Zn Co-doping Research Articles

Electronic structure and magnetic properties of Cr monodoped and (Cr, Al) codoped ZnO

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of Cr monodoped and (Cr, Al) codoped in ZnO. The results indicate that Cr monodoped in ZnO favors a spin-polarized state with a total magnetic moment of 7.50μ B per supercell and the magnetic moment mainly comes from the unpaired 3d electrons of Cr atoms. In addition, it was found that the ferromagnetic exchange interaction between Cr atoms is short-ranged in Cr monodoped ZnO. Interestingly, the ferromagnetic stability can be enhanced significantly by codoping Al Zn. We think that the enhancement of ferromagnetic stability should be attributed to the additional electrons introduced by Al Zn codoping.

Magnetic properties of Mn-doped GaN with defects: ab-initio calculations

According to first-principles density functional calculations, we have investigated the magnetic properties of Mn-doped GaN with defects, Ga1−x−yVGxMny N1−z−tVNzOt with Mn substituted at Ga sites, nitrogen vacancies VN, gallium vacancies VG and oxygen substituted at nitrogen sites. The magnetic interaction in Mn-doped GaN favours the ferromagnetic coupling via the double exchange mechanism. The ground state is found to be well described by a model based on a Mn3+−d5 in a high spin state coupled via a double exchange to a partially delocalized hole accommodated in the 2p states of neighbouring nitrogen ions. The effect of defects on ferromagnetic coupling is investigated. It is found that in the presence of donor defects, such as oxygen substituted at nitrogen sites, nitrogen vacancy antiferromagnetic interactions appear, while in the case of Ga vacancies, the interactions remain ferromagnetic; in the case of acceptor defects like Mg and Zn codoping, ferromagnetism is stabilized. The formation energies of these defects are computed. Furthermore, the half-metallic behaviours appear in some studied compounds.

Effects of codoping on the ferromagnetic enhancement in ZnO

The present work investigates the effect of the anionic codoping on the magnetic properties of the dilute magnetic semiconducting materials doped with transition-metal (TM) ions, namely, that of Zn(Mn,$X$)O, $X=\text{C}$, N, P, and S. The present findings are in complete analogy with the corresponding results for the cationic codoping of Zn(TM${}_{1}$,TM${}_{2}$)O as studied in our earlier works. For both the cationic and anionic codopings, we find that the magnetic impurities induce magnetic moments on their nearest neighbors, with the largest induced magnetic moment observed on the cation or the anion which mediates two magnetic impurities. The magnetic interaction in the case of the anionic codoping can be seen as a generalized double exchange or a superexchange type of mechanism similar to that found in the cationic codoping in which the magnetic interactions take the form of a double exchange or a superexchange between polarized tetrahedra each centered at one of the codopants.

Magneto-resistance, thermal conductivity, thermo-electric power and specific heat of superconductor Gd 0.95Pr 0.05Ba 2Cu 2.94M 0.06O 7−δ (M=Fe, Ni, Zn and Mn)

In the present paper, we present thermal and electrical transport properties of pristine and co-doped samples of high temperature superconductors Gd 0.95Pr 0.05Ba 2Cu 2.94M 0.06O 7−δ. It is found that all the samples, except the Mn co-doped sample, show metallic behavior in the normal state. It is observed that the upper critical field has a correlation with the substituent site of the co-dopant. Thermal conductivity κ ( T ) of all the samples, except the one with Zn co-doping, exhibits a hump like structure around their respective transition temperatures. A negative sign of the measured thermo-power ( S ) in Gd-123 indicates that electron-like carriers dominate the heat transport in the pristine sample; whereas a sign reversal in S , as a consequence of the change of dominant carrier upon doping, is observed. Specific heat ( C P ) measurements show a jump around the transition temperature ( T C ) for the pristine sample, however, such a jump in C P is strongly suppressed for the doped samples.

Optical conductivity of Mn-doped GaAs

The optical conductivity of the III–V diluted magnetic semiconductor compound (Ga, Mn)As was investigated theoretically. A direct comparison between the calculations and available experimental data is provided. We demonstrate that our model study is able to reproduce both qualitatively and quantitatively the measurements carried out by Burch et al (2006 Phys. Rev. Lett. 97 087208) and Singley et al (2002 Phys. Rev. Lett.89 097203, 2003 Phys. Rev. B 68 165204). It is found that an increase of the carrier density by up to one hole per Mn leads to a redshift of the broad conductivity peak located at approximately 200 meV in optimally annealed samples. Our study demonstrates that the non-perturbative treatment (beyond the valence band picture) is crucial for capturing these features. Otherwise a blueshift and an incorrect amplitude would result. We have calculated the Drude weight (order parameter) and have established the metal–insulator phase diagram. It is shown that (i) Mn-doped GaAs is indeed close to the metal–insulator transition and (ii) in both 5 and 7% doped samples, only 20% of the carriers are delocalized. Beyond the mobility edge, we found that the optical mass is mopt≈2me. Interesting new features of overdoped samples were found. The overdoped regime could be experimentally realized by Zn codoping. Detailed discussions and a careful analysis are provided.

Defect control for polarization switching in BiFeO3 single crystals

BiFeO₃ (BFO) single crystals were grown and the effects of Zn and Mn co-doping on the polarization and leakage current properties were investigated at 25 °C for establishing materials design based on defect chemistry. Although Zn doping or Mn doping led to a deterioration in the properties, Zn-Mn co-doping led to a large remanent polarization (36 μC/ cm²), a low coercive field (19 kV/cm), and a relatively low leakage current density (~10⁻⁸ A/cm²). It is proposed that defect dipoles composed of Zn²+ and Mn⁴+ act as effective nucleation sites for ferroelectric domains during polarization switching in BFO crystals.

Enhancement of upconversion luminescence of Y 2O 3:Er 3+ nanocrystals by codoping Li +–Zn 2+

The upconversion (UC) luminescence in sol–gel synthesized Li +, Zn 2+, or Li +–Zn 2+ codoped Y 2O 3:Er 3+ nanocrystals were investigated under the excitation of a 970 nm diode laser. Compared to undoped Y 2O 3:Er 3+ samples, proper doping of Li +–Zn 2+ leads to an drastic increase of the UC luminescence centered at 560 nm by a factor of 28. The UC luminescence enhancement is a result of the increased lifetime of the intermediate state 4 I 11/2 (Er). The intensity ratio of the green over red emissions (green/red) is also affected by the codoping of Zn 2+, Li + and Li +–Zn 2+ ions. Our results demonstrated that the Li +–Zn 2+ codoping in Y 2O 3:Er 3+ phosphors produced remarkable enhancement of the UC luminescence and green/red ratio, making this nanocrystal a promising candidate for photonic and biological applications.

Effect of Al and Sb doping on the magnetic properties of ZnMnO and ZnCoO

The bulk polycrystalline samples with nominal compositions Zn0.98−y(Mn∕Co)0.02(Al∕Sb)yO (y=0, 0.01, and 0.03) have been synthesized via solid state reaction route. The x-ray diffraction results reveal the presence of mainly wurtzite structure of pure ZnO in all the Al∕Sb codoped ZnMnO and ZnCoO samples. EDAX results confirm the presence of all elements in the samples in the stoichiometric ratio. While the Zn(Mn∕Co)O samples show paramagnetic behavior at room temperature, the Al∕Sb codoped ZnMnO samples exhibit room temperature ferromagnetism (RTFM) with saturation magnetization (Ms). The M-H curves of Al∕Sb codoped ZnCoO samples show almost linear variation with a small kink at the origin, indicating the presence of weak ferromagnetism in the Al∕Sb codoped ZnCoO samples. Possible mechanisms of the observed RTFM in Al∕Sb codoped Zn(Mn∕Co)O samples are discussed in this paper.

Influence of Nd:Zn codoping in near-stoichiometric lithium niobate

Near-stoichiometric lithium niobate (SLN) crystals doped with up to 1.6 mol % Zn and codoped with various Nd concentrations in the melt (0.2, 0.5, 0.9, and 1.5 mol %) (Nd:Zn:SLN) are grown from 58.6 mol % Li(2)O using conventional Czochralski technique. Crystals are pulled at the rate of 0.35 mmh with seed rotation at 9 rpm. Concentrations of Zn and Nd in the crystal are varied by adding appropriate amounts of ZnO and Nd(2)O(3) to the starting composition. Unit cell parameters of the grown crystals are calculated by Rietveld refinement method using FULLPROFF software. Domain structure studies are carried out by chemical etching followed by microscopic examination. Dielectric studies reveal the existence of piezoelectric resonance at high frequencies. Enhancement in dielectric constant and tan delta in Nd doped samples has been attributed to the space charge polarization. Nd doped samples exhibit reduction in the relative permittivity after oxygen annealing. Transmission spectra of Nd:Zn:SLN crystals in the UV region exhibit blueshift in the cutoff wavelength. In Mid Infrared (MIR) region crystals doped with 1.6 mol % Zn have shift in the OH absorption peak from 2873 to 2833 nm. Judd-Ofelt analysis carried out on the absorption spectra of codoped crystal yields the lifetime of 104 mus for the metastable state (4)F(32). The branching ratio for the electronic transition from (4)F(32) to (4)I(112) is high compared to that for (4)F(32) to (4)I(132), indicating a higher emission cross section for the former transition. Laser damage threshold evaluated using 532 nm, 5 ns pulsed neodymium doped yttrium aluminum garnet laser, shows an increase by two orders of magnitude for crystals doped with 1.6 mol % Zn. Photorefractive damage threshold for these crystals shows an enhancement of four orders of magnitude due to increase in the photoconductivity.

X-ray absorption spectroscopic and magnetic characterization of cobalt-doped zinc oxide nanocrystals prepared by the molten-salt method

The local atomic arrangement and electronic structure of the Co-doped Zn 1− x Co x O nanocrystal have been quantitatively examined along with its magnetic properties. According to our analysis using powder X-ray diffraction, electron microscopy, and Zn K-edge X-ray absorption spectroscopy (XAS), phase-pure wurzite-structured Zn 1− x Co x O nanocrystals have been successfully synthesized via the molten-salt method. The Co K-edge XAS analysis clearly demonstrates that all the Co 2+ ions are substituted for the tetrahedral Zn sites of the Wurzite structure with a coordination number of 3.9 and a bond distance of 1.97 Å, ruling out the presence of magnetic impurity phase and Co-metal cluster. Magnetization measurements reveal that the present Zn 1− x Co x O sample does not show any ferromagnetic transition down to 2 K. In this regard, we can conclude that Co-doped zinc oxide is not ferromagnetic but the previously reported ferromagnetism in this phase would be an extrinsic property.

Crystal chemistry of Co-doped Zn 7Sb 2O 12

Zn 7Sb 2O 12 forms a full range of Co-containing α solid solutions, Zn 7− x Co x Sb 2O 12, with an inverse-spinel structure at high temperature. At low temperatures for x<2, the solid solutions transform into the low temperature β-polymorph. For x=0, the β→ α transition occurs at 1225±25 °C; the transition temperature decreases with increasing x. At high x and low temperatures, α solid solutions are formed but are non-stoichiometric; the (Zn+Co):Sb ratio is >7:2 and the compensation for the deficiency in Sb is attributed to the partial oxidation of Co 2+ to Co 3+. From Rietveld refinements using ND data, Co occupies both octahedral and tetrahedral sites at intermediate values of x, but an octahedral preference attributed to crystal field stabilisation, causes the lattice parameter plot to deviate negatively from the Vegard's law. Sub-solidus compatibility relations in the ternary system ZnO–Sb 2O 5–CoO have been determined at 1100 °C for the compositions containing ⩽50% Sb 2O 5.

Effects of Cu and Zn co-doping on the electrical properties of Ni 0.5Mn 2.5O 4 NTC ceramics

The effects of Cu and Zn co-doping on Ni–Mn–O spinel-structured NTC ceramics were investigated. Dense Cu and Zn co-doped Ni 0.5Mn 2.5O 4 spinel-structured ceramics were prepared from mixed oxalate-derived powders. XPS analysis revealed that in the co-doped material Cu x Zn 1.0Ni 0.5Mn 1.5− x O 4, the majority of Cu ions resided at the B-sites due to the almost exclusive occupation of Zn ions at the A-sites, but in the material doped with Cu alone, Cu ions were situated at both A- and B-sites. The co-doped material exhibited a significant decrease in electrical resistivity without much decrease in the thermal constant. In comparison with the material doped with Cu alone, the co-doped material also showed much improved electrical stability upon annealing at 150 °C in air, which is attributed to its stable distribution of cations in the spinel.

Strong dependence of the interlayer coupling on the hole mobility in antiferromagneticLa2−xSrxCuO4(x&lt;0.02)

We have studied the interlayer coupling in the antiferromagnetic (AF) phase of Sr and Zn doped La2CuO4 by analyzing the spin flip transition in the magnetization curves. We find that the interlayer coupling strongly depends on the mobility of the hole charge carriers. Samples with the same hole content as well as the same Neel temperature but a different hole mobility, which we adjusted by Zn co-doping, can have a very different interlayer coupling. Our results suggest that only mobile holes can cause a strong frustration of the interlayer coupling.

Electron spin resonance study of Zn-codoping effect on the local structure of the Er-related centers in GaAs:Er,O

Electron spin resonance (ESR) measurements of Zn-codoped GaAs:Er,O grown by organometallic vapor phase epitaxy have been performed at 9.49GHz. Several anisotropic ESR signals (named as A, B, and C) are observed. The angular and temperature dependences of the A, B, and C signals are quite consistent with those observed in GaAs:Er,O. We found that the intensity of the C center decreases together with the decrease of the photoluminescence (PL) intensity by changing the doping amount of Zn, while the change of the ESR intensity of the B center is relatively small. This indicates that the Zn codoping selectively disturbed the formation of C center. Furthermore, the PL intensity seems to be scaled to the intensity of the signal C. This finding indicates that the C center plays an important role in the host excited PL of GaAs:Er,O.

Magnetism in cobalt-doped Cu2O thin films without and with Al, V, or Zn codopants

Thin films of 5% Co doped Cu2O were grown on single-crystal (001) MgO substrates by pulsed-laser deposition, without and with 0.5% codoping with Al, V, or Zn. Structural, electrical, and magnetic properties were studied. The films showed phase-pure character under the chosen optimum growth conditions. Spin-glass-like behavior was observed in Co-doped films without codoping. A clear ferromagnetic signal at room temperature was found only in the case of Co:Cu2O films codoped with Al.

EFFECT OF THE Fe AND Zn CODOPED ON THE INTERGRANULAR PROPERTIES FOR (Bi1.6Pb0.4)(Sr1.8Ba0.2)Ca2(Cu1-x-yZnxFey)3Oz SUPERCONDUCTING SYSTEM

Systematic measurements of the AC susceptibility as a function of temperature and magnetic field amplitude H ac for the Fe and Zn codoped ( Bi 1.6 Pb 0.4)( Sr 1.8 Ba 0.2) Ca 2( Cu 1-x-y Zn x Fe y)3 O z (x = 0.00;0.02 and y = 0.00;0.01) system are reported. Above H ac = 200 A/m, the linear dependence of T p for maximum of imaginary χ′′(T) peak as a function of AC field amplitude agree with Muller critical state model. The intergrain pinning force density decreases in doped and codoped samples.

Influence of co-doping of Zn(II)+Fe(III) on the photocatalytic activity of TiO 2 for phenol degradation

Titania nanopowders with co-doping of Zn(II) and Fe(III) were prepared through sol–gel processing. The photocatalytic activity was evaluated by the photocatalytic decomposition of phenol. The influence of some parameters such as pH, doping concentration on the activity was investigated. It was found that the photocatalytic activity of TiO 2 was obviously enhanced by the co-doping. In addition, a mechanism was proposed in order to account for the enhanced activity.

Effect of Si, Mg, and Mg–Zn doping on structural properties of a GaN layer grown by metalorganic chemical vapor deposition

We have studied the structural properties of undoped, Si-doped, Mg-doped, and Mg–Zn codoped GaN using high-resolution X-ray diffraction (HRXRD) and transmission electron microscopy. When compared with undoped GaN, the dislocation density at the surface of the GaN layer decreases with Si doping and increases with Mg doping. In addition, we observed a reduction of dislocation density by codoping with Zn atoms in the Mg-doped GaN layer. The full width at half maximum of HRXRD shows that Si doping and Mg–Zn codoping improve the structural quality of the GaN layer as compared with undoped and Mg-doped GaN, respectively.

Nonlinear optical effect of co-doped Mg, Zn: LiNbO3

Two damage-resistant impurity ions, Mg and Zn, were co-doped into LiNbO3 crystals with various concentrations. Infrared and UV-VIS transmission spectra were used to characterize the co-doped crystals. Experimental results show that such co-doping leads to the crystal fundamental absorption edge shifts to shorter wavelength direction and that 3.0 mol. % Mg and more than 2.0 mol. % Zn co-doping results in shifts of the OH transmission peak. Second harmonic generation efficiencies and phase matching temperatures were also studied experimentally.

Codoping characteristics of Zn with Mg in GaN

The doping characteristics of Mg–Zn codoped GaN films grown by metalorganic chemical vapor deposition are investigated. By means of the concept of Mg–Zn codoping technique, we have grown p-GaN showing a low electrical resistivity (0.72 Ω cm) and a high hole concentration (8.5×1017 cm−3) without structural degradation of the film. It is thought that the codoping of Zn atoms with Mg raises the Mg activation ratio by reducing the hydrogen solubility in p-GaN. In addition, the measured specific contact resistance of Mg–Zn codoped GaN film is 5.0×10−4 Ω cm2, which is one order of magnitude lower than that of Mg doped only GaN film (1.9×10−3 Ω cm2).