Charged Defect States Research Articles

Modulation of ferromagnetism through electron doping in Pd-doped β-Ga2O3

The incorporation of magnetism into semiconductor materials offers promising opportunities for expanding their applications in spintronics. In this study, first-principles calculations are employed to investigate the defect energetics and magnetic properties of Pd-doped β-Ga2O3, where Pd atoms substitute for Ga atoms. The results reveal that Pd substitution at tetrahedrally coordinated Ga sites incurs significantly higher energy, favoring octahedral coordination instead. Under n-type doping conditions, Pd tends to stabilize in a negatively charged state, contributing to its magnetic behavior. A detailed analysis of the interaction between Pd-Pd pairs demonstrates that ferromagnetism can be effectively controlled by tuning the carrier concentration. Specifically, at optimal n-type carrier densities, a pronounced ferromagnetic interaction is observed, while this effect diminishes with further increases in electron concentration. The findings provide valuable insights into the defect structures, charge states, and magnetic properties of Pd-doped β-Ga2O3, offering a pathway to manipulating magnetic behavior in semiconductors via carrier concentration modulation, thus paving the way for future spintronic device applications.

Charge Dynamics and Defect States under “Spot‐Light”: Spectroscopic Insights into Halide Perovskite Solar Cells

Halide perovskite solar cells (PSCs) have shown remarkable power conversion efficiencies. However, the inherent defect issues of perovskite materials still limit their performance and long‐term stability, resulting in lifespans far from commercial standards. With their noninvasive approach to probing the electronic and vibrational properties of materials, spectroscopic techniques have become crucial tools for uncovering and understanding defect states and complex charge carrier dynamics in halide perovskites. This review explores the application of various advanced spectroscopic techniques in PSCs to elucidate the complex behaviors of charge carriers within PSCs. These techniques reveal detailed temporal and spatial distributions of charge carriers, enabling precise analysis of defect impacts and interfacial charge transfer processes. By integrating spectroscopic data, it is possible to more accurately identify and mitigate defect‐induced nonradiative recombination and charge transfer, thereby enhancing the stability and efficiency of PSCs. This comprehensive spectroscopic understanding is crucial for developing innovative technologies to accelerate the commercial viability of PSCs.

First-principles calculations for intrinsic defects in MgO: Positron lifetime and momentum distribution of electron–positron pairs

We presented first-principles calculations of the formation energies for various charge states of intrinsic defects in magnesium oxide, including VO, VMg, VMg+O, VO+Mg+O, and VMg+O+Mg. These results can be used to predict their thermodynamic stability and detectability of these defects via positron annihilation spectroscopy. We employed two-component density functional theory to compute the positron annihilation properties, incorporating the effects of temperature and spin polarization. We then performed calculations of the momentum distributions of annihilating electron–positron pairs in these intrinsic defects in MgO. Using one-dimensional momentum distributions (Doppler-broadening spectra), we calculated the line-shape parameters Srel and Wrel. Our positron lifetime calculations were compared with experimental data. These research provides insight into the positron annihilation characteristics of defects in MgO.

The role of defect charge, crystal chemistry, and crystal structure on positron lifetimes of vacancies in oxides

Density functional theory based positron lifetime (PL) calculations for cation and oxygen monovacancies in a range of oxides-hematite, magnetite, hercynite, and alumina-have been conducted to compare the impact of defect chemistry and crystal structure on the predicted lifetimes. The role of defect charge state has also been examined. A comparison across the same type of crystalline structure but different composition shows that oxygen vacancies only induce a slight increase in the positron-electron overlap and thus barely modify the PL as compared to the bulk. A much more substantial increase of PL is observed for cation monovacancies, regardless of crystal structure or the elemental nature of the vacancy, which we ascribe to an enhanced localization of charge density around the vacant site. The structural and compositional richness of the oxide leads to longer defect PLs, with defected hercynite exhibiting the longest PLs. The charge state of cation monovacancies modifies only by a small percentage the positron localization, relegating to secondary importance the metal defect's oxidation state in modifying the lifetime of positrons within vacancy traps.

The defect chemistry and machine learning study 5d transition metal doped on graphitic carbon nitride for bifunctional oxygen electrocatalyst with low overpotential

Searching for highly efficient bifunctional oxygen evolution reaction/oxygen reduction reaction (OER/ORR) electrocatalysts for sustainable and renewable clean energy is vital. However, these electrocatalyst designs remain cost-prohibitive for both experimental and computational studies. This paper systematically investigates 5d transition metal (5d TM) doped into graphitic carbon nitride (g-C3N3) as potential bifunctional OER/ORR electrocatalysts by considering the defect charge states through the defect chemistry study. Our formation energies results show that 33 kinds of 5d TM doped g-C3N3 have better stabilities in the different charge states. Among them, IrN×@C54N53 (Ir occupation N), IrN•@C54N53, PtN×@C54N53 (Pt occupation N), and Iri•@C54N54 (Ir interstitial sites) exhibit lower overpotentials. They are great candidates for bifunctional OER/ORR electrocatalysts. This is because adjusting the charge states of 5d-TM@g-C3N3 can tune the interaction strength of the oxygenated intermediates. Furthermore, machine learning shows that the charge transfer of TM atoms (Qe) and total magnetic moment (μB0) are the two most essential descriptors for OER and ORR overpotential, respectively. The charged defect adjusting of the bifunctional OER/ORR activity would develop the potential electrocatalysts for energy conversion applications.

Thermodynamical stability of carbon-based defects in α boron from first principles

We report an exhaustive study of the formation enthalpy and charge states of carbon-based defects in rhombohedral α boron within the density functional theory (DFT) that enables us to derive rules about the formation of complex carbon defects. We have accounted for one and two interstitial carbon atoms, eventually combined with one substitutional carbon atom and/or one interstitial boron atom and varied several geometric parameters. We find that when positioned in the plane perpendicular to the [111] rhombohedral axis, two carbon atoms turn out to preferentially form a graphite-like hexagon with four boron atoms. When positioned instead along the [111] axis, the distance between them strongly affects the defect thermodynamic stability, and we find in particular that additional negative charges strongly stabilize the diatomic carbon–carbon chains.

Oxygen-related defects in 4H-SiC from first principles

We investigated the abundance, structures, energy levels, and spin states of oxygen-related defects in 4H-SiC on the basis of first-principles calculations. We applied a hybrid functional in the overall calculations, which gives reliable defect properties, and also considered relevant defect charge states. We identified the oxygen interstitial (O i,1), substitutional oxygen (OC), and oxygen-vacancy (OC V Si) complex as prominent defects in n-type conditions. Among them, OC V Si was predicted as a spin-1 defect with NIR emission in a previous study. On the basis of the obtained results, we discuss the possible spin decoherence sources when employing OC V Si as a spin-to-photon interface.

Investigating the electrical and optical characteristics of lead-free all-inorganic Cs3Sb2Br9 single crystals

In recent years, halide perovskites have emerged as promising materials for electronic devices. While lead-based perovskites have been extensively studied, exploration of lead-free variants has been limited. Here, we highlight the innovation of observing negative photoconductivity (NPC) induced by light, followed by self-recovery, in lead-free Cs3Sb2Br9 perovskite single crystals. Our study reveals the self-trapping of holes at the Vk center, corresponding to the Br2− dimer, within the midband states of this vacancy-ordered perovskite. This leads to the entrapment of photogenerated charge carriers by charged defect states denoted as Vk, creating an internal electrical field that counteracts the externally imposed field, resulting in NPC. We demonstrate the innovation through the construction of a prototype photodetector with notable sensitivity, featuring high responsivity (6.77 mA/W), detectivity (2.83 × 1012 Jones), and a dark-to-light current ratio of approximately 10. This recognition of retroactive photocurrent in optically active perovskite materials holds promise for advancing highly sensitive detectors.

Density functional theory study of Al, Ga and in impurities in diamond

As a consequence of its high atomic number density, diamond can incorporate a relatively limited range of impurities as distributed point-defects, chiefly N, B and H. A few other species can be grown-in, and other impurity species incorporated via implantation and annealing. For applications including electronic, electrical and quantum devices, the presence of states deep within the wide band-gap is of importance, and the list of potential colour centres available for exploitation continues to grow. Although B can be grown into diamond at high concentration, study of other group-13 elements is rather limited. In this paper we present the results of modelling of Al, Ga and In. We find all species readily form complexes with vacancies, and exhibit electronic structures that parallel those of the SiV complex. We report electronic structures, electrical levels, optical transitions and hyperfine interactions of the colour centres, as well as reflect upon the thermodynamics of the complexes. We suggest that co-implanting group-13 elements with nitrogen would give rise to the defect charge states with potential for quantum applications.

Nanoscale Probing of Electrical Memory Effects in van der Waals Layered PdSe2.

Tunable electronic materials that can be switched between different impedance states are fundamental to the hardware elements for neuromorphic computing architectures. This "brain-like" computing paradigm uses highly paralleled and colocated data processing, leading to greatly improved energy efficiency and performance compared to traditional architectures in which data have to be frequently transferred between processor and memory. In this work, we use scanning microwave impedance microscopy for nanoscale electrical and electronic characterization of two-dimensional layered semiconductor PdSe2 to probe neuromorphic properties. The local resolution of tens of nanometers reveals significant differences in electronic behavior between and within PdSe2 nanosheets (NSs). In particular, we detected both n-type and p-type behaviors, although previous reports only point to ambipolar n-type dominating characteristics. Nanoscale capacitance-voltage curves and subsequent calculation of characteristic maps revealed a hysteretic behavior originating from the creation and erasure of Se vacancies as well as the switching of defect charge states. In addition, stacks consisting of two NSs show enhanced resistive and capacitive switching, which is attributed to trapped charge carriers at the interfaces between the stacked NSs. Stacking n- and p-type NSs results in a combined behavior that allows one to tune electrical characteristics. As local inhomogeneities of electrical and electronic behavior can have a significant impact on the overall device performance, the demonstrated nanoscale characterization and analysis will be applicable to a wide range of semiconducting materials.

Identification and thermal healing of focused ion beam-induced defects in GaN using off-axis electron holography

Thermal healing of focused ion beam-implanted defects in GaN is investigated by off-axis electron holography in TEM. The data reveal that healing starts at temperatures as low as about 250 °C. The healing processes result in an irreversible transition from defect-induced Fermi level pinning near the VB toward a midgap pinning induced by the crystalline-amorphous transition interface. Based on the measured pinning levels and the defect charge states, we identify the dominant defect type to be substitutional carbon on nitrogen sites.

Manipulating solid-state spin concentration through charge transport

Solid-state defects are attractive platforms for quantum sensing and simulation, e.g., in exploring many-body physics and quantum hydrodynamics. However, many interesting properties can be revealed only upon changes in the density of defects, which instead is usually fixed in material systems. Increasing the interaction strength by creating denser defect ensembles also brings more decoherence. Ideally one would like to control the spin concentration at will while keeping fixed decoherence effects. Here, we show that by exploiting charge transport, we can take some steps in this direction, while at the same time characterizing charge transport and its capture by defects. By exploiting the cycling process of ionization and recombination of NV centers in diamond, we pump electrons from the valence band to the conduction band. These charges are then transported to modulate the spin concentration by changing the charge state of material defects. By developing a wide-field imaging setup integrated with a fast single photon detector array, we achieve a direct and efficient characterization of the charge redistribution process by measuring the complete spectrum of the spin bath with micrometer-scale spatial resolution. We demonstrate a two-fold concentration increase of the dominant spin defects while keeping the T2 of the NV center relatively unchanged, which also provides a potential experimental demonstration of the suppression of spin flip-flops via hyperfine interactions. Our work paves the way to studying many-body dynamics with temporally and spatially tunable interaction strengths in hybrid charge-spin systems.

The mechanism of the irradiation synergistic effect of silicon bipolar junction transistors explained by multiscale simulations of Monte Carlo and excited-state first-principle calculations.

Neutron and γ-ray irradiation damages to transistors are found to be non-additive, and this is denoted as the irradiation synergistic effect (ISE). Its mechanism is not well-understood. The recent defect-based model [Song and Wei, ACS Appl. Electron. Mater. 2, 3783 (2020)] for silicon bipolar junction transistors (BJTs) achieves quantitative agreement with experiments, but its assumptions on the defect reactions are unverified. Going beyond the model requires directly representing the effect of γ-ray irradiation in first-principles calculations, which was not feasible previously. In this work, we examine the defect-based model of the ISE by developing a multiscale method for the simulation of the γ-ray irradiation, where the γ-ray-induced electronic excitations are treated explicitly in excited-state first-principles calculations. We find the calculations agree with experiments, and the effect of the γ-ray-induced excitation is significantly different from the effects of defect charge state and temperature. We propose a diffusion-based qualitative explanation of the mechanism of positive/negative ISE in NPN/PNP BJTs in the end.

Manipulating the Charge State of Spin Defects in Hexagonal Boron Nitride.

Negatively charged boron vacancies (VB-) in hexagonal boron nitride (hBN) have recently gained interest as spin defects for quantum information processing and quantum sensing by a layered material. However, the boron vacancy can exist in a number of charge states in the hBN lattice, but only the -1 state has spin-dependent photoluminescence and acts as a spin-photon interface. Here, we investigate the charge state switching of VB defects under laser and electron beam excitation. We demonstrate deterministic, reversible switching between the -1 and 0 states (VB- ⇌ VB0 + e-), occurring at rates controlled by excess electrons or holes injected into hBN by a layered heterostructure device. Our work provides a means to monitor and manipulate the VB charge state, and to stabilize the -1 state which is a prerequisite for spin manipulation and optical readout of the defect.

First-principles studies of charged defect states in intrinsic ferromagnetic semiconductors: the case of monolayer CrI3

Defects play significant roles in spin-current-related physical processes in intrinsic ferromagnetic semiconductors (FMSs), which are great promise for spintronics applications. However, current defect calculation methods cannot be used to investigate charged defects in FMSs due to the spin polarization of both the charged defect states and ionized carriers, which is not well treated in current defect calculation methods. In order to solve this problem, we propose a spin-distinguishable charge correction (SDCC) method that uses spin-polarized band edge charge density instead of spin-unpolarized uniform background charge density as the compensating charge for charged defects. We apply our method to study the defect properties of CrI3 monolayer and find it can be doped n-type under the Cr-rich growth condition but difficult to be doped p-type. The SDCC method proposed here is generally suitable for all FMSs, which will be useful for the studies of defect properties of magnetic semiconductors.

Theory of magnetic 3d transition metal dopants in gallium nitride

Using first-principles density functional theory (DFT) methods and size-converged supercell models, we analyze the electronic and atomic structure of magnetic $3d$ transition metal dopants in cubic gallium nitride (c-GaN). All stable defect charge states for Fermi levels across the full experimental gap are computed using a method that correctly resolves the boundary condition problem (without a jellium approximation) and eliminates finite-size errors. The resulting computed defect levels are not impacted by the DFT band-gap problem, they span a width consistent with the experimental gap rather than being limited to the single-particle DFT gap. All defects with electronically degenerate (half-metal) ${T}_{d}$ ground states are found to have significant distortions, relaxing to ${D}_{2d}$ structures driven by the Jahn-Teller instability. This leads to insulating ground states for all substitutional $3d$ dopants, refuting claims in the literature that $+U$ or hybrid functional methods are required to avoid artificial half-metal results. Interpreting the ${d}^{n}$ atomic occupations within a crystal-field model and exchange splittings, we identify a systematic trend across the $3d$ transition metal series. Approaches to estimate excited-state energies as observed in photoluminescence from defect centers are assessed, ranging from a Koopmans-type single-particle energy interpretation to relaxed total energy differences in fully self-consistent DFT. The single-particle interpretations are found to be qualitatively predictive and the calculations are consistent with the limited available experimental data across the $3d$ dopant series. These results provide a baseline understanding to guide future studies and a conceptual framework within which to interpret new results.

Defect formation energy for various charge states of point defects in CdGa2S4

Defect formation energy for various charge states of point defects in CdGa2S4

Non-covalent Functionalization of Pristine and Defective WSe2 by Electron Donor and Acceptor Molecules

Two-dimensional transition-metal dichalcogenide (2D-TMD) monolayers have recently attracted growing interest, thanks to their excellent optoelectronic properties, especially a moderate direct band gap in the visible spectral range and an extremely strong light–matter interaction. Herein, by means of density functional theory (DFT) and ab initio molecular dynamics (AIMD), we systematically investigate the chemical doping of the WSe2 monolayer upon non-covalent attachment of electron donor and acceptor molecules, namely, fullerenes (C20, C26, and C60), tetrathiafulvalene (TTF), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). Our results confirm that the physisorbed molecules stack on WSe2 via a weak van der Waals interaction; this precludes any significant damage in the basal-plane structure of the monolayer. In turn, the modifications in the carrier density in the WSe2 monolayer due to organic dopants result in a change of the work function by up to 0.41 eV. By performing AIMD calculations, we show that the effect is more pronounced upon increasing the coverage density of physisorbed TTF and TCNQ molecules. The impact of Se-vacancy (VSe) defects on the electronic properties of the WSe2 monolayer and thermodynamic stability of physisorption (including the molecular density) is also considered. Interestingly, the molecules demonstrate an ability to modulate the degree of spatial localization of VSe trap states. Moreover, the shift of the Fermi level upon molecular adsorption also enables further stabilization of charged defect states associated to a VSe vacancy. The pronounced effect of molecular adsorption on the VSe defect behavior in WSe2 might open the door for potential engineering of defect states in 2D TMDs.

The charge effects on the hydrogen evolution reaction activity of the defected monolayer MoS2.

Doping engineering has proven to be an effective way to tune the hydrogen evolution reaction (HER) activity of MoS2. Introducing these defects could cause the overall charge imbalance of MoS2, which makes MoS2 charged. In order to understand the effect of charge on the HER activity of the defected MoS2, we systematically investigate the formation energies, hydrogen adsorption Gibbs free energy (), and electronic structures of 3d, 4d, and 5d transition metal (TM) doped monolayer MoS2 with S vacancies (Svac) based on the density functional theory (DFT) calculations. According to the formation energy calculation, Svac in the 0 and -1 charge states (S0vac and Svac1-) is found to be stable. of Svac1- is -0.16 eV, suggesting its HER catalytic activity is lower than that of Pt (), which is consistent with the experimental results. By substituting the Mo atom with TM atoms, we found that the TM atoms in groups VB-VIIB can promote the generation of Svac, forming defect complexes (TMMoSvac). is greatly affected by the charge state of defects; TMMoSvac defects (TM = V, Nb, Ta, Cr, W, Mn, and Re) in -1 charge states exhibited excellent HER activity (). Significantly, W and Re doping can promote the HER activity of MoS2 independent of the charge state and the Fermi level, which suggests that W and Re doping are most beneficial to improve the HER activity of MoS2. Therefore, the HER activity of defected MoS2 is not only influenced by as previously thought, but also by formation energies, charge state and Fermi level position of defects. The underlying physics could be deduced from the charge-induced changes in electronic structures. Our work highlights the defect charge effects on the electrochemical reactions and offers plausible mechanisms of defect charge effects.

Broadband ultrafast photogenerated carrier dynamics induced by intrinsic defects in <inline-formula><tex-math id="Z-20231102113631">\begin{document}$\boldsymbol\beta$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20231173_Z-20231102113631.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20231173_Z-20231102113631.png"/></alternatives></inline-formula>-Ga<sub>2</sub>O<sub>3</sub>

The ultra-wide bandgap semiconductor gallium oxide <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> with enhanced resistance to the irradiation and temperature is favorable for high-power and high-temperature optoelectronic devices. <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> also exhibits great potential applications in the field of integrated photonics because of its compatibility with the CMOS technique. However, a variety of intrinsic and extrinsic defects and trap states coexist in <i>β</i>-Ga<sub>2</sub>O<sub>3</sub>, including vacancies, interstitials, and impurity atoms. The defect-related carrier dynamics in <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> not only adversely affect the optical and electrical properties, but also directly limit the performance of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> based devices. Therefore, a comprehensive understanding of the carrier transportation and relaxation dynamics induced by intrinsic defects is very important. Supercontinuum-probe spectroscopy can provide a fruitful information about the carrier relaxation processes in different recombination mechanisms, and thus becomes an effective way to study the defect dynamics. In this work, we study the dynamics of carrier trapping and recombination induced by intrinsic defects in pristine <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> crystal by using wavelength-tunable ultrafast transient absorption spectroscopy. The broadband absorption spectra induced by the intrinsic defects are strongly dependent on the polarization of pump pulse and probe pulse. Particularly, two absorption peaks induced by the two defect states can be extracted from the transient absorption spectra by subtracting the absorption transients under two probe polarizations. The observed defect-induced absorption features are attributed to the optical transitions from the valence band to the different charge states of the intrinsic defects (such as gallium vacancy). The data are well explained by a proposed carrier capture model based on multi-level energies. Moreover, the hole capture rate is found to be much greater than that of the electron, and the absorption cross-section of the defect state is at least 10 times larger than that of free carrier. Our findings not only clarify the relationship between intrinsic defects and photogenerated carrier dynamics, but also show the importance in the application of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> crystals in ultrafast and broadband photonics.