Deep levels of intrinsic point defects and the nature of “anomalous” optical absorption in ZnGeP2

Deep levels of single vacancies and antisite defects in the structure of the ZnGeP₂ compound are investigated using the pseudopotential method and an extended unit cell. The data obtained are compared with those for the GaP isoelectronic analog. It is demonstrated that, in the case of the ZnGeP₂ crystal, deep levels (degenerate in the GaP structure) are substantially split as a result of lowering the lattice symmetry and anisotropy of the chemical bonding. In particular, the splitting of the V _P ⁰ (t ₂) level is equal to 1.58 eV. The averaged levels of defects in the ZnGeP₂ compound are in close agreement with the levels of defects in the GaP compound. The absorption coefficients for polarized light are calculated with allowance made for the neutral and charged states of the defects. The optical transitions responsible for the absorption peaks in the IR range of the spectrum of the ZnGeP₂ compound are revealed. It is shown that the first peaks are associated with the transitions of electrons from the valence band states located deep in the Brillouin zone to the V _Zn ^?1 and V _P ⁰ deep levels. This leads to a considerable shift (by ～0.3 eV) of these peaks toward the high-energy range as compared to the energy positions of the deep levels in the band gap with respect to the top of the valence band. The experimental data on the photoinduced EPR spectra of postgrowth and electron-irradiated ZnGeP₂ crystals are consistently interpreted by analyzing the electron density.     

EPR study of radiation-induced defects in carbonate-containing hydroxyapatite annealed at high temperature

Radiation-induced (γ or UV) paramagnetic defects in carbonate-containing hydroxyapatite (HAP) annealed at high (600–950 °C) temperature were studied by EPR. The complex spectra reveal the presence of different paramagnetic species. Their contributions were found to be strongly dependent on the annealing temperature as well as microwave power, thus, by the adjustment of experimental conditions some of the components can be eliminated that allowed to record EPR spectra caused by no more than two types of paramagnetic defects. All experimental spectra were analyzed using computer simulation. The parameters of the paramagnetic defects detected were determined, and the centers models were discussed. It was found that high-temperature annealing influences essentially the formation of radiation-induced defects in HAP. The СО₃³⁻, О⁻ centers and oxygen vacancy V_O⁻ were shown to be the main stable γ-induced defects in the HAP annealed at high temperatures. New paramagnetic defect with the parameters g_|| = 2.002, g_⊥ = 2.0135 was detected and tentatively identified as an O-related radical. The γ-induced EPR response from СО₃³⁻ radicals was found to be more intense than response from CO₂⁻ in non-annealed HAP. UV-irradiation was found to create smaller amounts of paramagnetic defects in comparison with γ-rays. Besides, oxygen vacancy V_O⁻ was not observed, while two other centers (СО₃⁻ and the center of unknown nature) appear in the UV-induced EPR spectra.     

Irradiation effects in II-VI compounds

Abstract

A summary of published results on electron energy threshold measurements in II–VI compounds, and their interpretation in terms of damage on the metal and chalcogenide sublattices, is given. EPR results in irradiated II–VI compounds are also reviewed. These include the F⁺ center in ZnS; ZnO, and BeO and new results on the zinc vacancy (V) in ZnSe. For the zinc vacancy, optical absorption bands at 4680Å and 8850Å are identified with V^? and a band at 5000Å is tentatively assigned to V^?. The activation energy for anneal of the vacancy is measured to be 1.26±.06 eV, and this is tentatively identified as the activation energy for vacancy motion. An EPR spectrum produced in a 20.4 °K irradiation is tentatively identified as a zinc vacancy-interstitial pair. Complete annealing occurs for this defect in the range 60–100 °K.     

The observation of high concentrations of arsenic anti-site defects in electron irradiated n-type GaAs by X-band EPR

N-type GaAs doped with sulphur (2.8 × 10¹⁸ cm^-3) has been subjected to 2 MeV electron irradiation in stages at room temperature and examined by the EPR technique. When the free carrier absorption is first eliminated no EPR signal is detected. After further irradiation, the spectrum of the As anti-site defect appears, grows and subsequently saturates at a concentration of about 10¹⁸ cm^-3. The saturation concentration is about one third of [n] in most samples. The defects are stable on annealing to 500°C but are not observed in various irradiated p-type samples. It is suggested that grown-in defects such as [V_Ga-As_Ga-V_Ga] capture Ga interstitials during the irradiation and are thereby converted to the simpler anti-site defect.     

Optically detected magnetic resonance of the zinc vacancy in ZnS

Optical detection of magnetic resonance (ODMR) is reported for the single negative charge state, V_Zn^?, of the isolated zinc vacancy in ZnS. Produced by 2.5 MeV electron irradiation, it is detected in a distant donor-acceptor (DA) pair luminescent band at 570 nm in which the vacancy acts as the acceptor. Excitation and emission spectral dependences of the V_Zn^? ODMR signals are analyzed in terms of a configurational coordinate model. We conclude that the double acceptor level (V_Zn⁼/V_Zn^?) is located ～1.1 eV from the valence band edge and that the trigonal Jahn-Teller relaxation energy for the V_Zn^? state is ～0.5 eV.     

Analysis of point defects in polycrystalline BaTiO3 by electron paramagnetic resonance

Several paramagnetic point defects in BaTiO₃ polycrystals were detected and identified using the electron paramagnetic resonance technique. Polycrystalline samples sintered in a reducing atmosphere showed a broad signal with a giromagnetic constant of 1.932. This signal, observed only in conducting ceramics, was assigned to Ti³⁺ and its related complexes. V_Ba and V_Ti defects were found at giromagetic constants of 1.974 and 2.004, respectively. The EPR intensity of these defects increases after oxidising at T>1000 °C. The appearance of the V_Ba and V_Ti signals in EPR spectra of oxidized ceramics correlates with the onset of the PTCR effect. At donor dopant levels ≥0.3 at.%, the concentration of V_Ba and V_Ti shows good agreement with the defect compensation mechanism of donor doped BaTiO₃. However, small amounts of V_Ba and V_Ti were also detected in undoped BaTiO₃, contrary to the generally accepted defect model.     

Identification of a metastable state of the VZnH2 defect in ZnO

An infrared absorption study of the Zn vacancy passivated by two hydrogen atoms (V_ZnH₂) is reported. The ground state of the defect V_ZnH₂ consists of the inequivalent O–H bonds, which are aligned parallel and perpendicular to the c-axis, respectively. A metastable state of the defect was detected with two equivalent O–H bonds oriented perpendicular to the c-axis (V_ZnH₂^?). V_ZnH₂^? has two local vibration modes at 3329.0 and 3348.4 cm⁻¹ which are the antisymmetric and symmetric combinations of the two O–H stretch modes. The metastable state of the defect is 75±9 meV above the ground state of V_ZnH₂. An activation energy of 0.96±0.12 eV for the transition from metastable to the ground state was determined.     

Divacancy complexes induced by Cu diffusion in Zn-doped GaAs

Positron annihilation spectroscopy was applied to investigate the nature and thermal behavior of defects induced by Cu diffusion in Zn-doped p-type GaAs crystals. Cu atoms were intentionally introduced in the GaAs lattice through thermally activated diffusion from a thin Cu capping layer at 1100 °C under defined arsenic vapor pressure. During isochronal annealing of the obtained Cu-diffused GaAs in the temperature range of 450?850 K, vacancy clusters were found to form, grow and finally disappear. We found that annealing at 650 K triggers the formation of divacancies, whereas further increasing in the annealing temperature up to 750 K leads to the formation of divacancy-copper complexes. The observations suggest that the formation of these vacancy-like defects in GaAs is related to the out-diffusion of Cu. Two kinds of acceptors are detected with a concentration of about 10¹⁶ ? 10¹⁷ cm^?3, negative ions and arsenic vacancy copper complexes. Transmission electron microscopy showed the presence of voids and Cu precipitates which are not observed by positron measurements. The positron binding energy to shallow traps is estimated using the positron trapping model. Coincidence Doppler broadening spectroscopy showed the presence of Cu in the immediate vicinity of the detected vacancies. Theoretical calculations suggested that the detected defect is V _Ga V _As-2Cu_Ga.     

Raman scattering investigations on Co-doped ZnO epitaxial films: Local vibration modes and defect associated ferromagnetism

Raman scattering spectroscopy has been performed on high quality Co-doped ZnO epitaxial films, which were grown on Al₂O₃ (0001) by oxygen-plasma assisted molecular beam epitaxy. Raman measurements revealed two local vibration modes (LVMs) at 723 and 699 cm⁻¹ due to the substitution of Co²⁺ in wurtzite ZnO lattice. The LVM at 723 cm⁻¹ is found to be an elemental sensitive vibration mode for Co substitution. The LVM at 699 cm⁻¹ can be attributed to enrichment of Co²⁺ bound with oxygen vacancy, the cobalt–oxygen vacancy–cobalt complexes, in Zn_1−xCo_xO films associated with ferromagnetism. The intensity of LVM at 699 cm⁻¹, as well as saturated magnetization, enhanced after the vacuum annealing and depressed after oxygen annealing.     

Native defects and Pr impurities in orthorhombic CaTiO3 by first-principles calculations

Formation energies of native defects and Pr impurities in orthorhombic CaTiO₃ are explored using the first-principles calculations. The Ca vacancy (V_Ca), Ti vacancy (V_Ti) and Ca antisite (Ca_Ti) are found to be energetically preferable. The Ti antisite (Ti_Ca) and O vacancy (V_O) are not energetically favorable in the wide range of Fermi level. In Pr-doped CaTiO₃, Pr substituting for Ca (Pr_Ca) is likely to form under condition A in which CaTiO₃ is in equilibrium with CaO and O₂. Under condition B (TiO₂, CaTiO₃ and O₂ are in equilibrium), Pr_Ti defect is energetically preferable depending on the Fermi levels. Several native defects and the two sites of Pr impurities in CaTiO₃ are coincided with several different defects in Pr-doped CaTiO₃ reported in the literature. Based on the present calculations, we can elucidate that the Ca deficiency design of the traditional formula Ca_1−xV_Ca(x/2)Pr_xTiO₃ is not the best for efficient red photoluminescence, which is realized via the experimental measurements.     

The electronic and magnetic properties with intrinsic defects in ZnO nanosheets: First-principles prediction

Using full-potential linearized augmented plane wave (FLAPW) method, we investigated the effects of intrinsic vacancies on electronic and magnetic properties in graphene-like ZnO nanosheets. The results show that the oxygen vacancy (V_O) has no influence on magnetism in ZnO nanosheet, whereas the Zn vacancy (V_Zn) lead to spin polarization of the nanostructures with a total magnetic moment of 2.0μ_B due to O-2p and Zn-3d hybridization. When the distance of two V_Zn defects increases to 6.499 Å, the system shows an intriguing half-metallic character with 100% spin-polarized carriers due to O(2p)–Zn(3d)–O(2p) coupling chain between two V_Zn defects.     

Thermally stimulated current study of electron-irradiation induced defects in semi-insulating InP obtained by multiple-step wafer annealing

Electron-irradiation induced defects in semi-insulating (SI) InP wafers with Fe concentration ranging from 1.5×10¹⁵ to 2.5×10¹⁵ cm⁻³, which have been obtained by multiple-step wafer annealing (MWA) under phosphorus vapor pressure, were studied using a thermally stimulated current (TSC) method. New traps, e₁, e₂, e₃, e₄ and e₅, with activation energies of 0.22, 0.28, 0.37, 0.44 and 0.46 eV, respectively, were observed. Based upon the annealing behavior of traps and the calculated defect levels, traps e₁ and e₅ produced by the irradiation with electron doses above 1×10¹⁵ cm⁻² were linked to In_P and P_In antisite defects, respectively, that probably form complexes. Traps e₃ and e₄ produced by the irradiation with doses above 1×10¹⁴ cm⁻² were associated with In and P vacancy related defects, respectively.     

Potential-induced sonoelectrochemical graphene nanosheets with vacancies as hydrogen peroxide reduction catalysts and sensors

Defective graphene nanosheets (dGN_4V) with 5-9, 5-8-5, and point defects were synthesised by a sonoelectrochemical method, where a potential of 4 V (vs. Ag/AgCl) was applied to drive the rapid intercalation of phosphate ions between the layers of the graphite foil as a working electrode. In addition to these vacancies, double vacancy defects were also created when the applied potential was increased to 8 V (dGN_8V). The defect density of dGN_8V (2406 μm⁻²) was higher than that of dGN_4V (1786 μm⁻²). Additionally, dGN_8V and dGN_4V were applied as catalysts for the hydrogen peroxide reduction reaction (HPRR). The mass activity of dGN_8V (1.31 × 10⁻² mA·μg⁻¹) was greater than that of dGN_4V (1.17 × 10⁻² mA·μg⁻¹) because of its high electrochemical surface area (ECSA, 1250.89 m²·g⁻¹) and defect density (N_D, 2406 μm⁻²), leading to low charge transfer resistance on the electrocatalytic interface. The ECSA and N_D of dGN_4V were 502.7 m²·g⁻¹ and 1786 μm⁻², respectively. Apart from its remarkable HPRR activity, the cost-effective dGN_8V catalyst also showed potential as an amperometric sensor for the determination of H₂O₂.     

Behavior of impurity copper in ZnGeP2 single crystals with diffusion doping

Results from a study of electrophysical and luminescent properties of zinc-germanium diphosphide single crystals, diffusion doped with copper, are presented. The nature of the dominant defects formed upon copper diffusion in ZnGeP₂ is determined using a thermodynamic analysis of defect formation processes in ZnGeP₂ and a ZnGeP₂: Cu solid solution performed within the framework of the quasichemical analysis method. It is demonstrated that by choosing the copper diffusion method the hole concentration in the ZnGeP₂ can be varied over the range 10¹²–10¹⁶ cm^–3. A retrograde character was observed in copper solubility in ZnGeP₂ compounds. Defect formation processes in ZnGeP₂ upon copper diffusion depend on the degree of atomic ordering in the cation sublattice at the diffusion annealing temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 64–69, February, 1985.     

Reduction of the annealing temperature of radiation-induced defects in ion-implanted MOS structures

Si-SiO₂ structures irradiated with 11-MeV electrons for 10 s and then implanted with B⁺ ions with an energy of 10 keV at a dose of 1.0×10¹² cm^-2 through the oxide were annealed at different temperatures. MOS capacitors including such oxide layers were studied by quasi-static C/V and thermally stimulated current (TSC) methods. A comparison of the radiation defect annealing of double-treated (electron-irradiated and ion-implanted) samples and of implanted-only samples was carried out. It is shown that a preceding low-dose high-energy electron irradiation of the samples leads to a lowering of the annealing temperature of radiation defects introduced by ion implantation. After annealing at 500 °C for 15 min, no TSC spectra for the double-treated samples were observed. The spectra of the other samples (which were not previously irradiated) showed that after the same thermal treatment only some of the radiation defects introduced by ion implantation are annealed. The difference between the annealed interface state density of previously electron-irradiated and current MOS structures is also demonstrated. A possible explanation of the results is proposed . PACS 61.82.Fk; 85.40.Ry; 61.80.Fe     

Electrical and optical properties of electron-irradiated ZnGeP2

Conclusions and summary The following conclusions are drawn from the reported study:The electrophysical properties of ZnGeP₂ crystals and their optical transparency in the range hG are attributable to the presence of a density-dominant (10¹⁷–10¹⁹ cm^–3) deep [E_v+(0.5–0.6) eV] growth defect associated predominantly with Zn vacancy clusters.Irradiation by high-energy electrons induces a shift of the Fermi level in the direction of E_G/2 and increases the resistivity of ZnGeP₂ to values of approximately 10¹² ·cm at 300 K. Irradiation with high-energy electrons is an effective technique for the optical bleaching of p-ZnGeP₂. The reversible modification of the optical absorption spectra of p-ZnGeP₂ in connection with irradiation and subsequent annealing indicates that the absorption step in the vicinity of h 0.6 eV is not attributable to light absorption by germanium inclusions, but to optical transition from the valence band to the growth-defect level E_v+(0.5–0.6) eV.Enhancement of the optical transmissivity of p-ZnGeP₂ in the range hG can be achieved in two wayss 1) as the result of a decrease in the density of centers with the level E_v+0.6 eV by variation of the growth conditions or subsequent annealing; 2) by shifting the Fermi level above the energy position E_v+0.6 eV through the irradiation-induced injection of compensating donor centers.The injection of radiation defects is an effective technique for controlling the electrical and optical parameters of the compound ZnGeP₂.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 122–130, August, 1986.     

EPR investigation of the trivalent chromium complexes in SrTiO3

The trivalent chromium centers were investigated by means of electron paramagnetic resonance (EPR) in SrTiO₃ single crystals grown using the Verneuil technique. It was shown that the charge compensation of the Cr³⁺-V_O dominant centers in octahedral environment is due to the remote oxygen vacancy located on the axial axis of the center. In order to provide insight into spin-phonon relaxation processes the studies of axial distortion of Cr³⁺-V_O centers have been performed as function of temperature. The analysis of the trigonal Cr³⁺ centers found in SrTiO₃ indicates the presence of the nearest-neighbor strontium vacancy. The next-nearest-neighbor exchange-coupled pairs of Cr³⁺ in SrTiO₃ has been analyzed from the angular variation of the total electron spin of S=2 resonance lines.     

Copper,the dominant acceptor in refined,undoped zinc telluride

Comparative measurements have been made of optical absorption and photoluminescence of refined undoped and Cu in-diffused ZnTe single crystals. Strong increases in a bound exciton BE line near 2.375 eV previously identified with the electrically dominant point defect acceptor ‘a’, with binding energy E_A ? 149 meV, suggests that this acceptor is substitutional Cu_Zn. Similarly strong increases in a relatively broad band at slightly higher energy suggests the simultaneous incorporation of shallow donors, possibly interstital Cu_I. These findings indicate that intrinsic defects such as V_Zn neither control the Fermi level in refined ZnTe nor produce shallow acceptors with E_A ? 250 meV, contrary to much previous speculation.     

Interplay of native defect-related photoluminescence response of ZnO nanosticks subjected to 80 keV Ar ion irradiation

We report on the effect of 80 keV Ar⁺ ion irradiation on the luminescence response of zinc oxide (ZnO) nanosticks synthesized using a simple microemulsion route. The formation of nanoscale rods was confirmed from the transmission electron microscopy, whereas the hexagonal wurtzite phase of the nanorods was detected in an X-ray diffraction pattern. The photoluminescence pattern of the nanorods was dominated by various native defect states of ZnO, which are responsible for the quenching of the typical band edge emission of ZnO. Under Ar⁺ ion irradiation at a fluence of 1×10¹³ ions/cm², the band edge emission was recovered owing to the suppression of oxygen vacancy defects. In addition, the formation of new zinc vacancy and ionized zinc interstitial defects were also evident. Conversely, the band edge emission was found to be quenched as a result of the creation of more oxygen vacancy (V_O) defects due to ion irradiation (fluence: 1×10¹⁵ ions/cm²). The nuclear energy loss of the Ar⁺ ions in ZnO is responsible for the formation of point (vacancy-related) defects, while relatively small amount of electronic energy loss of the Ar⁺ ion results in the ionization of the neutral zinc interstitial (Zn_i) defects. The energy deposition scheme of the energetic ions has been elaborated with the help of theoretical modeling that explains the observed features quite satisfactorily.     

Local structure and the electron paramagnetic resonance parameters for the Cr3+ ions at K+-vacancy site in Cr3+:KZnF3 laser crystal

The quantitative relationship between the electron paramagnetic resonance (EPR) parameters D,g_∥,g_⊥ and the local structure parameters of Cr³⁺ ion in KZnF₃ crystals is established. The local structure for Cr³⁺ paramagnetic center in KZnF₃:Cr³⁺ crystal has been determined from EPR parameters of Cr³⁺ ion. This work shows that the trigonal crystal field of Cr³⁺ ion in KZnF₃ crystals comes from following two origins: (1) the nearest-neighbor K⁺ vacancy caused by the charge compensation in the [1 1 1]-axis direction; and (2) the lattice distortions of the nearest-neighbor fluorine coordination caused by the K⁺ vacancy and the differences in mass, charge, and radius between Cr³⁺ ion and Zn²⁺ ion. The unified calculation of the EPR zero-field splitting and g factors, taking into account the K⁺ vacancy and the lattice distortions, has been carried out on the basis of the complete diagonalization procedure and the superposition crystal-field model, all calculation results are in excellent agreement with the experimental data. Although the main source of the trigonal crystal field comes from the K⁺ vacancy caused by the charge compensation, the contribution of the lattice distortion cannot be neglected.