Study of the electro-absorption spectrum of a nickel acceptor exciton in a ZnO:Ni crystal based on a calculation of vibrations associated with a Ni+1 impurity

Localized vibrations in ZnO crystals due to the substitution impurity Ni⁺¹ are modeled. The calculations were performed in the shell model using a recursive method for vibrations with A ₁ and E type symmetry. Numerical calculations allowed us to analyze the vibronic structure in the electro-absorption spectra for nickel acceptor excitons in ZnO:Ni. Fiz. Tverd. Tela (St. Petersburg) 41, 986–990 (June 1999)     

Fe掺杂对溶胶凝胶法制备的ZnO: Ni薄膜结构及发光特性的影响

采用溶胶凝胶法在玻璃基片上制备了ZnO及Ni, Fe共掺杂的Zn_0.95-xNi_0.05Fe_xO (x=0, 0.005, 0.01, 0.03, 0.05) 薄膜. 通过扫描电镜(SEM) 和X射线衍射(XRD) 研究了薄膜样品的表面形貌和晶体结构. 结果表明所有样品都具有(002) 择优取向, Fe掺杂导致ZnO: Ni薄膜的晶体质量变差, 晶粒尺寸减小, 但适当的Fe掺杂有利于获得致密、 均匀的薄膜. XPS测试结果表明样品中Ni离子的价态为+2价, Fe离子的价态为+2价和+3价.室温光致发光(PL) 测量表明, 所有样品均观察到较强的紫外发光峰, 蓝光双峰和绿光发光峰. ZnO: Ni薄膜的发光强度可以通过Fe掺杂进行有效调节. 进而我们讨论了Ni, Fe共掺杂ZnO样品的发光机理.     

Structural studies and Raman spectroscopy of forbidden zone boundary phonons in Ni‐doped ZnO ceramics

We present X‐ray diffraction and Raman spectroscopy studies of Ni‐doped ZnO (Zn_1−xNi_xO, x = 0.0, 0.03, 0.06, and 0.10) ceramics prepared by solid‐state reaction technique. The presence of the secondary phase along with the wurtzite phase is observed in Ni‐doped ZnO samples. The E₂(low) optical phonon mode is seen to be shifted to a lower wavenumber with Ni incorporation in ZnO and is explained on the basis of force‐constant variation of ZnO bond with Ni incorporation. A zone boundary phonon is observed in Ni‐doped samples at ∼130 cm⁻¹ which is normally forbidden in the first‐order Raman scattering of ZnO. Antiferromagnetic ordering between Ni atoms via spin‐orbit mechanism at low temperatures (100 K) is held responsible for the observed zone boundary phonon. Copyright © 2008 John Wiley & Sons, Ltd.     

Study of ZnO and Ni-doped ZnO synthesized by atom beam sputtering technique

Zinc oxide (ZnO) and Ni-doped zinc oxide (ZnO:Ni) films are prepared by atom beam sputtering with an intent of growing transparent conducting oxide (TCO) material and understanding its physical properties. The crystalline phases of the films are identified by the grazing angle X-ray diffraction (GAXRD) technique. Thicknesses of the films are measured by ellipsometry. Chemical states of the elements present in the films are investigated by X-ray photoelectron spectroscopy (XPS), which indicates the presence of Ni in the ZnO environment in a divalent state. Average transmission across the ZnO:Ni film was determined to be ∼83% in the visible region, which is less than that (∼90%) of undoped ZnO films. The resistivity measured by van der Pauw technique of the ZnO:Ni film (∼9×10^-3 Ω cm) is two orders of magnitude smaller as compared to its undoped counterpart (1 Ω cm). For ZnO:Ni film an average carrier concentration of ∼1.4×10¹⁹ cm^-3 was observed by Hall measurements. Two important mechanisms reported in the literature viz. influence of d–d transition bands and electron scattering from crystallites/grains are discussed as the possible causes for the increase in conductivity on Ni doping in ZnO. PACS 73.50.Bk; 78.66.Li; 79.60.Dp; 61.05.cp     

Surface plasmon-phonon polaritons of hexagonal zinc oxide

The anisotropy of surface plasmon-phonon polaritons of the first and second types in hexagonal zinc oxide is investigated at various electron concentrations and orientations of the C axis of the crystal relative to its surface. It is shown that surface plasmon-phonon polaritons of a third type are generated when the electron concentration in ZnO is greater than 2×10¹⁸ cm⁻³ and the orientation corresponds to K⊥C and xy⊥C. The spectrum of the surface plasmon-phonon polaritons of the third type are calculated, and the conditions for the existence of surface plasmon-phonon polaritons of the third type in ZnO single crystals are determined. Zh. Tekh. Fiz. 68, 58–62 (January 1998)     

Electrical and optical spectroscopy on ZnO:Co thin films

Magnetic oxide semiconductors, for example the highly transparent and intrinsically n-type conducting zinc oxide doped with the 3d transition metal Co (ZnO:Co), are promising for the emerging field of spintronics [1]. We investigated n-conducting ZnO:Co thin films with a Co content of nominal 0.02, 0.20, or 2.00 at. %. The substitution of Co cations in the tetrahedral sites of wurtzite ZnO with Zn was confirmed at low temperature by the 1.877 eV photoluminescence between crystal field split d-levels of Co²⁺ (d⁷) ions. Based on theoretical studies, it is predicted that the formation of electron levels with zinc interstitials (I_Zn) or hole levels with zinc vacancies (V_Zn) is necessary to induce ferromagnetism, whereas the formation of electron levels with oxygen vacancies (V_O) is detrimental for ferromagnetism in ZnO:Co [2]. Cobalt generates a hole level in ZnO [3]. We investigated the generation of electron levels in n-conducting ZnO:Co in dependence on the Co content by means of deep level transient spectroscopy (DLTS). However, because of the ambiguous categorization of deep defects in n-conducting ZnO (V_O, I_Zn), an optimization of defect-related ferromagnetism in ZnO:Co is not possible at the moment. PACS 78.30.Fs; 91.60.Ed; 91.60.Mk     

A vibronic-structure analysis of optical spectra in ZnO:Ni3+ crystals by localized-vibration simulation

A theoretical study is reported of the vibrations associated with a Ni³⁺ impurity charged with respect to the ZnO lattice. The calculations were made by a recursive method in terms of the shell model for vibrations with different symmetries. The vibronic structure observed in the spectra of d-d intracenter transitions in the Ni³⁺ impurity has been interpreted using model calculations. Fiz. Tverd. Tela (St. Petersburg) 41, 618–622 (April 1999)     

Synthesis and characterization of Ni-doped ZnO: A transparent magnetic semiconductor

We report synthesis of a transparent magnetic semiconductor by incorporating Ni in zinc oxide (ZnO) matrix. ZnO and nickel-doped zinc oxide (ZnO:Ni) thin films (∼60 nm) are prepared by fast atom beam (FAB) sputtering. Both undoped and doped films show the presence of ZnO phase only. The Ni concentration (in at%) as determined by energy dispersive X-ray (EDX) technique is ∼12±2%. Magnetisation measurement using a SQUID magnetometer shows that the Ni-doped films are ferromagnetic, having coercivity (H_c) values 192, 310 and 100 Oe and saturation magnetization (M_s) values of 6.22, 5.32 and 4.73 emu/g at 5, 15 and 300 K, respectively. The Ni-doped film is transparent (>80%) across visible wavelength range. Resistivity of the ZnO:Ni film is ∼2.5×10⁻³ Ω cm, which is almost two orders of magnitude lower than the resistivity (∼4.5×10⁻¹ Ω cm) of its undoped counterpart. Impurity d-band splitting is considered to be the cause of increase in conductivity. Interaction between free charges generated by doping and localized d spins of Ni is discussed as the reason for ferromagnetism in the ZnO:Ni film.     

Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method

Photoluminescence (PL) spectra of nitrogen-doped ZnO films (ZnO:N films) grown epitaxially on n-type ZnO single crystal substrates by using the plasma-assisted reactive evaporation method were measured at 5 K. In PL spectra, free exciton emission at about 3.375 eV was very strong and emissions at 3.334 and 3.31 eV were observed. These two emissions are discussed in this paper. The nitrogen concentration in ZnO:N films measured by secondary ion mass spectroscopy was 10^19-20 cm⁻³. Current-voltage characteristics of the junction consisting of an n-type ZnO single crystal substrate and ZnO:N film showed good rectification. Also, ultraviolet radiation and visible light were emitted from this junction under a forward bias at room temperature. It is therefore thought that ZnO:N films have good crystallinity and that doped nitrogen atoms play a role as acceptors in ZnO:N films to form a good pn junction. From these phenomena and the excitation intensity dependency of PL spectra, emissions at 3.334 and 3.31 eV were assigned to neutral acceptor-bound exciton (A⁰X) emission and a donor-acceptor pair (DAP) emission due to doped nitrogen, respectively.     

ESR study on crystallization of (Zn,Mn)O ceramics by Nd: YAG pulse laser annealing

Abstract

On the sintered ZnO involving a binding dopant of MnO below 10 mol%, Electron Spin Resonance (ESR) has been investigated before and after Nd:YAG pulse laser (λ=1.064 μm) annealing. As-sintered (Zn, Mn)O showed the resolved spectrum with the hyperfine structures for Mn²⁺ with 0.01 to 0.05 mol%, whereas above 1.0 mol%, a broad line appeared with the half-width of 500 Oe. By laser annealing, we found that the broad line was changed into the resolved spectrum. This result suggests that the Mn²⁺ clusterings located at grain boundaries in ZnO are diffused into crystal grains, in which the crystallization of ceramics is induced.     

Effect of Mn-doping on the growth mechanism and electromagnetic properties of chrysanthemum-like ZnO nanowire clusters

Chrysanthemum-like ZnO nanowire clusters with different Mn-doping concentrations are prepared by a hydrothermal process. The microstructure, morphology and electromagnetic properties are characterized by x-ray diffractometer high-resolution transmission electron microscopy (HRTEM), a field emission environment scanning electron microscope (FEESEM) and a microwave vector network analyser respectively. The experimental results indicate that the as-prepared products are Mn-doped ZnO single crystalline with a hexagonal wurtzite structure, that the growth habit changes due to Mn-doping and that a good magnetic loss property is found in the Mn-doped ZnO products, and the average magnetic loss tangent tanδ_m is up to 0.170099 for 3% Mn-doping, while the dielectric loss tangent tanδ_e is weakened, owing to the fact that ions Mn^{2 +} enter the crystal lattice of ZnO.     

Theoretical investigations on the g factors and local structures for Cu2+ in the ZnX (X= O,S and Se) nanocrystals

ABSTRACT

The g factors and local structures for Cu²⁺ in the ZnX (X = O, S and Se) nanocrystals at room temperature are theoretically investigated by the perturbation calculations for a tetragonally distorted tetrahedral 3d⁹ cluster in a consistent way, and the isotropic g factor is predicted for the ZnS:Cu²⁺ nanocrystals at room temperature. The bond angles θ between the four equivalent Cu²⁺?X^2? bonds and the C₄ axis are found to be about 1.26°, 1.24° and 1.07°, respectively, larger in the ZnO, ZnS and ZnSe nanocrystals than that (θ₀ ≈ 54.74°) for an ideal tetrahedron, inducing tetragonally compressed tetrahedra. The declining tendency (ZnO > ZnS > ZnSe) of the tetragonal angular distortion Δθ (= θ ? θ₀) can be ascribed to the decreasing strength of the dynamic Jahn–Teller effect via the vibration interactions of the [CuX₄]^6? groups due to the weakening Cu²⁺?X^2? bonding. The isotropic g factors are attributable to the appropriate Δθ due to the dynamic Jahn–Teller effect and the internal stress. The slightly increasing (ZnO < ZnS < ZnSe) g factors can be illustrated by the declining cubic field parameter Dq, angular distortion Δθ and covalency factor N of the systems.     

Microstructural and magnetic properties of Ax:Zn1-xO (A=Mn,Gd and Mn/Gd) nanocrystals

We report the microstructural and magnetic properties of transition (3d) and rare earth (4f) metal substituted into the A_x:Zn_1?xO (A=Mn, Gd and Mn/Gd) nanocrystal samples synthesized by solgel method. The structural properties and morphology of all samples have been analysed using X-ray diffraction (XRD) method and scanning electron microscopy. The impurity phase in the XRD patterns for all samples is not seen, except (Mn/Gd):ZnO sample where a very weak secondary phase of Gd₂O₃ is observed. Due to the large mismatch of the ionic radii between Mn²⁺ and Gd³⁺ ions, the strain inside the matrix increases, unlike the crystallite size decreases with the substitution of Mn and Gd into ZnO system. A couple of additional vibration modes due to the dopant have been observed in Raman spectrum. The magnetic properties have been studied by vibrating sample magnetometer. The magnetic hysteresis shows that Mn:ZnO and Gd:ZnO have soft ferromagnetic (FM) behaviour, whereas (Mn/Gd):ZnO has strong FM behaviour at room temperature (RT). The enhancement of ferromagnetism (FM) in (Mn/Gd):ZnO sample might be related to short-range FM coupling between Mn²⁺ and Gd³⁺ ions via defects potential and/or strain-induced FM coupling due to the expansion lattice by doping. The experimental results indicate that RTFM can be achieved by co-substitution of 3d and 4f metals in ZnO which can be used in spintronics applications.     

Synthesis and formation mechanism of Ag–Ni alloy nanoparticles at room temperature

Ag–Ni nanoparticles were prepared with a chemical reduction method in the presence of polyvinylpyrrolidone (PVP) used as a stabilizing agent. During the synthesis of Ag–Ni nanoparticles, silver nitrate was used as the Ag⁺ source while nickel sulfate hexahydrate was used as Ni²⁺ source. Mixed solutions of Ag⁺ source and Ni²⁺ source were used as the precursors and sodium borohydride was used as the reducing agent. Five ratios of Ag⁺/Ni²⁺ (9:1, 3:1, 1:1, 1:3, and 1:9) suspensions were prepared in the corresponding precursors. Ag–Ni alloy nanoparticles were obtained with this method at room temperature. Scanning electronic microscope (SEM), energy dispersive spectrum (EDS), high resolution transmission electron microscope (HRTEM) were used to characterize the morphology, composition and crystal structure of the nanoparticles. The crystal structure was also investigated with X-ray diffraction (XRD). In all five Ag/Ni ratios, two kinds of particle structures were observed that are single crystal structure and five-fold twinned structure respectively. Free energy of nanoparticles with different crystal structures were calculated at each Ag/Ni ratio. Calculated results revealed that, with identical volume, free energy of single crystal particle is lower than multi-twinned particle and the difference becomes smaller with the increase of particle size; increase of Ni content will lead the increase of free energy for both structures. Formation of different crystal structures are decided by the structure of the original nuclei at the very early stage of the reduction process.     

Calculation of the specific heat for the first order,tricritical and second order phase transitions in NH4C1

Abstract

This work presents our calculation for the specific heat C _v under an Ising model that uses our Raman frequencies of the _1/7TA (93cm ¹) and _1/5TO (144cm^?1) modes for NH₄C1. The specific heat calculation has been performed for first-order (P = 0kbar), tricritical (P=1.6kbar) and second-order (P = 2.8kbar) phase transitions in the NH₄C1 crystal. Our calculated C _v values are in good agreement with the experimentally observed Cp data from the literature for NH₄C1. This indicates that the NH₄C1 crystal can be adequately described in the lattice region by means of the Ising model studied here.     

Research on the defect structure and g factors for the tetragonal (CrO4)3− impurity center in ZrSiO4: Cr5+ crystal

ABSTRACT

The EPR g factors g// and g⊥ for the tetragonal (CrO₄)^3? impurity center in ZrSiO4: Cr⁵⁺ crystal are studied from the high-order perturbation formulas based on the two-mechanism (the crystal field and charge-transfer mechanisms) model. The studies indicate that differing from the tetragonally-elongated host (SiO4)^4- tetrahedron, the dominant defect structure of the substitutional (CrO₄)^3? tetrahedron is tetragonally- compressed with the ground state |d_z² due to the Jahn-Teller distortion. Furthermore, the agreement of g factors between calculation and experiment requires a small admixture of the first excited state |d_x²_?y² to the ground state |d_z² due to the vibrational motion of ligands, which leads a compressed (CrO₄)^3? tetrahedron to become a twinkling elongated one, These results are discussed.     

Theoretical calculations of spin-Hamiltonian parameters and defect structures for Cu2+ in trigonally-distorted tetrahedral sites of ZnO and GaN crystals

The spin-Hamiltonian (SH) parameters (g factors g _//, g ⊥ and hyperfine structure constants ⁶³ A _//, ⁶³ A ⊥, ⁶⁵ A _//, ⁶⁵ A ⊥) for Cu²⁺ ions in the trigonally-distorted tetrahedral sites of ZnO and GaN crystals are calculated from a complete diagonalization (of energy matrix) method (CDM) based on a two spin-orbit parameter model for d ⁹ ions in trigonal symmetry. In the method, the Zeeman and hyperfine interaction terms are added to the Hamiltonian in the conventional CDM. The calculated results are in good agreement with the experimental values. The calculated SH parameters are also compared with those using the traditional diagonalization method or perturbation method only within the ² T ₂ term. It appears that, for exact calculations of SH parameters of d ⁹ ions in trigonal tetrahedral clusters in crystals, the present CDM is preferable to the traditional diagonalization method or perturbation method within the ² T ₂ term. The local structures of Cu²⁺ centers (which differ from the corresponding structure in the host crystal) in ZnO : Cu²⁺ and GaN : Cu²⁺ are obtained from the calculations. The results are discussed.     

Phase-separation kinetics of 3He-4He solid solutions at various degrees of supersaturation

The phase-separation kinetics of ³He-⁴He solid solutions at various degrees of supersaturation Δx are studied using precise pressure measurements. It is found that the characteristic phase-separation times do not depend on the degree of supersaturation but are determined by the final temperature to which the crystal is cooled. The behavior of the effective diffusion coefficient, which governs the corresponding mass transfer, cannot be described within the impuriton model of the mobility of ³He atoms in a ⁴He crystal. Fiz. Tverd. Tela (St. Petersburg) 41, 2062–2064 (November 1999)     

The crystallinity and the photoluminescent properties of spray pyrolized ZnO phosphor containing Eu and Eu ions

A europium doped ZnO (ZnO:Eu) particle was directly synthesized by the spray pyrolysis method. The crystal structure of samples was designated by the europium ion and the synthesis temperature. We identified the coexistence of Eu²⁺ and Eu³⁺ ions in the as prepared ZnO, which was strongly influenced by the doping concentration and the synthesis temperature. With addition of a 0.5 mol% concentration of europium ions, only the Eu²⁺ ion existed in particles, while both Eu²⁺ and Eu³⁺ ions existed in sample using 1 mol% or higher concentration of europium ions. Changing the wavelength of the excitation source, we also found that both the blue and red luminescence can be obtained.     

Small polaron hopping conduction mechanism in Ni-doped LaFeO3

The electrical transport properties of LaFe_{1? x} Ni _x O₃ (0.1 ≤ x ≤ 0.6) bulk samples were investigated over a wide temperature range, i.e. 9–300 K. Powder x-ray diffraction patterns at room temperature showed that all samples were formed in a single phase. However, a structural transformation was observed from orthorhombic (Pnma) to rhombohedral crystal symmetry at x > 0.5 in Ni-doped samples, which is supported by the electrical transport analysis. Temperature-dependent resistivity data were fitted using Mott's variable-range hopping model for a limited range of temperatures to calculate the hopping distance and the density of states at Fermi level. It was found that all parameters vary systematically with an increase in Ni concentration. Moreover, the resistivity data were also fitted using the small polaron hopping (SPH) model. The non-adiabatic SPH conduction mechanism is followed up to 50% Ni concentration, whereas an adiabatic hopping conduction mechanism is active above it. Such a change in the conduction mechanism is accompanied by subtle electronically induced structural changes involving Fe³⁺–O–Fe³⁺ and Fe³⁺–O–Ni³⁺ bond angles and bond lengths. Thus, we suggest that the transport properties can be explained according to the additional delocalization of charge carriers induced by Ni doping.