Deep Defect States Research Articles

Engineering Dual Carbon and Nitrogen Vacancies in g-C3N4 for Enhanced Photodegradation of Tetracycline Hydrochloride.

Ultrathin g-C3N4 nanosheets with specific nitrogen vacancies and combined carbon and nitrogen dual vacancies were created by annealing g-C3N4 under different atmospheric conditions. The nanosheets with dual vacancies showed significant improvements in the photodegradation of tetracycline hydrochloride (TC-HCl) compared with both the pristine g-C3N4 and its nitrogen-deficient version. Various techniques, such as Raman spectroscopy, electron spin resonance (ESR), X-ray photoelectron spectroscopy, wavelength-dependent studies, electrochemical methods, and photoluminescence measurements, were used to identify vacancy defects, revealing that performance enhancement was particularly notable under visible light. Density functional theory calculations indicated that dual vacancies introduced a shallow defect state above the valence band, enhancing visible light absorption and reducing electron-hole pair recombination. Conversely, nitrogen vacancies alone formed a deep defect state, which extended light absorption but potentially trapped photoelectrons, limiting their contribution to photoreactions. Radical-scavenging experiments and ESR spin-trap spectra identified the superoxide radical (·O2-) as the primary reactive oxygen species responsible for TC-HCl degradation. A comprehensive degradation pathway for TC-HCl was proposed using liquid chromatography-mass spectrometry data. This research highlights a strategic approach to boost TC-HCl photodegradation by engineering the vacancies in g-C3N4.

Enhancement of electrical characteristics of SnGaO thin-film transistors via argon and oxygen plasma treatment

The impact of Ar or O2 plasma treatment on the electrical characteristics of SnGaO thin film transistors (TFTs) fabricated via solution method was examined. The findings of the study revealed that Ar plasma treatment for an optimal duration enhances the electrical properties of SnGaO TFTs, including the improvement of mobility, subthreshold swing (S·S.), and on-off ratio. This result is considered to be due to the increased concentration of oxygen vacancy as donor in the channel layer. Oxygen plasma treatment has an opposite effect on the change in oxygen vacancy concentration, thereby reducing the off-state current and facilitating the adjustment of threshold voltage. However, excessive Ar or O2 plasma treatment duration led to an increase in deep defect states and a significant “hump" phenomenon in the output characteristic curve.

Unveiling heavy heterovalent doping-modulated microstructure and thermoelectric performance in bulk hybrid perovskite single crystals

Organic-inorganic hybrid metal halide perovskites have emerged as a promising candidate for low-temperature thermoelectric conversion. However, challenges remain in heavily doping bulk perovskite single crystals without phase segregation or self-doping compensation issues to increase the carrier concentration. Previous efforts to enhance their thermoelectric performance rely on photoexcitation or surface/interface doping, which is not suitable for bulk devices to work in dark. This study achieves high monovalent and trivalent doping concentrations of up to 5 % in bulk single crystals of MAPbI3 and MAPbBr3 without phase segregation or defect formation. For monovalent doping, K+ and Li+ are incorporated into the perovskite lattice as interstitials. The resulting lattice distortion/microstrain enhances the electrical conductivity by 10 ∼ 100 times of undoped samples at 420 K. Trivalent doping using SbCl3, BiBr3, and BiI3 significantly enhances the carrier concentration and electrical conductivity, up to 1012 cm−3 and 0.038 S m−1 for MAPbBr3 with 10 % BiI3 at 465 K, respectively, which is 105 times of the undoped samples and 10 times of the best reports on doped MAPbBr3. Interestingly, the Bi3+ dopant oxidizes the deep defect state Pb0, resulting in Bi+ and Pb2+ formation and contributing to the enhanced electrical conductivity. Enhanced thermoelectric figure of merit (ZT) is achieved in MAPbBr3 with 5 % BiI3 at 315 K, almost 100 times of the best reports on doped MAPbBr3. This study provides fundamental understanding of doping-modulated microstructural, charge carrier, and thermoelectric properties of hybrid halide perovskites, offering potential doping strategies to realize the theoretically predicted promising thermoelectric properties in their bulk single crystals for practical thermoelectric devices. The doping-modulated carrier transport will also guide the optimization of optoelectronic applications of halide perovskites for solar cells, light emitters, and photodetectors.

How do the oxygen vacancies affect the photoexcited carriers dynamics in β-Ga2O3？

β-Ga2O3 is a rising star material for solar-blind photoelectronic detectors (PDs) since its excellent properties and lower cost than GaN and SiC. However, the high density of O vacancy (VO) strongly affect the performance of devices. In this work, we investigate the effect of VOs on carrier dynamics in β-Ga2O3 and its meaningful application in solar-blind PDs by the time-dependent density functional theory (TDDFT) and nonadiabatic molecular dynamics (NAMD). We find the three inequivalent VOs (VO1, VO2, and VO3) play different roles in carrier dynamics although they all introduce deep defect states in the bandgap, which significantly shorten the lifetime (τ) of carriers. VO1 and VO3 form nonradiative recombination centers, considerably decreasing the τ from 23.10 ns to 0.12 ns and 0.11 ns, respectively. VO2 forms shallow-level defects, and the τ is 0.47 ns, which weakens the decline of the carrier compared to VO1 and VO3. Our findings provide a different angle to understand the disputation on relationship between point defects and device performance based on β-Ga2O3, and to further improve the device performance by defect engineering.

On the Structure of Oxygen Deficient Amorphous Oxide Films.

Understanding defects in amorphous oxide films and heterostructures is vital to improving performance of microelectronic devices, thin-film transistors, and electrocatalysis. However, to what extent the structure and properties of point defects in amorphous solids are similar to those in the crystalline phase are still debated. The validity of this analogy and the experimental and theoretical evidence of the effects of oxygen deficiency in amorphous oxide films are critically discussed. The authors start with the meaning and significance of defect models, such as "oxygen vacancy" in crystalline oxides, and then introduce experimental and computational methods used to study intrinsic defects in amorphous oxides and discuss their limitations and challenges. To test the validity of existing defect models, ab initio molecular dynamics is used with a non-local density functional to model the structure and electronic properties of oxygen-deficient amorphous alumina. Unlike some previous studies, the formation of deep defect states in the bandgap caused by the oxygen deficiency is found. Apart from atomistic structures analogous to crystal vacancies, the formation of more stable defect states characterized by the bond formation between under-coordinated Al ions is shown. The limitations of such defect models and how they may be overcome in simulations are discussed.

Identification and characterization of deep nitrogen acceptors in β-Ga2O3 using defect spectroscopies

The ability to achieve highly resistive beta-phase gallium oxide (β-Ga2O3) layers and substrates is critical for β-Ga2O3 high voltage and RF devices. To date, the most common approach involves doping with iron (Fe), which generates a moderately deep acceptor-like defect state located at EC-0.8 eV in the β-Ga2O3 bandgap. Recently, there has been growing interest in alternative acceptors, such as magnesium (Mg) and nitrogen (N), due to their predicted deeper energy levels, which could avoid inadvertent charge modulation during device operation. In this work, a systematic study that makes direct correlations between the introduction of N using ion implantation and the observation of a newly observed deep level at EC-2.9 eV detected by deep-level optical spectroscopy (DLOS) is presented. The concentration of this state displayed a monotonic dependence with N concentration over a range of implant conditions, as confirmed by secondary ion mass spectrometry (SIMS). With a near 1:1 match in absolute N and EC-2.9 eV trap concentrations from SIMS and DLOS, respectively, which also matched the measured removal of free electrons from capacitance-voltage studies, this indicates that N contributes a very efficiently incorporated compensating defect. Density functional theory calculations confirm the assignment of this state to be an N (0/−1) acceptor with a configuration of N occupying the oxygen site III [NO(III)]. The near ideal efficiency for this state to compensate free electrons and its location toward the midgap region of the β-Ga2O3 bandgap demonstrates the potential of N doping as a promising approach for producing semi-insulating β-Ga2O3.

Threshold Modulative Artificial GABAergic Nociceptor.

Gamma-aminobutyric acid (GABA) is a crucial inhibitory neurotransmitter of the central nervous system. It modifies the signal threshold of the nociceptor, allowing it to react to external stimuli in various circumstances. Thus, GABAergic behaviors are critical characteristics of adaptive behavior in life. Here, a threshold-modulative artificial GABAergic nociceptor is reported for the first time at a Pt/Ti/Nb2 O5- x /Al2 O3- y /Pt/Ti (top to bottom) of the double charge trapping structure. The Al2 O3- y layer contains deep defect states that function similarly to the GABA neurotransmitter in modulating the signal threshold. Meanwhile, the Nb2 O5- x layer traps volatile charges and produces nociceptive behaviors. The combined dynamics of the two layers readily offer threshold-modulative GABAergic nociceptive behaviors. Based on these GABAergic behaviors, a method of implementing hot- and cold-sensitive thermoreceptors is demonstrated and shows its potential applications in advanced sensory devices.

Cs2TiI6 (Cs2TiIxBr6-x) Halide Perovskite Solar Cell and Its Point Defect Analysis.

This work presents a comprehensive numerical study for designing a lead-free, all-inorganic, and high-performance solar cell based on Cs2TiI6 halide perovskite with all-inorganic carrier transport layers. A rigorous ab initio density-functional theory (DFT) calculation is performed to identify the electronic and optical properties of Cs2TiI6 and, upon extraction of the existing experimental data of the material, the cell is designed and optimized to the degree of practical feasibility. Consequently, a theoretical power conversion efficiency (PCE) of 21.17% is reported with inorganic TiO2 and CuI as carrier transport layers. The calculated absorption coefficient of Cs2TiI6 reveals its enormous potential as an alternative low-bandgap material for different solar cell applications. Furthermore, the role of different point defects and the corresponding defect densities on cell performance are investigated. It is found that the possible point defects in Cs2TiI6 can form both the shallow and deep defect states, with deep defect states having a prominent effect on cell performance. For both defect states, the cell performance deteriorates significantly as the defect density increases, which signifies the importance of high-quality material processing for the success of Cs2TiI6-based perovskite solar cell technology.

Precise Defect Engineering on Graphitic Carbon Nitrides for Boosted Solar H2 Production.

Defect engineering has been regarded as an "all-in-one strategy" to alleviate the insufficient solar utilization in g-C3N4. However, without appropriate modification, the defect benefits will be partly offset due to the formation of deep localized defect states and deteriorated surface states, lowering the photocarrier separation efficiency. To this end, the defective g-C3N4 is designed with both S dopants and N vacancies via a dual-solvent-assisted synthetic approach. The precise defect control isrealized by the addition of ethylene glycol (EG) into precursor formation and molten sulfur into the pyrolysis process, which simultaneously induced g-C3N4. with shallow defect states. These shallow defect energy levels canact as a temporary electron reservoir, which are critical to evoke the migrated electrons from CB with a moderate trapping ability, thus suppressing the bulky photocarrier recombination. Additionally, the optimized surface states of DCN-ES arealso demonstrated by the highest electron-trapping resistance (Rtrapping) of 9.56×103 Ω cm2 and the slowest decay kinetics of surface carriers (0.057s-1), which guaranteed the smooth surface charge transfer rather than being the recombination sites. As a result, DCN-ES exhibited a superior H2 evolution rate of 4219.9µmol g-1 h-1, which is 29.1-fold higher than unmodified g-C3N4.

Fabrication and Light‐Emission Study of Ti‐δ‐Doped Quasi‐2D γ‐Alumina by Graphene‐Assisted Atomic Layer Deposition

Crystalline quasi‐2D Ti‐δ‐doped γ‐alumina (2D‐Ti:γ‐alumina) is synthesized by a facile, precursor‐alternating atomic‐layer‐deposition technique followed by a post‐growth air‐anneal at ≈800 °C. According to energy‐dispersive X‐ray spectroscopy maps, Ti atomic concentration remains uniform and approaches ≈1.5%. Subject to several UV‐light exposures, the samples are found to develop a full strength, stable emission in the ultrawide spectral range of ≈350–850 nm. The near‐infrared (NIR) emission band centered at ≈760 nm is assigned to Ti deep defect states introduced in the alumina host. According to photoluminescence excitation–emission studies, aggregate oxygen vacancies remain a primary excitation source of the NIR emission. In addition, a broad visible‐range emission band at ≈525 nm is registered in the temperature range of ≈108–348 K, but unlike the NIR band, its temperature dependence remains Arrhenius with the non‐radiative recombination activation energy of ≈73 ± 9 meV. The resultant 2D‐Ti:γ‐alumina holds promise for applications in low‐cost alumina‐based optoelectronics, photocatalytic remediation, and spectrally wide light sources.

Optically Active Telecom Defects in MoTe2 Fewlayers at Room Temperature.

The optical and electrical properties of semiconductors are strongly affected by defect states. The defects in molybdenum ditelluride (MoTe2) show the potential for quantum light emission at optical fiber communication bands. However, the observation of defect-related light emission is still limited to cryogenic temperatures. In this work, we demonstrate the deep defect states in MoTe2 fewlayers produced via a standard van der Waal material transfer method with a heating process, which enables light emission in the telecommunication O-band. The optical measurements show evidence of localized excitons and strong interaction among defects. Furthermore, the optical emission of defects depends on the thickness of the host materials. Our findings offer a new route for tailoring the optical properties of two-dimensional materials in optoelectronic applications.

Insights into the Efficiency Improvement for CZTSSe Solar Cells with over 12% Efficiency via Ga Incorporation

AbstractSuppressing the band tailing and nonradiative recombination caused by massive defects and defect clusters is crucial for mitigating open‐circuit voltage (Voc) deficit and improving the device performance of CZTSSe thin film solar cells. Cation substitution is one of the most commonly used strategies to address the above issues. The latest world record efficiency of 13.0% is obtained through this strategy (Ag substitution for Cu). Nevertheless, the importance of the approach to implementing metallic ion doping is easily overlooked by researchers. Here, different approaches are adopted to realize Ga doping and the differences in the efficacy and mechanism are thoroughly investigated. It is found that the secondary phase easily emerged when the physical‐based method is employed, and thus challenging to regulate the doping effect. In the case of the chemical‐based method, Ga doping can enlarge the depletion region widthand lower the defect activation energyand Urbach energy. Furthermore, GaSn defects located at grain boundaries can expand the energy band bending between GBs and grain interiors (GIs), thereby suppressing the deep defect states and nonradiative recombination. Consequently, power conversion efficiency as high as 12.12% with a Voc of 522 mV is achieved at 2% Ga doping.

Optical characterization of high-quality ZnO (0002) / Cu (111) epilayers grown by electrodeposition

Wurtzite ZnO thin films were grown on Cu substrates of three orientations – (111), (100), and (130), respectively – by a simple electrodeposition technique. Among them, the films deposited on the (111) and (130) substrates were epilayers with different crystallinity. Optical characterizations were therefore conducted for the (0002)ZnO/(111)Cu epilayers showing promising crystallinity while having different morphologies. Photoluminescence (PL) spectra of epilayers show a sharp UV near band edge (NBE) emission band, owing to excitonic transitions in ZnO layer, whereas a wide feature in visible spectral region is related to deep defect (O-vacancy) states in ZnO. The strong excitonic ZnO band (NBE) suppresses the defect (DEF) band intensity by ∼100 times, indicating good optical quality of ZnO epilayers. Using the Arrhenius-type equation, activation energies were determined as Ea = 23 meV and 27 meV, which appeared to be related with the Stokes shift between emission and absorption spectra. From the excitation-dependent (80–700 kW/cm2) PL measurements at 3 K, a linear behavior of ratio NBE:DEF was deduced, which indicated that excitonic- and defect-related radiative recombination paths are independent. Furthermore, from the dependence of the PL intensity on the laser power density, characterized with exponential factor of k ≃ 1.4, the donor-bound nature of the excitonic transitions (DoX) in the ZnO/Cu epilayers is suggested. Corroborative structural and optical findings with the bandgap values estimated (3.362 eV and 3.371 eV) in a very good agreement with the literature data for ZnO single crystal, confirm high-quality of the studied ZnO/Cu (111) epilayers.

Impact of Ga and N Vacancies at the GaN m -Plane on the Carrier Dynamics of Micro-Light-Emitting Diodes

Micro-light-emitting diodes ($\text{\ensuremath{\mu}}\mathrm{LEDs}$) based on $\mathrm{Ga}\mathrm{N}$ are a key component for next-generation displays, but serious surface $e$-$h$ recombination deteriorates the device efficiency. To gain microscopic insights into the surface recombination, we use hybrid functional first-principles calculations to investigate $\mathrm{Ga}$ (${V}_{\mathrm{Ga}}$) and $\mathrm{N}$ (${V}_{\mathrm{N}}$) vacancies on the $\mathrm{Ga}\mathrm{N}$ $m$ plane that can be created considerably during the production of one-dimensional $\mathrm{Ga}\mathrm{N}$ structures. We find that the surface ${V}_{\mathrm{Ga}}$ is not critical for nonradiative Shockley-Read-Hall (SRH) recombination in light of the large formation energy (>2 eV) and its shallow levels. In contrast, the surface ${V}_{\mathrm{N}}$ exhibits a low formation energy (2 eV) and develops a deep defect state near the midgap, indicating the possibility of becoming useful SRH recombination centers. By constructing configuration-coordinate diagrams, we demonstrate that the energy barriers for electron and hole capture on the surface ${V}_{\mathrm{N}}$ are small enough to cause significant capture coefficients due to strong electron-phonon coupling.

Low-Cost Antimony Selenosulfide with Tunable Bandgap for Highly Efficient Solar Cells.

About 10% efficient antimony selenosulfide (Sb2 (S,Se)3 ) solar cell isrealized by using selenourea as a hydrothermal raw material to prepare absorber layers. However, tailoring the bandgap of hydrothermal-based Sb2 (S,Se)3 film to the ideal bandgap (1.3-1.4eV) using the selenourea for optimal efficiency is still a challenge. Moreover, the expensive selenourea dramatically increases the fabricating cost. Here, a straightforward one-step hydrothermal method is developed to prepare high-quality Sb2 (S,Se)3 films using a novel precursor sodium selenosulfate as the selenium source. By tuning the Se/(Se+S) ratio in the hydrothermal precursor solution, a series of high-quality Sb2 (S,Se)3 films with reduced density of deep defect states and tunable bandgap from 1.31 to 1.71eV is successfully prepared. Consequently, the best efficiency of 10.05% with a high current density of 26.01 mA cm-2 is achieved in 1.35eV Sb2 (S,Se)3 solar cells. Compared with the traditional method using selenourea, the production cost for the Sb2 (S,Se)3 devicesis reduced by over 80%. In addition, the device exhibits outstanding stability, maintaining more than 93% of the initial power conversion efficiency after 30 days of exposure in the atmosphere without encapsulation. The present work definitely paves a facile and effective way to develop low-cost and high-efficiency chalcogenide-based photovoltaic devices.

Unique low-energy line defects and lateral heterostructures in phosphorene

Defect engineering and heterostructure construction are important approaches to modulate the properties of two-dimensional semiconductors. We introduced four phosphorene allotropes as the defective structures to construct the corresponding line defects and lateral heterostructures in black phosphorene. In all the constructed phosphorene systems, the P atoms at the boundaries will keep local threefold covalent bonding, forming clean one-dimensional interfaces and exhibiting a high stability. Electronic structure calculations show that all the constructed structures are semiconducting in absent of deep defect states and the band gap values can be regulated by introducing different defective structures. Distinct distributions of the electronic frontier states are found in the different line defect systems and both type-I and II band alignments can be formed in the semiconducting lateral heterostructures.

Defect-Induced Ultrafast Nonadiabatic Electron-Hole Recombination Process in PtSe2 Monolayer.

Defects are inevitable in two-dimensional materials due to the growth condition, which results in many unexpected changes in materials' properties. Here, we have mainly discussed the nonradiative recombination dynamics of PtSe2 monolayer without/with native point defects. Based on first-principles calculations, a shallow p-type defect state is introduced by a Se antisite, and three n-type defect states with a double-degenerate shallow defect state and a deep defect state are introduced by a Se vacancy. Significantly, these defect states couple strongly to the pristine valence band maximum and lead to the enhancement of the in-plane vibrational Eg mode. Both factors appreciably increase the nonadiabatic coupling, accelerating the electron-hole recombination process. An explanation of PtSe2-based photodetectors with the slow response, compared to conventional devices, is provided by studying this nonradiative transitions process.

The Role of Ionic Liquids in Performance Enhancement of Two‐Step Perovskite Photovoltaics

Much of the research efforts of late that are directed toward enhancement of the efficiency of perovskite‐based photovoltaics are focused on the application of ionic liquids (ILs) in a one‐step approach. On the contrary, the details of the alternative two‐step approach, such as the role of the ILs in the perovskite film solidification and its optoelectronic properties, remain poorly understood despite the increasing evidence that this latter method might offer considerable advantages, including better reproducibility and control over crystallization. Herein, the effect of IL methylammonium acetate (MAAc) introduced into the PbI2 layer by a sequential deposition process on the optoelectronic properties of the perovskite film and the performance of the ensuing photovoltaic devices are studied. The addition of MAAc lowers the MAAc–perovskite formation enthalpy, leading to an accelerated solidification process. Moreover, MAAc suppresses the formation of Pb0, thereby facilitating the perovskite formation while lowering the deep defect states caused by Pb0. In addition to grain boundary passivation, the acetate ions can diffuse into the bulk of the perovskite material, filling up the halide vacancies with reduced trap state density. As a result, a decent power conversion efficiency of 23.36% is achieved, with noticeably improved durability.

Tuning Defects in a Halide Double Perovskite with Pressure.

Dopant defects in semiconductors can trap charge carriers or ionize to produce charge carriers─playing a critical role in electronic transport. Halide perovskites are a technologically important semiconductor family with a large pressure response. Yet, to our knowledge, the effect of high pressures on defects in halide perovskites has not been experimentally investigated. Here, we study the structural, optical, and electronic consequences of compressing the small-bandgap double perovskites Cs2AgTlX6 (X = Cl or Br) up to 56 GPa. Mild compression to 1.7 GPa increases the conductivity of Cs2AgTlBr6 by ca. 1 order of magnitude and decreases its bandgap from 0.94 to 0.7 eV. Subsequent compression yields complex optoelectronic behavior: the bandgap varies by 1.2 eV and conductivity ranges by a factor of 104. These conductivity changes cannot be explained by the evolving bandgap. Instead, they can be understood as tuning of the bromine vacancy defect with pressure─varying between a delocalized shallow defect state with a small ionization energy and a localized deep defect state with a large ionization energy. Activation energy measurements reveal that the shallow-to-deep defect transition occurs near 1.5 GPa, well before the cubic-to-tetragonal phase transition. An analysis of the orbital interactions in Cs2AgTlBr6 illustrates how the bromine vacancy weakens the adjacent Tl s-Br p antibonding interaction, driving the shallow-to-deep defect transition. Our orbital analysis leads us to propose that halogen vacancies are most likely to be shallow donors in halide double perovskites that have a conduction band derived from the octahedral metal's s orbitals.

Effect of electron conduction on the read noise characteristics in ReRAM devices

The read variability of redox based resistive random access memory is one of the key characteristics with regard to its application in both data storage and novel computation in memory or neuromorphic architectures. While intrinsic noise limits the number of distinguishable states, it may be beneficial for probabilistic computing or to prevent overfitting. Thus, application and material system need to be carefully matched according to their read noise characteristics. Preceding density functional theory simulations suggested dividing oxides used in valence change memory into two categories based on the dominating conduction mechanism. We provide a comprehensive experimental study, which confirms the simulations and demonstrates how the conduction mechanism affects the variability. We analyze the signal-to-noise ratio (SNR) of five different switching oxides, revealing that oxides with shallow defect states (type 1) show high SNR whereas oxides with deep defect states (type 2) exhibit pronounced ionic noise. Thus, our results provide valuable input toward tuning of read noise characteristics by material design.