Shallow Acceptor Defects Research Articles

Unraveling the Oxygen Vacancy-Performance Relationship in Perovskite Oxides at Atomic Precision via Precise Synthesis.

Understanding the fundamental effect of the oxygen vacancy atomic structure in perovskite oxides on catalytic properties remains challenging due to diverse facets, surface sites, defects, etc. in traditional powder catalysts and the inherent structural complexity. Through quantitative synthesis of tetrahedral (LaCoO2.5-T), pyramidal (LaCoO2.5-P), and octahedral (LaCoO3) epitaxial thin films as model catalysts, we demonstrate the reactivity orders of active-site geometrical configurations in oxygen-deficient perovskites during the CO oxidation model reaction: CoO4 tetrahedron > CoO6 octahedron > CoO5 pyramid. Ambient-pressure Co L-edge and O K-edge XAS spectra clarify the dynamic evolutions of active-site electronic structures during realistic catalytic processes and highlight the important roles of defect geometrical structures. In addition, in situ XAS and resonant inelastic X-ray scattering spectra and density functional theory calculations directly reveal the nature of high reactivity for CoO4 sites and that the derived shallow-acceptor defect levels in the band structure facilitate the adsorption and activation of reactive gases, resulting in more than 23-fold enhancement for catalytic reaction rates than CoO5 sites.

Suppressed nonradiative electron–hole recombination in bismuth halide perovskite Cs3Bi2Cl9: Time domain ab initio analysis

Time-domain density functional theory, coupled with non-adiabatic molecular dynamics simulations, was employed to explore the defect characteristics and the associated nonradiative recombination processes in the bismuth halide perovskite Cs3Bi2Cl9. Our findings indicate that Cs3Bi2Cl9 inherently exhibits p-type semiconductor behavior, with vacancies at the Cs and Bi sites acting as predominant shallow acceptor defects. Although Cl vacancy and interstitial Cl defects introduce trap states within the bandgap of Cs3Bi2Cl9, the by-defect electron–hole (e-h) recombination is substantially impeded, which is attributed to the remarkable local structural deformations associated with the BiCl63− octahedral compression around the defects, which further results in decoupling between the defect state and the band edge state. As a result, the enhanced delocalization of defect states leads to a notably small wave function overlap between defect states and band edge states, as well as weak nonadiabatic couplings dominated by low-frequency phonons. Our study offers crucial insights into the mechanism of defect-mediated e-h recombination in bismuth-based perovskites and provides guidelines for designing efficient optoelectronic devices based on these materials.

Electronic Properties of F, Cl and H Atoms Adsorbed on CsPbI3(110) Surface

The electronic properties of the CsPbI3 surface are calculated by using first-principles calculations. Through calculating adsorption energy of CsPbI3 passivated by Cl, F and H atoms, it is found that adsorption energy values of CsPbI3 (110) surface passivated by H atoms fluctuate less, they are less sensitive to the adsorption position, and the adsorption is stable. Through calculating surface states of passivation system, it is found that F and H atoms have quite good passivation effect, and the Fermi level of passivation system increases and enters the forbidden band, indicating that the passivation effect is relatively ideal. Passivation effect of Cl atoms are poor, a new shallow acceptor defect states appeared at the bottom of the conduction band. This defect level could trap carriers and weaken the carrier migration ability, which is a disadvantage of Cl passivation system. Through the charge density difference and Bader charges, it is found that the F and H atoms get more electrons from the I atom, which is beneficial to the passivation of the surface states.

SIMULATION STUDIES OF CR DOPED CUO HETEROJUNCTION SOLAR CELL

1% and 3% Cr doped CuO thin films have been deposited on soda lime glass by spin coating method and then their structural, morphological and optical properties have been investigated by operating X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Ultraviolet-Visible Spectroscopy (UV-Vis) techniques, respectively. XRD patterns of CuO:Cr (1%) and CuO:Cr (3%) thin films demonstrate characteristics of monoclinic CuO structure with a C2/c space group. The morphology of coated film plays an important role in analyzing some optoelectronic properties. 1% Cr doped CuO thin film absorbs more photons compared to 3% Cr doped CuO in Vis and UV regions. The band gaps of 1% Cr and 3% Cr doped CuO thin films are to be 2.18 eV and 2.30 eV, respectively. The Mo/1% and 3% Cr doped CuO/n-ZnO/i-ZnO/AZO solar cell has modelled with SCAPS-1D simulation program. The photovoltaic (PV) parameters of solar cell deteriorated with some increase in the neutral defect density (N_t) value. As the shallow acceptor defect density (N_a) value is increased, J_SC is decreased, V_OC, FF and η are increased. PV performance of 1% Cr doped CuO solar cell were found to be better than that of 3% Cr doped CuO solar cell. The efficiency of 1% Cr doped CuO solar cell is increased with the use of SnO2 intermediate layer in 2 nm thickness at the heterojunction interface.

Thermal degradation and defect growth in CZTSSe photovoltaic devices

Thermal degradation of Kesterite thin film solar cells (CdS/CZTSSe) has been studied using three different time-dependent models which formulate the defect growth kinetics within the depletion width of CZTSSe/CdS junction. Generation and annihilation of a shallow acceptor defect at 0.55 eV under the stress of heating at 400 °C for 1000 h have been modeled. Three cases were modeled for defect growth rate by relating defect generation to electrons and hole density increment linearly (n) or quadratically (np or n2). The non-radiative recombination has been modeled for the three models and enabled the calculation of heat generation across the depletion width. The heat generation was also mapped through a coupled optical-electrical-thermal model in COMSOL for defect growth models at elevated stressing time. Heat generation is then stronger for the n-model and rises to double times higher by increasing the stressing time to 1000 h.

The role of native point defects and donor impurities in the electrical properties of ZnSb2O4: a hybrid density-functional study.

The ceramic material zinc antimony oxide ZnSb2O4 has promising electrical and magnetic properties, making it suitable for various applications such as electrochemical and energy storage. However, the effects of point defects and impurities on its electrical properties have never been revealed. Here, we employ hybrid density-functional calculations to investigate the energetics and electronic properties of native point defects and donor impurities in ZnSb2O4. The energetically favorable configurations of native point defects under selected growth conditions (O-rich and O-poor) are identified based on the calculated formation energies. The study finds no shallow donor and shallow acceptor defects with low formation energies. Still, the oxygen vacancy (VO) has the lowest formation energy among the donor-type defects under O-rich and O-poor conditions. However, it acts as a very deep acceptor, making it unlikely to provide free electron carriers to the conduction band. Moreover, electron carriers are likely to be compensated by the formation of zinc vacancies (VZn) and the zinc substituted for antimony (ZnSb), which behave as dominant acceptors. Our analysis of the charge neutrality conditions estimates that the Fermi level of undoped ZnSb2O4 would be pinned in a range that is 2.60 eV to 3.12 eV above the VBM for O-rich to O-poor growth conditions, respectively, suggesting that this material is semi-insulating. The possibility of enhancing free electron carriers by doping with Al, Ga, In, and F impurities is also investigated. Our results, however, indicate that high n-type conductivity is hindered by self-compensation in which the impurities additionally act as electron killers. Our results suggest that other impurity candidates and approaches may need to be considered to efficiently dope this material into n-type. Overall, this work paves the way for point defect engineering in this class of ternary oxides.

Li and group-III impurity doping in ZnSnN2 : Potential and limitations

II--IV nitrides and their alloys represent an earth-abundant and potentially cost-efficient alternative to the well-developed AlN-GaN-InN system. A major drawback with the II--IV nitrides is that $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_{2}$, the lowest band gap material, exhibits unfavorably high carrier concentrations for as-grown, stoichiometric material, limiting the material systems potential use in applications such as solar cells and light-emitting diodes. Lithium (Li) has been suggested as a shallow acceptor defect in $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_{2}$ if substituting for Zn, and hence doping with Li has been identified as a possible way to improve the electronic properties. Herein, theoretical calculations by hybrid functional density functional theory have been employed and extended to include defect complexes as well, which to this point remained unexplored. The calculations reveal that even though Li on the Zn site (the ${\mathrm{Li}}_{\text{Zn}}$) is an acceptor, the defect may easily complex with the ${\mathrm{Li}}_{i}$ donor, rendering the complex neutral. Our theoretical findings are supported by a Li-doping series of $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_{2}$, where a doping concentration ranging from $2.10\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ to $1.85\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ was obtained. The $\mathit{n}$-type carrier concentration was found to be unaffected by the doping concentration, and no systematic change in the absorption onset, probably affected by a Burstein-Moss shift, was observed. Possible group-III dopants, as have been found to yield interesting results for $\mathrm{Zn}\mathrm{Ge}{\mathrm{N}}_{2}$, such as In, Ga, Al, and B, have also been investigated as an alternative dopant in $\mathrm{Zn}\mathrm{Sn}{\mathrm{N}}_{2}$.

Hidden impurities in transparent conducting oxides: study of vacancies-related defects and impurities in (Cu–Ni) co-doped ZnO films

The effect of hydrogen and nitrogen impurities on the physical properties of transparent conductive oxides is investigated in this study. Therefore, 5 wt.% of copper and 5 wt.% of nickel co-doped zinc oxide ((Cu–Ni)/ZnO) films were prepared using the sol–gel method. The (Cu–Ni)/ZnO films were annealed in an oven at 500 °C for 2 h under air, vacuum, nitrogen, and argon atmospheres. The synthesized zinc hydroxide film was transformed to zinc oxide film during the annealing by evaporating {mathrm{H}}_{2}mathrm{O}. Films annealed under the mentioned atmosphere including as-prepared one were characterized by analyzing with UV–Vis and FTIR spectra in addition to the 2D mapping electrical conductivity of the surface measured by the 4-point probe. The annealed films under air, vacuum, and argon atmospheres led to generate H-related impurities bounded to the oxygen vacancy ({mathrm{H}}_{mathrm{O}}) which they act as shallow donor defects resulting in forming (Cu–Ni)/ZnO films into n-type materials. Whereas, the film annealed under a nitrogen atmosphere has N-related defects bounding to the zinc vacancy ({mathrm{N}}_{mathrm{Zn}}) which they act as shallow acceptor defects resulting in transforming the film from n-type to p-type. These defects affect the optical, electrical, and optoelectronic properties of the (Cu–Ni)/ZnO films.

Open-air printing of Cu2O thin films with high hole mobility for semitransparent solar harvesters

Cu2O is a promising p-type semiconductor for low-cost photovoltaics and transparent optoelectronics. However, low-cost and low-temperature fabrication of Cu2O films with good transport properties remains challenging, thus limiting their widespread adoption in devices. Here, we report Cu2O thin films of 20–80 nm thickness with hole mobility up to 92 cm2V−1s−1 using atmospheric-pressure spatial atomic layer deposition at temperatures below 260 °C, from a copper (I) hexafluoro-2,4-pentanedionate cyclooctadiene precursor. Raman spectroscopy indicates the presence of copper split vacancies and shows that the high hole mobility can be correlated to a low concentration of shallow acceptor defects. The optical bandgap of deposited films can be tuned between 2.08 eV and 2.5 eV, depending on the deposition temperature. All-oxide semitransparent Cu2O/ZnO solar harvesters are fabricated, showing efficiency values comparable to devices that incorporate much thicker Cu2O layers. Our work provides a promising approach towards cost-efficient, all-oxide solar harvesters, and for other (opto)electronic devices.

Shallow defects found in 2D semiconductors can ionize enough carriers to enable various applicationS

An investigation of the physical origin of intrinsic p-type conductivity in black phosphorene and black arsenene reveals large lattice-like intrinsic shallow acceptor defects.

Effect of Na doping on the performance and the band alignment of CZTS/CdS thin film solar cell

Alkali doping can suppress deleterious antisite defects in kesterite Cu2ZnSnS4 (CZTS) and improve the open-circuit voltage. In this study, the effects of light Na-doping on the performance and the band alignment of CZTS/CdS thin-film solar cells were investigated. CZTS:Na thin films were fabricated by the spin coating with 10% Na doping on the surface of CZTS. The Na-doping led to the narrower FWHM and larger grain size. The hole concentration and the conductivity were improved due to the NaZn shallow acceptor defects. In addition, Na-doping can improve the band alignment of absorber/buffer interface and inhibit SRH recombination by the Na passivation effect and the suppression of SnZn defects. The typical cliff-like conduction band offset (CBO) was reduced from 0.25 eV in CZTS:Na/CdS to 0.1 eV in CZTS/CdS heterojunction. CZTS:Na device exhibited a higher Voc of 653 mV than that of CZTS/CdS device. The maximum conversion efficiency reached 7.46%, increased by 44% after Na-doping. These results clarify the effect of Na-doping on the band structure of the heterojunction in CZTS solar cells and support a new aspect that synthesis of a surface-doping CZTS:Na absorber has great potential for future research.

Boron–Oxygen Complex Responsible for Light‐Induced Degradation in Silicon Photovoltaic Cells: A New Insight into the Problem

Results available in the literature on minority carrier trapping and light‐induced degradation (LID) effects in silicon materials containing boron and oxygen atoms are briefly reviewed. Special attention is paid to the phenomena associated with “deep” electron traps (J. A. Hornbeck and J. R. Haynes, Phys. Rev. 1955, 97, 311) and the recently reported results that have linked LID with the transformation of a defect consisting of a substitutional boron atom and an oxygen dimer (BsO2) from a configuration with a deep donor state into a recombination active configuration associated with a shallow acceptor state (M. Vaqueiro‐Contreras et al., J. Appl. Phys. 2019, 125, 185704). It is shown that the BsO2 complex is a defect with negative‐U properties, and it is responsible for minority carrier trapping and persistent photoconductivity in nondegraded Si:B+O samples and solar cells. It is argued that the “deep” electron traps observed by Hornbeck and Haynes are the precursors of the “slow” forming shallow acceptor defects, which are responsible for the dominant LID in boron‐doped Czochralski silicon (Cz‐Si) crystals. Both the deep and shallow defects are BsO2 complexes, transformations between charge states and atomic configurations of which account for the observed electron trapping and LID phenomena.

Mott type variable range hopping conduction and magnetoresistance in p-type CuIn3Te5 semiconductor compound

Variable range hopping (VRH) conduction of Mott type for a constant and non-vanishing density of states at the Fermi level is observed over a wide range of temperature between 45 and 210 K in the ordered defect compound CuIn3Te5 (CIT135). For this type of electrical conduction at a very high temperature, not reported before in elemental, II-VI, and I-III-VI2 compound semiconductors, we employed three different methods to analyze the data and to confirm this behavior. The occurrence of VRH at very high temperatures is explained as due to the presence of the electrically inactive (InCu+2 + 2 VCu−1) donor-acceptor defect pairs in CIT135. This defect-pair partially annihilates the shallow acceptor defect state Cu vacancy, which is responsible for the activated electrical conductivity observed in p-type ternary I-III-VI2 chalcopyrite compounds in the temperature range of liquid nitrogen. In such conditions, the only acceptor level available for electrical conduction in CIT135 is a deep acceptor state whose activation energy is about 200 meV and cannot be activated below about 200 K. Hence, the VRH conduction mechanism dominates the electrical properties of this material up to about 200 K. The study of the temperature and magnetic field dependence of the magnetoresistance (MR) up to 27 T is made by taking into consideration different theoretical models. To explain the negative MR at lower fields, the theory based on quantum interference is used. At higher magnetic fields, the MR becomes positive and is explained with the model based on the shrinkage of the wave function.

Ga acceptor defects in SnO 2 revisited: A hybrid functional study

Abstract SnO2 is one of the most interesting oxide semiconductors due to its wide band gap, good transparency, high thermal/chemical resistances, and low cost. As-grown SnO2 usually exhibits n-type conductivity with high carrier concentrations. Similar to the case of ZnO, it is difficult to dope SnO2 into p-type. Consequently, applications of SnO2 for optoelectronic as well as electronic devices are limited. In principle, substitution of group-III elements, including Al, Ga, and In, for Sn atom in SnO2 could give a hole carrier resulting in a p-type conductivity. Based on the HSE functional calculations (Varley et al., 2009) [13], it has been reported that these dopants are shallow acceptor defects. However, the calculations with PBE0 functional (Scanlon and Watson, 2012) [14] showed that these dopants are deep acceptors. In this work, Ga-doped SnO2 are revisited by using HSE functional. We found that GaSn defect is indeed a deep acceptor indicating that GaSn can serve only as a compensating acceptor defect in SnO2. Hoping to make the acceptor level shallower, we further study the effect of (compressive) strain on the acceptor level associated with GaSn defect by alloying SnO2 with Si atoms (SixSn1−xO2 alloy where x ~ 0.17). Our results showed that even with the application of strain, the acceptor level remains too deep to be useful.

Mechanisms forp-type behavior of ZnO,Zn1−xMgxO, and related oxide semiconductors

Possibilities of turning intrinsically n-type oxide semiconductors like ZnO and Zn$_{1-x}$Mg$_x$O into p-type materials are investigated. Motivated by recent experiments on Zn$_{1-x}$Mg$_x$O doped with nitrogen we analyze the electronic defect levels of point defects N$_{{\rm O}}$, v$_{{\rm Zn}}$, and N$_{{\rm O}}$-v$_{{\rm Zn}}$ pairs in ZnO and Zn$_{1-x}$Mg$_x$O by means of self-interaction-corrected density functional theory calculations. We show how the interplay of defects can lead to shallow acceptor defect levels, although the levels of individual point defects N$_{{\rm O}}$ are too deep in the band gap for being responsible for p-type conduction. We relate our results to p-type conduction paths at grain boundaries seen in polycrystalline ZnO and develop an understanding of a p-type mechanism which is common to ZnO, Zn$_{1-x}$Mg$_x$O, and related materials.

First principles study of the ternary complex model of EL2 defect in GaAs saturable absorber

First principles calculations are performed for the perfect GaAs crystal, the double Ga vacancies (VGa)₂, and the ternary complex defect (AsGaVAsVGa), using the state-of-the-art computational method with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional to correct the band gap and account for a proper description of the interaction between defects states and bulk states. Three shallow acceptor defect levels are found due to the creation of (VGa)₂ with nearest-neighbor As dangling bonds. However, for GaAs with the ternary complex defects (AsGaVAsVGa), the As antisite AsGa and the VAs'S nearest-neighbor Ga dangling bonds provoke several donor defect states. The lowest donor defect state locates at 0.85 eV below the bottom of conduction band, which is very close to the experimental observation of the EL2 defect level. In addition, structual evolution from (VGa)₂ defect to the ternary defect complex (AsGaVAsVGa) is simulated by ab initio molecular dynamic (MD) calculation at different temperatures. The MD results demonstrate that the ternary complex defect (AsGaVAsVGa) can be converted from the double Ga vacancies (VGa)₂ at room temperature, and it can exist stably at higher temperature. The present work is helpful to unravel the microstructure and the forming mechanism of the EL2 defect, to find out methods to improve the performance of the GaAs saturable absorber by changing the growth conditions of GaAs crystal.

The Special Features of the Hall Effect in GaMnSb Layers Deposited from a Laser Plasma

Epitaxial GaMnSb films with Mn contents up to about 10 at. % were obtained by deposition from a laser plasma in vacuum. The growth temperature T s during deposition was varied from 440 to 200°C, which changed the concentration of holes from 3 × 1019 to 5 × 1020 cm−3, respectively. Structure studies showed that, apart from Mn ions substituting Ga, the GaMnSb layers contained ferromagnetic clusters with Mn and shallow acceptor defects of the GaSb type controlled by the T s value. Unlike single-phase GaMnSb systems studied earlier with negative anomalous Hall effect values and Curie temperatures not exceeding 30 K, the films obtained in this work exhibited a positive anomalous Hall effect, whose hysteresis character manifested itself up to room temperature and was the more substantial the higher the concentration of holes. The unusual behavior of this effect was interpreted in terms of the interaction of charge carriers with ferromagnetic clusters, which was to a substantial extent determined by the presence of Schottky barriers at the boundary between the clusters and the semiconducting matrix; this interaction increased as the concentration of holes grew. The absence of this effect in semiconducting compounds based on III–V Group elements with MnSb or MnAs ferromagnetic clusters was discussed in the literature; we showed that this absence was most likely related to the low hole concentrations in these objects.

Dopants in Diamond Nanoparticles and Bulk Diamond : Density Functional Study of Substitutional B, N, P, SB, S, PN, O, NN, and Interstitial H

We show how calculated ionization potentials and electron affinities of doped diamond nanoparticles can be used to predict the donor and acceptor excitation energies of doped n-type and p-type semiconducting bulk diamond. The method uses good quality quantum chemical calculations on small clusters of diamond and doped diamond. Excitation energies are calculated based on differences in total energies of neutral and positively or negatively charged clusters. When charges are free in the bulk, as for an electron associated with substitutional a simple particle-in-a-box kinetic energy of cluster confinement is taken into account. Optical and thermal excitation energies are calculated for dopants using the B3LYP hybrid density functional method with the 6-31G basis set and substituted nanocrystals in the bulk structure and with structure optimization around the defects. Based on eight experimentally determined values, the errors of this method are 0.6 eV and less. Calculations at this level allow screening for potential shallow donor and shallow acceptor defects. A classical approach to estimating the cluster confinement energy based on charging the cluster in a polarized dielectric continuum is tested and shown to be less successful. © 2002 The Electrochemical Society. All rights reserved.

Growth-induced shallow acceptor defect and related luminescence effects in molecular beam epitaxial GaAs

We report a study of a defect responsible for the ‘‘g’’ bound exciton line at 1.5112 eV that is frequently detected in photoluminescence spectra of GaAs grown by molecular beam epitaxy (MBE). A direct correlation has been observed between this line and a transition at 1.4946 eV, which is shown to result from a conduction band-to-acceptor recombination involving a shallow, unidentified acceptorlike defect that is labeled ‘‘A.’’ The activation energy of the defect is 24.8±0.2 meV, about 1.7 meV lower than that of CAs acceptor. Upon hydrogenation the defect is passivated more extensively than any known shallow acceptor species in GaAs. This result is analyzed in terms of a passivation model, from which it can be inferred that the A defect is not due to a simple substitutional Group II impurity on a Ga site. Incorporation of the A defect strongly affects the luminescence properties of the material. An almost complete quenching of the donor-bound exciton lines, profound changes in the line shape and relative intensity of the free exciton recombination, and appearance of a sharp transition of unknown origin at 1.5138 eV were observed with increasing defect concentration. Apparently ‘‘donorless’’ low temperature exciton recombination spectra are reported for defect-rich p-type MBE GaAs layers with donor concentrations as high as 7×1014 cm−3 and compensation ratios of ∼0.3. The dependence of the defect incorporation on MBE growth parameters is discussed. The feasibility of MBE growth of high purity, nearly shallow defect-free p-type GaAs layers at marginally As-stabilized surface conditions over an about 1–5 μm/h range of deposition rates is demonstrated.

Native defects in undoped semi-insulating CdSe studied by photoluminescence and absorption.

Recombination of carriers in semi-insulating undoped CdSe was studied using infrared absorption and both the polarization and excitation power dependence of the photoluminescence spectra. The photoluminescence measurements showed that a deep donor and a shallow-acceptor state dominate the recombination process, which is slightly modified by a conduction-band tail. Polarization studies showed that the shallow-acceptor level is spin split by \ensuremath{\sim}1.7 meV, and that the band-tail states do not have the same symmetry as the maximum allowed symmetry of the crystal. The absorption spectra showed indirect absorption from energy levels of the shallow-acceptor defect, including a possible deep level at \ensuremath{\sim}1.3 eV. Deep-donor levels of the shallow-acceptor defect may be responsible for high-resistivity undoped CdSe and the self-compensation of p-CdSe.