Electronic Defect States Research Articles

Oxygen Vacancies Regulated S-Scheme Charge Transport Route in BiVO4-OVs/g-C3N4 Heterojunction for Enhanced Photocatalytic Performance.

Oxygen vacancies (OVs) are widely considered as active sites in photocatalytic reactions, yet the crucial role of OVs in S-scheme heterojunction photocatalysts requires deeper understanding. In this work, OVs at hetero-interface regulated S-scheme BiVO4-OVs/g-C3N4 photocatalysts are constructed. The Fermi-level structures of BiVO4 and g-C3N4 lead to a redistribution of charges at the heterojunction interface, inducing an internal electric field at the interface, which tends to promote the recombination of photogenerated carriers at the interface. Importantly, the introduction of OVs induces defect electronic states in the BiVO4 bandgap, creating indirect recombination energy level that serves as crucial intermediator for photogenerated carrier recombination in the S-scheme heterojunction. As a result, the photocatalytic degradation rate on Rhodamine B (RhB) and tetracyclines (TCs) for the optimal sample is 10.7 and 11.8 times higher than the bare one, the photocatalytic hydrogen production rate is also improved to 558 µmol g-1 h-1. This work shows the importance of OVs in heterostructure photocatalysis from both thermodynamic and kinetic aspects and may provide new insight into the rational design of S-scheme photocatalysts.

Advanced spectroscopic methods for probing in-gap defect states in amorphous SiNx for charge trap memory applications

Silicon nitride (SiNx) serves as the charge trap layer in current 3D NAND flash memory devices. The precise formation mechanism and electronic structure of localized defect trap states in SiNx remain elusive. Here, we present a refined experimental methodology to elucidate the in-gap defect states and the band gaps in amorphous SiNx thin films. Our approach integrates high-resolution reflection electron energy loss spectroscopy (REELS) and spectroscopic ellipsometry (SE) for comprehensive analysis. By systematical analysis, we aim to provide a robust method for determining in-gap electronic states in SiNx. We investigated two different SiNx films prepared by plasma-enhanced chemical vapor deposition and sputtering. Our analysis revealed several distinct in-gap states and determined band gap energies. This approach not only provide advanced spectroscopic methods to characterize the defect electronic states in SiNx, but also applicable to other large band gap semiconductors or dielectrics to predict device-level characteristics for future devices.

Low-Defect-Density Monolayer MoS2 Wafer by Oxygen-Assisted Growth-Repair Strategy.

Atomic chalcogen vacancy is the most commonly observed defect category in two dimensional (2D) transition-metal dichalcogenides, which can be detrimental to the intrinsic properties and device performance. Here a low-defect density, high-uniform, wafer-scale single crystal epitaxial technology by in situ oxygen-incorporated "growth-repair" strategy is reported. For the first time, the oxygen-repairing efficiency on MoS2 monolayers at atomic scale is quantitatively evaluated. The sulfur defect density is greatly reduced from (2.71 ± 0.65) × 1013 down to (4.28 ± 0.27) × 1012 cm-2, which is one order of magnitude lower than reported as-grown MoS2. Such prominent defect deduction is owing to the kinetically more favorable configuration of oxygen substitution and an increase in sulfur vacancy formation energy around oxygen-incorporated sites by the first-principle calculations. Furthermore, the sulfur vacancies induced donor defect states is largely eliminated confirmed by quenched defect-related emission. The devices exhibit improved carrier mobility by more than three times up to 65.2 cm2 V-1 s-1 and lower Schottky barrier height reduced by half (less than 20 meV), originating from the suppressed Fermi-level pinning effect from disorder-induced gap state. The work provides aneffective route toward engineering the intrinsic defect density and electronic states through modulating synthesis kinetics of 2D materials.

Ultrafast Electron Transfer from CuInS2 Quantum Dots to a Molecular Catalyst for Hydrogen Production: Challenging Diffusion Limitations.

Systems integrating quantum dots with molecular catalysts are attracting ever more attention, primarily owing to their tunability and notable photocatalytic activity in the context of the hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR). CuInS2 (CIS) quantum dots (QDs) are effective photoreductants, having relatively high-energy conduction bands, but their electronic structure and defect states often lead to poor performance, prompting many researchers to employ them with a core-shell structure. Molecular cobalt HER catalysts, on the other hand, often suffer from poor stability. Here, we have combined CIS QDs, surface-passivated with l-cysteine and iodide from a water-based synthesis, with two tetraazamacrocyclic cobalt complexes to realize systems which demonstrate high turnover numbers for the HER (up to >8000 per catalyst), using ascorbate as the sacrificial electron donor at pH = 4.5. Photoluminescence intensity and lifetime quenching data indicated a large degree of binding of the catalysts to the QDs, even with only ca. 1 μM each of QDs and catalysts, linked to an entirely static quenching mechanism. The data was fitted with a Poissonian distribution of catalyst molecules over the QDs, from which the concentration of QDs could be evaluated. No important difference in either quenching or photocatalysis was observed between catalysts with and without the carboxylate as a potential anchoring group. Femtosecond transient absorption spectroscopy confirmed ultrafast interfacial electron transfer from the QDs and the formation of the singly reduced catalyst (CoII state) for both complexes, with an average electron transfer rate constant of <kET> ≈ (10 ps)-1. These favorable results confirm that the core tetraazamacrocyclic cobalt complex is remarkably stable under photocatalytic conditions and that CIS QDs without inorganic shell structures for passivation can act as effective photosensitizers, while their smaller size makes them suitable for application in the sensitization of, inter alia, mesoporous electrodes.

Grain-Boundary Structural Relaxation in Sb2Se3 Thin-Film Photovoltaics

Grain boundaries play an important role in the efficiency of thin-film photovoltaics, where the absorber layer is invariably polycrystalline. Density-functional-theory simulations have previously identified a “self-healing” mechanism in Sb2Se3 that passivates the grain boundaries. During “self-healing,” extensive structural relaxation at the grain boundary removes the band-gap electronic defect states that give rise to high carrier recombination rates. In this work, lattice imaging in a transmission electron microscope is used to uncover evidence for the theoretically proposed structural relaxation in Sb2Se3. The strain measured along the [010] crystal direction is found to be dependent on the nature of the grain-boundary plane. For a (010) grain boundary, the strain and structural relaxation is minimal, since no covalent bonds are broken by termination of the grain. On the other hand, strains of up to approximately 4% extending approximately 2 nm into the grain interior are observed for a (041) grain boundary, where grain termination results in significant structural relaxation due to the ideal atomic coordination being disrupted. These results are consistent with theory and suggest that Sb2Se3 may have a high level of grain-boundary-defect tolerance. Published by the American Physical Society 2024

Effect of the bulkiness of alkyl ligands on the excited-state dynamics of ZnO nanocrystals.

Organic ligands on the surface of nanocrystals (NCs) are extremely important in influencing various physical properties, such as dispersibility, electrical properties, and optical properties. Recent studies have revealed that a slight difference in the molecular structure of aliphatic organic ligands significantly affects the dispersibility of the NCs. On the other hand, the effects of the difference in the molecular structure of ligands on the excited-state dynamics of NCs remain elusive. In this study, we synthesized a series of colloidal ZnO NCs capped with different alkyl phosphonic acids and investigated their photophysical properties using emission decay measurements and transient absorption spectroscopy. The spectral shape and lifetime of the emission originating from the surface oxygen defects of ZnO NCs are almost the same irrespective of the alkyl phosphonic ligands used, indicating that the electronic states of the surface oxygen defects are not affected by the bulkiness of the ligand. On the other hand, the emission quantum yield correlates with the rate of carrier trapping by oxygen defects, suggesting that the rate of carrier trapping reflects the number of oxygen defects. Revealing the detailed relationship between molecular structures of organic ligands and the optical properties of NCs is important for advanced photofunctional superstructures using semiconductor NCs.

Uncovering the Electronic State Interplay at Metal Oxide Electron Transport Layer/Nonfullerene Acceptor Interfaces in Stable Organic Photovoltaic Devices.

The implementation of sputter-deposited TiOx as an electron transport layer in nonfullerene acceptor-based organic photovoltaics has been shown to significantly increase the long-term stability of devices compared to conventional solution-processed ZnO due to a decreased photocatalytic activity of the sputtered TiOx. In this work, we utilize synchrotron-based photoemission and absorption spectroscopies to investigate the interface between the electron transport layer, TiOx prepared by magnetron sputtering, and the nonfullerene acceptor, ITIC, prepared in situ by spray deposition to study the electronic state interplay and defect states at this interface. This is used to unveil the mechanisms behind the decreased photocatalytic activity of the sputter-deposited TiOx and thus also the increased stability of the organic solar cell devices. The results have been compared to similar measurements on anatase TiOx since anatase TiOx is known to have a strong photocatalytic activity. We show that the deposition of ITIC on top of the sputter-deposited TiOx results in an oxidation of Ti3+ species in the TiOx and leads to the emergence of a new O 1s peak that can be attributed to the oxygen in ITIC. In addition, increasing the thickness of ITIC on TiOx leads to a shift in the O 1s and C 1s core levels toward higher binding energies, which is consistent with electron transfer at the interface. Resonant photoemission at the Ti L-edge shows that oxygen vacancies in sputtered TiOx lie mostly in the surface region, which contrasts the anatase TiOx where an equal distribution between surface and subsurface oxygen vacancies is observed. Furthermore, it is shown that the subsurface oxygen vacancies in sputtered TiOx are strongly reduced after ITIC deposition, which can reduce the photocatalytic activity of the oxide, while the oxygen vacancies in model anatase TiOx are not affected upon ITIC deposition. This difference can explain the inferior photocatalytic activity of the sputter-deposited TiOx and thus also the increased stability of devices with sputter-deposited TiOx used as an electron transport layer.

Tailoring Optoelectronic Properties of ZnO Films Through Synergistic Mg and B Co-Doping for Unprecedented High-Performance Transparent Electrode Applications

The quest for indium-free transparent conductive electrodes with enhanced optical band gaps and superior electrical conductivity is imperative for the advancement of flexible electrochromic devices. This study presents a groundbreaking approach wherein composite ZnO films, incorporating Mg and/or B dopants through a facile sol–gel spin coating method, are meticulously crafted and systematically scrutinized for their optoelectrical properties. Our findings reveal that Mg doping primarily influences the optical band gap, while B doping facilitates the augmentation of free electrons by modulating morphology and electronic defect states. Optimal performance is achieved with pure Mg doping at an atomic molar concentration of 0.2, resulting in a ZnMgO film boasting an exceptional average transmittance of 98.79% and an impressively low electrical resistivity of 15.3 Ω·cm. Although pure B doping compromises the crystalline quality, it significantly reduces electrical resistivity. Intriguingly, co-doping with Mg at an atomic molar concentration of 0.2 introduces challenges to crystalline quality but enriches the composite film with additional charge carriers, leading to a reduction in bandgap and a remarkable drop in resistance to 6.2 Ω·cm. This innovative work not only sheds light on the delicate balance between Mg and B doping in ZnO films but also paves the way for unparalleled opportunities in the development of high-performance transparent conductive electrodes for flexible electrochromic devices.

Tailoring SnO2 Defect States and Structure: Reviewing Bottom-Up Approaches to Control Size, Morphology, Electronic and Electrochemical Properties for Application in Batteries.

Tin oxide (SnO2) is a versatile n-type semiconductor with a wide bandgap of 3.6 eV that varies as a function of its polymorph, i.e., rutile, cubic or orthorhombic. In this review, we survey the crystal and electronic structures, bandgap and defect states of SnO2. Subsequently, the significance of the defect states on the optical properties of SnO2 is overviewed. Furthermore, we examine the influence of growth methods on the morphology and phase stabilization of SnO2 for both thin-film deposition and nanoparticle synthesis. In general, thin-film growth techniques allow the stabilization of high-pressure SnO2 phases via substrate-induced strain or doping. On the other hand, sol-gel synthesis allows precipitating rutile-SnO2 nanostructures with high specific surfaces. These nanostructures display interesting electrochemical properties that are systematically examined in terms of their applicability to Li-ion battery anodes. Finally, the outlook provides the perspectives of SnO2 as a candidate material for Li-ion batteries, while addressing its sustainability.

Proton radiation effects on electronic defect states in MOCVD-grown (010) β-Ga2O3

The impact of 1.8 MeV proton irradiation on metalorganic chemical vapor deposition grown (010) β-Ga2O3 Schottky diodes is presented. It is found that after a 10.8×1013cm−2 proton fluence the Schottky barrier height of (1.40±0.05 eV) and the ideality factor of (1.05±0.05) are unaffected. Capacitance–voltage extracted net ionized doping curves indicate a carrier removal rate of 268±10cm−1. The defect states responsible for the observed carrier removal are studied through a combination of deep level transient and optical spectroscopies (DLTS/DLOS) as well as lighted capacitance–voltage (LCV) measurements. The dominating effect on the defect spectrum is due to the EC-2.0 eV defect state observed in DLOS and LCV. This state accounts for ∼75% of the total trap introduction rate and is the primary source of carrier removal from proton irradiation. Of the DLTS detected states, the EC-0.72 eV state dominated but had a comparably smaller contribution to the trap introduction. These two traps have previously been correlated with acceptor-like gallium vacancy-related defects. Several other trap states at EC-0.36, EC-0.63, and EC-1.09 eV were newly detected after proton irradiation, and two pre-existing states at EC-1.2 and EC-4.4 eV showed a slight increase in concentration after irradiation, together accounting for the remainder of trap introduction. However, a pre-existing trap at EC-0.40 eV was found to be insensitive to proton irradiation and, therefore, is likely of extrinsic origin. The comprehensive defect characterization of 1.8 MeV proton irradiation damage can aid the modeling and design for a range of radiation tolerant devices.

Controlling electronic defect states in stacked graphene using Moire structure

Controlling electronic defect states in stacked graphene using Moire structure

Ligand-variable metal clusters charge transfer in Ce-Por-MOF/AgNWs and their application in photoelectrochemical sensing of ronidazole.

A photoelectrochemical sensing platform based on ligand-variable metal clusters charge transfer was established for the quantitative assay of ronidazole (RNZ) using Ce-porphyrin-metal-organic frameworks/silver nanowires (Ce-Por-MOFs/AgNWs). Rod-like Ce-Por-MOFs and well-dispersed sub-50nm AgNWs were prepared using a hydrothermal method and polyol strategy, and then through simple drop coating to yield Ce-Por-MOFs/AgNWs nanocomposites. We investigated the intrinsic semiconducting properties of the composites. More importantly, it was found that the variable-valence metal node can provide electronic defect states similar to those caused by multi-metal doping, synergizing with the surface plasmon effect of AgNWs, which significantly improved the photoelectric conversion efficiency, thereby resulting in excellent optoelectronic properties. In combination with molecular imprinting, a competitive type trace photoelectrochemical sensor for RNZ was constructed using Fe2+ as the electron donor and probe. Under optimal conditions, the sensor response is proportional to the logarithm of RNZ concentration in the range 0.1-104nM with a detected limit of 0.038nM. The recoveries ranged from 87.2 to 116% with relative standard deviations (RSDs) < 6.5% (n = 3) in milk sample. This work reveals the charge-transfer process of variable-valence metal nodes in MOFs during photoelectrochemical processes, which will provide new insights for the sensing application of variable-valence metal MOFs.

Study of the Electronic Defect States of One-Dimensional Comb-like Quantum Wires Structure with Resonator Defect by Using the Transfer Matrix Method

In this paper using the transfer matrix method (TMM), we consider an electronic comb-like waveguides system composed by the periodicity of segment semiconductor (GaAs type) of length and grafted in its extremity by one semiconductor resonator (GaAlAs type) of length . These segments and resonators are considered quantum wires. The perfect system in question presents the electronic pass bands and electronic band gaps which allow to control and manipulate the electrons waves whose energy is identical to the energy of the gaps. We insert the defect at the resonator level in the middle of this system in question. Hence, very narrow localized defect states are created in the electronic band gaps, with probably high transmission rate and very important quality factor. These localized defect states shift to low energy by increasing the resonator defect length, while is move to high energy when the resonator defect concentration increases. In this study, we consider that the segments and resonators lengths are very small in front of their sections, so that the propagation of electronic waves occurs only in a single dimension.

Electronic transport mechanism and defect states for p-InP/i-InGaAs/n-InP photodiodes

We have studied the defect states and electronic transport mechanisms in the mismatched p-InP/i-InGaAs/n-InP photodiode grown by molecular beam epitaxy (MBE) using deep-level transient spectroscopy (DLTS) and current–voltage (I–V) measurements. Two defect states were observed in the p-InP layer, which could have originated from Zn diffusion, and another one detected in i-InGaAs acted as a dislocation defect. The total defect density of the device estimated by DLTS and space-charge limited current (SCLC) was about 4 × 1013, which was much lower than the device grown by other methods. I–V measurements pointed out that the leakage current in this photodiode was caused by the existence of defect states and the small conduction band offset. Therefore, to suppress the leakage current in the devices, it is urgent to reduce the trap density and optimize the thickness of the p-InP layer. The study of defect state and electronic transport mechanism in p-InP/i-InGaAs/n-InP PIN diodes could help to develop a strategy to improve the SWIR sensing technology in the future.

Quantum embedding methods for correlated excited states of point defects: Case studies and challenges

A quantitative description of the excited electronic states of point defects and impurities is crucial for understanding materials properties, and possible applications of defects in quantum technologies. This is a considerable challenge for computational methods, since Kohn-Sham density functional theory (DFT) is inherently a ground-state theory, while higher-level methods are often too computationally expensive for defect systems. Recently, embedding approaches have been applied that treat defect states with many-body methods, while using DFT to describe the bulk host material. We implement such an embedding method, based on Wannierization of defect orbitals and the constrained random-phase approximation approach, and perform systematic characterization of the method for three distinct systems with current technological relevance: a carbon dimer replacing a B and N pair in bulk hexagonal BN (${\mathrm{C}}_{\text{B}}{\mathrm{C}}_{\text{N}}$), the negatively charged nitrogen-vacancy center in diamond (${\mathrm{NV}}^{\ensuremath{-}}$), and an Fe impurity on the Al site in wurtzite AlN (${\text{Fe}}_{\text{Al}}$). In the context of these test-case defects, we demonstrate that crucial considerations of the methodology include convergence of the bulk screening of the active-space Coulomb interaction, the choice of exchange-correlation functional for the initial DFT calculation, and the treatment of the ``double-counting'' correction. For ${\mathrm{C}}_{\text{B}}{\mathrm{C}}_{\text{N}}$ we show that the embedding approach gives many-body states in agreement with analytical results on the Hubbard dimer model, which allows us to elucidate the effects of the DFT functional and double-counting correction. For the ${\mathrm{NV}}^{\ensuremath{-}}$ center, our method demonstrates good quantitative agreement with experiments for the zero-phonon line of the triplet-triplet transition. Finally, we illustrate challenges associated with this method for determining the energies and orderings of the complex spin multiplets in ${\text{Fe}}_{\text{Al}}$.

The role of Si vacancies in the segregation of O, C, and N at silicon grain boundaries: An ab initio study.

Grain boundaries (GBs) are defects originating in multi-crystalline silicon during crystal growth for device Si solar cell fabrication. The presence of GBs changes the coordination of Si, making it advantageous for charge carriers to recombine, which brings a significant reduction of carrier lifetimes. Therefore, GBs can be highly detrimental for device performances. Furthermore, GBs easily form vacancies with deep defect electronic states and are also preferential segregation sites for various impurity species, such as C, N, and O. We studied from first principles the correlation between structural, energetics, and electronic properties of the Σ3{111} Si GB with and without vacancies, and the segregation of C, N, and O atoms. C and O atoms strongly increase their ability to segregate when vacancies are present. However, the electronic properties of the Σ3{111} Si GB are not affected by the presence of O, while they can strongly change in the case of C. For N atoms, it is not possible to find a clear trend in the energetics and electronic properties both with and without vacancies in the GB. In fact, as N is not isovalent with Si, as C and O, it is more flexible in finding new chemical arrangements in the GB structure. This implies a stronger difficulty in controlling the properties of the material in the presence of N impurity atoms compared to C and O impurities.

Thickness induced modifications in the valence, conduction bands and optoelectronic properties of undoped and Nb-doped anatase TiO2 thin films

Nb-doped anatase TiO2 (NTO) is a promising wide band gap transparent semiconductor as it is an inexpensive, environment-friendly and chemically stable compound. However, its overall electronic structure and optoelectronic properties are tailored by tuning the microstructure, which are effected by varying process conditions. To this end, here series of thickening NTO and undoped anatase TiO2 (TO) films are deposited on quartz substrate in RF magnetron sputtering, followed by vacuum annealing at 823 K. Following fabrication, detailed electronic structure, defect states, microstructure and optoelectronic characterization of all these films are performed. Here, energy positions of the valence and conduction band edges are correlated with the oxygen and titanium content of these films. Whereas, TO films are found to be highly insulating, NTO films show highly conducting characteristics. The lowest electrical resistivity of 4.80 × 10−3 Ω cm is obtained for ~74 nm thick NTO film. Additionally, relative positions and intensities of the shallow donor level defect states of anatase are analyzed and compared with ZnO to gain fundamental insight into the differences in their electrical behavior. Along with visible region, these films also exhibit high transmittance in NIR range, thereby making these promising transparent conductors in the entire spectral range.

Driven electronic bridge processes via defect states in Th229 -doped crystals

The electronic defect states resulting from doping $^{229}$Th in CaF$_2$ offer a unique opportunity to excite the nuclear isomeric state $^{229m}$Th at approximately 8 eV via electronic bridge mechanisms. We consider bridge schemes involving stimulated emission and absorption using an optical laser. The role of different multipole contributions, both for the emitted or absorbed photon and nuclear transition, to the total bridge rates are investigated theoretically. We show that the electric dipole component is dominant for the electronic bridge photon. In contradistinction, the electric quadrupole channel of the $^{229}$Th isomeric transition plays the dominant role for the bridge processes presented. The driven bridge rates are discussed in the context of background signals in the crystal environment and of implementation methods. We show that inverse electronic bridge processes quenching the isomeric state population can improve the performance of a solid-state nuclear clock based on $^{229m}$Th.

Structural defects in MBE-grown CdTe-basing heterojunctions designed for photovoltaic applications

Structural defects in the p-ZnTe/i-CdTe/n-CdTe single-crystalline heterojunctions designed for photovoltaic applications have been investigated by transmission electron microscopy (TEM) and deep-level transient spectroscopy (DLTS). Lattice parameters and misfit strain in the undoped cadmium telluride (CdTe) absorber layers of the heterojunctions, grown by the molecular-beam epitaxy technique on two different substrates, GaAs and CdTe, have been determined with high-resolution x-ray diffractometry. A dense network of misfit dislocations at the lattice-mismatched CdTe/GaAs and ZnTe/CdTe interfaces and numerous threading dislocations and stacking faults have been shown by the cross-sectional TEM imaging of the heterojunctions. The DLTS measurements revealed five deep-level traps in the heterojunctions grown on GaAs and only three of them in the heterojunctions grown on CdTe. Four of the traps have been attributed to the electronic states of extended defects, presumably dislocations, on the grounds of their logarithmic capture kinetics of charge carriers. Two of these traps, displaying the largest values of their capture cross-section and the properties characteristic of bandlike electronic states, have been ascribed to the core states of dislocations. It is argued that they are most likely responsible for decreased lifetime of photo-excited carriers resulting in a low energy conversion efficiency of solar cells based on similarly grown heterojunctions.

Role of Chemistry and Crystal Structure on the Electronic Defect States in Cs-Based Halide Perovskites.

The electronic structure of a series perovskites ABX3 (A = Cs; B = Ca, Sr, and Ba; X = F, Cl, Br, and I) in the presence and absence of antisite defect XB were systematically investigated based on density-functional-theory calculations. Both cubic and orthorhombic perovskites were considered. It was observed that for certain perovskite compositions and crystal structure, presence of antisite point defect leads to the formation of electronic defect state(s) within the band gap. We showed that both the type of electronic defect states and their individual energy level location within the bandgap can be predicted based on easily available intrinsic properties of the constituent elements, such as the bond-dissociation energy of the B–X and X–X bond, the X–X covalent bond length, and the atomic size of halide (X) as well as structural characteristic such as B–X–B bond angle. Overall, this work provides a science-based generic principle to design the electronic states within the band structure in Cs-based perovskites in presence of point defects such as antisite defect.