Charge Transition Levels Research Articles

Defect levels of 3dn transition-metal series in wide-gap oxide and fluoride insulators: A first-principles study

The defect levels produced in insulators doped with transition metal (TM) ions are important for a variety of defect-related optical processes and magnetic properties. Here, we performed a systematical first-principles study of the defect levels in the band gap for all the $3{d}^{n}$ TM ions occupying octahedral sites in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}, {\mathrm{MgAl}}_{2}{\mathrm{O}}_{4}$, MgO, ${\mathrm{KMgF}}_{3}$, and ${\mathrm{MgF}}_{2}$ based on density functional theory. The spin-polarized hybrid density functional with a dielectric-dependent Hartree-Fock mixing parameter was adopted to improve the band gap. The defect levels, i.e., the charge transition levels, were obtained by calculating the total energy differences of relaxed geometric structures for various valence states, considering image-charge and potential alignment corrections. These results were rationalized with available experimental optical spectra related to the charge transfer or photoionization transitions. The calculation scheme is appropriate for predicting the valence states of TM ions and their defect levels in insulators. The trends of variation of the defect levels across the $3d$ TM series may help interpreting experimental results and aid in the design and optimization of optical materials.

First-principles study of transition metal dopants as spin qubits

Defect centers in solid-state materials have played an important role in the development of quantum information science as spin qubits. In recent years, many studies using optically detected magnetic resonance (ODMR) to manipulate the ground state spin of transition metal dopants have been reported. In this work, the tetrahedral ${d}^{2}$ and the octahedral ${d}^{3}$ transition metal ion systems are considered for the potential spin qubits, based on the ligand field theory and the basic requirements of general $\mathrm{\ensuremath{\Lambda}}$-type spin qubits. Then a computational scheme based on density functional theory is performed on the transition metal ligand structures $M{X}_{n}(M=\mathrm{V},\mathrm{Cr},\mathrm{Mn},\mathrm{Fe};X=\mathrm{N},\mathrm{O},\mathrm{C};\phantom{\rule{4pt}{0ex}}n=4, 6)$ in several typical hosts to analyze quantitatively their feasibility. The results on the charge transition levels, excitation, and excited-state structure relaxation energies of $M{X}_{n}$ show that some of them have the proper valence states to form the required ${d}^{2}$ or ${d}^{3}$ electronic systems and the appropriate excited-state level structure with the spin-selective transition. The results on the spontaneous emission lifetime show that the ${\mathrm{MnO}}_{4}$ structure is mostly suitable for spin polarization due to the fastest optical transition rates among these systems considered. Finally, some suggestions on determining potentially feasible systems activated by transition metal ions in solids are given.

Electrical charge control of h-BN single photon sources

Colour centres of hexagonal boron nitride (h-BN) have been discovered as promising and practical single photon sources due to their high brightness and narrow spectral linewidth at room-temperature. In order to realize h-BN based photonic quantum communications, the ability to electrically activate the single photon fluorescence using an external electric field is crucial. In this work, we show the electrical switching of the photoluminescence from h-BN quantum emitters, enabled by the controllable electron transfer from the nearby charge reservoir. By tuning the Fermi level of graphene next to the h-BN defects, we observed luminescence brightening of quantum emitters upon the application of a voltage due to the direct charge state manipulation. In addition, the correlation measurement of the single photon sources with the graphene’s Raman spectroscopy allows us to extract the exact charge transition level of quantum emitters, providing the information on the crystallographic nature of the defect structure. With the complete on-off switching of emission intensity of h-BN quantum emitters using a voltage, our result paves the way for the van der Waals colour centre based photonic quantum information processing, cryptography and memory applications.

Quantum confinement effect on defect level of hydrogen doped rutile VO2 nanowires

Accurate description of solubility and defect ionization energies in low dimensional nanostructures is critical for electronic applications of semiconductors with improved functionalities. Here, we present quantum confinement effect driven strategies for tuning defect level of hydrogen doping in the core region of rutile VO2(R) nanowires. The inverse dependence of a bandgap with a diameter (∝d−0.48) confirms the presence of quantum confinement effect in nanowires. The hydrogen doping in both interstitial and substitution at the O site behaves as a deep donor in low diameter nanowires, where the effect of quantum confinement is significant. The position of a donor charge transition level becomes increasingly shallower with increased nanowire diameters. The ionization energies of hydrogen defects decrease for larger-diameter nanowires due to the dielectric screening effect increment. This indicates the possibility of achieving n-type dopability with large diameter VO2(R) nanowires. This study prescribes the strategies for optimizing doping and the defect level for extensive applications of highly correlated 1D nanostructured materials.

First-principles predictions of qubits in defective MgS

In this paper, MgS is evaluated to be a suitable qubit host material due to its wide bandgap, being nuclear spin free, and its weak spin-orbit coupling. The point defects including vacancies, substitutions, and defect pairs were systematically investigated using first-principles calculations. Results show that the neutral ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}$ defect pair has ${C}_{3v}$ symmetry with the defect level sequence of ${E}_{1}>{E}_{exy}>{E}_{2}$. In addition, the net spin and defect levels of ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}$ can be tuned by charge states. The ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2+}$ and ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2\ensuremath{-}}$ defects are relatively stable with the charge transition level of \ensuremath{\sim}2.53 eV. The four charged defect states of ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{+}$, ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2+}$, ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{\ensuremath{-}}$, and ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2\ensuremath{-}}$ may be plausible qubits with nonzero spin, and their zero-phonon lines indicate the corresponding fluorescence wavelengths fall within the infrared band. Furthermore, the zero field splitting of the ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{+}$ and ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{\ensuremath{-}}$ shows that the values of parameter $D$ are in the range of microwaves. The hyperfine constants are roughly proportional to the spin difference charge density and exponentially decay as a function of the distance from the P site.

Cost-effective calculation of defects in Si using hybrid density functional with downsampled reciprocal grids

A deep understanding of defects in terms of stability and optoelectronic properties is essential for optimizing semiconductor devices. For faster and more accurate investigation of defects, we examined whether the hybrid density functional theory calculation can be performed cost-effectively by downsampling the k-point meshes for Fock exchange potential. The P dopant and Si self-interstitial defects were used to check the convergence of the total energy. Our calculation shows that the defect properties like the formation energy and the charge transition levels can be cost-effectively calculated by using downsampled k-point meshes. We also investigated the intrinsic vacancy defects and obtained consistent results with previous studies.

Giant Stokes shift for charged vacancies in monolayer SnS

We perform first-principles density-functional theory calculations to determine the equilibrium defect structures, formation energies, charge transition levels, and electronic structures of Sn and S vacancies in monolayer SnS. Both Sn and S vacancies exhibit multiple charge transition levels and in-gap defect states, indicating that they may be stable in different charge states depending on the Fermi level in the system. Depending on the charge state of the vacancy, the easily distorted SnS lattice undergoes different relaxations, and in some cases, symmetry-breaking reconstructions, creating defect states within the gap that electrons can occupy at a lower energetic cost. Due to significant atomic relaxations between the equilibrium defect structures in different charge states, optical charge transitions involving both types of vacancies exhibit significant Stokes shifts of over 1 eV, which may provide opportunities for increased efficiency in light emission diode, solar cell, and solar concentrator applications.

Quantum point defects in 2D materials - the QPOD database

Atomically thin two-dimensional (2D) materials are ideal host systems for quantum defects as they offer easier characterisation, manipulation and read-out of defect states as compared to bulk defects. Here we introduce the Quantum Point Defect (QPOD) database with more than 1900 defect systems comprising various charge states of 503 intrinsic point defects (vacancies and antisites) in 82 different 2D semiconductors and insulators. The Atomic Simulation Recipes (ASR) workflow framework was used to perform density functional theory (DFT) calculations of defect formation energies, charge transition levels, Fermi level positions, equilibrium defect and carrier concentrations, transition dipole moments, hyperfine coupling, and zero-field splitting. Excited states and photoluminescence spectra were calculated for selected high-spin defects. In this paper we describe the calculations and workflow behind the QPOD database, present an overview of its content, and discuss some general trends and correlations in the data. We analyse the degree of defect tolerance as well as intrinsic dopability of the host materials and identify promising defects for quantum technological applications. The database is freely available and can be browsed via a web-app interlinked with the Computational 2D Materials Database (C2DB).

First-principles study of radiation defects in silicon

The study of the properties of defects in silicon forming under irradiation condition has been carried out for many years, however, many open questions remain. Particularly, there are not comprehensive results for variety of radiation centers calculated at the same level of theory. For example the previously calculated transition levels and formation energies show a large scatter. In this work, we focus on the important radiation-induced defect complexes in Si: double vacancy, vacancy-oxygen, vacancy-phosphorus. Additionally, the phosphorus–vacancy-oxygen complex was studied. Formation energy, charge transition levels and binding energy were calculated from first-principles using the hybrid exchange–correlation functional HSE06. Spin-polarized calculations and large supercells allows us to obtain charge transition levels which agree well with experimental measurements.

Universal machine learning framework for defect predictions in zinc blende semiconductors

SummaryWe develop a framework powered by machine learning (ML) and high-throughput density functional theory (DFT) computations for the prediction and screening of functional impurities in groups IV, III–V, and II–VI zinc blende semiconductors. Elements spanning the length and breadth of the periodic table are considered as impurity atoms at the cation, anion, or interstitial sites in supercells of 34 candidate semiconductors, leading to a chemical space of approximately 12,000 points, 10% of which are used to generate a DFT dataset of charge dependent defect formation energies. Descriptors based on tabulated elemental properties, defect coordination environment, and relevant semiconductor properties are used to train ML regression models for the DFT computed neutral state formation energies and charge transition levels of impurities. Optimized kernel ridge, Gaussian process, random forest, and neural network regression models are applied to screen impurities with lower formation energy than dominant native defects in all compounds.

Charged vacancy defects in monolayer phosphorene

Two-dimensional semiconductor phosphorene has attracted extensive research interests for potential applications in optoelectronics, spintronics, catalysis, sensors, and energy conversion. To harness phosphorene's potential requires a better understanding of how intrinsic defects control carrier concentration, character, and mobility. Using density functional theory and a charge correction scheme to account for the appropriate boundary conditions, we conduct a comprehensive study of the effect of structure on the formation energy, electronic structure, and charge transition level of the charged vacancy point defects in phosphorene. We predict that the neutral vacancy exhibits a 9-5 ring structure with a formation energy of 1.7 eV and transitions to a negatively charged state at a Fermi level 1.04 eV above the valence band maximum. The corresponding optical charge transitions display sizable Frank-Condon shifts with a large Stokes shift of 0.3 eV. Phosphorene vacancies should become negatively charged in $n$-doped phosphorene, which would passivate the dopants and reduce the charge carrier concentration and mobility.

Theoretical investigation of charge transfer between the NV− center in diamond and substitutional N and P

The nitrogen-vacancy (NV) lattice defect in diamond, consisting of an N substitutional atom and an adjacent C vacancy, is commonly observed in two charge states, negative (NV−) and neutral (NV0). The NV− defect exhibits spin state-dependent fluorescence and is, therefore, amenable to optical methods for spin-state readout, while the NV0 is not. Hence, the NV− defect is much more useful for quantum sensing and quantum information processing. However, only NV0 electroluminescence has been observed, even from centers showing NV− in photoluminescence. In the present work, we use first-principles electronic structure calculations to determine adiabatic charge transition levels for the conversion of NV− to NV0 in the presence of substitutional N or P impurities, which provide the charge of the NV− center. We find that the adiabatic charge transition levels in the presence of such impurities lie at energies close to or lower than the zero-phonon line of the NV− center and that these energies only decrease as the concentration of N donors increases. This work, therefore, elucidates the absence of observed electroluminescence from the NV− and proposes a path toward observation of the phenomenon.

Impact of carbon–carbon defects at the SiO2/4H-SiC (0001) interface: a first-principles calculation

Poor quality of the SiO2/4H-SiC (0001) interface is a long-standing issue limiting the performance of silicon carbide (SiC)-MOSFETs. However, the origin of the interface defects is still not fully understood. In this article, five types of carbon–carbon defects introduced to the SiO2/4H-SiC (0001) interface are systematically investigated by first-principle calculations. The thermal oxidation process of 4H-SiC is analyzed through the variation of the chemical potentials based on the chemical reaction equation. The trend of the formation energy with different oxidation conditions show that oxygen-poor condition is effective to suppress the formation of carbon–carbon defects. We obtain the accurate electronic structure with hybrid density functional and the distribution of electron states for the carbon–carbon defects. The interfacial carbon–carbon defects in SiC tend to cause gap states near the conduction band minimum (CBM). Furthermore, the calculated charge transition levels (CTLs) also show that the 0/− CTLs of the carbon–carbon defects are close to the CBM acting as acceptors. All the theoretical calculation results provide insight into understanding the atomic structures and electronic properties for the carbon–carbon defects at the SiO2/4H-SiC (0001) interface, which are fundamental to improve the performance of n-channel SiC-MOSFETs.

Exploring Intrinsic Electron-Trapping Centers for Persistent Luminescence in Bi3+-Doped LiREGeO4 (RE = Y, Sc, Lu): Mechanistic Origin from First-Principles Calculations.

Revealing the nature of intrinsic defects that act as charge-carrier trapping centers for persistent luminescence (PersL) in inorganic phosphors remains a crucial challenge from an experimental perspective. It was recently reported that Bi3+-doped LiREGeO4 (RE = Sc, Y, Lu) compounds displayed strong ultraviolet-A PersL at ∼360 nm with a duration of tens of hours at room temperature. However, the mechanistic origin of the PersL remains to be unveiled. Herein, we carried out a systematic study on optical transitions, formation energies, and charge-transition levels of dopants and intrinsic point defects in these compounds using hybrid density functional theory calculations. The results show that the efficient charging by 254 nm is due to the D-band transition of Bi3+ and hence the charge carriers pertinent to PersL are electrons originating from the dopants which are involved in the trapping and detrapping processes. The main electron-trapping centers are antisite defects GeLi0, interstitial defects Lii0, and dopants Bi2+, with the former one responsible for the strong PersL and the latter two for its long-time duration. These findings are further confirmed by comparison with calculated results for isostructural NaLuGeO4 and LiLuSiO4, based on which the roles of Li and Ge elements in forming intrinsic defects with appropriate trap depths for PersL are clarified. Our results not only assist in the understanding of experimental observations but also provide a theoretical basis for the rational design of novel PersL phosphors containing lithium and germanium in the host compound.

M-center in 4H-SiC: Isothermal DLTS and first principles modeling studies

We report on a bistable defect known as M-center, here introduced in n-type 4H-SiC by 2 MeV He ion implantation. Deep levels of the M-center are investigated by means of junction spectroscopy techniques, namely, deep level transient spectroscopy (DLTS) and isothermal DLTS. In addition to previously reported three deep levels arising from the M-center (labeled as M1, M2, and M3), we provide direct evidence on the existence of a fourth transition (labeled as M4) with an activation energy of 0.86 eV. Activation energies and apparent capture cross sections for all four metastable defects are determined. From first-principles calculations, it is shown that the observed features of the M-center, including the charge state character, transition levels, bi-stability dynamics, and annealing, are all accounted for by a carbon self-interstitial.

First-principles study of the Li(Y/Lu)SiO4:Ce3+,Sm3+ storage phosphor

First-principles calculations are carried out to study the native point defects and dopants (Ce3+, Sm3+) in Li(Y/Lu)SiO4 for revealing the mechanism of the optical excitation energy storage properties. The calculated excitation and emission energies, the Stokes shifts as well as the positions of 4f and 5d levels of Ce3+ relative to host band edges show great consistent with the experimental results. The calculated formation energies reveal that the Li vacancies (VLi) and the antisite defects LiY and YLi (or LiLu and LuLi) are always much more energetically favorable than Y or Lu vacancies (VY or VLu) and O vacancies (VO) in Li(Y/Lu)SiO4 under reductive atmospheres. Moreover, according to the calculated charge transition levels of the native point defects and the dopants, the antisite defects YLi (or LuLi) and the dopants Sm3+ (SmY or SmLu) are deemed as electron traps that provide suitable charge transition levels for information storage in Li(Y/Lu)SiO4. Based on our calculations, a mechanism diagram of charge carrier storage and recombination during irradiation and thermal or optical readout is constructed for the storage phosphors Li(Y/Lu)SiO4:Ce3+,Sm3+.

Hydrogen states in mixed-cation CuIn(1-x)GaxSe2 chalcopyrite alloys: a combined study by first-principles density-functional calculations and muon-spin spectroscopy

ABSTRACT First-principles calculations were performed jointly with muon-spin (μSR) spectroscopy experiments in order to examine the electrical activity of hydrogen in mixed-cation chalcopyrite Cu(In,Ga)Se (CIGS) alloys and other related compounds commonly used as absorbers in solar-cell technology. The study targeted the range of Ga concentrations most relevant in typical solar cells. By means of a hybrid-functional approach the charge-transition levels of hydrogen were determined and the evolution of the defect pinning level, E(+/–), was monitored as a function of the Ga content. The use of E(+/–) as a metric of the charge-neutrality level allowed the alignment of band structures, thus providing the band offsets between the CuInSe compound and the CIGS alloys. The μSR measurements in both thin-film and bulk CIGS materials confirmed that the positively charged state is the thermodynamically stable configuration of hydrogen for p-type conditions. The interpretation of the μSR data further addressed the existence of a metastable quasi-atomic neutral configuration that was resolved from the calculations and led to a formation model for muon implantation.

Deep levels related to the carbon antisite–vacancy pair in 4H-SiC

Photo-induced current transient spectroscopy (PICTS) and electron paramagnetic resonance (EPR) are used to study irradiation-induced defects in high-purity semi-insulating (HPSI) 4H-SiC. Several deep levels with the ionization energy ranging from 0.1 to ∼1.1 eV have been observed in irradiated and annealed samples by PICTS. Among these, two deep levels, labeled E370 and E700 at ∼0.72 and ∼1.07 eV below the conduction band, respectively, are detected after high-temperature annealing. The appearance and disappearance of these two deep levels and the EPR signal of the positive C antisite–vacancy pair (CSiVC+) in the sample annealed at 1000 and 1200 °C, respectively, are well correlated. Based on data from PICTS and EPR and the energies predicted by previous calculations for different charge states of dominant intrinsic defects, the E370 and E700 levels are suggested to be related to the charge transition levels (0|–) and (+|0), respectively, of the C antisite–vacancy pair. The activation energy of Ea ∼ 1.1 eV in commercial HPSI 4H-SiC materials is, therefore, reassigned to be related to the single donor (+|0) level of CSiVC.

Towards ab initio identification of paramagnetic substitutional carbon defects in hexagonal boron nitride acting as quantum bits

Paramagnetic substitutional carbon (${\mathrm{C}}_{\text{B}},{\mathrm{C}}_{\text{N}}$) defects in hexagonal boron nitride are discussed as candidates for quantum bits. Their identification and suitability are approached by means of photoluminescence (PL), charge transitions, electron paramagnetic resonance, and optically detected magnetic resonance (ODMR) spectra. Several clear trends in these are revealed by means of an efficient plane wave periodic supercell ab initio density functional theory approach. In particular, this yields insight into the role of the separation between ${\mathrm{C}}_{\text{B}}$ and ${\mathrm{C}}_{\text{N}}$. In most of the cases, the charge transition between the neutral and a singly charged ground state of a defect is predicted to be experimentally accessible, since the charge transition level position lies within the band gap. A posteriori charge corrections are also discussed. A near-identification of an experimentally isolated single spin center as the neutral ${\mathrm{C}}_{\text{B}}$ point defect was found via comparison of results to recently observed PL and ODMR spectra.

Atomic-scale understanding on the physics and control of intrinsic point defects in lead halide perovskites

Lead halide perovskites (LHPs) have attracted considerable attention as promising materials for photovoltaic and optoelectronic applications. Intrinsic point defects play an important role in determining the performance of semiconductor devices. LHPs exhibit strong ionic character and unique electronic structure; thus, their defect properties are quite different from conventional covalent bond semiconductors. Understanding the defect science is crucial to the performance optimization of LHP-based devices. State-of-the-art first-principles calculation methods enable one to explore atomistic mechanisms of various defect-related processes, and tremendous efforts from theoretical simulations have provided invaluable insights to the defect physics and defect control of LHPs. In this review, we summarize recent progress, made with the help of theoretical modeling, on atomic-scale understanding about intrinsic point defects and related processes in LHPs. The fundamental properties of intrinsic point defects in LHPs are first introduced, including defect formation energy, charge transition level, and defect tolerance and its origin. A particular emphasis is given to the effects of band edge position on calculated defect properties. The impact of these defects on structural properties, carrier dynamics, and photoluminescence of LHPs is then presented. Advanced strategies to engineer the defects in LHPs are also reviewed, such as growth condition, defect passivation, and doping. Finally, we discuss open issues and outline directions toward a better understanding of defects of LHPs from a theoretical perspective. The goal of the review is to provide a comprehensive summary of atomic-scale understanding of intrinsic point defects in LHPs and to help further related research in the perovskite community.