Defect Complexes Research Articles

Laser manufacturing of spatial resolution approaching quantum limit

Atomic and close-to-atom scale manufacturing is a promising avenue toward single-photon emitters, single-electron transistors, single-atom memory, and quantum-bit devices for future communication, computation, and sensing applications. Laser manufacturing is outstanding to this end for ease of beam manipulation, batch production, and no requirement for photomasks. It is, however, suffering from optical diffraction limits. Herein, we report a spatial resolution improved to the quantum limit by exploiting a threshold tracing and lock-in method, whereby the two-order gap between atomic point defect complexes and optical diffraction limit is surpassed, and a feature size of <5 nm is realized. The underlying physics is that the uncertainty of local atom thermal motion dominates electron excitation, rather than the power density slope of the incident laser. We show that the colour centre yield in hexagonal boron nitride is transformed from stochastic to deterministic, and the emission from individual sites becomes polychromatic to monochromatic. As a result, single colour centres in the regular array are deterministically created with a unity yield and high positional accuracy, serving as a step forward for integrated quantum technological applications.

First-principles calculations of proton defect properties in Ca-doped YPO4.

YPO4 has a xenotime-type structure with one-dimensional percolating channels along the c-axis, and Ca-doped YPO4 exhibits proton conduction. In the present study, using first-principles calculations, we investigate the behaviors of proton solutions in 3 mol% Ca-doped YPO4 in the intermediate temperature range on the basis of point-defect formation energies, defect concentrations, and migration barriers. Although the charge-neutrality condition is mainly satisfied by and defects within the defect formation energy diagrams, the defect complex has the lowest formation energy and the highest concentration among examined defects under various temperature and partial-pressure conditions. The migration barriers of isolated protons in the [100] and [001] directions, as obtained from nudged elastic band (NEB) calculations, are 0.49 and 0.17 eV, respectively, confirming that protons in the YPO4 crystal exhibit anisotropic diffusion and are likely to migrate along the c-axis channels. The activation energy estimated by the sum of the migration energy of the isolated proton and the association energy for the defect complex is comparable to the reported experimental value. Based on concentration calculations and diffusion-property analyses, we identified the Ca doping conditions and temperature ranges that govern proton conduction in YPO4 and elucidated the diffusion pathways.

Defect Isolation from Whole to Local Field Separation in Complex Interferometry Fringe Patterns through Development of Weighted Least-Squares Algorithm

In this paper, based on Gaussian 1σ-criterion and histogram segmentation, a weighted least-squares algorithm is applied and validated on digital holographic speckle pattern interferometric data to perform phase separation on the complex interference fields. The direct structural diagnosis tool is used to investigate defects and their impact on a complex antique wall painting of Giotto. The interferometry data is acquired with a portable off-axis interferometer set-up with a phase-shifted reference beam coupled with the object beam in front of the digital photosensitive medium. A digital holographic speckle pattern interferometry (DHSPI) system is used to register digital recordings of interferogram sequences over time. The surface is monitored for as long as it deforms prior to returning to its initial reference equilibrium state prior to excitation. The attempt to separate the whole vs. local defect complex amplitudes from the interferometric data is presented. The main aim is to achieve isolation and visualization of each defect’s impact amplitude in order to obtain detailed documentation of each defect and its structural impact on the surface for structural diagnosis purposes.

Dependence of Proton Diffusivity on Dopant Concentration in Yttrium-Doped Barium Zirconate; First-Principles Study

Yttrium-doped barium zirconate (BaZrO3) has high proton conductivity and thermal stability, attracting attention to apply solid electrolytes to medium-temperature fuel cells. It has been reported that the proton conductivity decreases as the concentration of Y in BaZrO3increases, and it is desired to elucidate its origin. In order to perform a theoretical analysis associated with such a high-concentration solid solution state, it is necessary to consider a solid solution model that simultaneously includes two or more point defect complexes. Though the number of atomic arrangements is huge, only the structures along the path with the lowest activation energy contribute to proton conduction. In this study, we found the structures that contribute to proton conduction by pathfinding based on the combination of first-principles calculations and graph theory. From the structures on the conduction path with the lowest activation energy, we extracted the parameters that contribute to the improvement of proton conduction by performing the feature analysis.The solution models of the 2×2×2 supercell of BaZrO3 with 12.5 and 25 mol% Y were constructed in which one and two atoms of dopant element Y were substituted to Zr sites, respectively, and the same number of protons were introduced. The total energy calculation was performed for all the symmetrically independent structures. The conduction path search based on the graph theory was performed using the obtained energies of structures, and the association and activation energies of conduction in long-range diffusion were evaluated. A feature regression analysis of total energies was conducted to extract the dominant factors of energies of the structures along the conduction path.The association and activation energies were evaluated from the structural energies of the structures along the obtained conduction path. The difference between the highest and lowest energies with protons at the stationary positions along the searched path is considered the association energy and the energy difference between the highest energy on the path and the highest energy with the proton at the stationary positions is considered to correspond to the mobile activation energy of the proton. The mobile activation and association energies were increased with the increase of Y concentration, which is in good agreement with the experiments.To clarify the parameters which affect the mobile activation and association energies, we performed regression analysis for the energy of the Y 25 mol% using the random forest to extract the dominant factor of the structural energy along the diffusion path in the heavily doped system. The regression parameters of rotation and distortion of octahedra were considered as well as direct descriptors of H-Y configurations. Though the parameters such as a distance of H-Y directly related to the configuration of H and Y appeared to be less important, the parameters related to the rotation of Zr(Y)O6 octahedra, are the first and second highest important parameters. They have an almost linear relationship with high negative correlation coefficients around -0.7 for the structures along the searched diffusion path including H at the transition positions. It is suggested that the rotation of octahedra declines the energy and contributes to the stabilization of the structures along the searched diffusion path. To confirm the rotation effect on the structural energy of Y contents, we calculated the energies as a function of octahedron rotation for the Y 12.5 and 25 mol% models. The larger the Y concentration, the larger the energy change concerning octahedron rotation, suggesting that the Y 25 mol% model is stiffer in terms of octahedron rotation than Y 12.5 mol%. This stiffening of octahedron rotation with Y addition is a key factor of mobile activation and association energies, which are dominant in proton conduction.This work was supported by the Japan Science and Technology Agency CREST (JPMJCR18J3) and JSPS KAKENHI (JP22H05146 and JP22K14465). This work was also supported by MEXT Program: Data Creation and Utilization Type Material Research and Development Project Grant Number JPMXP1122683430.

Defects engineered ferroelectricity and electrocaloric effect in Pb0.7Ba0.3ZrO3 ceramics

Point defects play a versatile role in modulating ferroelectricity and piezoelectricity, however, the decisive role of point-defect concentration/species in controlling macroscopic performance remains unknown. In this work, defects are implanted by reduced atmosphere annealing and/or Mn doping in Pb0.7Ba0.3ZrO3 ceramics. i) defects induce lattice distortion, internal lattice strain and accordingly, phase structure is changed (Williamson-Hall analysis and Rietveld refinement); ii) ferroelectricity is enhanced for 0 MN and 1 MA sample with contribution of defect polarization (first-order reversal curves). iii) Joule heat is depressed as an enhanced resistivity by carrier annihilation/forming defect complex; iv) ECE behaves distinctly among 0 MN, 1 MA and 1 MH, respectively. 0 MN sample exhibits a ΔTmax of 0.39 K and 1 MA possesses a ΔTmax of 0.34 K. Interestingly, a broad temperature span with a low instability of ∼20 % within 288 K–423 K is obtained in 1 MH sample. This work not only provides a guideline for defects enhancing ferroelectricity and ECE, but also clarifies the reduction-resistant properties of (Pb, Ba)ZrO3 system that are suitable for constructing base-metal multilayer ceramic capacitors in solid-state cooling devices.

Thermoluminescence Studies of Proton-Irradiated Cr-, Mg-Codoped β-Ga2O3.

Chromium-doped Ga2O3, with intense Cr3+-related red-infrared light emission, is a promising semiconductor material for optical sensors. This work constitutes a comprehensive study of the thermoluminescence properties of Cr-, Mg-codoped β-Ga2O3 single crystals, both prior to and after proton irradiation. The thermoluminescence investigation includes a thorough analysis of measurements with different β- irradiation doses used to populate the trap levels, with preheating steps to disentangle overlapping peaks (TM-TSTOP and initial rise methods) and finally by computationally fitting to a theoretical expression. At least three traps with activation energies of 0.84, 1.0, and 1.1 eV were detected. By comparison with literature reports, they can be assigned to different defect complexes involving oxygen vacancies and/or common contaminants/dopants. Interestingly, the thermoluminescence signal is enhanced by the proton irradiation while the type of traps is maintained. Finally, the pristine glow curve was recovered on the irradiated samples after an annealing step at 923 K for 10 s. These results contribute to a better understanding of the defect levels in Cr-, Mg-codoped β-Ga2O3 and show that electrons released from these traps lead to Cr3+-related light emission that can be exploited in dosimetry applications.

High performance vertical organic phototransistor doped with MXene

Organic phototransistors are leading the development of next-generation wearable, monitoring, imaging, and sensing technologies due to their light weight, low cost, high yield, compatibility with flexible substrates, and customizable methods for synthesizing optoelectronic properties. However, the long channel length of conventional planar structures, usually in the micron range, greatly reduces the carrier transport efficiency and leads to an increased probability of defect trapping and complex recombination of photogenerated carriers. Here, an organic phototransistor with a vertical channel structure has been demonstrated, in which the active layer is blended with Mxene. Due to the short channel length of vertical structure, the recombination possibility of photogenerated excitons is reduced, and the high ultraviolet sensitivity of Mxene increases the bandwidth of detection. The device exhibits excellent photodetection performance with the photosensitivity of 4.28 × 106, photoresponsivity of 2.39 × 104 A/W, detectivity of 1.04 × 1017 Jones under irradiation with weak light of 8 μW cm−2 at 365 nm. This work paves the way for the study of transistors scale and high-performance organic photodetection in the future.

Bridging the gap between theory and experiment in vacancy concentration, C/N/O diffusivity, and divacancy interaction in tungsten: Role of vacancy-C/N/O interaction

Light element solutes such as carbon (C), nitrogen (N), and oxygen (O) are impurities commonly seen in tungsten alloys, often exerting great impact on materials properties due to their strong interaction with defects. In this work, we conducted a comprehensive investigation into the atomistic properties of small vacancy-impurity defect complexes in tungsten using rigorous first-principles calculations. Through the integration of a statistical approach, we meticulously examined the concentration distributions of these complexes across various temperatures and impurity concentrations. Our findings reveal that the introduction of C/N/O impurities significantly augments the concentration of vacancy-type defects to a level well above the thermal equilibrium vacancy concentration in pure tungsten. Moreover, we conducted a thorough assessment of the diffusivity of C/N/O atoms, conclusively demonstrating that the inclusion of the vacancy trapping effect is paramount in comprehending their unexpectedly low diffusivity observed in experiments. Finally, in conjunction with an object kinetics Monte Carlo method, we revisited the experimental determination of the divacancy binding energy in tungsten. We showed that the presence of C/N/O impurities may have conceivably impacted the experimentally ascertained divacancy binding energy. This work provides accurate data on evaluating vacancy-impurity interaction in tungsten, unveils the pivotal role played by C/N/O in affecting vacancy concentration and divacancy binding energy, and sheds new insights toward understanding the diffusion behavior of impurities in tungsten.

Analysis of radiation defects in gallium nitride using deep level transient spectra and first principles methods

Radiation defects in gallium nitride devices were analyzed by combining experiments with simulation methods. Various forms of native defects, impurities, and these hydrogenated defect complexes in GaN were studied with first-principles calculations employing hybrid functionals which exhibit a reference data base for experimental analysis. We consider hydrogen complexes made of the combinations of single point defects (VGa, VN), di-vacancies (VGa-VGa, VN-VN, VGa-VN) and impurities (C, O, Si). Formation energies were computed for configurations with different charge states after full geometry optimizations. From the calculated formation energies, thermodynamic transition levels were evaluated, which are related to the thermal activation energies observed in experimental techniques such as deep level transient spectroscopy (DLTS). Hydrogen interstitial atoms were identified by the combination of DLTS and first-principles calculations. The transition level measured by DLTS was EC-0.5 eV, which is consistent with the calculation value. The signal of EC-0.9 eV peak in DLTS may be consisting of [VGa]-3/-2 and [VN-VN]+1/+2. According to the degradation of the electrical performance of the AlGaN/GaN high electron mobility transistors and the calculation results, the hydrogen passivation may be caused by [VGa]-3, [VGa-VGa]-2 and [VGa-VN]-1.

Native defects association enabled room-temperature p-type conductivity in β-Ga2O3

The room temperature hole conductivity of the ultra-wide bandgap semiconductor β-Ga2O3 is a pre-requisite for developing the next-generation electronic and optoelectronic devices based on this oxide. In this work, high-quality p-type β-Ga2O3 thin films grown on r-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD) exhibit ρ = 5 × 104 Ω·cm resistivity at room temperature. A low activation energy of conductivity as Ea2 = 170 ± 2 meV was determined, associated to the VO++−VGa−native acceptor defect complex. Further, taking advantage of cation (Zn) doping, the conductivity of Ga2O3:Zn film was remarkably increased by three orders of magnitude, showing a long-time stable room-temperature hole conductivity with the conductivity activation energy of around 86 meV. VO++−ZnGa− defect complex has been proposed as a corresponding shallow acceptor.

Atomistic models of In and Ga diffusion in Cu(In,Ga)Se2

To improve the performance of Cu(In,Ga)Se2 thin-film photovoltaic devices, a robust understanding of the dominant diffusion pathways of the alloy species In and Ga is needed. Here, the most probable defect complexes and mechanisms for In and Ga diffusion are identified with the aid of density functional theory. The binding energies and migration barriers for these complexes are calculated in bulk CuInSe2 and CuGaSe2. Analytic models and kinetic lattice Monte Carlo simulations are employed to predict the diffusivity of In and Ga under variations in composition and temperature. We find that a model based on coulombic interactions between group III antisites and vacancies on the Cu-sublattice produces results that match well with experiment.

Influence of neutron and gamma irradiation on the electrocaloric properties of Mn-doped 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 ceramics

The influence of neutron and gamma irradiation on the low- and high-field dielectric and electrocaloric (EC) properties of Mn-doped 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 (PMN–10PT) ceramic is studied. Upon exposure to neutron fluences of up to 1017 cm−2 and gamma-ray doses of up to 1200 kGy the Mn-doped PMN–10PT exhibits a lower saturated polarization, increased internal bias field and reduced EC temperature change. In comparison, the respective properties of the undoped PMN–10PT remain almost unchanged upon exposure to neutrons and gamma rays. In Mn-doped PMN–10PT, the acceptor-oxygen vacancy defect complexes, introduced via doping, contribute to the lowering of the threshold radiation dose that the material survives without noticeable changes in properties. Radiation-induced degradation of the EC response of Mn-doped PMN–10PT can be partially healed by annealing at 450 °C. The study provides guidance for designing EC ceramic materials for solid-state cooling applications in environments of high ionizing radiation, such as the medical field or space technologies.

Overcoming the compensation of acceptors in GaN:Mg by defect complex formation

In GaN:Mg, the MgGa acceptor is compensated extensively by the formation of nitrogen vacancies (VN) and Mg interstitials (Mgi). However, we show that such compensation can be overcome by forming two kinds of Mg-rich complexes: one that contains VN and the other that contains only MgGa and Mgi. Such complexing not only neutralizes VN and Mgi but also forms better complex acceptors that have lower formation energies and smaller hole localization energies than isolated MgGa. Our results help explain the different doping behaviors in samples grown by different methods.

Investigation of the Boron removal effect induced by 5.5 MeV electrons on highly doped EPI- and Cz-silicon

This study focuses on the properties of the BiOi (interstitial Boron–interstitial Oxygen) and CiOi (interstitial Carbon–interstitial Oxygen) defect complexes by 5.5MeV electrons in low resistivity silicon. Two different types of diodes manufactured on p-type epitaxial and Czochralski silicon with a resistivity of about 10 Ω⋅cm were irradiated with fluence values between 1 × 1015cm-2 and 6 × 1015cm-2. Such diodes cannot be fully depleted and thus the accurate evaluation of defect concentrations and properties (activation energy, capture cross-section, concentration) from Thermally Stimulated Currents (TSC) experiments alone is not possible. In this study we demonstrate that by performing Thermally Stimulated Capacitance (TS-Cap) experiments in similar conditions to TSC measurements and developing theoretical models for simulating both types of BiOi signals generated in TSC and TS-Cap measurements, accurate evaluations can be performed. The changes of the position-dependent electric field, the effective space charge density Neff profile as well as the occupation of the BiOi defect during the electric field dependent electron emission, are simulated as a function of temperature. The macroscopic properties (leakage current and Neff) extracted from current–voltage and capacitance–voltage measurements at 20°C are also presented and discussed.

A density functional theory study of the CiN and the CiNOicomplexes in silicon

Nitrogen (N) is an important impurity in silicon (Si), which associates with impurities as well as with other defects to form defect complexes. The knowledge of the properties and behavior of defect structures containing carbon (C), N and oxygen (O) is important for the Si–based electronic technology. Here, we employ density functional theory (DFT) calculations to investigate the association of nitrogen with carbon and oxygen defects to form the CiN and CiNOidefects. We provide evidence of the formation of these defects and additional details of their structure, their density of states (DOS) and Bader charges. Therefore, CiN and CiNOidefects are now well characterized.

Identification of native defects of 1T-HfTe2

Defects have significant impact on material properties. Identifying the atomic structure is the foundation for defect studies. Combining scanning tunneling microscopy/spectroscopy and density functional theory simulations, we have studied native defects in the transition-metal dichalcogenide 1T-HfTe2. We find that the dominant single-atom defects are the Te vacancy, Te substitution by residual iodine, and Te substitution by residual oxygen. The defect concentrations have been estimated from the scanning tunneling microscopy images and are explained using the defect formation energies. In addition to those single-atom defects, we have also identified two types of defect complexes, which are composed of Te vacancies. Our work clarifies the defect information of 1T-HfTe2 and thus paves the way for future studies.

Interplay of hydrogen and point defects in B2-type PdCu: A density functional theory study

To improve the performance of hydrogen-permeable alloys, quantitatively elucidating the metal–hydrogen interactions and identifying the controlling factors that govern the bottleneck step of the hydrogen permeation process are essential. In this study, based on the density functional theory, we characterized the interactions of hydrogen with point defects (such as antisite atoms and vacancies) in B2-type non-stoichiometric PdCu alloys, which are promising candidate materials for low-temperature working membranes for hydrogen purification. We proposed a “double defect” mechanism that produces a new type of defect complex with an atomic configuration similar to the Cu vacancy in B2-type PdCu alloys. We found that both Cu vacancies and double defects can act as strong hydrogen-trapping sites and are thus responsible for the marked decrease in hydrogen diffusivity. In contrast, antisite Cu atoms only repel H atoms and do not significantly hinder hydrogen diffusion. Whereas the binding energy of hydrogen to Cu vacancies and double defects exceeded 0.3 eV when relatively few (one or two) H atoms were trapped, the binding energy of hydrogen to Pd vacancies was substantially smaller (0.02 eV at most). Therefore, in Cu-rich non-stoichiometric PdCu alloys, the double defects may be the major hydrogen trapping sites. Furthermore, the effects of hydrogen on the formation of Pd vacancies, Cu vacancies, and double defects in B2-type PdCu alloys were clarified. Notably, double-defect formation from Pd vacancies in B2-type PdCu was significantly enhanced by the presence of nearby H atoms. This phenomenon, in which vacancies in B2-type intermetallic compounds transform into defect complexes in the presence of hydrogen, is postulated for the first time in this study. The findings of this study contribute to further understanding the driving forces for the formation of vacancy-hydrogen complexes in PdCu alloys at the atomic level and provide theoretical support for identifying the lattice defects that inhibit hydrogen permeability in Pd-based membranes.

Computational study of defect complexes in β-LiGaO2 and their relation to the donor–acceptor-pair recombination

Hybrid functional calculations are presented for defects in LiGaO2 with the fraction of non-local exchange adjusted to reproduce the recently reported exciton gap of 6.0 eV. We study how the defect transition levels of the main native defects change with respect to the band edges compared to earlier calculations which assumed a smaller bandgap near 5.1 eV. In addition, we consider defect complexes formed by combining the main native donor GaLi with the main acceptors, VLi and LiGa antisites as a function of their relative position. These results are used to tentatively identify the photoluminescence bands previously assigned to donor–acceptor-pair recombination.

Optical and laser characteristics of Сo-doped magnesium-aluminum spinel crystals grown using molybdenum crucible

Single crystals of Co-doped magnesium-aluminum spinel (MALO:Co) are used for Q-switching of laser radiation at λ = 1.54 μm. Usually, they are grown by the Czochralski method from iridium crucibles that are too expensive. This investigation demonstrates the possibility of obtaining MALO:Co single crystals by the horizontal directional crystallization method using a molybdenum crucible in Ar + (CO, H2) protective reducing medium.The obtained MALO:Co crystals have the spinel structure of MgO‧xAl2O3 with 1 ≤ x < 1.15, and the impurity of molybdenum in them does not exceed 0.019 wt%. Co2+ ions replace Mg2+ ions in the [MgO4] tetrahedral environment. This is confirmed by the broad absorption bands (λmax ≈ 600 nm and λmax ≈ 1400 nm) in Vis-IR spectra. The absorption bands in the UV region of the spectrum are shown to originate from the complexes of point defects that can be eliminated by subsequent annealing in oxidizing/reducing environment. Experimentally shown is the possibility to obtain monoimpulses with the energy of ∼ 1–2 mJ and ∼10–15 ns duration at a wavelength of ∼1.54 μm in passive Q-switches based on MALO:Co crystals.

γ‐Irradiation Damage Mechanism of InGaAs/InP p–i–n Focal Plane Array Investigated by Spatially Resolved and Temperature‐Dependent Photoluminescence

InGaAs infrared photodetectors subjected to irradiation environments undergo microstructural modifications and concomitant degradation, yet the underlying microscopic mechanism has not been fully studied. Herein, the influence of γ irradiation (total dose of 20 krad(Si)) on an In0.53Ga0.47 As/InP p–i–n focal plane array is studied by spatially resolved and temperature‐dependent (3–290 K) photoluminescence (PL) measurements. By comparative PL studies of pre‐irradiation and post‐irradiation, the spatially resolved PL results of irradiation indicate that the in‐plane uniformity of all PL features presents bigger fluctuations, meanwhile, the results of temperature‐dependence PL demonstrate that the PL integral intensity related to impurities and interface‐bound states is significantly weakened after irradiation. This can be attributed to the enhanced migration and reaction of defects caused by γ irradiation. Some mobile defects tend to migrate to lower energy regions, such as interfaces, and form defect complexes. In addition, some impurities combine with mobile defects and form inactive impurity–defect complexes. The findings reveal the effects of low‐dose γ irradiation on InGaAs devices and may provide useful information for enhancing radiation resistance.