Charged Defect States Research Articles

New insights into the process of intrinsic point defects in PuO2.

Intrinsic point defects are known to play a crucial role in determining the physical properties of solid-state materials. In this study, we systematically investigate the intrinsic point defects, including vacancies (VPu and VO), interstitials (Pui and Oi), and antisite atoms (PuO and OPu) in PuO2 using the first-principles plane wave pseudopotential method. Our calculations consider the whole charge state of these point defects, as well as the effect of oxygen partial pressure. This leads to a new perspective on the process of intrinsic point defects in PuO2. We find that the antisite atoms OPu and PuO are more likely to appear in O-rich and O-deficient environments, respectively. Interestingly, the most energetically favorable type of Schottky defect is {2VPu3-: 3VO2+} in an O-rich environment, while {4VO1+: VPu4-} is preferred in an O-deficient environment. These results differ from the commonly known {VPu4-: 2VO2+} type of Schottky defect. Moreover, under O-deficient conditions, we predict that the stable cation Frenkel defect is {VPu4+: Pui4+}, while the most stable anion Frenkel defect is {VO2+: Oi2-} under O-rich conditions. Lastly, we find that the only two types of antisite pairs that can appear are {OPu5-: PuO5+} and {OPu6-: PuO6+}, with the latter being the more stable configuration. These unconventional defect configurations provide a new viewpoint on the process of intrinsic point defects in PuO2 and lay theoretical foundations for future experiments.

Electronic structures and optical spectra of KDP crystals with Sp doping defects: a first-principles study.

In order to further clarify the effect of sulfur doping on the laser damage threshold of potassium dihydrogen phosphate (KDP), the properties of sulfur substituting for phosphorus doping defects (SP) in KDP crystals with paraelectric (PE) and ferroelectric (PE) phases are studied in this article. More accurate defect transition levels were obtained by band edge correction, and the band edge corrected values were 1.28 eV and 1.88 eV for the PE and FE phases, respectively. The defect formation energies with four different defect charges (0, +1, +2, and-1) were obtained using the finite size correction scheme. The stable defect charge states were (+2 charge state) (+1 charge state) and (-1 charge state) in turn when the Fermi level moved from the valence band maximum (VBM) to the conduction band minimum (CBM). Moreover, by considering the electron-phonon coupling, the optical absorption and emission spectra were obtained. The absorption peak for the state of the PE phase at 4.63 eV was close to the experimental value. We predicted that the absorption peak at 4.50 eV belongs to the state with the FE phase. The emission peaks at 0.10 eV and 1.36 eV were related to the PE and FE phases, accordingly. The absorption may affect the application of S-KDP crystals and reduce the laser damage threshold.

Structural and charge carrier dynamics study of Dy stabilized La6MoO12 ionic conductors

Room temperature single phase stabilization of different metals (Zr, Cd, and Ca) doped La6MoO12 compound was achieved by incorporating Dy2O3 using the solution combustion method. Rietveld refined X-ray diffraction profiles showed cubic fluorite (Fm3¯m) structure for all the compositions. The optical bandgap, calculated from ultraviolet-visible spectra, exhibited blue shifts with decreasing dopant ionic radius. The photoluminescence spectra confirmed the charge carrier's various trapping or defect states. The complex impedance plot exhibited non-Debye-type relaxation with a negative temperature coefficient of resistance (NTCR). It also indicated that the dopant might act as an acceptor or donor depending upon its valency. The ac conductivity was found to follow the small polaron hopping mechanism. A thermally activated single relaxation process was observed. The composition La5.4Dy0.4Zr0.2MoO12.1 showed the highest conductivity, 1.05 × 10−4 Ω−1 cm−1 at 680 °C. The scaling of spectra indicated temperature and composition independent charge carrier mechanism.

Origin of Ga vacancy-related YL center in n-type GaN: A first-principles study

The widely existed yellow luminescence (YL) in n-type GaN is attributed to Ga vacancy-related point defects. However, the nature of Ga vacancy-related YL centers, especially the identification of defect types, is still unclear. Based on first-principles calculations, we systematically investigated the defect formation energies, “0/+“-related optical transition processes and the hole non-radiative trapping of VGa-ON, VGa-VN, VGa-3H and VGa-ON-2H defects in n-type GaN. By static coupling method, the calculated capture cross-sections of VGa-ON and VGa-VN at 300 K are 3.52 × 10−14 cm2 and 6.22 × 10−14 cm2, respectively, which are in good agreement with the hole capture cross-sections of YL center measured by deep-level transient spectroscopy (DLTS) experimentally. While the calculated capture cross-section of 2.46 × 10−13 cm2 for VGa-ON-2H is three times of the maximum experimental data, and the value 1.02 × 10−12 cm2 for VGa-3H is 1–2 orders of magnitude larger than the experimental results. The calculated hole capture cross-sections of these defects centers are quite different, but the transitions between 0 and + charge states of the above defects are all consistent with the experimental YL emission process. This work provides the correlation between the VGa-related defects and the YL center in n-type GaN, bringing essential information to a long-standing controversy.

Native and radiation induced point defects in AlN and Sc-doped AlN

We have performed first-principles calculations to investigate the electronic structure, configurations, formation, and binding energies of native and radiation induced point defects in pristine and Sc-doped wurtzite AlN. For the native defects, the nitrogen vacancy has the lowest formation energy in $p$-type material while the aluminum vacancy has the lowest formation energy in $n$-type material which is consistent with the previous studies. Several interstitial defect structures were modeled for Al, N, and Sc atoms. The effects of charge state on their relative stability were investigated. The binding energy of Sc with point defects was calculated and found to be dependent strongly on the defect type and charge state. The results obtained are discussed in light of the possible Sc effects on the radiation damage evolution in AlN. Thus the attraction of Sc atom to N vacancy and both Al and N interstitials reduces their mobility and increases Frenkel pair recombination distance.

High performance of ZnSnO thin-film transistors engineered by oxygen defect modulation

Thin film transistor (TFT) with low cost and high performance is significantly important for practical applications. In this work, In-free ZnSnO (ZTO) TFT with high performance is achieved by modulating the annealing atmosphere to manipulate the amount and charge state of oxygen defect, and then the effects of annealing atmosphere are systematically investigated by experiment and theoretical calculation. Results show that, O2 condition results in an inferior electron mobility although the formation of oxygen defect is suppressed. The oxygen vacancies created in N2 ambience are most probably dominated by neutral ones that limit the electron transport. The ZTO TFT dealt with vacuum condition exhibits the best performances, i.e. μlin, Vth, SS and Ion/Ioff are 19.80 cm2/Vs, 0.05 V, 0.46 V/dec and 108, respectively, which can be attributed to the cooperation of the decreased RC and RSH, the charge-state transition of oxygen defect. Further investigation indicates that, doubly charged oxygen vacancy not only results in the elevation of electron content and thus reduce the conduction barrier, but also shrinks the supercell volume to maintain the highly dispersed conduction band, which is crucially important for the electron transport. The findings may facilitate the ability to engineer other high performance AOS materials and devices.

Magnetic Order, Electrical Doping, and Charge-State Coupling at Amphoteric Defect Sites in Mn-Doped 2D Semiconductors.

Two-dimensional (2D) dilute magnetic semiconductors (DMSs) are attractive material platforms for applications in multifunctional nanospintronics due to the prospect of embedding controllable magnetic order within nanoscale semiconductors. Identifying candidate host material and dopant systems requires consideration of doping formation energies, magnetic ordering, and the tendency for dopants to form clustered domains. In this work, we consider the defect thermodynamics and the dilute magnetic properties across charge states of 2D-MoS2 and 2D-WS2 with Mn magnetic dopants as candidate systems for 2D-DMSs. Using hybrid density functional calculations, we study the magnetic and electronic properties of these systems across configurations with thermodynamically favorable defects: 2D-MoS2 doped with Mn atoms at sulfur site (MnS), at two Mo sites (2MnMo), on top of a Mo atom (Mn-top), and at a Mo site (MnMo). While the majority of the Mn-defect complexes provide trap states, MnMo and MnW are amphoteric, although previously predicted to be donor defects. The impact of cluster formation of these amphoteric defects on magnetic ordering is also considered; both MnMo-MnMo (2Mn2Mo) and MnW-MnW (2Mn2W) clusters are found to be stable in ferromagnetic (FM) ordering. Interestingly, we observed the defect charge state dependent magnetic behavior of 2Mn2Mo and 2Mn2W clusters in 2D-TMDs. We investigate that the FM coupling of 2Mn2Mo and 2Mn2W clusters is stable in only a neutral charge state; however, the antiferromagnetic (AFM) coupling is stable in the +1 charge state. 2Mn2Mo clusters provide shallow donor levels in AFM coupling and deep donor levels in FM coupling. 2Mn2W clusters lead to trap states in the FM and AFM coupling. We demonstrate the AFM to FM phase transition at a critical electron density nce = 3.5 × 1013 cm-2 in 2D-MoS2 and 2D-WS2. At a 1.85% concentration of Mn, we calculate the Curie temperature of 580 K in the mean-field approximation.

Site-Dependent Properties of Quantum Emitters in Nanostructured Silicon Carbide

Deep defects in silicon carbide (SiC) possess atom-like electronic, spin and optical properties, making them ideal for quantum-computing and -sensing applications. In these applications, deep defects are often placed within fabricated nanostructures that modify defect properties due to surface and quantum confinement effects. Thus far, theoretical studies exploring deep defects in SiC have ignored these effects. Using density functional theory, this work demonstrates site-dependence of properties of bright, negatively-charged silicon monovacancies within a SiC nanowire. It is shown that the optical properties of defects depend strongly on the hybridization of the defect states with the surface states and on the structural changes allowed by proximity to the surfaces. Additionally, the analysis of the first principles results indicates that the charge-state conversion and/or migration to thermodynamically-favorable undercoordinated surface sites can deteriorate deep-defect properties. These results illustrate the importance of considering how finite-size effects tune defect properties, and of creating mitigating protocols to ensure a defect's charge-state stability within nanostructured hosts.

Understanding the charge transfer and defect states in ZnO/In2O3 composite nanostructures (CNs)

Understanding the charge transfer and defect states in ZnO/In2O3 composite nanostructures (CNs)

Electrical detection of TV2a-type silicon vacancy spin defect in 4H-SiC MOSFETs

Color centers in silicon carbide (4H-SiC) are potentially usable as spin defects for quantum sensing and quantum information technology. In particular, neutral divacancies (the P6/P7centers, VSiVC0) and a certain type of silicon vacancies (the TV2a center, VSi- at the k site) are promising for addressing and manipulating single spins. Although the TV2a spin is readable at room temperature, the readout techniques have been limited to luminescence-based ones (e.g., optically detected magnetic resonance). In this study, we demonstrated electrical detection of TV2a-type silicon vacancies at room temperature by using electrically detected magnetic resonance on 4H-SiC metal–oxide–semiconductor field effect transistors (MOSFETs). TV2a spin defects were embedded in the channel region of well-defined 4H-SiC MOSFETs via controlled proton irradiation. The number of detected TV2a spins was estimated to be ∼105. We also found that the charge state of the TV2a spin defect can be controlled by varying the gate voltage applied to the MOSFET.

Photoluminescence of sapphire irradiated by low-energy electrons and ions

Photoluminescence (PL) of sapphire single crystals as received and preirradiated by low energy Ar+ ions and electrons has been studied to reveal a relationship between sapphire charging under electron beam irradiation and radiation-induced defect formation. The photoluminescence spectra were obtained using a confocal microscope excitation wavelength of 445 nm as well as by a nonconfocal method with excitation at a wavelength of 355 nm. The lines observed in PL spectra for all samples are associated with both intrinsic and impurity defects. It has been established that preliminary ion irradiation leads to disordering of the near-surface region of the sample resulting in a significant increase in the photoluminescence intensity. Preliminary electron irradiation can lead to a change in the charge state of defects that initially exist in the crystal. Keywords: radiation-stimulated defects, sapphire photoluminescence, ion and electron irradiation.

Фотолюминисценция сапфира, облученного электронами и ионами низких энергий

Photoluminescence (PL) of sapphire single crystals as received and preirradiated by low energy Ar+ ions and electrons has been studied to reveal a relationship between sapphire charging under electron beam irradiation and radiation-induced defect formation. The photoluminescence spectra were obtained using a confocal microscope wavelength of 445 nm as well as by a nonconfocal method with excitation at a wavelength of 355 nm. The lines observed in PL spectra for all samples are associated with both intrinsic and impurity defects. It has been established that preliminary ion irradiation leads to disordering of the near-surface region of the sample resulting in a significant increase in the photoluminescence intensity. Preliminary electron irradiation can lead to a change in the charge state of defects that initially exist in the crystal.

Non-excitonic defect-assisted radiative transitions are responsible for new D-type blinking in ternary quantum dots.

This work addresses the issue of dark states formation in QDs by cooperative excitonic and intrinsic defect-assisted radiative transitions. Here we refer to the observed blinking as D-type to distinguish it from purely excitonic types. It is shown experimentally that defect-assisted radiative relaxations in a single I-III-VI QD result in atypical blinking characteristics that cannot be explained on the basis of charged exciton models. In addition to the excitonic channel, it has been proposed that defect-assisted kinetics can also form blinking patterns. Two conditions for the formation of dark states have been identified which are related to correlation and competition when considering photons emitted from bright defects. Two transition schemes have therefore been proposed. The first transition scheme includes time-correlated trapping of more than one electron at a single trap centre. This is used to simulate variations in the defect's charge state and switching between radiative/nonradiative transitions. The latter scheme, on the other hand, involves uncorrelated trapping and radiative relaxations from two different types of defects (competition). Both schemes are seen to play an equal role in radiative processes in I-III-VI QDs. Considered together, the proposed models can reflect the experimental data with very good accuracy, providing a better understanding of the underlying physics. An important implication of these schemes is that dark states formation doesn't have to be limited to mechanisms that involve charged excitons, and it may also be observed for independent defect assisted kinetics. This is especially valid for highly defected or multinary QDs.

Formation and Diffusion of Intrinsic Point Defects in Bulk and Monolayer MoS 2 : First‐Principles Study

The formation and diffusion of intrinsic defects in bulk and monolayer MoS2 are studied. For the first time, the formation energy of a split sulfur and molybdenum Frenkel pair in bulk (6.56 and 12.6 eV) and monolayer (3.95 and 12.1 eV) MoS2 is determined without using chemical potential. For the intrinsic defects, a complete calculation of the diffusion coefficient (pre-exponential factor and activation barrier) is performed and the effect of the defect charge state on the diffusion coefficient is studied.

Light-driven permanent transition from insulator to conductor

The transition from insulator to conductor can be realized in some materials but requires modification of both the arrangement of atoms and their electronic configurations. This is often achieved by doping. Here we reveal a different mechanism the lattice may adopt to induce such a transition. Experiments showed the surprising finding that limited exposure to subband-gap light caused a permanent transition from an insulator state to a conductor state in the insulating oxide ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, with a nine orders of magnitude increase in electronic conduction. Furthermore, annealing up to 400 \ifmmode^\circ\else\textdegree\fi{}C did not suppress or decrease the induced conductivity. Photoexcitation by light-induced modification in the charge state of defects and subsequent lattice distortion around them was suggested to be the underlying mechanism behind this transition. Density functional theory calculations confirmed that modifying the charge state of defects leads to redistribution of the localized electrons and massive structural distortion in the surrounding lattice, causing large shifts in the density of states and introducing new states with shallower energy levels. Both experimental and theoretical results revealed the introduction of stable shallow energy levels, explaining the mechanism behind the transition from an insulator to a conductor state by light. We suggest that this mechanism may occur in other wide band-gap metal oxides leading to drastic modification in their electronic properties.

Photoluminescence and excitation energy transfer in non-stoichiometric silicon nitride

Photoluminescence of non-stoichiometric silicon nitride films has been studied at excitation range 3.35–5.16 eV. Several bands with energies 3.15, 3.4, 3.62, 3.8, and 3.95 eV have been observed in the PL spectra. The bands at 3.62, 3.8, 3.95 eV assigned to exciton annihilation. Emission at energies 3.15 and 3.4 eV shifts in red spectrum region with excitation energy decreasing. The shift is explained by photo-induced neutralization of charged defect states and the emission can be associated with radiative recombination of charge carriers localized on donor-acceptor pairs. Energy-resolved photoluminescence method reveals the excitation energy transfer channels. It has been found that excitons participate in process of energy transfer to donor-acceptor pairs, providing 3.4 eV band position invariant to the excitation energy.

Voltage-induced modulation in the charge state of Si-vacancy defects in diamond using high voltage nanosecond pulses

Silicon-vacancy defects have been identified as a promising optical transition for quantum communications, quantum control, and quantum information processing. In the work presented here, we demonstrate a voltage-controlled mechanism by which the photoluminescent (PL) emission from silicon-vacancy (Si-V) defects in diamond can be modulated. In particular, we can selectively produce emission from the negatively charged state of this defect (i.e., Si-V−), which exhibits narrow (Γ = 4 nm) emission at 738 nm at low laser power. This approach uses high voltage (2–5 kV) nanosecond pulses applied across top and bottom electrodes on a 0.5 mm thick diamond substrate. In the absence of high voltage pulses, we observe no emission at 738 nm. This feature increases monotonically with peak pulse voltage, pulse repetition rate (i.e., frequency), and incident laser intensity. We observe saturation of the PL intensity for pulse voltages above 3.2 kV and frequency above 100 Hz. Based on electrostatic simulations, we estimated the local electric field intensity near the tip of the Cu electrode to be 2.8 ×106 V/cm at these voltages. However, as a function of laser power, we observe a linear dependence of PL intensity without saturation. These saturating and non-saturating behaviors provide important insight into the voltage-induced charging mechanisms and kinetics associated with this process.

Modeling the Charge State of Monatomic Hydrogen and Other Defects with Arbitrary Concentrations in Crystalline Silicon

A rigorous physical‐mathematical model for predicting the charge states of both mono‐ and multivalent defects with arbitrary concentrations in Si in both thermal equilibrium and non‐equilibrium steady‐state conditions is presented. The model avoids the assumption that the defect concentration is much lower than the doping level, which is common in previous approaches. It is shown that the general occupancy ratio given by α = (kn1 + p)/(kn + p1) is still valid under these more general conditions. However, more caution needs to be taken when calculating the equilibrium and excess carrier densities in the presence of larger defect concentrations, because of: a) the non‐negligible contribution to n0 and p0 from defect ionisation, and b) unequal Δn and Δp caused by the change in the occupied states at the defect level from equilibrium to non‐equilibrium. Modeled examples of monatomic H and interstitial Fe in Si are shown and the effects of defect concentration, temperature, and carrier injection are discussed.

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.

Nonradiative Carrier Recombination Enhanced by Vacancy Defects in Ionic II-VI Semiconductors

Nonradiative-recombination-related defects are significant for optoelectronic semiconductor devices. Here, we analyze nonradiative-recombination processes in ionic semiconductors using first-principles density-functional theory. In ionic group II-VI semiconductors, we find that large lattice relaxations of anion vacancies caused by strong Coulomb interactions between different charged defect states can significantly enhance recombination processes through a two-level recombination mechanism. Specifically, we show that the defect level of the 2+ charged anion vacancy $\mathrm{(}{V}_{\mathrm{Se}}^{2+})$ in group II-VI $\mathrm{Zn}\mathrm{Se}$ is close to the conduction-band minimum and easily captures an electron to form a metastable 1+ charged state $\mathrm{(}{V}_{\mathrm{Se}}^{+})$; then, the large lattice relaxation, on account of the change in Coulomb interactions locally in the different charged states, rapidly changes this metastable state to a stable one and simultaneously move the defect level of ${V}_{\mathrm{Se}}^{+}$ closer to that valence-band maximum, and thus, increases the hole-capture rate. Compared with the Shockley-Read-Hall nonradiative-recombination theory based on a single defect level, this two-level recombination mechanism involving anion vacancies can greatly increase the nonradiative-recombination rate in ionic group II-VI semiconductors. This understanding is expected to be useful for the study of the nonradiative-recombination process in ionic semiconductors for applications in the field of optoelectronic devices.