Electronic structure and ferromagnetism of polycrystalline Zn1−xCoxO (0≤x≤0.15)

The electronic structure of polycrystalline ferromagnetic Zn_1−xCo_xO (0.05≤x≤0.15) and the oxidation state of Co in it, have been investigated. The Co-doped polycrystalline samples are synthesized by a combustion method and are ferromagnetic at room temperature. XPS and optical absorption studies show evidence for Co²⁺ ions in the tetrahedral symmetry, indicating substitution of Co²⁺ in the ZnO lattice. However, powder XRD and electron diffraction data show the presence of Co metal in the samples. This give evidence to the fact that some Co²⁺ ion are incorporated in the ZnO lattice which gives changes in the electronic structure whereas ferromagnetism comes from the Co metal impurities present in the samples.     

Formation of GaAs1−xNx nanofilm on GaAs by low energy N2 implantation

Nitridation of GaAs (1 0 0) by N₂⁺ ions with energy E_i = 2500 eV has been studied by Auger- and Electron Energy Loss Spectroscopy under experimental conditions, when electrons ejected only by nitrated layer, without contribution of GaAs substrate, were collected. Diagnostics for quantitative chemical analysis of the nitrated layers has been developed using the values of NKVV Auger energies in GaN and GaAsN chemical phases measured in one experiment, with the accuracy being sufficient for separating their contributions into the experimental spectrum. The conducted analysis has shown that nanofilm with the thickness of about 4 nm was fabricated, consisting mainly of dilute alloy GaAs_1−xN_x with high concentration of nitrogen x ∼ 0.09, although the major part of the implanted nitrogen atoms are contained in GaN inclusions. It was assumed that secondary ion cascades generated by implanted ions play an important role in forming nitrogen-rich alloy.     

Structural studies of evaporated CoxCr1−x/Si (1 0 0) and CoxCr1−x/glass thin films

Series of Co_xCr_1−x thin films have been evaporated under vacuum onto Si (1 0 0) and glass substrates. Chemical composition and interface properties have been studied by modelling Rutherford backscattering spectra (RBS) using SIMNRA programme. Thickness ranges from 17 to 220 nm, and x from 0.80 to 0.88. Simulation of the energy spectra shows an interdiffusion profile in the thickest films, but no diffusion is seen in thinner ones. Microscopic characterizations of the films are done with X-ray diffraction (XRD) measurements. All the samples are polycrystalline, with an hcp structure and show a 〈0 0 0 1〉 preferred orientation. Atomic force microscopies (AFM) reveal very smooth film surfaces.     

Formation of room-temperature ferromagnetic Zn1−xCoxO nanocrystals

Optical and magnetic properties of Co²⁺-doped ZnO nanocrystals were studied. Optical measurements confirm the incorporation of Co²⁺ in ZnO lattice with tetrahedral geometry. Optical absorption spectra also reveal the partial bleaching of the excitonic feature attributable to an increase in electron concentration. Magnetization measurements indicate the ferromagnetic ordering in Co²⁺-doped ZnO nanocrystals with saturation magnetization . No structural changes were observed in lightly doped ZnO nanocrystals. The present investigations are important in obtaining the ferromagnetic Zn_1−xCo_xO nanocrystals.     

Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn_1−xCo_xO films. Zn_1−xCo_xO films were prepared on (0001)-oriented Al₂O₃ substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp-d exchange interactions and typical d-d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn_1−xCo_xO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co²⁺ for Zn²⁺ without changing the wurtzite structure.     

Optical properties of bulk Zn1−xCoxO magnetic semiconductors

Polycrystalline Zn_1−xCo_xO (x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn_1−xCo_xO samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co²⁺ substituting Zn²⁺ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.     

Structural and photoluminescence analysis of Zn1 − xMnxO nanocrystalline powders

This Letter reports on structural and photoluminescence properties of Zn_1 − xMn_xO nanocrystalline powders, which were synthesized by using oxalate precursor decomposition method. From the XRD features, we have noticed that all samples exhibit wurtzite crystal structure. The origin of photoluminescence properties of Mn doped and undoped ZnO have been discussed.     

Study of Zn1−xCoxO (0.02≤x≤0.08) dilute magnetic semiconductor prepared by mechanosynthesis route

Single-phase Zn_1−xCo_xO (0.02≤x≤0.08) dilute magnetic semiconductor is prepared by mechanical milling process. The shift of XRD peaks towards the higher angle and a redshift in the band gap compared to the undoped ZnO ensure the incorporation of Co²⁺ ions in the semiconductor host lattice. Pure Zn_xCo_1−xO phases show the paramagnetic behavior in the temperature range 80 K≤T≤300 K. The room temperature volume magnetic susceptibility (χ_v) estimated in case of Zn_0.96Co_0.04O is ∼10⁻⁵ emu/Oe cm³. The temperature dependence of susceptibility χ_v can be fitted well with Curie law confirming the paramagnetic interaction. The observed crystal-field splitting of 3d levels of Co²⁺ ions inside Zn_1−xCo_xO has been successfully interpreted using Curie law.     

On the short-range order of the SiOx (0 ≤ x ≤ 2) surface

Fine (oscillating) structure (FS) in the elastically scattered electron spectra (ESES) [O. Bondarchuk, S. Goysa, I. Koval, P. Melnik, M. Nakhodkin, Surf. Sci. 258 (1991) 239; O. Bondarchuk, S. Goysa, I. Koval, P. Melnik, M. Nakhodkin, Surf. Rev. Lett. 4 (1997) 965] was used to investigate surface structure of the SiO_x (0 ≤ x ≤ 2). SiO_x surface with different stoichiometry was prepared by implantation of 500 eV oxygen ions into a silicon wafer. Fourier transformation of the FS ESES contains one peak at 2.32 Å for Si, two peaks at 1.62 Å and 2.65 Å for a-SiO₂ and three peaks centered at 1.6-1.7 Å, 2.1-2.2 Å and 2.65-3.04 Å for SiO_x. Peaks at 1.62 Å and 2.65 Å are assigned to Si-O and O-O nearest distances correspondently. Ratio of the area under the peak at 2.65 Å to the area under the peak at 1.62 Å turned out to be not constant but grows linearly with the composition parameter x. The latter is considered to prove validity of the Random Bond Model to describe short-range order on the surface of non-stoichiometric silicon oxide.     

First-principles and Monte Carlo combinational study on Zn1−xCoxO diluted magnetic semiconductor

The electronic structures and magnetic properties of Zn_1−xCo_xO (x=5.55%,8.33%,12.5%) are studied using first-principles calculations in combination with Monte Carlo (MC) simulation. The combinational method makes possible a complete simulation from the microscopic magnetic interaction to macroscopic magnetic behavior. The calculated results from first principles indicate that the ferromagnetic ground state is stabilized by a half-metallic electronic structure which originates from the strong hybridization between Co 3d electrons and O 2p electrons. With the magnetic coupling strengths obtained from first-principles calculations, the MC simulation predicts the ferromagnetism of Zn_1−xCo_xO (x=5.55%,8.33%,12.5%) with , which is consistent with the experimental facts.     

Room temperature ferromagnetism in Zn1−xCoxS thin films with wurtzite structure

The magnetic properties of Zn_1−xCo_xS (x=0.025 and 0.05) thin films grown on α-quartz substrates at different temperatures (T_S) of 200, 400 and 600 °C by means of pulsed laser deposition are presented. The films are crystallized with wurtzite structure. Optical absorption and transmission electron microscopy measurements indicate that Co ions are substituted to Zn on tetrahedral sites. Their magnetic response is composed of ferromagnetic and paramagnetic components of which respective strengths depend on T_S and Co concentration. This behavior is interpreted as due to fluctuations in the magnetic ordering, depending on grain size and site location in grain boundaries or in crystal cores.     

An ab initio-based approach to adsorption-desorption behavior of Si adatoms on GaAs(1 1 1)A-(2 × 2) surfaces

The adsorption-desorption behavior of Si adatoms on GaAs(1 1 1)A-(2 × 2) surfaces is investigated using our ab initio-based approach, in which adsorption and desorption behavior of Si adatoms is described by comparing the calculated desorption energy obtained by total-energy electronic-structure calculations with the chemical potential estimated by quantum statistical mechanics. We find that the Si adsorption at the Ga-vacancy site on the (2 × 2) surfaces with As adatoms occurs less than 1140-1590 K while the adsorption without As adatom does less than 630-900 K. The change in adsorption temperature of Si adatoms by As adatoms is due to self-surfactant effects of As adatoms: the promotion of the Si adsorption triggered by As adatoms is found to be interpreted in terms of the band-energy stabilization. Furthermore, the stable temperature range for Si adsorbed surfaces with As adatoms agrees with the experimental results. The obtained results provide a firm theoretical framework to clarify n-type doping processes during GaAs epitaxial growth.     

Effect of Cr on the magnetic properties of evaporated CoxCr1−x/Si(1 0 0) and CoxCr1−x/glass thin films

Series of Co_xCr_1−x thin films have been evaporated under vacuum onto Si(1 0 0) and glass substrates. Thickness ranges from 17 to 220 nm, and x from 0.80 to 0.88. Alternating gradient field magnetometer (AGFM) measurements provided saturation magnetization values ranging from 220 to 1200 emu/cm³. Values of squareness exceeding 0.8 have been measured. Coercive field may reach values up to 700 Oe, depending on the percentage of chromium, as well as the substrate nature and the direction of the applied magnetic field. The saturation magnetization value decreases as the Cr content increases. In order to study their dynamical magnetic properties, Brillouin Light Scattering (BLS) measurements have been performed on these samples. Stiffness constant value and anisotropy magnetic field were adjusted to fit the experimental BLS spectra. These results are analyzed and correlated.     

Effect of substrate temperature on the morphology, structural and optical properties of Zn1−xCoxO thin films

Zn_1−xCo_xO thin films with c-axis preferred orientation were deposited on sapphire (0 0 0 1) by pulsed laser deposition (PLD) technique at different substrate temperatures in an oxygen-deficient ambient. The effect of substrate temperature on the microstructure, morphology and the optical properties of the Zn_1−xCo_xO thin films was studied by means of X-ray diffraction (XRD), atomic force microscopy (AFM), UV-visible-NIR spectrophotometer, fluorescence spectrophotometer. The results showed that the crystallization of the films was promoted as substrate temperature rose. The structure of the samples was not distorted by the Co incorporating into ZnO lattice. The surface roughness of all samples decreased as substrate temperature increased. The Co concentration in the film was higher than in the target. Emission peak near band edge emission of ZnO from the PL spectra of the all samples was quenched because the dopant complexes acted as non-radiative centers. While three emission bands located at 409 nm (3.03 eV), 496 nm (2.5 eV) and 513 nm (2.4 eV) were, respectively, observed from the PL spectra of the four samples. The three emission bands were in relation to Zn interstitials, Zn vacancies and the complex of V_O and Zn_i (V_OZn_i). The quantity of the Zn interstitials maintained invariable basically, while the quantity of the V_OZn_i slightly decreased as substrate temperature increased.     

Structural, electrical, optical and magnetic properties of Co0.2AlxZn0.8−xO films

Co_0.2Al_xZn_0.8−xO films prepared with different molar ratio of aluminum nitrate to zinc acetate were deposited on substrates by the sol-gel technique. X-ray diffraction, photoluminescence and ferromagnetism measurements were used to characterize the Co_0.2Al_xZn_0.8−xO diluted magnetic semiconductors. The authors found that the intensity of the acceptor-related photoluminescence increased with increasing aluminum concentration and an increase in the number of the acceptor-like defects (zinc vacancies especially) in the Co_0.2Al_xZn_0.8−xO film might lead to the enhancement of the magnetic properties. This implies that controls of the aluminum concentration and the number of the acceptor-like defects are important factors to produce strong ferromagnetism Co_0.2Al_xZn_0.8−xO films prepared by the sol-gel method.     

Annealing of thin Zr films on Si1−xGex(0≤x≤1): X-ray diffraction and Raman studies

X-ray diffraction experiments have been combined with Raman scattering and transmission electron microscopy data to analyze the result of rapid thermal annealing (RTA) applied to Zr films, 16 or 80 nm thick, sputtered on Si_1−xGe_x epilayers (0≤x≤1). The C49 Zr(Si_1−xGe_x)₂ is the unique phase obtained after complete reaction. ZrSi_1−xGe_x is formed as an intermediate phase. The C49 formation temperature T_f is lowered by the addition of Ge in the structure. Above a critical Ge composition close to x=0.33, a film microstructure change was observed. Films annealed at temperatures close to T_f are continuous and relaxed. Annealing at T>T_f leads to discontinuous films: surface roughening resulting from SiGe diffusion at film grain boundaries occurred. Grains are ultimately partially embedded in a SiGe matrix. A reduction in the lattice parameters as well as a shift of Raman lines are observed as T exceeds T_f. Both Ge non-stoichiometry and residual stress have been considered as possible origins for these changes. However, as Ge segregation has never been detected, even by using very efficient techniques, it is thought that the changes originate merely from residual stress. The C49 grains are expected to be strained under the SiGe matrix effect and shift of the Raman lines would indicate the stress is compressive. Some simple evaluations of the stress values indicate that it varies between −0.3 and −3.5 GPa for 0≤x≤1 which corresponds to a strain in the range (−0.11, −1.15%). X-ray and Raman determinations are in good agreement.     

Effect of electric field on electrocaloric effect in Pb(Zr1 − xTix)O3 solid solution

A thermodynamic analysis is employed to investigate the intrinsic electrocaloric effect of Pb(Zr_1 − xTi_x)O₃ solid solution system under the different electric field. Theoretical analysis indicates that Pb(Zr_1 − xTi_x)O₃ system has the giant electrocaloric coefficient and the large adiabatic temperature change near its ferroelectric Curie temperature. The applied electric field decreases not only the electrocaloric coefficient but also its temperature dependence. Furthermore, it increases the adiabatic temperature change as well as its dependence of temperature. The temperature corresponding to the maximum of electrocaloric coefficient and adiabatic temperature change increases with the enhancement of electric field because of its first-order phase transition between ferroelectric phase and paraelectric phase.     

Nanoscale semiconductor Pb1−xSnxSe (x = 0.2) thin films synthesized by electrochemical atomic layer deposition

In this paper the fabrication and characterization of IV-VI semiconductor Pb_1−xSn_xSe (x = 0.2) thin films on gold substrate by electrochemical atomic layer deposition (EC-ALD) method at room temperature are reported. Cyclic voltammetry (CV) is used to determine approximate deposition potentials for each element. The amperometric I-t technique is used to fabricate the semiconductor alloy. The elements are deposited in the following sequence: (Se/Pb/Se/Pb/Se/Pb/Se/Pb/Se/Sn …), each period is formed using four ALD cycles of PbSe followed by one cycle of SnSe. Then the deposition manner above is cyclic repeated till a satisfactory film with expected thickness of Pb_1−xSn_xSe is obtained. The morphology of the deposit is observed by field emission scanning electron microscopy (FE-SEM). X-ray diffraction (XRD) pattern is used to study its crystalline structure; X-ray photoelectron spectroscopy (XPS) of the deposit indicates an approximate ratio 1.0:0.8:0.2 of Se, Pb and Sn, as the expected stoichiometry for the deposit. Open-circuit potential (OCP) studies indicate a good p-type property, and the good optical activity makes it suitable for fabricating a photoelectric switch.     

BiFeO3/Zn1−xMnxO bilayered thin films

BiFeO₃/Zn_1−xMn_xO (x = 0-0.08) bilayered thin films were deposited on the SrRuO₃/Pt/TiO₂/SiO₂/Si(1 0 0) substrates by radio frequency sputtering. A highly (1 1 0) orientation was induced for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO thin films demonstrate diode-like and resistive hysteresis behavior. A remanent polarization in the range of 2P_r ∼ 121.0-130.6 μC/cm² was measured for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO (x = 0.04) bilayer exhibits a highest M_s value of ∼15.2 emu/cm³, owing to the presence of the magnetic Zn_0.96Mn_0.04O layer with an enhanced M_s value.