Donor Defects Research Articles

Hierarchical Bi2S3 nanothorn on ZnO core-branch photoelectrode: A promising heterostructure for enhanced photoelectrochemical water splitting

The zinc oxide semiconductor associated with defects and their recombination effect restricts the development of photoelectrode for hydrogen evolution. The combination of semiconductor hetero-junction and hierarchical interface of ZnO/Bi2S3 photoelectrode fabricated. In this study, heterostructure ZnO/Bi2S3 were studied as a photoanode with their impact of oxygen vacancy in ZnO nano rods. The trace of the Bi2S3 on the ZnO was studied and compared with pristine and oxygen annealed ZnO nano rods. The photon-luminescence studies reveal that shallow donor and acceptor defect in ZnO and restricted by Bi2S3 heterostructure. The less defect contemplations in the photoanodes accelerates up electron and hole migration leading to significant built-in potential and photocurrent generation. The appropriate method has been followed to architype less interfacial defect in ZnO(300)/Bi2S3 photoanode and boosted the photo-redox reactions for efficient hydrogen evolution. The photoanode exhibits substantial properties of photocurrent density 0.33 mA/cm2, charge transfer resistance of 700 Ω cm2 and higher inbuilt potential of −0.88V vs Ag/AgCl with 0.17 % applied bias photon to electron conversion efficiency and 0.11 % solar to hydrogen conversion efficiency.

Mechanism of Defect Passivation in Sb2Se3 Solar Cells via Buried Selenium Seed Layer

AbstractQuasi‐1D antimony selenide (Sb2Se3) is known for its stable phase structure and excellent light absorption coefficient, making it a promising material for high‐efficiency light harvesting. However, the (Sb4Se6)n ribbons align horizontally, increasing defect interference and limiting vertical carrier transport. Herein, a novel strategy of burying selenium (Se) seed layers to reduce lattice mismatch at the heterojunction interface, promote crystal orientation, mitigate deep donor defects, increase P‐type carrier concentration, and purify the PN junction, is proposed. Admittance spectroscopy reveals that Sb2Se3 solar cells with Se seed layers have higher activation energies for defect states and significantly lower defect densities (1.2 × 1014, 2.7 × 1014, and 1.3 × 1015 cm−3 for D1, D2, and D3) compared to an order of magnitude higher densities in Sb2Se3 solar cells without a Se seed layer. First‐principles calculations support these findings, showing that Se seed layers create a Se‐rich environment, reducing selenium vacancies (VSe), antimony on selenium sites (SbSe), and interface defects. This dual passivation mechanism suppresses defect formation and activation, increasing carrier concentration and open‐circuit voltage (VOC). Ultimately, employing this novel method, a VOC of 498.3 mV and an efficiency of 8.42%, the highest performance reported for Sb2Se3 solar cells prepared via vapor transport deposition (VTD), are achieved.

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.

Comprehensive study of photoluminescence and device properties in Cu2Zn(Sn1−xGex)S4 monograins and monograin layer solar cells

Ge-alloying of Cu2ZnSnS4 is a promising strategy for producing wide-bandgap absorber materials suitable for use in tandem structures or indoor solar cell applications. Incorporating Ge can suppress Sn-related defects while maintaining the kesterite structure and p-type conductivity throughout the entire range of composition. In this study, Cu2Zn(Sn1−xGex)S4 monograins were synthesized in molten salt by the synthesis-growth method, where value x was varied from 0 to 1, with a step of 0.2. The inclusion of Ge into the crystals was confirmed by energy-dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray diffraction analysis. The bandgaps of Cu2Zn(Sn1−xGex)S4 solid solutions were determined as Eg = 1.50–2.25 eV by external quantum efficiency measurement. A detailed study of temperature and laser power-dependent photoluminescence (PL) for powder crystals with x = 0, x = 0.2 and x = 0.4 revealed that the dominant recombination mechanisms originate from defect clusters involving a shallow acceptor and deep donor defect. Ge-alloying helped to suppress the harmful SnZn donor defects, but at the same time shifted the defects' energy levels deeper into the bandgap. The obtained activation energies indicate that the acceptor defect becomes shallower with the inclusion of Ge. At the mid-temperature range (T = 60–200 K), the presence of two different recombinations was revealed in the system, originating from very closely located PL emission bands.

Effects of B2O3 Dopants on the Electrical Properties of Ta‐Doped SnO2 Varistors

As a new core component of surge arresters, it is crucial for manganese dioxide varistors to solve the challenges of reducing the leakage current and lowering the residual voltage ratio. In this study, the electrical characteristics of tin dioxide varistors using B2O3 as the acceptor dopant and Ta2O5 as the donor dopant are investigated. Several tin dioxide based varistors doped with different amounts of B2O3 are prepared and characterized by the researchers. X ray diffraction (XRD) is used to analyze the structural features of the samples and to observe the phases involved. It is found that the B2O3 dopant provides oxygen vacancies, reduces the grain size, increases the barrier height, and decreases the residual pressure ratio. The Ta2O5 dopant provides donor defects and free electrons needed to form the barrier. When balancing the concentrations of B2O3 and Ta2O5, the optimum dopant concentrations is found to be 0.1 mol% of B and 0.05 mol% of Ta; these concentrations result in a leakage current of 0.25 μA cm−2, a residual pressure ratio of 1.95, a nonlinear coefficient of 25, and a voltage gradient of 394 V mm−1 for the varistor.

Donor Defect Induced Ferromagnetism in Nb‐Doped ZnO Thin Films Grown by RF Magnetron Sputtering

The effect of Nb doping concentration (0, 1, 2, and 4 at. %) on donor defect‐induced ferromagnetism in ZnO thin films is investigated. The films are deposited on Si(111) substrates utilizing the radio frequency magnetron sputtering. X‐ray diffraction pattern unveils that the films show a pronounced orientation along the (002) direction. The relative intensities of defect‐related bands with that of the ultraviolet band from photoluminescence (PL) spectra show that 2 at. % Nb doping results in a greater number of donor defects (Zni+ and VO+) in the ZnO lattice. The parameters extracted from the electron paramagnetic resonance spectra follow a similar trend. The results from vibrating sample magnetometer measurement indicate that pure ZnO displays diamagnetic nature, whereas Nb‐doped ZnO exhibits a ferromagnetic nature. The saturation magnetization value is found highest for 2 at. % Nb doping, which correlates with the presence of a greater number of donor defects, as supported by the PL and electron paramagnetic resonance results. Images obtained from atomic force microscopy show that the surface roughness of the ZnO thin film reduces upon Nb doping. X‐ray photoelectron spectroscopy validates that Nb is doped in 2 at. % Nb‐doped ZnO thin film with Nb oxidation state of +5.

General algorithm for characterization of donor-acceptor pair recombination processes in solid-state materials

Radiative recombination processes can occur in solid-state systems through the pairing of donor and acceptor defects of the lattice. Recently, donor-acceptor pairs (DAP) have been proposed as promising candidates for quantum applications, and their signature has been observed in emerging low-dimensional materials. Therefore, the identification of such processes is gaining interest and requires methods to efficiently and reliably characterize them. Here, we introduce a general algorithm to identify DAP processes starting from the experimental photoluminescence (PL) emission spectrum and basic material parameters, including the lattice structure and dielectric constant. The algorithm recognizes possible DAP transitions from the emission pattern in the spectrum and returns the characteristic energy of the DAP transition and the separation between the donor and acceptor sites. By testing the algorithm on the photoluminescence spectrum of hexagonal boron nitride (hBN), we show that our method is robust against experimental errors and adds new capabilities to the investigation toolbox of semiconductors and their optical properties.

Incorporation and Interaction of Co‐Doped Be and Mg in GaN Grown by Metal‐Organic Chemic Vapor Deposition

Despite recent advances in growth and characterization of GaN:Be, reliable conductive p‐type GaN:Be remains elusive. In this work, GaN is co‐doped with Be and Mg to improve the incorporation and ionization efficiency of both Be and Mg. Be and Mg are found to interact in complex ways rendering GaN:Be,Mg semi‐insulating. Rather than improving Be and Mg incorporation efficiency, there is an apparent mutual inhibition of incorporation when Be and Mg are co‐dopants. Furthermore, photoluminescence measurements indicate that the BeGa acceptor is reduced in particular, relative to the total [Be]. The same effect is not observed for Mg. From this, it is concluded that Mg preferentially incorporates into Ga lattice sites over Be, and excess Be that cannot incorporate into substitutional sites instead occupies interstitial sites. The interstitial Be acts as donor defect, which compensate Mg acceptors. This provides an explanation for the observed reduction in BeGa‐related luminescence intensity (without an associated decrease in [Be]) in GaN:Be with significant [Mg] content.

DFT study of the role and possible contribution of defects to lithium metasilicate [formula omitted] luminescence

Lithium metasilicate (Li2SiO3) has recently been shown to display outstanding stimulated luminescence properties, which are governed by defects and/or impurities. Here, the possible contribution and character of native point and complex defects to stimulated luminescence of lithium metasilicate have been investigated employing calculations based on the density functional theory (DFT). Under high concentrations of acceptor defects, isolated lithium interstitials were found to be the defects with the lowest formation energy (0.41 eV) but it is shown that they cannot contribute to luminescence. On the other hand, Oi−2 are the most favorable and stable defects under high concentrations of donor defects. F-centers and oxygen peroxides were identified as first- and second-order processes of point defects, which would explain experimental observations in the literature. Bridging oxygen vacancies were shown to introduce shallow electron traps able to trap up to four electrons. On the contrary, non-bridging oxygen vacancies lead to the formation of deep color centers with the capability of trapping up to two carriers/center, which could be the final state for the radiative process of luminescence. These results provide guidance for the interpretation of experimental measurements of luminescence.

New finger reconstruction technologies using 3D printing

Introduction The use of 3D printing technology in finger reconstruction improves accuracy of the procedure minimizing the donor defect and optimizing the appearance and function of the finger. The use of this technology in the finger reconstruction with an osteocutaneous radial forearm flap with axial blood supply and lengthening of the digital stumps and metacarpals remains poorly explored.The objective of the study was to demonstrate new methods of preoperative planning for finger reconstruction and improve surgical outcomes.Material and methods Outcomes of five patients treated with original methods based on 3D technology were retrospectively evaluated during preoperative planning, reconstruction of the thumb using an osteocutaneous radial forearm flap with axial blood supply, relocation of the stump of the third finger and lengthening of the stumps of the first and second metacarpals. The patients could achieve consolidation of interpositional bone allografts following lengthening of the finger stumps, stability of the bone base of the finger, organotypic restructuring of the marginal allograft during plastic surgery with an osteocutaneous radial forearm flap, and a functional position of the reconstructed thumb using the middle finger stump.Results and discussion An individual device for planning finger reconstruction allows identification of the optimal size and position of the finger in three planes, which is essential for patients with severe hand deformities to avoid corrective procedures. An individual guide was used to osteotomize the radius to harvest a vascularized graft providing a cutout of a given size and shape and a cortical-cancellous allograft being identical in shape and size to replace the donor bone defect. The combined use of Masquelet technology and distraction of the finger stump or a metacarpal improved conditions for consolidation and restructuring of the interpositional allograft preventing fractures and infection.Conclusion The use of 3D technology in finger reconstruction using an osteocutaneous radial forearm flap with axial blood supply and distraction of the finger stumps and metacarpals can improve surgical outcomes.

The role of interface energetics in Sb2Se3 thin film solar cells

We comprehensively simulated the interface energetics at the Sb2Se3/CdS interfaces and showed its impact on device performance. The interface discontinuity, band bending at interface and energy level alignment generates interfaces issues and must be optimized for an optimal device performance. The design parameters for controlling interface. Metal contact work function preferably higher than electron affinity (EA) and Fermi level (EF) combined (EA + EF), should result in near Ohmic behaviour of contact. Secondly electron affinity of buffer could be tuned to achieve small positive conduction bandoffset (spike barrier) at absorber/buffer interface which lowers the chances of recombination through interface states. A pn + configuration with highly doped buffer layer, as compared to p-absorber, is favourable as it will extend depletion in absorber, providing additional drift to photo-generated carriers. Lastly, acceptor defect at Sb2Se3-CdS interface generate surface inversion and detrimental to performance. Donor defects occupying interface states are preferred condition for optimal device performance. We have compiled the optimal ranges for these controlling parameters, to achieve theoretically ideal values of energy level alignment and energetics, leading to optimal performance.

WITHDRAWN: Vacancies and antisites investigation at InAlAs/InGaAs hetero-interface: A first-principles study

The vacancies and antisites at InAlAs/InGaAs hetero-interface have been investigated by first-principles calculation based on hybrid density functional (HSE06). We propose an approach to obtain the chemical potentials based on the growth process of hetero-junction. The valence-band maximum of hetero-interface supercell is aligned with the reference of the average electrostatic potential of bulk material. The VIn, VGa, and VAl defects are deep acceptors, while VAs-1, VAs-2, and VAs-3 defects would behave as donor defects. The antisites are amphoteric defects, whereas they will only behave as donor defects due to the Fermi-level should be below the conduction-band minimum of InGaAs side. For InAlAs/InGaAs devices, the impact of the cation vacancies (acceptors) and antisites (donors) is associated to the bias conditions, while it is bias-independent for As vacancies (donors).

Competing conduction mechanisms for two-dimensional electron gas at LaTiO3/SrTiO3 heterointerfaces

In pursuit of understanding the mechanism of two-dimensional electron gas (2DEG), artificial heterostructures based on SrTiO3 have sparked a wealth of exciting experimental and theoretical results. However, to date, the physical origin of this phenomenon still remains controversial. In our endeavor to unravel the mystery, we have synthesized a series of LaTiO3+δ/SrTiO3 films by controlling growth temperature and oxygen pressure, respectively. X-ray absorption results identify the transition of Ti ions from Ti3+ to Ti4+. It is proved that electrical transport properties in the LaTiO3/SrTiO3 film are dominated by oxygen vacancies in SrTiO3. In addition, the metallic behavior observed in the amorphous LaTiO3/SrTiO3 film suggests contributions from growth-induced donor defects to 2DEG. Our work presents direct evidence of competing conduction mechanisms for the observed 2DEG at the LaTiO3/SrTiO3 interface and highlights the important role of oxygen diffusion from the substrate SrTiO3. Meanwhile, we emphasize the significance of LaTiO3 as an oxygen scale meter to investigate interfacial properties of oxide heterostructures.

Two-step synthesis of antimony sulfide thin films: enhancement in physical properties through sulfurization

Recently, there has been an increase in the use of antimony sulfide (Sb2S3) in Si-based tandem solar cells as a potential absorber material for top sub-cells. The choice of the material stems from the favoured properties such as appropriate bandgap, simple binary composition, nontoxic elements, and long-term stability. However, the physical properties and practical applicability of these materials depend largely on their synthesis conditions. In this work, we investigate the role of sulfurization on the structural, morphological, compositional, and optical properties of Sb2S3 thin films deposited on soda-lime glass via a thermal evaporation technique. Sulfurization was performed on the as-prepared thin films in a customized Chemical Vapor Deposition (CVD) chamber at five different temperatures. Analysis of the crystallinity of the film using the x-ray diffraction technique illustrates the transformation of the film from impure, poor crystalline phase to phase-pure, and highly crystalline orthorhombic structure due to sulfurization. Scanning electron microscopic investigations of the samples revealed better grains with nanorods on the surface at a temperature of 400 °C. For the samples investigated here, the energy values estimated via density functional theory (DFT) calculations agreed well with the experimental data obtained from UV-visible absorption spectral studies. Additionally, it was observed that the desired near-stoichiometric Sb2S3 thin films could be achieved via sulfurization, and the presence of Sb2S3 in all samples was confirmed via Raman spectroscopic studies. Additionally, the defects and trap states of the prepared films were investigated using photoluminescence studies, and donor and acceptor defects were identified. Our study revealed that sulfur rich Sb2S3 films prepared at a sulfurization temperature of 400 °C produced the desired structure, morphology, and optical qualities for future photovoltaic applications.

Conduction mechanism and shallow donor defects in Nb-doped β-Ga2O3 single crystals

Electrical properties, electronic defects, and photoluminescence (PL) of Nb-doped β-Ga2O3 crystals grown by the floating zone method have been studied in the temperature range from 10 to 350 K. The activation energies of shallow and deep traps were obtained and compared with their counterparts. The 0.25-Nb-doped Ga2O3 crystal has a metallic behavior at room temperature and a semiconducting behavior at low temperatures. This behavior was interpreted by the quantum corrections to the electrical resistivity resulting from the weak localization and Coulomb interaction. In addition, the 0.25-Nb-doped Ga2O3 crystal has a shallow donor ionization energy of 0.03 eV, indicating that it requires less energy to enable n-type conductivity. Hence, Nb is a good choice for improving the electrical conductivity of the β-Ga2O3 crystal. Three defect trap states were found at EC-0.81 eV, EC-0.49 eV, and EC-0.12 eV below the conduction band edge. The PL spectra of the undoped sample show two characteristic bands in the UV and blue regions under the excitation wavelength of 256 nm. On the other hand, in the Nb-doped β-Ga2O3 sample, a novel UV emission band with a shorter wavelength was observed.

Intrinsic point defects investigation in InAlAs with extrapolated defect transition level

The intrinsic point defects in InAlAs have been studied by first-principles calculations, with a simplified approach to rescale the charge transition levels from the semilocal to the hybrid functional level. Both antisite defects and vacancy defects exhibit a high sensitivity to growth conditions. For Al-poor, In-poor and As-poor growth conditions, AsAl, AsIn and VAs demonstrate the lowest defect formation energy, respectively. All antisite defects including AsIn, AsAl, InAs and AlAs, exhibit donor-like behavior in the band gap as well as VAs. Both VIn and VAl are amphoteric defects, which can behave as donor defects or acceptor defects.

Defect engineering and alloying strategies for tailoring thermoelectric behavior in GeTe and its alloys

GeTe exhibits excellent p-type medium-temperature thermoelectric properties with low toxicity and good mechanical characteristics, making it highly promising for development in the thermoelectric field. However, GeTe is prone to producing Ge vacancies, leading to high p-type carrier concentration, which results in elevated electronic thermal conductivity and a low Seebeck coefficient. This study systematically analyzes intrinsic and extrinsic defects in GeTe and its alloys, focusing on reducing p-type carrier concentration through first-principles calculations. The results reveal that substituting Ge-sites with Bi (BiGe) yields lower donor defect formation energy, effectively reducing p-type carrier concentration of GeTe and its alloys compared to other elemental doping. Additionally, alloying with certain elements, such as Pb, proves favorable for decreased p-type carrier concentration due to lowered energy levels of valence band maximum (VBM). Inspired by this, screening divalent elements for alloying on Ge-sites reveals that Sr, Ba, Eu, and Yb substantially reduce the VBM of GeTe. Further calculations for Ba and Yb-alloyed GeTe confirm changes in formation energies for donor (favorable) and acceptor (unfavorable) defects. Our work provides a systematic investigation of intrinsic and various extrinsic doping defects in GeTe and its alloys, shedding light on possible strategies of optimizing carrier concentration in these compounds.

Controlled evolution of surface microstructure and phase boundary of ZnO nanoparticles for the multiple sensitization effects on triethylamine detection.

In ZnO gas sensors, donor defects (such as zinc interstitials and oxygen vacancies) are considered active sites for the chemical adsorption and ionization of oxygen on the surface of ZnO, which can significantly enhance the sensor's response. However, the influence of the surface microstructure and phase boundaries of ZnO nanoparticles on the chemical adsorption and ionization of surface oxygen has rarely been explored. In this study, we developed a mixed-phase ZnO nanoparticle gas sensor with a rich phase boundary showing 198-50 ppm improvement in response to triethylamine at 340 °C. This is attributed to the generation of defects originating from lattice mismatch at the ZnO - zincite phase boundaries, which providing more active sites for adsorption of oxygen and triethylamine molecules. This work demonstrates a feasible method of combining surface microstructure regulation with pyrolysis strategies to develop ZnO sensors with significantly enhanced gas response performance.

Numerical analysis of the thin film solar cell modelled based on In doped CdS semiconductor

In this study, pure and 1%, 2% and %3 In-doped CdS thin films were produced by spray pyrolysis method. CdS is an n-type (II-VI group) semiconductor material and used as a buffer layer in solar cells. By doping In into CdS thin film, it was investigated how optical and crystalline behavior of thin film are changed. Using Moss and Herve&Vandamme and Ravindra relations, refractive indices and dielectric coefficients were investigated depending on the band gap of the obtained CdS sample. It has been observed that In element decreases the band gap of CdS thin film, improved its crystal structure and reduced its roughness. Therefore, 3% In doped CdS has gained a more ideal feature for use as an n-type semiconductor in solar cells. CIGS/In doped CdS solar cell was modelled and analysed by SCAPS-1D simulation program by using the physical parameters of the semiconductor layers that make up solar cells as imputs of program. Photovoltaic parameters of solar cell based on donor defect density, the neutral interface defect density and Auger electron/hole capture coefficient which were calculated by using In %3 doped CdS thin film, which has the most ideal n-type semiconductor properties.

Regulate defects and energy levels for perovskite solar cells by co-modification strategy

The under-coordinated bonds and deep-level defects on the perovskite surface always act as non-radiative recombination centers and lead to energy loss. The usual surface modification efficiency is limited because of the diversity of surface terminals and defects. Here, a co-modification strategy is proposed for more-thorough modification by reconstructing and post-modifying perovskite surface defects. In detail, the tetrahydrofuran (THF) is firstly implemented to remove the organic component and expose the perovskite surface with uniform Pb terminals and defects, and then the 4-Methylbenzyl Mercaptan (MBM) is used to form strong R-S-Pb bonding with them, which achieves the increase and decrease of under-coordinated Pb donor defects. Correspondingly, the surface energy level of perovskite is observed a consistent change with defects. THF/MBM co-modification changes the perovskite surface energy level from n-type toward p-type, which enhances the holes’ transport efficiency. As a result, the best power conversion efficiency (PCE) increases from 22.23% of the control device to 24.17% of the device with THF/MBM co-modification. Notably, a full-coverage and strong-bonding protective layer is formed by THF/MBM co-modification, which facilitates excellent device stability (Retains 93.7%, after 3360 h in N2). This strategy shows great potential in high-performance PSCs by reconstructing defects and energy level of perovskite surface.