Defect Levels Research Articles

Influence of Mo doping on the structural, Raman scattering, and magnetic properties of NiO nanostructures

In this work, the Ni1-xMoxO, (x = 0.000, 0.025, 0.050, 0.075, 0.100, and 0.150) nanoparticles were prepared employing the coprecipitation method. The X-ray diffraction (XRD) confirmed that all the samples have a face-centered cubic (FCC) structure with no secondary phases by the effect of the Mo-doping. The Mo-dopants yielded smaller crystallites, reaching a size of 9 nm with x = 0.150. The transmission electron microscope (TEM) images revealed agglomerated NiO nanoparticles with nearly spherical shapes varied to elliptical-like shapes upon increasing Mo concentration. The energy dispersive X-ray (EDX) confirmed the purity of the synthesized samples. The XPS analysis confirmed the valence states of the presented elements in the samples as Ni2+, Ni3+, Mo6+, and O2− ions. The XPS detected the reduction of the nickel and oxygen vacancies, by studying the ratio of Ni2+/Ni3+ and lattice oxygen (OL) to vacant oxygen (OV) peaks. The Raman analysis demonstrated the active vibrational modes of NiO, for all the samples, along with stretching Mo = O bonds for the doped samples. The Photoluminescence (PL) spectroscopy was employed to study the near band edge and deep level emissions, giving insight to the defect levels within the band gap. The PL affirmed the decrease of the oxygen vacancies upon Mo-doping. Besides, the magnetic hysteresis measurements at room temperature revealed the superparamagnetic contribution embedded in the antiferromagnetic matrix of NiO. The magnetization was tuned by Mo doping concentration, where it affected the saturation magnetization, coercivity, and remnant magnetization. Mo dopant can modify the magnetic property of NiO nanoparticles and can be a potential candidate in biomedical field and data storage applications.Graphical

Particle formation: Promise and challenges

Particle formation establishes fundamental particle attributes at the initiation of an overall particle production process. These attributes include particle size distribution, particle morphology, crystal form, and defect level. These fundamental properties, in turn, affect downstream particle processing and product performance properties, including rate of drying, filterability, flowability, caking tendency, rate of dissolution, and for biologically active products bioavailability. Almost always, the goal is that the properties established at formation will provide excellent performance all the way through downstream processing, product formulation, and ultimate use of the product. However, meeting this goal is often not possible for a variety of reasons. First, it may be difficult, prohibitively expensive, or even impossible to achieve the desired properties during formation. Second, there are often conflicting requirements at different stages of production and use. For example, large equant particles may be desired for good filterability and flowability through downstream process steps, but very small particles may be required in the ultimate product application. Examples from the chemical and pharmaceutical industries are presented: both cases successfully producing particles with properties meeting the needs throughout the chain of downstream processing and final application, as well as cases where it was impossible to meet those requirements. For the latter, the additional processing or adaptations required for the product to succeed are outlined. The focus here is on additive particle formation, e.g. crystallization and precipitation from solution, and not subtractive particle formation, e.g. milling. Legacy of Reg DaviesI was fortunate enough, as a budding industrial crystallization engineer, to be invited by Reg to join the newly formed PARticle Science And Technology PARSAT group at DuPont. Reg recognized and documented the importance of particle technology to a large, diverse chemical and materials company like DuPont and PARSAT was to become a leading industrial particle technology group. Reg's vision was to combine the fundamental particle unit operations into a unified, comprehensive technology organization that would address the formation and processing of particles from process concept through end use.This paper describes the first step in many particle processes: particle formation. Reg's legacy vis a vis particle formation is the recognition that particle formation is a critical, foundational process in an overall process train that ultimately produces useful particulate products and intermediates, but that particle formation is nonetheless but one part of the overall process. It may be able to produce particles exactly as needed for end use performance, or additional processes may be needed to tailor particles for their ultimate use. In addition, intermediate processes may be required to preserve useful properties developed during formation but subject to degradation in subsequent operations.Finally, Reg served as inspiration and mentor for everyone in his orbit, and we are far better scientists and engineers for it.

Behavior of defects in GaN avalanche photodiodes grown on GaN substrates

GaN avalanche photodiodes grown on GaN substrates were successfully fabricated. These devices displayed a low dark current, measuring <80 pA at a reverse bias of 82.0 V. Notably, the response spectrum of the devices showed new out-of-band response peaks with increasing reverse bias. Moreover, at high reverse bias, the devices emitted visible light. These phenomena were attributed to inherent defects within the materials. The defect level fitted from the tunneling currents closely matched the experimental value, indicating that the defect-assisted tunneling effect, with a defect level at 0.127 eV relative to the conduction band, contributed to the out-of-band response peak in the response spectrum. The Franz–Keldysh effect led to a redshift in the response spectrum. Additionally, the Mg-related deep energy level situated approximately 0.498 eV above the valence band, facilitated radiative recombination at high reverse bias. Meanwhile, the device’s luminescent image displayed a consistently square shape, suggesting uniform avalanche breakdown throughout the device.

Revealing the degradation pathways of layered Li-rich oxide cathodes.

Layered lithium-rich transition metal oxides are promising cathode candidates for high-energy-density lithium batteries due to the redox contributions from transition metal cations and oxygen anions. However, their practical application is hindered by gradual capacity fading and voltage decay. Although oxygen loss and phase transformation are recognized as primary factors, the structural deterioration, chemical rearrangement, kinetic and thermodynamic effects remain unclear. Here we integrate analysis of morphological, structural and oxidation state evolution from individual atoms to secondary particles. By performing nanoscale to microscale characterizations, distinct structural change pathways associated with intraparticle heterogeneous reactions are identified. The high level of oxygen defects formed throughout the particle by slow electrochemical activation triggers progressive phase transformation and the formation of nanovoids. Ultrafast lithium (de)intercalation leads to oxygen-distortion-dominated lattice displacement, transition metal ion dissolution and lithium site variation. These inhomogeneous and irreversible structural changes are responsible for the low initial Coulombic efficiency, and ongoing particle cracking and expansion in the subsequent cycles.

Additive engineering for efficient wide-bandgap perovskite solar cells with low open-circuit voltage losses.

High-performance wide-bandgap (WBG) perovskite solar cells are used as top cells in perovskite/silicon or perovskite/perovskite tandem solar cells, which possess the potential to overcome the Shockley-Queisser limitation of single-junction perovskite solar cells (PSCs). However, WBG perovskites still suffer from severe nonradiative recombination and large open-circuit voltage (Voc) losses, which restrict the improvement of PSC performance. Herein, we introduce 3,3'-diethyl-oxacarbo-cyanine iodide (DiOC2(3)) and multifunctional groups (C=N, C=C, C-O-C, C-N) into perovskite precursor solutions to simultaneously passivate deep level defects and reduce recombination centers. The multifunctional groups in DiOC2(3) coordinate with free Pb2+ at symmetric sites, passivating Pb vacancy defects, effectively suppressing nonradiative recombination, and maintaining considerable stability. The results reveal that the power conversion efficiency (PCE) of the 1.68eV WBG perovskite solar cell with an inverted structure increases from 18.51% to 21.50%, and the Voc loss is only 0.487V. The unpackaged device maintains 95% of its initial PCE after 500h, in an N2 environment at 25°C.

CROSSLINK DENSITY AND ITS DISTRIBUTION IN HEAT AND OIL RESISTANT ELASTOMERS BY DOUBLE QUANTUM NUCLEAR MAGNETIC RESONANCE

ABSTRACT Double quantum (DQ) nuclear magnetic resonance (NMR) was used to characterize the crosslink density, crosslink density distribution, and defect level in a series of heat and oil resistant elastomers. A wide range of defect levels, crosslink densities, and crosslink density distributions was measured, and results depended on elastomer type and compound formulations, including the vulcanization system. The sol fraction defect level generally correlated with the concentration of added plasticizer in the formulation. The presence of polar side chains appeared to cause additional dynamic contributions to the dangling chain end fraction. The large differences in elastomer composition and rubber formulations prevented meaningful correlation of the measured crosslink densities with the low strain modulus. Fast Tikhonov regularization and log normalization fitting of the corrected DQ build-up curve was extremely useful to provide insight into the modality and widths of the crosslink density distributions. A high degree of heterogeneity of the crosslink network of heat and oil resistant elastomers was found. Crosslink density distributions were explained in terms of the polymer chain structure comprised of monomer sequencing coupled with the position of the crosslinking sites. The type of vulcanization system had a lesser effect of the nature of the crosslink density distribution. The primary polymer chain crosslinking sites may become segregated from the continuous phase due to polarity differences seen in the microstructure of oil and heat resistance elastomers. The development of such micromorphologies can favor curative partitioning. The sole use of DQ NMR can provide valuable insight into the nature of the polymer chain structure and crosslink network in rubber.

Microstructural analysis and defect characterization of additively manufactured AA6061 aluminum alloy via laser powder bed fusion

AA6061 is a widely used aluminum alloy with significant applications in the aerospace and automotive industries. Despite its popularity, the utilization of additively manufactured AA6061 through the laser powder bed fusion (LPBF) process has been hindered by the pronounced formation of pores and cracks during rapid solidification. This study quantitatively investigated defects, including pores and cracks, and microstructures, including texture, grain size, subgrain structure, and precipitates, of LPBF-manufactured AA6061 across a broad spectrum of laser power and speed combinations. A high relative density of more than 99 % was achieved with a low-power and low-speed condition, specifically 200 W and 100 mm/s, with minimal cracks. Large pores, akin to or exceeding melt pool dimensions, emerged under either low or high energy densities, driven by the lack of fusion and vaporization/denudation mechanisms, respectively. Solidification cracks, confirmed by the fractography, were propagated along grain boundaries and are highly dependent on laser scanning speed. Elevated power and speed exhibited finer grain size with refined subgrain cellular structures and increased precipitates at interdendritic regions. The cooling rate and thermal gradient estimated from thermal analytical solutions explain the microstructures’ characteristics. Nano-sized Si-Fe-Mg enriched precipitates are confirmed in both as-built and heat-treated conditions, whereas T6 heat treatment promotes a uniform distribution with coarsening of those precipitates. The low-power and low-speed conditions demonstrated the highest yield strength, consistent with defect levels. A minimum of 102.3 % increase in yield strength with reduced ductility was observed after heat treatment for all examined conditions. This work sheds light on printing parameters to mitigate the formation of pores and cracks in additively manufactured AA6061, proposing a process window for optimized fabrication and highlighting the potential for enhanced material properties and reduced defects through process control.

Surface integrity and high-cycle fatigue life of direct laser metal deposited AISI 431 alloys modified by plasticity ball burnishing

Laser metal deposition (LMD) as an additive manufacturing (AM) is widely used to repair and extend wear and fatigue life of the critical components. This paper has investigated the application of ball burnishing (BB) to improve surface integrity and high-cycle fatigue resistance of LMDed AISI 431 alloys. Results showed that the BB treated samples exhibited significant surface finish improvement by lowering roughness by 91 %. Microhardness increased from 490 to 530 HV0.1, an increase by 10 % with a modified depth of 400 µm from the top surface. XRD results showed a peak shift and increase in FWHM by up to 17 %. This had been corroborated by EBSD exhibiting a 20 % increase in dislocation density and 24 % increase in localised misorientation within microstructure. As a result, the overall high cycle fatigue strength of the burnished sample increased by 50 %, and the cracks initiated from sub-surface level defects at a depth of 350 μm below the top surface, delaying the crack propagation and fracture failure. The findings clearly highlight that the burnishing treatment can be a plausible approach in improving the dynamic fatigue resistance and the overall service life of LMDed AISI 431 steel alloys components in engineering applications.

Oxygen vacancies boosted Bi2WO6 for photoelectrochemical water oxidation

In this work, bismuth tungstate (Bi2WO6) films enriched with oxygen vacancies were fabricated via the hydrogen treatment with an excellent catalytic activity. The photocurrent density of Bi2WO6 H-15 is 0.18 mA/cm2 at 1.23 V (vs. RHE), which is 11.25 times higher than that of pure Bi2WO6. The introduction of oxygen vacancy not only increases the charge concentration, facilitates the charge transport, promotes the water absorption ability, but also extends the light absorption of Bi2WO6. The density of states (DOS) calculated by the density functional theory (DFT) showed that a new defect level was formed and increased DOS at the valance band maximum (VBM). This work manifests the effective use of oxygen vacancies in regulating carriers transport, which is of great significance for the PEC oxygen evolution reaction.

Oxygen vacancy defect engineering in MoO2/Mo-doped BiOCl Ohmic junctions for enhanced photocatalytic antibiotic elimination

Vacancy defect engineering is an effective strategy for improving photocatalytic activity, but controlling surface vacancies precisely remains a challenge. In this study, MoO2/Mo doped BiOCl (Mo-BiOCl) composites enriched with oxygen vacancies (OVs) were prepared by a facile defect engineering strategy with MoO2 hollow spheres as a precursor and molybdenum sources, where the slowly released Mo4+ ions in the reaction system resulted in the tailored formation of Mo-BiOCl. The optimized MoO2/Mo-BiOCl composite (BOM-3) demonstrated significantly improved photocatalytic degradation efficiency for Tetracycline hydrochloride (TC) under visible light, with an apparent rate constant (k) 4.7 and 120 times higher than that of BiOCl and MoO2, respectively. The enhanced photocatalytic activity of BOM-3 can be attributed to the presence of abundant OVs, which extend the light response range by creating intermediate defect level from OVs. Additionally, the formation of ohmic heterojunction between Mo-BiOCl and MoO2 with close interface contacts facilitates rapid separation of interfacial charge carries and efficient migration of photogenerated electrons from Mo-BiOCl to MoO2. The possible degradation pathway of TC using BOM-3 is proposed, and toxicity evaluation indicates that most intermediates have lower toxicity than TC. This work presents a straightforward approach for improving photocatalytic antibiotic degradation through the construction of a novel Ohmic-junction with tuned surface OVs.

Tin Lodide Thin Films Growth via Plasma Enhanced Chemical Vapor Deposition and its Optimization Using V–I Probe Impedance Analyser for Optoelectronic Applications

AbstractTin(ii) iodide (SnI2) faces significant challenges in photodetector applications, primarily due to its sensitivity to moisture and degradation over time. Achieving uniform, high‐quality films with low impurity and defect levels is also a challenge. Potential solutions include advanced deposition techniques to improve film quality and stability, surface passivation and encapsulation, doping and alloying. In this study, SnI2 thin films have been deposited for the first time using plasma enhanced chemical vapour deposition (PECVD) technique to the best of our knowledge. Process parameters like deposition pressure and RF‐power have been optimised via non‐intrusive in‐situ V−I probe impedance analyser. SnI2 thin films have been deposited on glass &amp; transparent conducting oxide (TCO) and p‐Si wafer at various RF‐power to make SnI2/p‐Si heterojunction followed by metallization to make Ag/SnI2/p‐Si/Ag heterojunction photodetector. Characterization techniques like thin film thickness measurement, UV‐Vis‐NIR spectroscopy, Photoluminescence spectroscopy, glancing incidence x‐ray diffraction (GIXRD), SEM and I−V measurements were carried out to study its optical, structural and electronic properties. Fabricated devices, Ag/SnI2/p‐Si/Ag heterojunction photodiode exhibits best critical performance for the film deposited at 150 W having rectifying ratio of 6.9×104 at 1.0 V and photo‐sensitivity of 1.6×104 at 100 mW/cm2 light intensity.

Investigation of optical properties of Ce and Eu-doped Gd2SiO5 insights from GGA + U calculations

Gadolinium silicate, (Gd2SiO5) co-doped with Ce and Eu has been found to exhibit enhanced luminescence efficiency, which makes it a promising material for use in scintillators and phosphors. It has excellent scintillation properties such as high density and high Zeff. In this study, we used density functional theory (DFT) calculations within the Wien2k software to investigate the effect of Ce and Eu concentration and native defects on the electronic structure and optical properties of Ce and Eu co-doped Gd2SiO5. We utilized the DFT + U method to treat the localized 4f electrons of Ce and Eu. Our results indicate that the electronic structure and optical properties of Ce and Eu co-doped Gd2SiO5 are significantly affected by the concentration of the dopants and presence of native defects. We found that increasing the concentration of Ce and Eu dopants leads to a shift in the bandgap to lower energies, resulting in enhanced absorption and emission spectra. Moreover, our calculations reveal that presence of oxygen vacancies and Gd interstitials can induce new defect levels in the bandgap, which may affect the luminescence properties of the material. Our study provides valuable insights into the atomic-level mechanisms that govern the luminescence properties of Ce and Eu co-doped Gd2SiO5 which can aid in the design and optimization of luminescent materials for various applications.

Analisis Pengendalian Kualitas Menggunakan Statistical Process Control untuk Mengurangi Produk Cacat pada Mebel UD Sadewo Jagat di Ngawi

UD Sadewo Jagat is a company engaged in furniture production, this company provides sales of finished, semi-finished products and maintenance for various types of teak wood furniture. Finished products include tables, chairs, shelves and various other types of displays. However, in this study, we only analyzed defective products for wooden box table products because the production process of wooden box table products because the production process of wooden box tables experienced problems, namely the number of defects exceending 5% of the company’s standard provisions. The aim of this research is to analyze product quality control techniques using the Statistical Process Control method in reducing defective products, and to find out what factors cause defects in products produced by UD Sadewo Jagat. This research uses the Statistical Process Control (SPC) method, which is one of the analytical methods used to analyze the causes of defects that occur, both in terms of product quality and in terms of the quality of the production process. Quality control analysis is carried out using. The results of the check sheet analysis show an average of 9,6% product defects per month. From the results of the Pareto diagram, it can be seen that the highest level of defects is crack defects, with a total of 80 units or 44,94% of total product defects in 2023. Meanwhile, the results of the control chart that has been made can be seen that quality control is uncontrolled and not according to standards. From the results of the fishbone diagram, it is concluced that the factors causing product defects are human factors, materials, environment, and methods.

Over 10% efficient Cu2ZnSn(S, Se)4 Thin-Film cells prepared by aluminium doping

Cu2ZnSn(S,Se)4(CZTSSe) solar cell is of excellent development tendency because of lower pollution and outstanding power conversion efficiency(PCE). Yet, current PCE of CZTSSe is limited because of a high open-circuit voltage deficit, primarily put down to severe band tailing and carrier recombination. In this paper, absorber films were obtained by solution method and the influences of different aluminum doping concentrations on the properties of CZTSSe thin film cell were studied. Analysis about X-ray diffraction (XRD) and Raman spectroscopy confirms which Al has incorporated into CZTSSe thin film lattice and replaced Sn. Hall effect measurements reveal the improvement of carrier concentration on thin film absorber layer attribute to the formation of AlSn acceptor defects. Furthermore, X-ray photoelectron spectroscopy (XPS) reveals that Al enter into the CZTSSe lattice and Al is in the appropriate state of + 3 valence without generating additional harmful defects. More important, the generation of deep energy level defects and defect clusters associated with Sn were suppressed by replacing Sn with Al. Finally, the photovoltaic performance of different Al doped CZTSSe cells were characterized by conducting J-V and external quantum efficiency (EQE) tests. Remarkably, the CZTSSe solar cell with PCE as high as 10.59 % is obtained under a doping level of 6 % Al, which has an open circuit voltage of 522 mv. Compared with reference CZTSSe solar cells with an efficiency of 7.29 %, which indicates with improvement of 45 % in efficiency. Consequently, Al doping process is considered as a prospective tactic to elevate the PCE of CZTSSe solar cells.

16.48% Efficient solution-processed CIGS solar cells with crystal growth and defects engineering enabled by Ag doping strategy

Solution-processed Cu(In,Ga)Se2 (CIGS) solar cells suffer from serious carrier recombination and power conversion efficiency (PCE) loss because of the poor film properties and easy formation of defects. Herein, we propose Ag&Se co-selenization strategy to enhance the crystallization and passivate harmful defects of the CIGS films. The formation of Ag-Se phase during the selenization process enables the formation of large grains and suppresses the deep level defects. It is found that Ag doping can enlarge the depletion region width, lower the Urbach energy and prolong the carrier lifetime. As a result, a champion solution-processed CIGS solar cell presents a high efficiency of 16.48% with the highly improved open-circuit voltage (VOC) of 662 mV and fill factor (FF) of 75.8%. This work provides an efficient strategy to prepare high quality solution-processed CIGS films for high-performance CIGS solar cells.

High-Volume SiC Epitaxial Layer Manufacturing-Maintaining High Materials Quality of Lab Results in Production

Typically, research and development (R&amp;D) results of epitaxial layer growth show superior properties of the grown layers compared to high volume results. Layer uniformities are excellent and achieved defect densities are low compared to typical results. In particular, the conversion of basal plane dislocations (BPD) from the silicon carbide (SiC) substrate is in focus to reduce bipolar degradation of p-n-junctions. It is a great challenge to maintain those excellent results in high-volume manufacturing considering all the factors that impact the properties of the epilayer. Thus, quality of the layers, high throughput and low cost have to be assessed to find a compromise between these key factors. In this paper we present results on the growth of epitaxial layers on 150 mm and 200 mm 4° off-oriented 4H-SiC substrates using warm-wall multi-wafer chemical vapor deposition (CVD) systems. Single wafer data of the key epitaxial layer parameters, thickness, doping and defect densities, are compared to batch and lot results, as well as to statistical data of several hundreds of wafers produced. Improvements in wafer-to-wafer (w-t-w) doping uniformity could be achieved for instance by implementation of an on-wafer temperature measurement. Substrate impact on defect levels is shown comparing X-ray topography (XRT) results of bare substrate wafers and defect analysis of epilayers on sister wafers from the same crystal. Statistical defect data and resulting predicted yield loss also show a dependence on substrate suppliers. For the first time we show w-t-w and run-to-run (r-t-r) results of doping and thickness measurements on 200 mm substrates. Also, defect results of epilayers on 200 mm wafers are compared to results on 150 mm.

Effect of access to natural shade on scrotal thermoregulatory capacity, integrity of the testicular parenchyma and sperm morphology of Nelore (Bos indicus) and Canchim (Bos taurus x Bos indicus) bulls.

The objective of this work was to evaluate the effect of using naturally shaded pastures on scrotal thermoregulatory capacity, testicular echotexture, and sperm morphology of Nelore (Bos indicus) and Canchim (5/8 Bos taurus x 3/8 Bos indicus) bulls in a tropical climate region. Sixty-four adult Nelore and Canchim bulls were used, equally allocated in Full Sun (FS, n = 32) or Crop-Livestock-Forestry (CLF, n = 32) pasture systems. During five consecutive climate seasons, the bulls underwent monthly breeding soundness evaluations and the biometeorological variables in the systems were continuously monitored. Microclimate was significantly different between systems. CLF system had lower BGHI than FS throughout the experimental period. No triple interaction (Season x Breed x Treatment, P > 0.05) was observed for any of the variables. Animals in CLF showed lower body temperature in Summer (FS:39.41 ± 0.05 vs. CLF:39.30 ± 0.05°C; P = 0.005) and in Autumn (FS:39.54 ± 0.05 vs. CLF:39.35 ± 0.05°C; P = 0.005). Access to shading did not determine differences in the evolution of scrotal biometry, temperatures, and scrotal thermal gradients (P > 0.05). Regardless of breed, animals in CLF showed greater right testicular volume (FS:247.5 ± 5.7 vs. CLF:259.0 ± 5.7cm³; P < 0.05), more suitable parenchyma echotexture, and fewer microlithiasis spots in the Spring and Summer. Testosterone concentration was higher in FS (FS:2.6 ± 0.2 vs. CLF:2.1 ± 0.2 ng/mL; P = 0.035). Canchim bulls presented higher total sperm defects during the Autumn and Winter (P = 0.010), but the total defects levels for Canchim and Nelore bulls were in normal range for adult bulls. Thus, the natural shade in CLF system was effective in improving the microclimate of pastures and minimizing adverse environmental effects on some reproductive features of interest in beef cattle.

Highly sensitive and selective NO2 detection using face-centered cubic Zn2SnO4 nanostructures

This study reports the synthesis and modification of hexagonal ZnO to face-centered cubic (Fcc) Zn2SnO4 via a single hydrothermal method. We systematically investigated the synthesized materials' structural, morphological, surface area, and sensing properties. X-ray diffraction (XRD) analysis confirmed the Fcc structure of the Sn-modified ZnO samples. X-ray photoelectron spectroscopy (XPS) was employed to elucidate the influence of Sn modified on defect levels in these chemical sensors. Remarkably, Zn2SnO4 exhibited outstanding NO2 sensing properties, including high response, selectivity, and recovery characteristics. The sensor demonstrated a rapid response time of less than 18s and an optimal operating temperature of 260 °C. Furthermore, the cubic face-centered Zn2SnO4 displayed a wide detection range of 1–100 ppm for NO2. The synergistic effects of chemical sensitization and catalytic activity are responsible for the enhanced sensing mechanism in Zn2SnO4.

Study on the effects caused by defect LaK in KH2PO4 crystal

Based on first-principles calculations, we have investigated the effects of different charged states of LaK on the defect formation energies (DFE), lattice distortions, electronic structures, and optical properties of paraelectric phase (PE-KDP) and ferroelectrical phase (FE-KDP) crystals. Our calculations indicate that LaK·· is readily formed in both crystal structures, and the degree of H–O bond distortion is more pronounced in PE-KDP. This could explain the formation of small growth mounds on the surface of La-doped crystals observed in the experiments, as well as the large decrease in the hardness of La-doped crystals. Furthermore, the band gap value of La-doped crystals is smaller than that of perfect crystals. This can be attributed to the fact that the 5d orbitals of La affect the 2p orbitals of O located in the conduction band minimum (CBM), resulting in a splitting of the energy levels and the formation of a defective energy level that enters the forbidden band. The optical spectra have been obtained considering the electron-phonon coupling. Large stokes redshift and nonradiative jump will lower the crystal laser damage threshold. The La-doped PE-KDP has an absorption band at 3.03 eV (409 nm) in good agreement with the experimentally observed absorption band at 390 nm.

Study of native point defects in Al0.5Ga0.5N by first principles calculations

To explore the formation mechanism of native point defects in high Al content AlGaN film, the first principles methods are applied to study the native point defects in Al0.5Ga0.5N. The different kinds of vacancies, interstitials, and antisites are investigated. The formation energies of Al0.5Ga0.5N with native point defects under different charge states and growth conditions are analyzed and compared. Then, the preferable charge state, donor and acceptor properties of Al0.5Ga0.5N with different native point defects are explicitly obtained. The results show that AlN, GaN, Ali, and VN exhibit donor properties in p-type condition while VGa, VAl, VN, and NGa exhibit acceptor properties and can play roles as compensating center in n-type Al0.5Ga0.5N under metal rich condition. For N rich condition,VGa, VAl, NGa, and NAl are favorable acceptors in n-type Al0.5Ga0.5N. Meanwhile, the charge distribution and bonding state of Al0.5Ga0.5N with native point defects are explored. It is found the N atoms in Ni and NGa form covalent bond and ionic bond with atoms in Al0.5Ga0.5N. Moreover, the band calculation reveals that the removal of N atom in Al0.5Ga0.5N makes the conduction band fatter and the effective masses of electrons increase while the introduction of N point defects make the valence band flatter, increasing the effective masses of holes. Furthermore, the corresponding thermodynamic transition energy levels of defects under different charge states are summarized. It is found that the thermodynamic transitions for VN, Ni, and NAl may likely to happen under certain conditions. The above studies yield a detailed and quantitative description of native point defects in Al0.5Ga0.5N, which helps to get a deeper insight to the growth and doping of AlGaN.