Antisite Defects Research Articles

Combinatorial tuning of structure and optoelectronic properties of Zn-Ga-O thin films for deep ultraviolet photodetection

Ternary complex oxides exhibit strong compositional dependence on their structure and functionalities. While the profound impact of the cation stoichiometry on the physical properties of ZnGa2O4 is widely recognized, how it affects the ultraviolet photodetection behavior remains not clear. Here we apply a combinatorial pulsed laser deposition technique to establish graded composition in ZnxGa1-xO (0 ≤ x ≤ 1) across a 2-inch sapphire substrate. In combination with structural and optical characterizations, a systematic investigation of the compositional effect on the phase, crystallinity and bandgap for the ZnxGa1-xO system is performed, and their further impact on the photodetection performance is compared by fabricating multiple metal–semiconductor-metal (MSM) photodetectors based on the graded ZnxGa1-xO films. Decent photodetection behavior is seen for the photodetectors with x ≤ 0.33, above which the performance is largely limited by the conductive phase formation. An optimized compositional range of x = 0.18–0.27 is identified to deliver the best device performance, which includes a responsivity of 12.88 A/W and a photo-to-dark current ratio of 2.8 × 108. This slight zinc deficiency is considered beneficial for the formation of moderate level of oxygen vacancies or antisite defects that boosts carrier transport. This study sheds light on the composition-dependent evolution of structure and properties in ternary ultrawide-bandgap semiconductors for high-performance ultraviolet photodetection.

Identifying the intrinsic anti-site defect in manganese-rich NASICON-type cathodes

Identifying the intrinsic anti-site defect in manganese-rich NASICON-type cathodes

Optical, surface, and structural studies of InN thin films grown on sapphire by molecular beam epitaxy

A series of indium nitride (InN) thin films have been grown on sapphire substrates by molecular beam epitaxy (MBE) technology under different growth conditions of temperature and plasma power. Their structural, surface, and optical properties are studied by a variety of techniques of scanning electron microscopy, Hall effect, x-ray diffraction, photoluminescence (PL), Raman scattering, x-ray photoelectron spectroscopy (XPS), synchrotron radiation x-ray absorption near edge structure (XANES), and so on. The lower carrier concentration in InN can be obtained with a higher MBE growth temperature and a lower plasma power. As the plasma power increases, the PL peak energy is observed to shift toward the higher energy side and the Raman E2 (high) and A1 (LO) modes are shifted to the lower frequency. The residual compressive strain in epitaxial InN is relaxed. The lower plasma power and the higher growth temperature are preferred for the MBE growth of high-quality InN films. The influencing factors on the InN PL peak and band gap Eg have been revealed. It is evidenced that the InN PL peak and Eg can be shifted from high down to ∼0.65 eV with the carrier concentration down to low E19 cm−3 and the plasma power down to ∼80 W. Both the XPS and N K-edge XANES revealed the antisite defect of N on the In site, NIn. The XANES In L-edge measurements on the In L3-edge of InN films with various carrier concentrations has indicated the fourfold InN intermediate crystal structures. These obtained results are significant and useful to deepen the understanding and promote further investigation in InN and III-N materials.

Oxidation-promoting strategy boosts highly ordered Co-free Ni-rich layered oxides

Nickel-rich cobalt-free cathodes are considered as an attractive candidate for next-generation lithium-ion batteries in terms of economic efficiency, environmental friendliness, and energy density. However, high-nickel and zero-cobalt contents are prone to induce layered oxide cathode active materials (CAMs) with undesirable anti-site defects (Li+/Ni2+ mixing) and a defective interface (rock-salt phase layer). Herein, a highly ordered structural LiNi0.80Mn0.17Al0.03O2 CAM was successfully synthesized via the active Li2O2 boosted method. The significant reduction in Li+/Ni2+ mixing (5.5%→2.9%) and thickness of the rock-salt phase interface (5 nm→1.2 nm) vigorously verify the feasibility of this strategy. The evolution of the Ni2+/Ni3+ proportion and primary particle size can further illustrate the co-oxidation mechanism of this strategy. In-situ XRD also shows that the degree of H2-H3 phase transition in the modified sample is relieved, which alleviates the volume change. As a result, the optimized sample displays enhanced cyclability (162.60 mAh/g at 1C over 100 cycles) and advanced rate performance (124.29 mAh/g at 5C).

Enhancing structural stability of γ′-Co3(Al, Nb) phases: The key role of anti-site defects

The design of the composition for two-phase superalloys of γ/γ′ has always been key to achieving a breakthrough in their mechanical performance. However, the underlying mechanism of the anti-site defects in γ′ precipitates that severely affect stability and performance have rarely been explored. The present work therefore specifically investigated the structural stability and mechanical properties of strictly stoichiometric and off-stoichiometric γ′-Co3(Al, Nb) phases by using the first-principles calculations. The results show that the change in the Al/Nb ratios in the compounds could induce the instability of the L12 structure, while an increase of Nb atoms leads to compounds with predominantly covalent bonds exhibiting stronger mechanical properties. After evaluating the Gibbs free energy of these γ′-Co3(Al, Nb) phases at finite temperature, it is found that Co29Al2Nb1 and Co29Al1Nb2 compounds with anti-site defects can be thermodynamically stable at greater than 710 and 905 K, respectively. Accordingly, Co-10Al-2Nb and Co-2.5Al-3Nb alloys were successfully prepared experimentally, further confirming that the γ′-Co3(Al, Nb) phase with anti-site defects can precipitate preferentially and remain stable during the 168 h annealing treatment.

Threading dislocation and lattice stress modulation of Si based GaN material with AlPN nucleation layer

An innovative AlPN nucleation layer is proposed for the growth of high quality GaN on Si. AlPN nucleation layer is prepared on Si substrate by metal organic chemical vapor deposition to explore the stress and dislocation adjustment mechanism. It is found that there are stress-related antisite defects in the P-doped lattice. In addition, the incorporation of P makes the dislocation densities of screw dislocations and edge dislocations have different trends, which can realize the independent control of the different types of dislocation densities. A hexagonal-cubic-phase mixed structure model is proposed to analyze and explain the variation trends of dislocation densities and stress. This work not only puts forward a new type of nitride material, which provides an idea for the growth of high-quality GaN on Si. On the other hand, it is of great significance to perfect the independent regulation mechanism of dislocations.

Energetics and diffusion kinetics of point defects in MnCr2O4 spinel from first principles

MnCr2O4 spinel is a component of the multilayered oxide film which forms atop austenitic steel alloys under high-temperature corrosion conditions. In this work, the thermodynamics and kinetics of cationic and anionic point defects in this spinel are examined through density functional theory calculations. To model the physical conditions of the corrosion process more closely, temperature is accounted for by adjusting the chemical potential of each species to fit experimentally determined values. We find that manganese is the most mobile species - migrating through the lattice via a vacancy mediated mechanism. Oxygen migration can occur via vacancy mediated or three-body interstitial mechanisms with similarly low barrier heights, highlighting the possibility of oxide growth at the film/alloy interface due to the high oxygen permeability. Furthermore, a number of more complex migration mechanisms involving substitution and antisite defects are evaluated for their potential contribution to cation mobility.

Magnetic and transport properties in metallic and disordered Ru2VAl and Ru2VGa

In this work, we report an elaborate study on the structural, magnetic and thermoelectric properties of two Ru-based Heusler alloys, viz., Ru2VAl and Ru2VGa, having valence electron count 24. While Ru2VGa exhibits a nonmagnetic ground state, in Ru2VAl a short range ferromagnetic interaction is developed below 10 K due to the structural antisite defects and disorder, that also manifests its signature in the specific heat and electrical resistivity measurements at low temperatures. Both the Seebeck coefficient (S) and Hall measurements suggest the dominance of hole-type carrier for electronic transport in Ru2VAl whereas Ru2VGa is found to be a n-type material. The linear nature and small absolute values of S, as well as non-zero values of electronic contribution to specific heat γS (γSRu2VAl = 4.9 mJ/mol-K2 and γSRu2VGa = 5.5 mJ/mol-K2) point towards a metallic ground state for both these alloys. Both the compounds exhibit lower thermal conductivity at room temperature in comparison to Fe-based Heusler alloys due to the presence of heavier element Ru.

INFLUENCE OF HEATING RATE ON THE STRUCTURAL AND OPTICAL PROPERTIES OF SILVER AND GERMANIUM CO-DOPED CZTS THIN FILM

The effect of heating rate on the structural and optical properties of Ag+Ge co-doped CZTS thin film were investigated and compared with the undoped CZTS sample. The undoped and Ag+Ge co-doped CZTS samples obtained by two-stage technique consisting of the sequential deposition of the precursor stacks by sputtering systemand sulfurization of these layers at elevated temperature in the RTP system by employing heating rate of 1°C/s, 2°C/s and 3°C/s. Ag and Ge co-doped precursor stack as well as undoped stack demonstrated Cu-poor, Zn-rich composition. In addition, the dopant ratio of the Ag+Ge co-doped stack was close to the targeted content considering to EDS measurement. Regardless of the employed heating rate or the doping process, all of the samples crystallized in a kesterite structure. However, it was confirmed by XRD measurements that high heating rates caused phase separation in kesterite phase formation. On the other hand, The Raman peaks assigned to Cu-vacancy and CuZn antisite defects formation inhibited with incorporating Ag and Ge into the CZTS structure. Ag and Ge co-doped CZTS sample produced with a heating ramp rate of 1°C/s showed better structural and optical results among them.

Effect of crystal growth conditions on carrier lifetime and lattice defects in the solar absorber material ZnSnP2

We investigated the minority carrier lifetime and behavior of lattice defects in ZnSnP2 bulk crystals through experiments on carrier recombination and defect properties. Advanced deep level transient spectroscopy (DLTS) revealed that an electron trap with a short time constant at 0.2 eV below the conduction band minimum edge may contribute to the short minority carrier lifetime evaluated by time-resolved photoluminescence (TRPL). The temperature dependence of steady-state photoluminescence suggested that the carrier recombination through the electron trap was nonradiative around room temperature, which supports the fact of the short carrier lifetime and lower current density in ZnSnP2 solar cells. Previously reported theoretical calculation suggests that such a trap comes from the antisite defect of Sn from the viewpoint of the thermodynamic transition level. We, thus, prepared ZnSnP2 crystals by the solution growth method under conditions with a higher chemical potential of Zn, and we achieved the enhancement of the carrier lifetime compared to that under other growth conditions. In this case, the evaluation of the liquidus temperature and chemical potentials by a thermodynamic model indicated that the formation of Sn antisite was effectively suppressed by a lower precipitation temperature in addition to the effect of chemical potentials. Finally, we demonstrated the improvement of current density in ZnSnP2 solar cells using crystals with a longer lifetime, especially in the longer wavelength range.

Mitigation of Power Factor Degradation in Bi2Te3 Thin Films

For thin-film thermoelectric modules with electrodes deposited on the sides of the thermoelectric material, the diffusion of atoms from the electrode affects the module’s performance. Long-term aging is crucial when the module is for practical applications. In this study, Bi2Te3 thin films with a highly preferred orientation were fabricated by a co-sputtering deposition method. Cu was used as the electrode because of its high electrical conductivity; it migrates through the Bi2Te3 film via surface and grain boundary diffusion. The diffusing Cu segregates at grain boundaries and forms a Cu2–xTe intermetallic compound on the sample surface. Contact resistivity at the interface between the Cu electrode and Bi2Te3 thin films substantially increases with aging time. Long-term aging unavoidably degrades the power factor (PF) of the pristine Bi2Te3 since n-type Bi2Te3 converts into p-type by the formation of antisite defects. The study shows that the diffusing Cu from the electrodes of the Cu/Bi2Te3/Cu module significantly mitigates the degradation of the PF after long-term aging.

Robustness of semimetallic transport properties of TaAs against off-stoichiometric disorder

TaAs single crystals were grown by a standard chemical vapor transport method. The single-crystallinity and homogeneous distribution of elements were confirmed by transmission electron microscope and x-ray diffraction observations. Positron annihilation measurements revealed that the atomic vacancy concentration was kept below 10−5 at. %. However, inductively coupled plasma analysis showed an As-deficient (7–9 at. %) off-stoichiometry. First-principles calculations implied that the off-stoichiometry could be compensated for with excess Ta antisite defects, thereby inducing metallic states. Nevertheless, excellent semimetallic transport properties of a well-suppressed carrier density (≲1018 cm−3), ultrahigh carrier mobility (≳106 cm2/V/s), and large transverse magnetoresistance (>200000% at 9 T) with the quantum oscillation were obtained at 1.7 K. This indicated the robustness of semimetallic transport properties against the off-stoichiometric disorder and the quenching of metallic conduction associated with excess Ta atoms. The negative longitudinal magnetoresistance, which is considered evidence of a Weyl semimetal (chiral anomaly), was not observed. These data were discussed with theoretical calculations.

Defect and spectroscopy properties of Eu-doped RbBaPO4 phosphors: A computer simulation study

The effects of Eu incorporation in the RbBaPO4 structure was investigated using atomistic simulation based on lattice energy minimization combined with crystal field models. The defect study reveals that the most favorable intrinsic defect type is formed by the anti-site defect. The results revealed that Eu3+ prefers to enter the Ba site with a RbBa' anti-sites as charge compensation. Crystal field theory shows that the substitution of the Eu3+ ion for the Ba2+ ion in the anti-site defect accompanied by the local lattice deformation induces a local symmetry without the inversion center of the slightly distorted C4 type at the position of the impurity Eu3+ ion. The 7F1 and 7F2 energy sublevels as well as the 7F1 manifold were satisfactorily reproduced with a set of phenomenological charge factors and non-null crystal field parameters.

Defect formation and migration in MAlB (M=Mo, W) and N2AlB2 (N=Cr, Fe) : A first-principles study

The exceptional properties of MAB phases (where M = transition metal, A = aluminum, and B = boron) make them a promising option for various applications. However, their usage in most fields requires an understanding of the defect formation and migration processes, which can aid in designing durable and functional materials. Through density functional theory calculations, we investigated the stability and mobility of the most prominent point and extended defects in MAlB (where M = Mo and W) and ${\mathrm{N}}_{2}{\mathrm{AlB}}_{2}$ (where N = Cr and Fe) MAB phases under different chemical conditions. Our findings indicate that ${\mathrm{V}}_{\mathrm{B}}/{\mathrm{N}}_{\mathrm{Al}}$ is the easiest defect to form under M/N-rich conditions. We observed that vacancy and antisite defects form more easily than interstitial ones in all studied systems. We also investigated different extended structural defects and stacking faults and revealed the stability of relevant compositional defects and tilt/rotational boundaries in MAlB and ${\mathrm{N}}_{2}{\mathrm{AlB}}_{2}$. Our results suggest that these systems can likely be synthesized by tuning the experimental conditions. We additionally observed grain boundary formation in ${\mathrm{N}}_{2}{\mathrm{AlB}}_{2}$ and compared our findings with relevant experiments.

Chemical Trend of Nonradiative Recombination in Cu(In,Ga)Se2 Alloys

Understanding nonradiative recombination is important for improving semiconductor devices. For Cu(In,Ga)Se${}_{2}$ (CIGS) solar cells, antisite defects have long been considered the main recombination centers, yet the underlying mechanism has remained elusive. Here first-principles calculations show that these ``killer centers'' themselves cannot capture holes efficiently for effective recombination. However, internal conversion to the distorted neutral DX center does open an efficient hole-capture pathway, and DX's stability in CIGS increases with Ga concentration, which resolves the longstanding issue of why the efficiency of CIGS solar cells decreases at high Ga concentration.

Defect engineering in ZnIn2X4 (X=S, Se, Te) semiconductors for improved photocatalysis

ZnIn2S4 has emerged as a material of interest for semiconductor-based chalcogenide photocatalysts due to its visible light absorption, chemical and thermal stability, and low cost. However, the photocatalytic activity of ZnIn2S4 is affected by the limited range of visible light absorption and ultrafast recombination of solar light-induced holes and electrons. While previous studies have considered the consequences of metal doping, metal deposition, and vacancy engineering on the photocatalytic activity of ZnIn2S4, a comprehensive understanding of native point defects and how they affect electronic and photocatalytic properties remains elusive. Here, we present a density functional theory (DFT) investigation of defect energetics in ZnIn2X4 (X=S, Se, Te) compounds in both bulk and ultrathin phases. Using both semi-local and hybrid DFT functionals, properties of interest such as the electronic band gap and band edges, optical absorption spectra, and carrier mobilities are first computed for defect-free structures. Although ultrathin ZnIn2S4 shows lower absorption compared to other chalcogenides, it exhibits sufficient overpotential for oxidation and reduction reactions for photocatalytic water splitting. Formation energies of all possible vacancies, self-interstitials, and anti-site substitutional defects are then computed for all structures, as a function of chemical growth conditions, charge state, and Fermi level (EF), which leads to the identification of the lowest energy acceptor and donor type defects and their corresponding shallow or deep level nature. DFT results show that these metal sulfide photocatalysts are prone to ZnIn and InZn anti-site substitutions, which pin the equilibrium EF close to the conduction band edge, indicative of n-type conductivity. While ZnIn does not create deep defect levels in ZnIn2X4, most of the stable native defects do create deep levels, which could adversely affect solar absorption. Finally, we report the influence of defects on the photocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on ultrathin ZnIn2X4. Our results suggest that a metal interstitial defect could substantially boost HER and OER on the surface of ultrathin ZnIn2X4. Overall, this systematic first principles investigation can help drive the experimental design and defect engineering of ZnIn2X4 compounds for a variety of photocatalytic applications.

Effect of Magnetic Field and Hydrostatic Pressure on Metamagnetic Isostructural Phase Transition and Multicaloric Response of Fe49Rh51 Alloy

The effect of a high magnetic field up to 12 T and a high hydrostatic pressure up to 12 kbar on the stability of the metamagnetic isostructural phase transition and the multicaloric effect of Fe49Rh51 alloy has been studied. The phase transition temperature shifts under the magnetic field and the hydrostatic pressure on with the rates of dTm/μ0dH = −9.2 K/T and dTm/dP = 3.4 K/kbar, respectively. The magnetocaloric and multicaloric (under two external fields) effects were studied via indirect method using Maxwell relations. The maximum of the entropy change is increasing toward the high temperature region from ∆S~2.5 J/(kg K) at 305 K to ∆S~2.7 J/(kg K) at 344 K under simultaneously applied magnetic field of 0.97 T and hydrostatic pressure of 12 kbar. The obtained results were explained using the first-principle calculations of Gibbs energies and the phonon spectra of the ferromagnetic and the antiferromagnetic phases. Taking into account the low concentration of antisite defects in the calculation cells allows us to reproduce the experimental dTm/dP coefficient.

Cation-Disorder-Enhanced Unintentional Doping in MgSnN2

${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_{2}$ has recently attracted significant interest as a promising candidate for applications in green light-emitting diodes and photovoltaic absorbers. However, the experimentally synthesized ${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_{2}$ samples suffer from a high concentration of unintentionally doped electrons, and their origin is not fully understood yet. By performing first-principles calculations, we investigate the properties of intrinsic point defects and oxygen impurities in cation-ordered and -disordered wurtzite ${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_{2}$. It is found that the cation antisite defect ${\mathrm{Sn}}_{\mathrm{Mg}}$ is the predominant donor defect, contributing an electron concentration as high as ${10}^{17}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ to cation-ordered ${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_{2}$. However, this value is 2--3 orders of magnitude lower than experimental observations (${10}^{19}\ensuremath{-}{10}^{20}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$). We further show that cation disorder significantly decreases the formation energies of defects, leading to a much higher electron concentration (${10}^{19}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$) than in the case of the cation-ordered phase. The reduced formation energy can be understood by the energy gain due to the creation of energetically favorable local motif structures after the formation of defects. Hence, reducing the degree of cation disorder is beneficial for decreasing the densities of defects and self-doped carriers.

An Electrostatically Embedded QM/MM Scheme for Electrified Interfaces.

Atomistic modeling of electrified interfaces remains a major issue for detailed insights in electrocatalysis, corrosion, electrodeposition, batteries, and related devices such as pseudocapacitors. In these domains, the use of grand-canonical density functional theory (GC-DFT) in combination with implicit solvation models has become popular. GC-DFT can be conveniently applied not only to metallic surfaces but also to semiconducting oxides and sulfides and is, furthermore, sufficiently robust to achieve a consistent description of reaction pathways. However, the accuracy of implicit solvation models for solvation effects at interfaces is in general unknown. One promising way to overcome the limitations of implicit solvents is going toward hybrid quantum mechanical (QM)/molecular mechanics (MM) models. For capturing the electrochemical potential dependence, the key quantity is the capacitance, i.e., the relation between the surface charge and the electrochemical potential. In order to retrieve the electrochemical potential from a QM/MM hybrid scheme, an electrostatic embedding is required. Furthermore, the charge of the surface and of the solvent regions has to be strictly opposite in order to consistently simulate charge-neutral unit cells in MM and in QM. To achieve such a QM/MM scheme, we present the implementation of electrostatic embedding in the VASP code. This scheme is broadly applicable to any neutral or charged solid/liquid interface. Here, we demonstrate its use in the context of GC-DFT for the hydrogen evolution reaction (HER) over a noble-metal-free electrocatalyst, MoS2. We investigate the effect of electrostatic embedding compared to the implicit solvent model for three contrasting active sites on MoS2: (i) the sulfur vacancy defect, which is rather apolar; (ii) a Mo antisite defect, where the active site is a surface bound highly polar OH group; and (iii) a reconstructed edge site, which is generally believed to be responsible for most of the catalytic activity. According to our results, the electrostatic embedding leads to almost indistinguishable results compared to the implicit solvent for the apolar system but has a significant effect on polar sites. This demonstrates the reliability of the hybrid QM/MM, electrostatically embedded solvation model for electrified interfaces.

Amino-acid-type alkylamine additive for high-performance wide-bandgap perovskite solar cells

A wide-bandgap (WBG) perovskite top component cell is vital for all perovskite-based tandem solar cells. However, such WBG perovskite solar cell (PSC) suffers from inferior crystallinity, huge voltage loss and poor photostability. Herein, we report a amino-acid-type alkylamine, 5-aminolevulinic acid hydrochloride (ALH) additive to address these issues to enhance the performance of WBG PSCs. It is found that the ALH effectively modulates the perovskite crystallization via chemical complexation between the ALH and PbI2, and enhances the crystallinity of the perovskite film. Moreover, the amino acid ALH not only passivates negatively charged deep-level IFA and IPb anti-site defects via –NH3+ terminals to reduce voltage loss, but also suppresses photo-induced phase segregation via the coordination between –COO− groups and halide vacancies. With these merits, the ALH-based PSC delivers an excellent power-conversion efficiency (PCE) of 21.13%, one of the highest values for inverted WBG PSCs. In addition, the air and thermal stability are also significantly improved. Our findings provide a feasible approach to prepare highly efficient and stable WBG PSCs.