Structure and energy level of native defects in as-grown and electron-irradiated zinc germanium diphosphide studied by EPR and photo-EPR

The properties of defects in as-grown p-type zinc germanium disphosphide (ZnGeP₂) and the influence of electron irradiation and annealing on the defect behavior were studied by means of electron paramagnetic resonance (EPR) and photo-EPR. Besides the well-known three native defects (V_Zn, V_P, Ge_Zn), an S=1/2 EPR spectrum with an isotropic g=2.0123 and resolved hyperfine splitting from four equivalent I=1/2 neighbors is observed in electron-irradiated ZnGeP₂. This spectrum is tentatively assigned to the isolated Ge vacancy. Photo-EPR and annealing treatments show that the high-energy electron irradiation-induced changes in the EPR intensities of the zinc and phosphorus vacancies are caused by the Fermi level shift towards the conduction band. Annealing of the electron-irradiated samples induces a shift of the Fermi level back to its original position, accompanied by an increase of the EPR signal associated with the V_Zn⁻ and a proportional increase of the EPR signal assigned to the V_P⁰ under illumination (λ<1 eV) as well as generation of a new defect. The results indicate that the EPR spectra originally assigned to the isolated V_Zn⁻ and V_P⁰ are in fact associated defects and the new defect is probably the isolated phosphorus vacancy V_Pi.     

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.     

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.     

A selective hole burning method applied to determine distances between paramagnetic species in photosystems

A method of selective hole burning in EPR spectra was applied to determine the distances from a radical to the acceptor quinone-iron in bacterial and plant photosystems. A low amplitude hole burning 180° pulse and high amplitude 90° and 90° pulses applied to detect ESE of P870⁺ inRb. Sphaeroides and the distance from the primary electron donor P870⁺ to the acceptor Q _A ^? Fe²⁺ was determined to be 26±2 Å from the dipolar broadening of the burned hole in P870⁺ EPR. This result is consistent with that given by X-ray analysis and susceptibility measurement. In plant photosystem II the same method was applied to the EPR spectrum of tyrosine D⁺, but the effect of crystalline field splitting of Fe²⁺ ion was taken into consideration. The effective spin value for the ferrous iron in PS II was found to be 0.8 and the distance between the radical and the non-heme iron was obtained to be 42±2 Å.     

Electron spin resonance of donor centers in beryllium oxide

The ESR spectra of two deep centers found in irradiated BeO crystals and powders, respectively, are reported and analyzed. They are due to electrons trapped at substitutional boron (B⁺⁺⁺) and fluorine (F^?) ions as concluded from the hf interaction of the unpaired electron with the respective nuclei. The boron center revealed a static, the fluorine center a dynamic Jahn-Teller effect between 77°K and 300°K. In both cases, the unpaired electron was found to be localized in nonaxial orbitals, which is shown to be consistent with the direction of the polar field of the crystal structure. The strong localisation of the electron is contrasted to the behavior of donor electrons in other II–VI compounds, which are highly delocalized.     

Electron-hole plasma in pulse photoexcited single quantum wells

We report photoluminescence studies of MOCVD grown, GaAsAl_xGa_1?xAs single quantum wells which were intensly excited with a pulse dye laser at T=2K. For a well width of d～40Å, the spectra are interpreted as due to the radiative recombination of a hot electron-hole plasma confined to the well. The density of charge carriers and their temperature depend upon the excitation intensity, and vary in the range of 10¹¹–10¹³ cm^?2 and 100–500K for an absorbed photon flux of 10¹³–10¹⁶ photons-cm^?2 per pulse, respectively. The observed spectral features are identified as the e1-hh1 and e1-lh1 transitions and two additional bands which are tentatively assigned to transitions involving virtual bound states of either the electron or the hole. The electron-hole plasma spectra of the d～40Å sample are strongly polarized perpendicular to the well quantization axis. For wider wells (d～80 and 150Å) smaller photoexcited carrier densities were observed for the same absorbed photon flux. It is thus concluded that the capture efficiency of the well is small.     

Spin Hamiltonian parameters and local structures for tetragonal and orthorhombic Ir2+ centers in AgCl

The perturbation formulae of the spin Hamiltonian parameters (the anisotropic g factors, hyperfine structure constants and superhyperfine parameters) are established for a 5d⁷ ion in an orthorhombically elongated octahedron based on the cluster approach. These formulae are applied to the theoretical studies of the EPR spectra and the local structures for the tetragonal and orthorhombic Ir²⁺ centers in AgCl. For the tetragonal Ir²⁺ center, the uncompensated substitutional [IrCl₆]⁴ cluster is found to experience a relative elongation of about 0.08 Å along the C ₄ axis due to the Jahn–Teller effect. For the orthorhombic center, the ligand octahedron also suffers Jahn–Teller elongation (by about 0.08 Å) along the [001] (or Z) axis. Meanwhile, the ligand Cl intervening in the impurity Ir²⁺ and the next nearest neighbor silver vacancy V_Ag along the [100] (or X) axis may undergo an inward displacement of 0.004 Å towards the center of the octahedron due to electrostatic repulsion of the V_Ag. The calculated spin Hamiltonian parameters based on the above local structures show good agreement with experimental data for both centers.     

Presence of Jahn-Teller distortions in a novel six-coordinate Ag(II) complex: temperature dependent EPR,optical and magnetic susceptibility measurements

Single-crystal variable temperature EPR, optical and polycrystalline magnetic susceptibility studies have been made on a novel six-coordinate Ag(II) complex. Temperature dependent EPR studies on pure single crystals of this compound reveal that dynamic Jahn-Teller distortion operates above 230 K, between 230 K and 120K static Jahn-Teller distortion sets in and below 110 K there is evidence of exchange interaction. Crystal g values were obtained by least-squares fitting with the data obtained from the orientation dependent EPR spectra of the undiluted single crystal of this complex at 300 K and 77 K. From an optical study the Jahn-Teller stabilization energy is found to be ～2250cm^?1. Comparison of Abs_max values for other silver(II) compounds enables us to conclude that the formal geometry of this complex is a tetragonally distorted octahedral. Infrared spectra of this complex were also recorded over a wide range of temperature. Magnetic susceptibility measurements over a wide range of temperature on the powder sample of this compound reveal that the complex is antiferromagnetically coupled in the temperature range 5–40 K with 2J = 0.906cm^?1, and above 40K it is ferromagnetically coupled with 2J = +7.4cm^?1. The effective magnetic moment (μ_eff) of this complex has been compared with that of a series of other silver(II) complexes available in the literature. Finally, the spectral and magnetic data of this complex have been compared with those of a corresponding isostructural and isomorphous copper(II) complex.     

Quantum effects in the low temperature reorientation of N2− in KCl and KI

The electron paramagnetic resonance (EPR) spectra of KCl:N₂^? and KI:N₂^? were studied as a function of temperature and applied uniaxial stress in the temperature range 2–35°K. The molecular reorientation rates were determined for both 60 and 90° reorientations from the motional broadening and narrowing effects in the observed spectra. Although thermally activated in the range 15–35°K for KCl:N₂^?, the parameters deduced from the data are not consistent with a simple classical model of the motion. In both hosts the 90° rotation is markedly faster than the 60° reorientation, in conflict with simple models, but explicable in terms of the suppression of the tunneling matrix elements by polarization of the host crystal. In KI:N₂^?, the 90° reorientation is still apparent at 1·7°K and the spectra clearly show the influence of the symmetry of the nuclear spin states upon the reorientation kinetics, an effect predicted by Sussman.     

Annealing of γ-ray irradiated lithium compensated p-type silicon

Abstract

Annealing behavior of electrical properties and photoluminescence spectra both at 77 °K in electron-irradiated melt-grown n-GaAs were investigated. Defects electrically active in the Hall mobility and carrier removal anneal through two stages centered at 250° and 460 °K. From the temperature dependence of carrier concentration the existence of a defect level located near 0.15 eV below the conduction band is supposed. Several emission bands are resolved at 1.51, 1.47, 1.415, 1.305 and ～1.2 eV in photoluminescence experiments. Electron irradiation (1.5–2.0 MeV) causes a remarkable decrease in emission intensity of 1.51 and ～1.2 eV bands. Recovery of emission intensity occurs remarkably when samples are annealed to 520 °K which would correspond to the 460 °K annealing stage for carrier concentration and Hall mobility. The 250 °K annealing stage is not observed in photoluminescence experiments. The 1.415 eV peak appears clearly after irradiation and grows remarkably with the 520 °K annealing, especially in Si-doped samples, resulting in large reverse annealing. This band is tentatively speculated to be a complex of Si on As site with As vacancy. Moreover, in samples doped with Te a new emission band at 1.305 eV (9500 Å) is observed after 470°–620 °K annealing.     

Experimental determination of U diffusion in α-Ti

α-spectrometry was used in order to measure the diffusion of U in bulk α-Ti in the temperature range 863–1123?K (540–850?°C). A straight Arrhenius plot was found, giving diffusion parameters Q?=?297?kJ/mol and D ₀?=?5?×?10^?3?m²/s, which are similar to the α-Ti self-diffusion ones, when measured in Ti samples with a similar impurity content than presently. This behaviour is compatible with the hypothesis of U diffusing via a vacancy-assisted mechanism in the α-Ti lattice and contrasts with older results in which the activation energy is almost a third the self-diffusion one, even lower than the vacancy formation energy.     

Temperature dependence of the fundamental absorption edge in CuGaSe2

The temperature dependence of the fundamental absorption edge in CuGaSe₂ single crystals was determined in the temperature range from 15 to 300 K. Above about 120 K the gap energy changes linearly with temperature with dE_g/dT = ? (2.1 ± 0.1) eV K^?1. The downshift in dE_g/dT of the I–III–VI₂ compounds compared to their II–VI analogs is discussed accounting for the p-d hybridization of the uppermost valence band.     

Effective charge in the II–VI and III–V compounds with zincblende or wurtzite type structure

The effective charge in the II–VI and III–V compounds was analyzed by using a linear chain model. On the assumption that the ionic lattice is immersed in a cloud of valence electrons with the dielectric constant of ?(∞) = n²?₀ (n: refractive index, ?₀ = 8·85 × 10^?12f/m), the effective charge on an ion is equal to 2e in the II–VI compounds and to e in the III–V compounds (e: electronic charge), respectively. These values of the effective charge are just n times the Szigeti charge.Although direct connection between neighboring atoms is the main part of the binding force, the fact can not be neglected that the second nearest neighbor atoms are connected by sharing some of valence electrons between them. The electrons in valence bonds contribute to the refractive index and are estimated to be e and 2e per atom in the II–VI and III–V compounds, respectively.     

Studies of the defect structure from the calculations of optical and electron paramagnetic resonance spectra for Ni<Superscript>2+</Superscript> centre in <Emphasis Type="Italic">α</Emphasis>-LiIO<Subscript>3</Subscript> crystal

By calculating the optical spectrum band positions and EPR parameters (g factors, g‖, g⊥ and zero-field splitting D) by diagonalizing the complete energy matrix of 3d⁸ ions in trigonal symmetry, the defect structure of Ni²⁺ centre in α-LiIO₃ crystal is studied. It is found that to reach the good fits of optical and EPR data between calculation and experiment, the Ni²⁺ ion should shift by Δz ≈ 0.298 Å along C₃-axis and the O^2? ions between the Ni²⁺ ion and Li⁺ vacancy (V _Li) should be displaced away from the V _Li by Δx ≈ 0.097 Å because of the electrostatic interaction. The results are discussed.     

Point-defect properties of and sputtering events in the {001} surfaces of Ni3Al I. Surface and point-defect properties

Constant-area and fully relaxed molecular dynamics methods are employed to study the properties of the surface and point defects at and near {001} surfaces of bulk and thin-film Ni, Al and Ni₃Al respectively. The surface tension is larger than the surface energy for all {001} surfaces considered in the sequence: Al (1005?mJ?m^?2)<?Ni₃Al (mixed Ni–Al plane outermost, 1725?mJ?m^?2)<?Ni₃Al (all-Ni-atoms plane outermost, 1969?mJ?m^?2)<?Ni (1993?mJ?m^?2). For a surface of bulk Ni₃Al crystal with a Ni–Al mixed plane outermost, Al atoms stand out by 0.0679?Å compared with the surface Ni atoms and, for the all-Ni-atoms surface, Al atoms in the second layer stand out by 0.0205?Å compared with Ni atoms in the same layer. Vacancy formation energies are about half the bulk values in the first layer and reach a maximum in the second layer where the atomic energy is close to the bulk value but the change in embedding energy of neighbouring atoms before and after vacancy formation is greater than that in the bulk. Both the vacancy formation energy and the surface tension suggest that the fourth layer is in a bulk state for all the surfaces. The formation energy of adatoms, antisite defects and point-defect pairs at and near {001} surfaces of Ni₃Al are also given.     

The high pressure phase transitions in KF and RbF

The high pressure B1 ? B2 transitions in KF at ~ 40 kbar and RbF at ~ 30 kbar have been studied using hydrostatic X-ray diffraction. No other transitions have been observed. The addition of the $\text{ΔV}\text{V}$ for these transitions to the already existing body of literature on B1–B2 transitions in alkali halides permits an extension of Pauling's theory to larger values of radius ratios. It also permits the modified Born criterion for predicting phase transitions to be further verified. Values of ionic radii for 8 coordination we suggest are 1.33 Å for F^?, 1.84 Å for Cr^?, 2.00 Å for Br^? and 2.27 Å for I^?.     

Calculation of shuffle 60° dislocation width and Peierls barrier and stress for semiconductors silicon and germanium

The dislocation width for shuffle 60° dislocation in semiconductors Si and Ge have been calculated by the improved P-N theory in which the discrete effect has been taken into account. Peierls barrier and stress have been evaluated with considering the contribution of strain energy. The discrete effect make dislocation width wider, and Peierls barrier and stress lower. The dislocation width of 60° dislocation in Si and Ge is respectively about 3.84 Å and 4.00 Å (～1b, b is the Burgers vector). In the case of 60° dislocation, after considering the contribution of strain energy, Peierls barrier and stress are increased. The Peierls barrier for 60° dislocation in Si and Ge is respectively about 15 meV/Å and 12–14 meV/Å, Peierls stress is about 3.8 meV/ų (0.6 GPa) and 2.7–3.3 meV/ų (0.4–0.5 GPa). The Peierls stress for Si agrees well with the numerical results and the critical stress at 0 K extrapolated from experimental data. Ge behaves similarly to Si.     

Copper and native defects in zinc telluride

Zinc telluride crystals were grown from tellurium-rich solutions containing 10¹⁷–10²⁰ cm^?3 atoms of copper. The copper concentrations in these crystals were measured by activation analysis. Hall effect and resistivity measurements were performed. Photoluminescence spectra were also determined. Our interpretation of the different results is that copper brings about both acceptor defects Cu_Zn with a 0·12–0·13 eV ionization energy, and donor defects. The second acceptor level of the zinc vacancy was found to be at 0·15 eV.     

Theoretical studies of the dependence of EPR g-factors on local structure for the trigonal Er3+–VK centres in KMgF3 and KZnF3

The dependence of the EPR g-factors on the local structural parameter for a 4f¹¹ configuration ion Er³⁺ in a trigonal crystal-field has been studied by diagonalizing the 364×364 complete energy matrices. Our studies indicate that the EPR spectra of the trigonal Er³⁺–V_K centers in KMgF₃ and KZnF₃ may be attributed to the translation of the cubic Kramers doublet Γ₇. Furthermore, the EPR g-factors of the trigonal Er³⁺–V_K centers may be interpreted reasonably by the shifts ΔZ≈0.340 Å and ΔZ≈0.303 Å of the Er³⁺ ions toward the charge compensator V_K along the C₃ axis for the KMgF₃:Er³⁺ and the KZnF₃:Er³⁺ systems respectively.