Charge-state Transition Research Articles

Effects of synthesis methods and different concentrations of Eu3+ ions on the emission properties of Sr2 SiO4 phosphors.

Sr2-x SiO4 -xEu (x = 0.01, 0.02, 0.035, and 0.05) phosphors were synthesized at 1000-1,300°C for 2 h in two different processes, the solid-state reaction (SSR) method and a two-step (TS) method, and these results revealed three important findings. The secondary Sr3 SiO5 phase was observed in 1,300° C-TS-synthesized Sr2-x SiO4 -xEu powder, but no raw materials or secondary phases were observed in the SSR-prepared Sr2-x SiO4 -xEu powders. The concentration quenching effect of Eu3+ ions was really observed in TS-prepared Sr2-x SiO4 -xEu phosphors, which was not observed in SSR-prepared Sr2-x SiO4 -xEu phosphors. High emission intensity of charge transition state (CT) band was observed in the photoluminescence excitation spectra, for that the 265 and 393 nm were used as the excitation wavelengths of Sr2-x SiO4 -xEu phosphors. Sr2-x SiO4 -xEu phosphors under different synthesis processes and excitation wavelengths would have different main emission peaks in the photoluminescence emission spectra. In this study, we also well discussed and explored the relationships of photoluminescence properties with the dipole transitions (electric or magnetic) of Eu3+ ions and the different coordination structures of Sr+2 ions.

First principles study on low valence states photoluminescence in Bi-doped M2B5O9Cl crystals

The multi-valence states of bismuth ions diversify the photoluminescence in bismuth doped materials and it is of great significance to analyze the main characteristic properties of different valence states. First principles calculations are carried out for Bi3+, Bi2+, Bi1+ and Bi0 valence states of substitutional and interstitial bismuth defects. The formation energies and charge-state transition levels results both confirm that all these valence states can be stable under appropriate chemical circumstances. The excitation and emission energies are calculated for various transition processes, and compared to experimental results. Furthermore, the calculations clearly show the trend of red shift of the emission energy with lowering the valence state, which is of significance for understanding the luminescent properties of low valence states of bismuth in M2B5O9Cl (M = Ba, Sr, Ca) crystals and in other hosts with suitable environments.

Acid–base reaction of a cationic hydration probe in vicinity of anionic interface of AOT reverse micelles

The excited-state proton transfer (ESPT) of a cationic super-photoacid, N-methyl-7-hydroxyquinolium, has been studied within the water pool of an aerosol-OT reverse micelle (RM). Time-resolved emission spectra were obtained and analyzed to reveal the effect of environmental heterogeneity on the ESPT process. The ESPT was found to involve two main emissive species, namely the excited cationic and keto forms of the photoacid and was facilitated by scenarios that expose them to an abundance of water molecules. The dynamic charge-state transition (from positive to neutral) induced by the ESPT dictated the location and diffusion of the probe with the modulation of the Coulombic interaction between the probe and anionic interface of the RM. The non-exponential behavior of the ESPT was observed using time-resolved intensity profiles and rationalized in terms of the heterogeneity along the reaction coordinates involving the hydrogen-bond network, polarity, and viscosity of the confined water pool within the RM. It was found that the disruption of the hydrogen-bond network in the vicinity of the interface resulted in the decrease of polarity that retarded the overall ESPT by modulating the energetics of the reactant and the product in the ESPT. Solvation dynamics was also considered and found to occur on timescales faster than the ESPT process, profoundly for larger water pools.

Detection of charge states of an InAs nanowire triple quantum dot with an integrated nanowire charge sensor

A linear triple quantum dot (TQD) integrated with a quantum dot (QD) charge sensor is realized. The TQD and the charge sensor are built from two adjacent InAs nanowires by the fine finger gate technique. The charge state configurations of the nanowire TQD are studied by measurements of the direct transport signals of the TQD and by detection of the charge state transitions in the TQD via the nanowire QD sensor. Excellent agreement in the charge stability diagrams of the TQD obtained by the direct transport measurements and by the charge-state transition detection measurements is achieved. It is shown that the charge stability diagrams are featured by three groups of charge state transition lines of different slopes, corresponding to the changes in the electron occupation numbers of the three individual QDs in the TQD. It is also shown that the integrated nanowire QD sensor is highly sensitive and can detect the charge state transitions in the cases where the direct transport signals of the TQD are too weak to be measurable. Tuning to a regime, where all three QDs in the TQD are close to being on resonance with the Fermi level of the source and drain reservoirs and co-existence of triple and quadruple points becomes possible, has also been demonstrated with the help of the charge sensor in the region where the direct transport signals of the TQD are hardly visible.

Primary intrinsic defects and their charge transition levels in β–Ga2O3

A steady-state photocapacitance (SSPC) setup directly connected to the beamline of a MeV ion implanter is utilized to study primary intrinsic defects in $\ensuremath{\beta}\text{--}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ generated by He implantation at cryogenic temperatures (120 K). At low temperatures, the migration of defects is suppressed, and hence the generation of primary intrinsic defects is expected to prevail. SSPC measurements reveal defect-related optical transitions in halide vapor-phase epitaxy (HVPE) -grown $\ensuremath{\beta}\text{--}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films with onset energies at 1.3 $({T}_{1}),$ 1.7 $({T}_{2}),$ 1.9 $({T}_{3}),$ 2.6 $({T}_{4}),$ 3.7 $({T}_{5}),$ and 4.2 eV $({T}_{6})$. ${T}_{2},$ ${T}_{4},$ ${T}_{5},$ and ${T}_{6}$ were observed in as-received HVPE-grown $\ensuremath{\beta}\text{--}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ thin films, whereby ${T}_{2}$ is only sporadically observed. The introduction rates for ${T}_{3},$ ${T}_{4},$ as well as ${T}_{6}$ indicate an origin related to primary intrinsic defects. Notably, ${T}_{1}$ and ${T}_{3}$ are only observed after He implantation at cryogenic temperatures. Hybrid-functional calculations were performed to estimate the optical absorption cross-section spectra for the gallium $({\mathrm{Ga}}_{\mathrm{i}})$ and oxygen $({\mathrm{O}}_{\mathrm{i}})$ interstitials as well as the corresponding vacancies (${V}_{\mathrm{Ga}}$ and ${V}_{\mathrm{O}}$, respectively), and compared with the measured onsets for optical absorption found by SSPC measurements. Indeed, we propose ${T}_{3}$ to be associated with ${\mathrm{Ga}}_{\mathrm{i}}^{(+/+2)}$ and/or ${V}_{\mathrm{GaI}}^{(\ensuremath{-}3/\ensuremath{-}2)}$, while ${T}_{4}$ is suggested to be related to ${V}_{\text{O}K}^{(0/+)}$ $(K=\mathrm{I}, \mathrm{II}, \mathrm{III})$ and/or ${V}_{\mathrm{GaII}}^{(\ensuremath{-}3/\ensuremath{-}2)}$. Additionally, several further charge-state transition levels associated with ${V}_{\mathrm{GaI}}$ and ${V}_{\mathrm{GaII}}$ may contribute to ${T}_{4}$ and ${T}_{6}$. We further studied the kinetics of the defects created with He implantation by exposing the sample to room temperature. The kinetics observed for ${T}_{3}$ and ${T}_{4}$ further support the proposed assignments of the corresponding defect signatures.

First-principles Study of the Impact of Hydrogen Passivation on the Charge State Transition Levels of the CiOi(Sii)n Defect Complexes in Silicon

Density functional theory (DFT) with the Heyd-Scuseria-Ernzerhof hybrid functional has been used to predict the stability, formation energy, geometric structure, and the charge state transition levels of the CiOi(Sii)n (i = interstitial) defect complex in silicon for n = 0, 1, 2. The effect of hydrogen passivation on the properties of the CiOi(Sii)n defect complex is also investigated. The results for the binding energies confirm the stability of the defect complexes at neutral charge state. The charge state transition levels show that CiOi induces a shallow donor level and a deep acceptor level. The CiOiSii and CiOi(Sii)2 however induced deep donor levels. After hydrogen passivation, the acceptor level of the CiOiSii and the CiOi(Sii)i defect complexes disappear and the donor levels for all the defect complexes move deeper in the band gap.

Carbon doping of GaN: Proof of the formation of electrically active tri-carbon defects

Carbon doping is used to obtain semi-insulating GaN crystals. If the carbon doping concentration exceeds 5 × 1017 cm−3, the carbon atoms increasingly form triatomic clusters. The tri-carbon defect structure is unambiguously proven by the isotope effect on the defects' local vibrational modes (LVMs) originally found in samples containing carbon of natural isotopic composition (∼99% 12C, ∼1% 13C) at 1679 cm−1 and 1718 cm−1. Number, spectral positions, and intensities of the LVMs for samples enriched with the 13C isotope (∼99% and ∼50%) are consistently interpreted on the basis of the harmonic oscillator model taking into account the probability of possible isotope combinations. Including the polarization dependence of the LVM absorption, we show that the tri-carbon defects form a triatomic molecule-like structure in two crystallographically different configurations: a basal configuration with the carbon bonds near the basal plane and an axial configuration with one of the carbon bonds along the c axis. Finally, the disappearance of the LVMs under additional below-bandgap illumination is interpreted as defect recharging, i.e., the tri-carbon defects possess at least one charge state transition level within the bandgap and contribute to optical absorption as well as to the electrical charge balance.

Prospects for n-type doping of (AlxGa1−x)2O3 alloys

We systematically explore the properties of group-IV (C, Si, Ge, and Sn) and transition metal (Hf, Zr, and Ta) dopants substituting on the cation site in (AlxGa1−x)2O3 (AlGO) alloys using first-principles calculations with a hybrid functional. In Ga2O3, each of these dopants acts as a shallow donor. In Al2O3, they are deep defects characterized by the formation of either DX centers or positive-U (+/0) levels. Combining our calculations of dopant charge-state transition levels with information of the AlGO alloy band structure, we estimate the critical Al composition at which each dopant transitions from being a shallow to a deep donor. We identify Si to be the most efficient dopant to achieve n-type conductivity in high Al-content AlGO alloys, acting as a shallow donor over the entire predicted stability range for AlGO solid solution alloys.

Triple group-V donors in ZnO

Triple donors have been explored in a few semiconductor materials; however, the conventional effective mass theory treatment fails at short length scales due to the high degree of localization implied by a 3+ nuclear charge. Using density functional theory, we consider the various charge states of group-V elements substituting for the Zn sublattice in ZnO under oxygen-rich conditions. For the case of Sb and Bi substitutional impurities, the (1+/0) charge state transition is shallow and has strong similarities to a (1+/0) charge transition of the more common shallow group III donors such as Ga and Al. We compare these calculations with extensive photoluminescence (PL) measurements that now exist for the Sb-related donor bound exciton in ZnO, which is known to contain substitutional Sb on Zn sites. We present new experimental data on the magneto-PL properties of the Sb-related donor bound exciton. These data confirm the strong similarity of the (+1/0) charge state transition of this center to the common group III shallow donors in ZnO. We propose that the very low binding energy (40.2 meV) of the neutral Sb donor is due to a combination of increased screening due to the two inner donor electrons, as well as the exclusion principle, resulting in a repulsive central cell potential close to the defect core.

Ab initio calculations on oxygen vacancy defects in strained amorphous silica**Project supported by the Science Challenge Project, China (Grant No. TZ2016003-1-105) and the CAEP Microsystem and THz Science and Technology Foundation (Grant No. CAT201501).

The effects of uniaxial tensile strain on the structural and electronic properties of positively charged oxygen vacancy defects in amorphous silica (a-SiO2) are systematically investigated using ab-initio calculation based on density functional theory. Four types of positively charged oxygen vacancy defects, namely the dimer, unpuckered, and puckered four-fold (4 ×), and puckered five-fold (5 ×) configurations have been investigated. It is shown by the calculations that applying uniaxial tensile strain can lead to irreversible transitions of defect structures, which can be identified from the fluctuations of the curves of relative total energy versus strain. Driven by strain, a positively charged dimer configuration may relax into a puckered 5× configuration, and an unpuckered configuration may relax into either a puckered 4× configuration or a forward-oriented configuration. Accordingly, the Fermi contacts of the defects remarkably increase and the defect levels shift under strain. The Fermi contacts of the puckered configurations also increase under strain to the values close to that of center in a-SiO2. In addition, it is shown by the calculations that the relaxation channels of the puckered configurations after electron recombination are sensitive to strain, that is, those configurations are more likely to relax into a two-fold coordinated Si structure or to hold a puckered structure under strain, both of which may raise up the thermodynamic charge-state transition levels of the defects into Si band gap. As strain induces more puckered configurations with the transition levels in Si band gap, it may facilitate directly the development of oxide charge accumulation and indirectly that of interface charge accumulation by promoting proton generation under ionization radiation. This work sheds a light on understanding the strain effect on ionization damage at an atomic scale.

Ab initio study of the effect of hydrogen passivation on boron-oxygen-carbon related defect complexes in silicon

We present hybrid density functional theory (DFT) study of defect complexes formed by oxygen, boron and carbon, in ion-implanted and electron-irradiated Czrolaski (CZ) silicon containing boron. The effect of hydrogen passivation on the defect complexes is also studied. The defects considered are the interstitial boron-interstital oxygen (BiOi) complex as well as interstitial boron-substitutional carbon (BiCs) complex, which are found in ion-implanted Czochralski silicon. Other defect complexes which are found in electron-irradiated CZ silicon and have been studied are the interstitial boron (Bi), substitutional boron-interstitial oxygen (BsOi), substitutional boron-interstitial boron (BsBi), interstitial boron-interstitial oxygen dimer (BiO2i), and substitutional boron-interstitial oxygen dimer (BsO2i). We found the defects to be stable in neutral charge state with the stability of the Bi, BiOi, BsBi, BiO2i and BsO2i increasing after hydrogen passivation. However, the stability of BsOi and BiCs defect complexes decreases after the hydrogen passivation. For the BsBi defect complex in particular, a deep donor level appeared after hydrogen passivation. Furthermore, we found a shift in the charge state transition levels in to the valence band for the donor levels, and a shift in to the conduction band for the acceptor levels for all the defects after hydrogen passivation. We compare our results with available theoretical and experimental studies.

P-Type conductivity mechanism and defect structure of nitrogen-doped LiNbO3 from first-principles calculations.

Most metal-doped lithium niobates (LiNbO3, LN) exhibit n-type conductivity. The absence of p-type conductive LiNbO3 limits its application. Based on the finding that p-type conductive LiNbO3 can be realized by doping with a non-metallic element N, we investigate the most stable defect configurations and formation energies of LiNbO3 doped with non-metal nitrogen (LN:N) by first-principles calculations. Nitrogen substitution, interstitial and quasi-substitution point defects in different sites and their effects were explored. The results show that N prefers to occupy the oxygen site with only little lattice distortion. Ab initio molecular dynamics (AIMD) simulations confirm the structural stability of an N ion occupying the O site. The charge-state transition level ε(0/-1) slightly above the valence band maximum (VBM) indicates that N point defects would contribute to p-type conductivity of LiNbO3. The analysis of the band structure reveals that the partially filled impurity levels can accommodate electrons that jump from valence bands and result in holes to become the main charge carriers. The calculation not only explains the occurrence of p-type conductivity in LN:N but also provides a simple and efficient way to discover p-type conductive candidates in numerous doped LiNbO3 crystals.

Hybrid functional study of hydrogen passivation in carbon-oxygen related defect complexes in silicon

Density functional theory (DFT) with the Heyd-Scuseria-Ernzerhof hybrid functional was used to predict the structure, formation energy, stability and the charge state transition energy levels induced by carbon-oxygen related complexes as a result of hydrogen passivation in silicon. Our results show that, in the neutral charge state the hydrogen interstitial (Hi) interacts with Oi, Ci, CsOi, CiOi, and CsCi to form a stable OiHi, CiHi, CsOiHi, CiOiHi, and CsCiHi defect complexes. Predicted charge state transition levels showed reasonable agreement with the experiment. The charge state transition energy levels were found to shift to the conduction band as a result of hydrogen passivation.

Negative-Uand polaronic behavior of the Zn-O divacancy in ZnO

Hybrid functional calculations reveal the Zn-O divacancy in ZnO, consisting of adjacent Zn and O vacancies, as an electrically active defect exhibiting charge states ranging from $2+$ to $2-$ within the band gap. Notably, the divacancy retains key features of the monovacancies, namely the negative-\textit{U} behavior of the O vacancy, and the polaronic nature of the Zn vacancy. The thermodynamic charge-state transition levels associated with the negative-\textit{U} behavior $\varepsilon$($0$/$2-$), $\varepsilon$($-$/$2-$) and $\varepsilon$($0$/$-$) are predicted to occur at 0.22, 0.42 and 0.02 eV below the conduction band minimum, respectively, resulting in \textit{U} = $-$0.40 eV. These transition levels are moved closer to the conduction band and the magnitude of \textit{U} is lowered compared to the values for the O vacancy. Further, the interaction with hydrogen has been explored, where it is shown that the divacancy can accommodate up to three H atoms. The first two H atoms prefer to terminate O dangling bonds at the Zn vacancy, while the geometrical location of the third depends on the Fermi-level position. The calculated electrical properties of the divacancy are in excellent agreement with those reported for the E4 center observed by deep-level transient spectroscopy, challenging the O vacancy as a candidate for this level.

Disappeared deep charge-states transition levels in the p-type intrinsic CsSnCl3 perovskite

Lead-free inorganic perovskites are promising for optoelectronic applications. Understanding their phase diagram and defect properties is beneficial to predict the stable phase and applications. Here, taking CsSnCl3 as an example, the stability and defect properties are investigated systemically. The results show that the stoichiometric CsSnCl3 can only be grown in a narrow area determined by Sn and Cs chemical potentials. No matter what the Sn condition is, both acceptor defects (Cs- and Sn-vacancies with low charge states) are formed spontaneously with negative formation energies, and the donor defects are difficult to be formed with high formation energies. Interestingly, these charge-state transition levels induced by such stable defects are in the valence band. Meanwhile, no deep acceptor and donor states are formed in the bandgap. In addition, although the Fermi levels can vary in a wide range, the Fermi levels are pinned in the valence band, irrespective of the atmospheric conditions, leading to p-type CsSnCl3 with high hole density and low electron density. Such electronic characters are elucidated in detail by the atomic orbitals and structural deformations. Our studies provide an insight view of the defect properties of CsSnCl3 and provide a valuable guideline for CsSnCl3 fabrication and further modulation.

Surface doping of ZnO nanowires with Bi: Density-functional supercell calculations of defect energetics

Defect calculations using the density and hybrid functionals in combination with the supercell approach are employed to characterize the electrical properties of a number of ZnO nanowires of various thicknesses doped with Bi atoms occupying surface sites. The variation of the differences between the total energies of charged and neutral supercells with the supercell size is studied, which led the authors to devise an extrapolation procedure to obtain reliable defect energetics in the dilute defect limit. The calculated defect formation energies indicate that although the substitution of Bi into Zn or O sites can take place spontaneously under suitable thermodynamic conditions, the substitution into Zn sites is generally more likely. The defect (charge-state) transition energies are computed and parameterized as a function of the nanowire thickness. It is revealed that the substitution of Bi into O (Zn) sites on the surface of ZnO nanowires yields deep acceptor (shallow donor) levels (except for extremely thin nanowires). It is therefore concluded that the incorporation of Bi into the surface of ZnO nanowires results in $n$-type doping.

A SCANNING TUNNELING MICROSCOPY STUDY OF MONOLAYER AND BILAYER TRANSITION-METAL DICHALCOGENIDES GROWN BY MOLECULAR-BEAM EPITAXY

This review presents an account of some recent scanning tunneling microscopy and spectroscopy (STM/S) studies of monolayer and bilayer transition-metal dichalcogenide (TMD) films grown by molecular-beam epitaxy (MBE). In addition to some intrinsic properties revealed by STM/S, defects such as inversion domain boundaries and point defects, their properties and induced effects, are presented. More specifically, the quantum confinement and moiré potential effects, charge state transition, quasi-particle interference and structural phase transition as revealed by STM/S are described.

Transition levels of acceptor impurities in ZnO crystals by DFT-LCAO calculations

Large scale ab-initio calculations are carried out to study the charge state transition levels of nitrogen and phosphorus impurity defects in zinc oxide crystals using the DFT-LCAO approximation as implemented into the CRYSTAL computer code. It is shown that at a high concentration of defects (close location of defects) their formation energy is underestimated due to a significant delocalization of the charge within the supercell. After inclusion the energy offset correction and defect-defective interaction, the formation energy is improved, in a comparison with that calculated in a large supercell. The optical transition levels obtained by a direct calculation confirm the experimental observation: nitrogen and phosphorus impurities are deep acceptor centers with large formation energy in a charged state and, therefore, cannot serve as the effective source of hole charge. The obtained results are in good agreement with the previous theoretical work, in which other calculation methods were used, and are capable of qualitatively describing the energy characteristics of the charged defects.

Zn vacancy-donor impurity complexes in ZnO

Results from hybrid density functional theory calculations on the thermodynamic stability and optical properties of the Zn vacancy ($V_{\text{Zn}}$) complexed with common donor impurities in ZnO are reported. Complexing $V_{\text{Zn}}$ with donors successively removes its charge-state transition levels in the band gap, starting from the most negative one. Interestingly, the presence of a donor leads only to modest shifts in the positions of the $V_{\text{Zn}}$ charge-state transition levels, the sign and magnitude of which can be interpreted from a polaron energetics model by taking hole-donor repulsion into account. By employing a one-dimensional configuration coordinate model, luminescence lineshapes and positions were calculated. Due to the aforementioned effects, the isolated $V_{\text{Zn}}$ gradually changes from a mainly non-radiative defect with transitions in the infrared region in \textit{n}-type material, to a radiative one with broad emission in the visible range when complexed with shallow donors.

Influence of annealing atmosphere on formation of electrically-active defects in rutile TiO2

Electronic states in the upper part of the bandgap of reduced and/or hydrogenated n-type rutile TiO2 single crystals have been studied by means of thermal admittance and deep-level transient spectroscopy measurements. The studies were performed at sample temperatures between 28 and 300 K. The results reveal limited charge carrier freeze-out even at 28 K and evidence the existence of dominant shallow donors with ionization energies below 25 meV. Interstitial atomic hydrogen is considered to be a major contributor to these shallow donors, substantiated by infrared absorption measurements. Three defect energy levels with positions of about 70 meV, 95 meV, and 120 meV below the conduction band edge occur in all the studied samples, irrespective of the sample production batch and the post-growth heat treatment used. The origin of these levels is discussed in terms of electron polarons, intrinsic point defects, and/or common residual impurities, where especially interstitial titanium atoms, oxygen vacancies, and complexes involving Al atoms appear as likely candidates. In contrast, no common deep-level defect, exhibiting a charge state transition in the 200–700 meV range below the conduction band edge, is found in different samples. This may possibly indicate a strong influence on deep-level defects by the post-growth heat treatments employed.