Zn incorporation and (CuIn)1−xZn2xSe2 thin film formation during the selenization of evaporated Cu and In precursors on Al:ZnO coated glass substrates

CuInSe₂ thin films with typical 1.0 eV gap energy and tetragonal chalcopyrite structure have been obtained on soda–lime glass substrates by the reaction of sequentially evaporated Cu and In layers with elemental selenium vapor, at 500 °C in flowing Ar. When analogous deposition and reaction processes were performed on Al:ZnO coated glasses, some increment in the band gap energy and diminution in the crystalline interplanar spacings have been detected for the resulting films with an extent that depends on the Cu/In atomic ratio of the evaporated precursor layers. This fact has been related to Zn incorporation into the selenized film, with quaternary (CuIn)_1−xZn_2xSe₂ compound formation that is influenced by the presence of copper selenide phases during the reaction process. Such deductions are supported by the optical, structural and compositional characterizations that have been performed comparatively on samples prepared by selenization of evaporated metallic precursors with two different Cu/In ratios (0.9 and 1.1) on bare and Al:ZnO coated glass substrates.     

Surface morphology and growth mechanism of catalyst‐free ZnO and Mgx Zn1–x O nanorods

High‐resolution transmission electron microscopy was employed to investigate morphologies and catalyst‐free growth mechanism of ZnO/Mg_x Zn_1–x O ‘multi‐quantum well’ and ‘core‐shell’ nanorod heterostructures as well as ZnO nanorods. The one‐dimensional growth mechanism and the hexagonal faceting of ZnO nanorod were explained by the surface energy anisotropy. The morphology change by alloying with Mg was successfully explained by investigating the energy gain by adatom adsorption and the reduction in the surface energy anisotropy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of lower growth temperature on C incorporation in GeC epilayers on Si(0 0 1) grown by MBE

Effect of lower growth temperature T_s on C incorporation to substitutional sites in Ge_1−xC_x/Si(0 0 1) grown by molecular beam epitaxy was investigated. To enhance the non-equilibrium growth condition, the temperature T_s was lowered from 600°C down to 300°C. The C incorporation into substitutional sites of GeC epilayers was very sensitive to T_s. X-ray diffraction (XRD) measurement indicated that the substitutional C composition x increased with decrease in T_s from 600°C to 400°C. At T_s350°C, the estimation of x by the XRD analysis was impossible because of polycrystallization. The Raman shift measurement enables to estimate x for T_s350°C, as consequently larger x than that grown at T_s=400°C was verified. The enhancement of non-equilibrium growth condition by decreasing T_s was important to increase x.     

Microstructure and thermoelectric properties of Zn1−xAlxO ceramics fabricated by spark plasma sintering

Al-doped ZnO powders were synthesized via solid reaction between Zn(OH)₂ and Al(OH)₃ and consolidated by spark plasma sintering (SPS) to fabricate fine-grained Zn_1−xAl_xO ceramics as a thermoelectric material. X-ray diffraction and spectrophotometer experiments revealed that Al doping into ZnO is enhanced by the present process, and consequently the SPS-processed Zn_1−xAl_xO samples show significantly improved electrical conductivity as compared with those prepared via mixing ZnO and Al₂O₃ oxide powders. Because of the combined effect of Al doping and grain refinement, the present Zn_1−xAl_xO ceramics show much lower thermal conductivity, which also results in an enhanced dimensionless figure of merit (ZT), than un-doped ZnO oxides prepared also by SPS.     

Magnetic transitions and magnetoresistance of -type () compounds

Magnetic transitions and magnetotransport properties of polycrystalline Tb_1−xGd_xMn₆Ge₆ (x=0.2–1.0) compounds have been investigated by magnetic property and resistivity measurements in an applied magnetic field up to 50 kOe. The cell parameter a,c and cell volume V of compounds (x=0.2–1.0) increase with an increasing Gd content. The compounds (x=0.2–1.0) show a rich variety of magnetic behavior, such as antiferromagnetic, ferrimagnetic and paramagnetic state with increasing temperature. Their Curie temperatures increase almost linearly with an increasing Gd content from 460 K for x=0.2 to 484 K for x=1.0. The compounds (x=0.2–1.0) display the field induced metamagnetic transitions, and the threshold fields first increase and then decrease with an increasing Gd content. The magnetoresistance curves of the Tb_0.4Gd_0.6Mn₆Ge₆ compounds in an applied magnetic field up to 50 kOe are presented and the magnetoresistance effects are related to the metamagnetic transitions.     

Influence of vacancies on the electrical properties of the ZnCr2−xNixSe4 spinels

The effect of vacancies on the electronic transport in the ZnCr_2−xNi_xSe₄ paramagnetic single crystals is considered. For this purpose, the structure refinements, the high temperature electrical conductivity and the thermoelectric power measurements as well as the calculations of the vacancy model parameters were used. The electrical measurements have been done in the temperature range from 290 to 520 K for single crystals with x=0.001, 0.05, and 0.065. The above investigations provide evidence for polaron conduction in defective spinel materials. In particular: (1) at high temperatures a linear dependence between thermopower S and the electrical resistivity (ln ρ), a characteristic of small polarons, was observed, (2) an origin of small polarons in this case could be associated with a crystalline distortion, which is characterized by larger values of the anion parameter than u=0.375, which describes an ideal spinel structure, and (3) a large defectiveness, which is identified by the large value of the vacancy parameter of about β=11.5%. These effects are explained in terms of a polaron mechanism of the electrical conductivity including structural defects.     

Magnetic and magnetocaloric properties of Pr0.6−xEuxSr0.4MnO3 manganese oxides

Studies of the structural, magnetic and magnetocaloric properties of polycrystalline Pr_0.6−xEu_xSr_0.4MnO₃ (0≤x≤0.15) perovskite manganites were carried out. Substitution for praseodymium with europium, with smaller ionic radius, induces local distortion in the 〈Mn–O–Mn〉 bonds and consequently causes a random distribution in the magnetic exchange interactions. The competition between magnetic interactions leads to the appearance of an inhomogeneous magnetic state in our samples. Pr_0.6−xEu_xSr_0.4MnO₃ (x=0, 0.05, 0.1 and 0.15) polycrystalline samples were prepared using the solid–solid reaction method at high temperature. The compounds yielded are single phase and crystallize in the orthorhombic system with the Pnma space group. The substitution of Eu for Pr leads to a decrease of the Curie temperature T_C from 303 K for x=0.00 to 260 K for x=0.15. All of our compounds exhibit a large magnetic entropy change with a maximum around 2.2 J/kg K under a magnetic applied field change of 2 T for all compounds.     

Preparation and photocatalytic activity of nitrogen-doped TiO2 hollow nanospheres

TiO₂ hollow nanospheres were prepared using silicon oxide as a template. N-doped titanium oxide hollow spheres, TiO_2−xN_x were synthesized by reacting TiO₂ hollow spheres with thiourea at 500 °C. XRD and XPS data showed that oxygen was successfully substituted by nitrogen through the nitrogen-doping reaction, and finally N-doped TiO₂ hollow spheres were formed. The N-doped TiO₂ hollow spheres showed new absorption shoulder in visible light region so that they were expected to exhibit photocatalytic activity in the visible light. The photocatalytic activity of N-doped TiO₂ hollow spheres under visible light was similar to that of normal spherical TiO_2−xN_x in spite of the structural difference.     

IR and Raman investigation on the structure of (100-x)B2O3-x[0.5 BaO-0.5 ZnO] glasses

Glasses with molar composition of (100-x)B₂O₃-x[0.5 BaO-0.5 ZnO], x=40, 50, 60, 70 were prepared from the melts of ZnO, BaCO₃ and H₃BO₃ mixture. The structure and thermal behavior were characterized by IR and Raman spectroscopy, DSC and Dilatometer. The investigation shows that the transition of the structural unit [BO₄] (B^IV) to [BO₃] (B^III) happens when BaO and ZnO content x increases in the borate glass, resulting in fewer B^III-O-B^IV bonds and more B^III-O-B^III bonds. At the same time, the diborate groups, which are found to be the predominant structural group of the glass with high B₂O₃ content, gradually changes into ring-type metaborate, pyro- and orthoborate groups. With increasing ZnO and BaO content x, the glass transition temperature (T_g) and the softening point (T_f) decreases, while linear expansion coefficient (α) increases, that comes from the weakening of the glass network.     

Low-field transport properties of (1−x)La0.7Ca0.2 Sr0.1MnO3+x(ZnO) composites

Composite samples (1−x)La_0.7Ca_0.2Sr_0.1MnO₃(LCSMO)+x(ZnO) with different ZnO doping levels x have been investigated systematically. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between LCSMO and ZnO grains, and that ZnO segregates mostly at the grain boundaries of LCSMO. The magnetic properties reveal that the ferromagnetic order of LCSMO is weakened by addition of ZnO. The results also show that ZnO has a direct effect on the resistance of LCSMO/ZnO composites, especially on the low-temperature resistance. With increase of the ZnO doping level, T_P shifts to a lower temperature and the resistance increases. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 3 kOe. The maximum MR appears at x=0.1. The low field magnetoresistance (LFMR) results from spin-polarized tunneling. However, around room temperature, the enhanced MR of the composites is caused by magnetic disorder.     

Investigation of the solid solution series 2(MnX)-CuInX2 (X=S, Se)

(2MnX)_x(CuInX₂)_1−x with X=S and Se were prepared by solid state reaction from the end members α-MnS, β-MnS and CuInS₂ in the range 0<x≤0.2 (≤0.6 for β-MnS) as well as MnSe and CuInSe₂ in the range 0<x≤0.1. Mixed crystals with 0≤x≤0.1 crystallize in the tetragonal chalcopyrite type structure, (2α-MnS)_x(CuInS₂)_1−x samples with 0.1<x≤0.2 and (2β-MnS)_x(CuInS₂)_1−x samples up to x=0.6 consist of two phases, occuring as tetragonal domains (x∼0.1 for X=S) within a cubic matrix with zinc-blende type structure (x∼0.4 for X=S), indicating a miscibility gap. For tetragonal single phase samples the band gap energy, the lattice constants and the anion parameter have been determined. The first and the latter ones show a different composition dependent behaviour caused by the modification of the MnS (α-MnS with NaCl type structure, β-MnS with zinc-blende type structure) used during the synthesis. Additionally a CuMn_xIn_1−xS₂ powder sample, in which Mn substitutes the M^III site, was investigated. The SQUID measurements revealed a well-distinct magnetic transition between 15 and 16 K as well as ferromagnetic-like hysteresis loops pronounced for temperatures below the transition temperature. Below this temperature a clear splitting between the zero field cooling (ZFC) and the field cooling (FC) curves indicate to the existence of a long-range magnetic ordering phenomenon. This behaviour was not found in the other samples were Mn substitutes both sites M^I as well as M^III.     

Luminescence from ZnO/MgO nanoparticle structures prepared by solution techniques

The optical and structural properties of mixed ZnO/MgO particles prepared by solution techniques are investigated by the cathodoluminescence and electron microscopy techniques. The samples annealed at 400–1000 °C show well crystalline wurtzite structure of the ZnO (MgZnO) particles with the size in range of 10–100 nm. Annealing at high temperatures (>700 °C) leads to Mg diffusion in ZnO and Mg_xZn_1−xO alloy formation. The blue shifts of the near-band-edge emission as a result of the alloy band gap widening and quantum confinement effect for the small size particles are demonstrated.     

Optical properties and mechanical characteristics of transparent nanostructured Zn1−xMnxO thin films

Uniform and transparent thin films of Zn_1−xMn_xO (0?x?0.10) were fabricated by a sol-gel spin coating method. XRD results indicated the hexagonal structure of ZnO as the primary phase at all concentrations (x) of Mn. However, at x?0.035, Mn₃O₄ (tetragonal) is observed as the secondary phase, which was confirmed by selected-area electron diffraction patterns. SEM and TEM results showed a tendency of grains to arrange into wire-shaped morphologies, leading to elongated needle-like structures at high Mn addition. Increasing Mn content in the range 0?x?0.10 led to quenching of photoluminescence, increase in the band gap (E_g) from 3.27 to 3.33 eV, and increase in film thickness, refractive index and extinction coefficient of Zn_1−xMn_xO thin films. The residual stress evaluated was compressive in all cases and found to increase by an order of magnitude with addition of Mn. Furthermore, an overall increase in microhardness and yield strength of Zn_1−xMn_xO thin films at higher Mn concentrations is attributed to change in microstructures, presence of secondary phase and increase in film thickness.     

Growth of InN thin films on ZnO substrate by plasma-assisted molecular beam epitaxy

We have studied the microstructure property of InN epitaxial films grown on ZnO substrate by plasma-assisted molecular beam epitaxy. We found that the In₂O₃ compound was produced on ZnO substrate and many pits were formed on the InN films when InN was directly grown on ZnO substrate with the N/In flux ratio less than 40. We demonstrated that the quality of InN film was significantly improved when the In₂O₃ layer was used as a buffer to prevent the reaction between In and the ZnO substrate.     

Raman investigations of Zn1−xCoxO nanocrystals: role of starting precursors on vibrational properties

The Raman spectra of sol–gel derived Co‐doped ZnO nanoparticles (NPs) in the spectral range 100–1500 cm⁻¹ were investigated. In the sol–gel method, three different series of Co‐doped ZnO particles, i.e. Zn_1−xCo_xO (x = 0.05, 0.10, 0.15, and 0.20), were obtained using three different starting precursors, viz. cobalt chloride hexahydrate, cobalt acetate tetrahydrate, and cobalt nitrate hexahydrate, respectively. It has been observed that cobalt acetate is a better precursor in comparison to cobalt chloride and cobalt nitrate to obtain single‐phase Co‐doped ZnO NPs. As for cobalt acetate‐derived NPs, no hidden secondary phase of Co₃O₄ was observed for the lower (x = 0.05) Co concentration. The Fröhlich interaction associated with the longitudinal modes was found to be destroyed with increasing Co concentration due to structural disorder and defects induced by the dopant. In addition to ZnO and Co₃O₄ vibrational modes, a few additional modes near 550 and 715 cm⁻¹ were also observed in all cases, which could be attributed to the modes due to Co doping in ZnO. Copyright © 2011 John Wiley & Sons, Ltd.     

Microwave dielectric properties of high quality factor La(Mg0.5−xCaxSn0.5)O3 ceramics

The microwave dielectric properties of La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics were examined with a view to their exploitation for wireless communications. The La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics contained Ca₂SnO_4, CaSnO₃, and La₂O₃. The amount of Ca₂SnO₄ increased with increasing sintering temperature. However, the relative amount of CaSnO₃ decreased with increasing sintering temperature. An apparent density of 6.52 g/cm³, a dielectric constant (ε_r) of 20.2, a quality factor (Q×f) of 80,500 GHz, and a temperature coefficient of resonant frequency (τ_f) of −79 ppm/°C were obtained for La(Mg_0.4Ca_0.1Sn_0.5)O₃ ceramics that were sintered at 1500 °C for 4 h.     

Fabrication of p-CuGaS2/n-ZnO:Al heterojunction light-emitting diode grown by metalorganic vapor phase epitaxy and helicon-wave-excited-plasma sputtering methods

Greenish-white electroluminescence (EL) was observed from the heterojunction light-emitting diodes (LEDs) composed of p-type (001) CuGaS₂ chalcopyrite semiconductor epilayers and preferentially (0001)-oriented polycrystalline n-type ZnO thin films. The CuGaS₂ layers were grown on a (001) GaP substrate by metalorganic vapor phase epitaxy and the ZnO films were deposited by the surface-damage-free helicon-wave-excited-plasma sputtering method. The n-ZnO/p-CuGaS₂ LED structure was designed to enable an electron injection from the n-type wider band gap material forming a TYPE-I heterojunction. The EL spectra exhibited emission peaks and bands between 1.6 and 2.5 eV, although their higher energy portions were absorbed by the GaP substrate. Since the spectral lineshape resembled that of the photoluminescence from identical CuGaS₂ epilayers, the EL was assigned to originate from p-CuGaS₂.     

The effect of Fe substitution on the electrical and thermal conductivity and thermopower of Ca3(FexCo1−x)4O9 synthesised by a sol–gel process

The effect of Fe substitution for Co on direct current (DC) electrical and thermal conductivity and thermopower of Ca₃(Co_1−xFe_x)₄O₉ (x = 0, 0.05, 0.08), prepared by a sol–gel process, was investigated in the temperature range from 380 down to 5K. The results indicate that the substitution of Fe for Co results in an increase in thermopower and DC electrical resistivity and substantial (14.9–20.4% at 300K) decrease in lattice thermal conductivity. Experiments also indicated that the temperature dependence of electrical resistivity ρ for heavily substituted compounds Ca₃(Co_1−xFe_x)₄O₉ (x = 0.08) obeyed the relation lnρ ∝ T^−1/3 at low temperatures, T < ~55K, in agreement with Mott’s two-dimensional (2D) variable range hopping model. The enhancement of thermopower and electrical resistivity was mainly ascribed to a decrease in hole carrier concentration caused by Fe substitution, while the decrease of thermal conductivity can be explained as phonon scattering caused by the impurity. The thermoelectric performance of Ca₃Co₄O₉ was not improved in the temperature range investigated by Fe substitution largely due to great increase in electrical resistivity after Fe substitution.     

Ferromagnetic nature of (Ga, Cr)As epilayers revealed by magnetic circular dichroism

A systematic study of magnetic circular dichroism (MCD) was carried out in a wave length range 500-960 nm for (Ga_1−x, Cr_x)As epilayers with x=2.38% and 4.59% grown by the low temperature molecular beam epitaxy (LT-MBE) technique. Hysteresis characteristics showed up indeed in the magnetic field dependence of both MCD and magnetization measured by the superconductor quantum interference device (SQUID). The Curie temperature of the (Ga_1−x, Cr_x)As epilayer was determined to be about 12 K by the Arrott approach. The present result provides evidences that there is strong coupling of the Cr spins to the GaAs host band structure in (Ga_1−x, Cr_x)As samples. That affects the critical point of the semiconductor host, and makes the magnetization behavior in a plot of M∼B/T (magnetic field divided by temperature) substantially different from standard superparamagnetism.     

Far-infrared dielectric functions and phonon spectra of alloys determined by spectroscopic ellipsometry

We have used spectroscopic ellipsometry to determine the complex dielectric function of a series of ternary Be_xZn_1−xTe thin films grown by molecular beam epitaxy. The II–VI semiconductor alloys were grown on InP substrates that had an InGaAs buffer layer. After the growth, X-ray diffraction experiments were performed in order to determine the alloy concentration. A standard inversion technique was used to obtain the dielectric functions from the measured ellipsometric spectra, obtained between 2000 nm (5000 cm⁻¹) and 40,000 nm (250 cm⁻¹). By modelling the dielectric function as a collection of oscillators, representing longitudinal and transverse optical phonons of the Be_xZn_1−xTe lattice, we were able to recover the phonon spectra for this alloy system. It is argued that the additional phonon modes that are obtained from ellipsometry are best understood from the recently-proposed percolation model.