance of specimens. Increasing the sintering temperature decreased the defect concentration with enhanced taenite formation, aiding to higher densification. Taenite formed completely in Fe50Ni50 after sintering at 1250 °C. Tribological test revealed the maximum wear resistance for Fe70Ni30 specimen due to the presence of both kamacite and taenite in significant proportions. The formation of taenite as well as the decrease in defect concentration improves the corrosion resistance of the specimens significantly in 1M HCl solution. A maximum corrosion protection efficiency of around ∼87% was achieved in acidic medium for Fe50Ni50, sintered at 1250 °C.

MANIPULATING THE STRUCTURAL AND ELECTRICAL PROPERTIES OF ZINC OXIDE THIN FILMS BY CHANGING THE SPUTTERING POWER OF RADIO FREQUENCY MAGNETRON SPUTTERING

Oxygen-deficient zinc oxides thin films with different levels of defects were prepared by using radio frequency magnetron sputtering method with sintered zinc oxide disk as target at different sputtering powers. The composition, structure and electrical properties of the prepared films were investigated. Under the present conditions, all the obtained films possessed würtzite structure, which were growing preferentially along the [Formula: see text]-axis. The thickness of the films, the size of the zinc oxide grains and the content of Zn atoms increased with increasing sputtering power. In the films deposited at a sputtering power from 52[Formula: see text]W to 212[Formula: see text]W, the main defect was interstitial zinc. With increasing sputtering power, due to the enhanced number of interstitial zinc in the films, their room-temperature electrical resistivity would decrease, which was controlled by electron conduction. At increasing measurement temperature, their electrical resistivity would increase, owing to the decrease of defect concentration caused by oxidization.

Aluminosilicate glasses structure under electron irradiation: An EPR study

To understand the complex oxide glasses behavior under irradiation, 13 simplified aluminosilicate glasses were elaborated, composed of SiO2, Al2O3, Na2O and CaO oxides, which were distributed in 3 series to investigate the influence of Na2O/CaO, Al2O3/SiO2 ratios and Al2O3 content. 2.5MeV Electron irradiations were performed to obtain bulk irradiated samples by electronic collisions at low stopping power. The influence of dose (from 1.105 up to 4.109Gy) and chemical composition on the formation of paramagnetic centers have been investigated by electron paramagnetic resonance (EPR) spectroscopy. It has shown that electronic collisions induce production of mainly 3 types of point defects (OHC, Al-OHC, E’) in the glassy network, whose content increase up to 1GGy. The production efficiency of Al-OHC defects is increased when the calcium cations are in charge compensator role around aluminum cations instead of sodium cations. A significant decrease of defects concentration was observed between 1 and 4GGy for all studied glasses.

Evolution of the Helium-Related Defect in Polycrystalline Tungsten

The evolution of the helium-related defect in polycrystalline tungsten was studied by Doppler Broadening Positron Annihilation Spectroscopy and Scanning Electron Microscope as functions of annealing temperature and implantation fluence. The experimental results showed that the defect type was not changed at lower annealing temperature, and the decline of S parameter manifested that the lower temperature annealing led the decrease of defect concentration. In addition there existed the same defect type with the increment of the implantation fluence, thus the rise of S parameter implied the increase of the defect concentration.

Structural, morphological, ferromagnetic and photoluminescence properties of Fe-doped ZnO, prepared by hydrothermal route

Fe doped ZnO particles of flower-like shape, hexagonal prisms and hybrid structures flower-prisms have been synthesized by hydrothermal technique, and their luminescence and magnetic properties have been investigated as a function of the morphology changes due to iron dopant (0–0.03 at.% Fe3+). The X-ray diffractograms of Zn1-xFexO powders indicated a hexagonal wurtzite polycrystalline structure. SEM images reveal the change of Zn1-xFexO grains shape from flower-like to hexagonal prisms as the Fe concentration (x) increases from 0 to 3 at.% Fe. Undoped ZnO shown weak room temperatures ferromagnetism, with high coercivity (Hc = 107 Oe) and saturation magnetization Ms of 1.5·10−3 emu/g. ZnO doped with 1 and 3 at.% Fe presented a significant increase of the magnetization in comparison with the undoped ZnO. For ZnO doped with 3 at.% Fe, Ms = 32.5·10−3 emu/g and Mrem = 0.78·10−3 emu/g. Compared with other reports on magnetic properties of undoped and Fe doped ZnO, these results indicated higher coercivity and smaller magnetizations. The drop in the intensity of characteristic green-yellow photoluminescence band of ZnO at about 550–600 nm was attributed to the decrease of the number of oxygen vacancies and interstitial oxygen. By increasing the Fe concentration, the electron paramagnetic resonance (EPR) signal of undoped ZnO decreases due to the decrease of defects concentration.