Studies on electrical switching behavior and optical band gap of amorphous Ge–Te–Sn thin films

Amorphous thin film Ge₁₅Te_85−x Sn _x (1≤x≤5) and Ge₁₇Te_83−x Sn _x (1≤x≤4) switching devices have been deposited in sandwich geometry using a flash evaporation technique, with aluminum as the top and bottom electrodes. Electrical switching studies indicate that these films exhibit memory type electrical switching behavior. The switching fields for both the series of samples have been found to decrease with increase in Sn concentration, which confirms that the metallicity effect on switching fields/voltages, commonly seen in bulk glassy chalcogenides, is valid in amorphous chalcogenide thin films also. In addition, there is no manifestation of rigidity percolation in the composition dependence of switching fields of Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x amorphous thin film samples. The observed composition dependence of switching fields of amorphous Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x thin films has been understood on the basis of Chemically Ordered Network model. The optical band gap for these samples, calculated from the absorption spectra, has been found to exhibit a decreasing trend with increasing Sn concentration, which is consistent with the composition dependence of switching fields.     

Photoluminescence measurements in the phase transition region of Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S films

Thin films of Zn_1−x Cd _x S (0.1 ≤ x ≤ 0.5) were prepared by using pulsed laser ablation technique on corning glass substrates. Phase transition from cubic to hexagonal in Zn_1−x Cd _x S films is determined by X-ray diffraction analysis. We observed a lowering in the phase transition temperature with increase in the cadmium concentration. Transmission electron microscopy suggests the crystalline nature of thin films with average particle size of 15 nm. The grown Zn_1−x Cd _x S samples show the high peak intensity ratio of the near band edge emission to the defect center luminescence even at room temperature, which indicates the small concentration of complex defects in the samples. Photoluminescence measurement show stoichiometric dependence of the energy band gap and is found to have quadratic dependence on x.     

Tailoring the optical properties of MgxZn1?xO thin films by nitrogen doping

Thin films of Mg _x Zn_1−x O and Mg _x Zn_1−x O doped with nitrogen were deposited by Radio Frequency plasma beam assisted Pulsed Laser Deposition (RF-PLD) in oxygen or oxygen-nitrogen discharge with different nitrogen/oxygen ratios. A Nd:YAG laser working at a wavelength of 266 nm, having a 10 Hz repetition rate was used for the depositions. The energy density of the incident beam was 3 J/cm² and the RF power was set to 100 W for all the samples. X-ray Diffraction (XRD) and Spectroscopic Ellipsometry (SE) were employed to investigate the samples. The degree of crystallinity is fount to decrease with increasing the Mg concentration, while the solubility of Mg in ZnO increases by 30% in the N-doped Mg _x Zn_1−x O thin films grown by RF-PLD. Segregation of MgO phase at a Mg concentration of 30% for Mg _x Zn_1−x O thin film is detected both by XRD and SE. The band gap of the samples increases from 3.37 up to 3.57 eV with increasing the Mg concentration and the nitrogen/oxygen ratio for each Mg concentration. A dependence of the dielectric function (refractive index) on both stoichiometry and degree of crystalinity is also found, the refractive index having values between 1.7 and 2 in visible spectral range.     

Melting temperature variation with concentration of alkali halides in mixed crystal systems

Diffusion-vibration theory of melting (Sharmaet al 1991) has been extended to study the variation in the melting temperature of mixed ionic crystals with concentration. The melting temperature varies non-linearly with concentration in the KCl _x Br_1−x , RbCl _x Br_1−x , K _x Rb_1−x Br and NaCl _x Br_1−x mixed alkali halides and shows a sharp increase in melting temperature for values ofx>0.5 which is in good agreement with the experimental values. This behaviour has been explained on the basis of present propounded theory.     

Advances in single crystal growth and annealing treatment of electron-doped HTSC

High quality electron-doped HTSC single crystals of Pr_2−xCe_xCuO_4+δ and Nd_2−xCe_xCuO_4+δ have been successfully grown by the container-free traveling solvent floating zone technique. The optimally doped Pr_2−xCe_xCuO_4+δ and Nd_2−xCe_xCuO_4+δ crystals have transition temperatures T _c of 25 K and 23.5 K, respectively, with a transition width of less than 1 K. We found a strong dependence of the optimal growth parameters on the Ce content x. We discuss the optimization of the post-growth annealing treatment of the samples, the doping extension of the superconducting dome for both compounds as well as the role of excess oxygen. The absolute oxygen content of the as-grown crystals is determined from thermogravimetric experiments and is found to be ≥ 4.0. This oxygen surplus is nearly completely removed by a post-growth annealing treatment. The reduction process is reversible as demonstrated by magnetization measurements. In as-grown samples the excess oxygen resides on the apical site O(3). This apical oxygen has nearly no doping effect, but rather influences the evolution of superconductivity by inducing additional disorder in the CuO₂ layers. The very high crystal quality of Nd_2−xCe_xCuO_4+δ is particularly manifest in magnetic quantum oscillations observed on several samples at different doping levels. They provide a unique opportunity of studying the Fermi surface and its dependence on the carrier concentration in the bulk of the crystals.     

Influence of thermal treatment in oxidizing and reducing atmospheres on the composition and properties of solid solutions based on bismuth vanadate: (Bi,La)4(V,Me)2O11?y (Me = Ga, Zr, Cu)

Ceramic solid solutions (Bi_{1 − y} La _y )₄(V_{1 − x} Me _x )₂O_{11 − z} (x, y < 0.2: Me = Zr, Ga, Fe, Cu) were prepared by solid-state reaction. It was shown that the annealing (973 K, reducing atmosphere H₂/Ar (20/80)) of the samples whose compositions belong to the stability domains of α, β, and γ′ polymorphs increases their electronic conductivity by six orders of magnitude. The samples with low concentrations of dopant cations exhibited good compositional stability and a reversible change in their structure parameters. At the same time, the solid solutions with a high concentration of lanthanum cations and/or copper cations (y, x ∼ 0.1) underwent partial decomposition.     

Free carrier concentration and conductivity in chlorine doped lead telluride

Plasma resonance in the IR reflection spectra is used to measure the concentration and relaxation time of free charge carriers along with the conductivity in PbTe_{1 − x} Cl _x solid solutions. It is found that with increasing the chlorine concentration, the electron concentration and conductivity increase and reach saturation at x = 0.03 (n = 5.5 × 10¹⁹ cm⁻³, σ = 3750 Ohm⁻¹ · cm⁻¹). The relaxation time decreases with increasing the chlorine concentration and reaches the minimum value of 2.2 × 10⁻¹⁴ s at x = 0.03; then, it almost does not change.     

Effect of Mg doping and substrate temperature on the properties of pulsed laser deposited epitaxial Zn1?xMgxO thin films

In this paper, we report on the pulsed laser deposition of epitaxial (0002) oriented Zn_1−x Mg _x O thin films onto (0001) sapphire substrate in O₂ ambient at different deposition temperatures. Pulsed laser deposited Zn_1−x Mg _x O films showed (0002) oriented hexagonal wurtzite structure up to 34% of Mg concentration. The bandgap of Zn_1−x Mg _x O thin films is successfully tuned from 3.3 to 4.2 eV by adjusting the Mg concentration x=0.0 to x=0.34. Pulsed laser deposited Zn_1−x Mg _x O thin films were characterized by XRD, AFM, SEM, PL and UV–VIS spectrometer. We have also studied the effect of deposition temperature on to the structure, surface morphology and optical properties of Zn_1−x Mg _x O thin films.     

Magnetic aftereffects in nickel ferrites

We have studied the magnetic aftereffects in the Ni _x Fe_3−x−ΔO₄ system, for 0≦x≦1 and 10⁻⁵≦Δ≦2×10⁻¹, between 80 and 500 K. The samples were obtained by sintering at 1400°C in an appropriate gas atmosphere. The measurements are based on the deviation from equilibrium that is produced in a Maxwell-Wien bridge when the self-induction of a coil with ferrite core varies because of the phenomenon of magnetic aftereffects. The numerical analysis of the results shows the presence of relaxation processes at 300 K (III), 330 K (IIa), and above 500 K (I). The Processes III and IIa are related to the concentration of nickel,x, and of vacancy, Δ. It is seen that the IIa peak can be attributed to a process of diffusion of Ni ions in the spinel lattice by means of vacancies on octahedral sites.     

Critical concentrations in Ba-doped incipient ferroelectric SrTiO3

Temperature-induced variations of light refraction and dielectric permittivity in single-crystal Sr_1−x Ba_xTiO₃ (x=0.02, 0.05, 0.07, and 0.14), Sr_1−x Ca_xTiO₃ (x=0.014), and in nominally pure strontium titanate have been studied within the 17–300 K temperature range. The spontaneous polar contribution to the refractive index has been isolated. It was used to calculate the temperature and concentration dependences of the polarization autocorrelation function 〈P _s ^2〉 in the Sr_1−x Ba_xTiO₃ system. For x⩽0.07, the polarization P _s=〈P _s ² 〉^1/2 varies proportional to (x−x _g)^1/2, where x _g=0.0027 is the new critical concentration in Sr_1−x Ba_xTiO₃, below which short-range polar order vanishes. Fiz. Tverd. Tela (St. Petersburg) 39, 704–710 (April 1997)     

Structural and magnetic properties of zinc- and aluminum-substituted cobalt ferrite prepared by co-precipitation method

Spinal ferrites having the general formula Co_{1 − x} Zn _x Fe_{2 − x} Al _x O₄ (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were prepared using the wet chemical co-operation technique. The samples were annealed at 800°C for 12 h and were studied by means of X-ray diffraction, magnetization and low field AC susceptibility measurements. The X-ray analysis showed that all the samples had single-phase cubic spinel structure. The variation of lattice constant with Zn and Al concentration deviates from Vegard’s law. The saturation magnetization σ _s and magneton number n _B measured at 300 K using high field hysteresis loop technique decreases with increasing x, suggesting decrease in ferrimagnetic behaviour. Curie temperature T _C deduced from AC susceptibility data decreases with x, suggesting a decrease in ferrimagnetic behaviour.      

Microwave-assisted synthesis of Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S–MWCNT heterostructures and their photocatalytic properties

The multi-walled carbon nanotubes (MWCNTs) wrapped with hexagonal wurtzite Zn _x Cd_1−x S nanoparticles with a uniform and small diameter have been prepared to form Zn _x Cd_1−x S–MWCNT heterostructures by microwave-assisted route using Zn(Ac)₂, Cd(NO₃)₂, and thioacetamide as the reactants_. The heterostructures have been characterized by X-ray powder diffraction, scanning and transmission electron microscopy, high-resolution transmission electron microscopy, photoluminescence (PL) and PL excited lifetime. Despite the analogous size and configuration, the Zn _x Cd_1−x S–MWCNT (x = 0, 0.2, 0.5, 0.8, 1) with different Zn concentration exhibit composition-dependent absorption properties in the visible zone. The PL peak positions of Zn _x Cd_1−x S–MWCNT change gradually from ZnS–MWCNT to CdS–MWCNT. The Zn _x Cd_1−x S–MWCNT shows different photocatalytic activity towards the photodegradation of fuchsin acid under visible light illumination, photocatalytic activity of the Zn _x Cd_1−x S–MWCNT decreases gradually with the increase in the Zn concentration, the Zn_0.2Cd_0.8S–MWCNT possessed the best photocatalytic activity. After recycling thrice, the photocatalysts still have about 85% efficiency.     

Silicon and transition metal-silicides implanted with57Fe

Si(111) single crystals were implanted with⁵⁷Fe in a broad dose range in order to overlap the concentration range of bulk amorphous Fe _x Si_1−x samples. At high (≥10¹⁶ atoms/cm²) doses the measured hyperfine interaction values were found to be the same as in the bulk amorphous samples, suggesting the same Fe−Si bonding and a very similar structure for the two amorphous phases produced by different methods. A comparison of the isomer shift (δ) and quadrupole splitting (ΔE) values with the values of the stoichiometric crystalliine phases showed the same δ but different δE values indicating similar Fe−Si bonding but different atomic arrangement around the iron atom.     

Microscopic indium distribution and electron localization in zinc blende InGaN alloys and InGaN/GaN strained quantum wells

In order to give an atomic level understanding of the light emission mechanism and seek In distribution patterns closely related to the elusive electron localization centers, we optimize the crystal structure of zinc blende In _x Ga_1−x N (0≤x≤1) alloys with different In distributions and investigate their electronic structures using first-principles calculations. Our results show that In _x Ga_1−x N forms a random alloy, in which several-atom In–N clusters and In–N chains can exist stably with a high concentration due to their small formation energy. These In–N clusters and chains form more easily in zinc blende structure than in wurtzite structure. The band gap of zinc blende In _x Ga_1−x N alloys insensitively depends on the In distribution. Moreover, we find that both small In–N clusters and straight In–N chains with three or more In atoms, acting as radiative recombination centers, highly localize the electrons of the valence band maximum state and dominate the light emission of Ga-rich In _x Ga_1−x N alloys. The strains of In _x Ga_1−x N layers can enhance the electron localization in In _x Ga_1−x N/GaN strained quantum wells. Our results are in good agreement with experiments and other calculations.     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

Growth mechanism, microstructure and transport properties of Sr1−x La x CuO2 (x=0.10−0.15) thin films

Sr_1−x La _x CuO₂ (x=0.10−0.15) thin films with an infinite-layer type structure were grown on BaTiO₃ buffered (001) SrTiO₃ substrates by pulsed laser deposition (PLD). The evolution of the growth front was monitored, in-situ, by high-pressure reflection high-energy electron diffraction (RHEED), while the surface morphology was analyzed by means of atomic force microscopy (AFM), ex-situ. X-ray diffraction (XRD) was used to determine the evolution of the film structure with deposition and cooling parameters, as well as to study the type and level of epitaxial strain in the Sr_1−x La _x CuO₂ films. The RHEED data showed that the Sr_1−x La _x CuO₂ films grow on BaTiO₃/SrTiO₃ following a 2D or Stranski-Krastanov mechanism, depending on the La doping level. The transition point (critical thickness d _c) from layer-by-layer like (2D) to island (3D) growth depends on the film stoichiometry: decreasing the La doping concentration x from 0.15 to 0.10, the critical thickness d _c increases from ∼45 nm to ∼75 nm. In order to induce superconductivity, the Sr_1−x La _x CuO₂ films were cooled down under reduction conditions. The as-deposited films showed semiconducting or metallic behavior, the resistivity decreasing with increasing La concentration. Post-deposition vacuum annealing resulted in a superconducting transition onset (but no zero resistance down to 4.2 K) only for some of the x=0.15 Sr_1−x La _x CuO₂ films.     

Influence of nickel on iron-deficient spinel ferrites

A detailed study of the electric and magnetic properties of ceramic nickel-ferrites, Ni _x Fe_3−x−ΔO₄ withX≧0.8 and Δ≦10⁻² is presented. The physical interpretation of the results, the x-ray structural analysis and the magnetic aftereffects spectra allow us to define the substitution mechanism Ni↔Fe and explain the progressive variation of the properties in the samples with increasing concentrations of nickel.     

Anomalies of the electronic properties of alloys in the presence of variation of the local magnetic moments

The concentration dependences of the electronic properties (residual electrical resistivity, diffusion thermoelectric power, normal Hall effect, and low-temperature specific heat) and the magnetic characteristics (magnetization and paramagnetic susceptibility) of quasibinary (Pd_xPt_1−x )₃Fe, Pt₃Mn_xFe_1−x , (Pd_xAu_1−x )₃Fe, and Sc_xTi_1−x Fe₂ alloys are investigated. A relationship is established between the anomalous behavior of the kinetic properties and the variation of the local magnetic moments. The absence of corresponding anomalies in the concentration dependence of the specific heat indicates that the density of states at the Fermi level does not change significantly and, therefore, that the conventional Mott two-band model cannot be used to describe the anomalies in the properties of the alloys in question. A single interpretation of the sum-total of the experimental results is given on the basis of the theory of local fluctuations of the electron spin density in metal magnets. Fiz. Tverd. Tela (St. Petersburg) 39, 1257–1262 (July 1997)     

Electrical and magnetic properties of [(CoFeZr)x(Al2O3)1 ? x/(α-SiH)]n multilayer structures

The concentration dependences of the electrical resistivity and complex permeability of [“(Co₄₅Fe₄₅Zr₁₀) _x (Al₂O₃)_{100 − x} ”/“α-Si: H”] _n multilayer structures and (Co₄₅Fe₄₅Zr₁₀) _x (Al₂O₃)_{100 − x} composites have been studied. It has been established that introduction of a semiconductor interlayer into the (Co₄₅Fe₄₅Zr₁₀) _x (Al₂O₃)_{100 − x} composites substantially decreases the electrical resistivity of [“(Co₄₅Fe₄₅Zr₁₀) _x (Al₂O₃)_{100 − x} ”/“α-Si: H”] _n multilayer structures. The concentration dependences of the real and imaginary parts of the complex permeability of the [“(Co₄₅Fe₄₅Zr₁₀) _x (Al₂O₃)_{100 − x} ”/“α-Si: H”] _n nanomultilayer structures substantially differ from those of the (Co₄₅Fe₄₅Zr₁₀) _x (Al₂O₃)_{100 − x} composites. The real part of the complex permeability of the [“(Co₄₅Fe₄₅Zr₁₀) _x (Al₂O₃)_{100 − x} ”/“α-Si: H”] _n nanomultilayer structures follows the curve with a minimum near the percolation threshold of the composite, and the imaginary part smoothly decreases as the ferromagnetic phase concentration increases. The results obtained are explained by the increase in the bifurcation temperature due to the conduction electrons of the semiconductor interlayer, which favor magnetic ordering of ferromagnetic grains.