The variation of normalized electrical resistivity in the system of glasses Ge_{15}Te_{85-x}Sn_{x} with (1 ≤ x ≤ 5) has been studied as a function of high pressure for pressures up to 9.5 GPa. It is found that with the increase in pressure, the resistivity decreases initially and shows an abrupt fall at a particular pressure, indicating the phase transition from semiconductor to near metallic at these pressures, which lie in the range 1.5-2.5 GPa, and then continues being metallic up to 9.5 GPa. This transition pressure is seen to decrease with the increase in the percentage content of tin due to increasing metallicity of tin. The semiconductor to near metallic transition is exactly reversible and may have its origin in a reduction of the band gap due to high pressure.
The problem of the effective ohmic junction and the question of the barrier height for thin film structures of Al/a-Si:H/n^{+}c-Si/Al and Al/a-Si:H/n^{+}c-Si/Mo are studied. Current-voltage and temperature characteristics were measured and possible mechanisms of conductivity were extracted and discussed.
Te_{42}As_{36}Ge_{10}Si_{12} chalcogenide composition was prepared by conventional melt-quenching. The ac conductivity and the dielectric properties were carried out in the frequency range 0.5×10^{3}-4×10^{6} Hz and temperature range 300-423 K. The analysis of the experimental results of the frequency dependence of ac conductivity σ_{ac}(ω) indicates that σ_{ac}(ω) is proportional to ω^{s} where s> 1. The temperature dependence of both ac conductivity and the parameter s is reasonably well interpreted by the correlated barrier hopping model. The maximum barrier height W_{m} calculated from ac conductivity and the density of localized states were determined. Values of dielectric constant ε_{1} and dielectric loss ε_{2} were found to decrease with frequency and increase with temperature. The analysis of dielectric loss leads to determine the barrier height W_{m} and agrees with that proposed by the theory of hopping of charge carriers over potential barrier between charged defect states as suggested by Elliott in case of chalcogenide glasses.
In the (Bi,Pb)-Sr-Ca-Cu-O system, materials of various electrical and superconducting properties may be produced by a glass-ceramic method. Phase composition, structure, microstructure and also electrical properties of the material change as a result of heat treatment. Depending on the heat treatment conditions, either a superconductor with the critical temperature between 8 and 105 K or material without a superconducting transition may be obtained. The properties of the material change so much because during annealing three oxide superconductors belonging to the bismuth family are formed. (Bi_{0.8}Pb_{0.2})_4Sr_3Ca_3Cu_4O_x glass was prepared by a standard technique of quenching homogenized and melted substrates. The glass-ceramic samples were obtained by annealing of the glass beneath melting temperature. Structure and microstructure of glass-ceramic samples were studied with scanning electron microscopy and X-ray diffraction method. Superconducting properties were studied by means of electrical conductivity and magnetization measurements. In this paper we present the influence of the phase composition, structure and microstructure on the electrical conductivity in the normal state and the superconducting properties of (Bi,Pb)_4Sr_3Ca_3Cu_4O_x glass-ceramics.
Cd_{5}Se_{95-x}Zn_{x} (x=0, 2, 4, 6) chalcogenide semiconductors were prepared by conventional melt-quenching and were characterized by X-ray diffraction, scanning electron microscopy, and Fourier transform infrared studies. Ac conductivity of Cd_{5}Se_{95-x}Zn_{x} chalcogenide semiconductor has been investigated in the frequency range of 1 kHz-1 MHz and in the temperature range of 290-370 K. The analysis of the experimental results indicates that the ac conductivity is temperature, frequency and concentration dependent. Ac conductivity is found to obey the power law ω^{s} where s < 1. A strong dependence of ac conductivity and exponent s can be well interpreted in terms of correlated barrier hopping model. The maximum barrier height W_{m} were calculated from the results of dielectric loss according to the Guintini equation that agree with the theory of hopping of charge carriers over potential barrier as suggested by Elliot in case of chalcogenide semiconductors.
(Bi,Pb)-Sr-Ca-Cu-O glasses, annealed in proper conditions, transform into a granular metal and superconductor. As a result of annealing oxide superconductors belonging to the bismuth family (Bi,Pb)_2Sr_2CuO_x, (Bi,Pb)_2Sr_2 CaCu_2O_x, and (Bi,Pb)_2Sr_2Ca_2Cu_3O_x crystallize. (Bi,Pb)-Sr-Ca-Cu-O glass-ceramic samples were obtained by annealing the amorphous solid at temperatures between 500°C and 870°C. Their microstructure was studied with scanning electron microscopy, atomic force microscopy, and X-ray diffraction. The temperature dependence of resistivity in annealed samples was studied in a temperature range from 3 K to 300 K. In this work we present the influence of the microstructure on the electrical properties of the granular and disordered material composed of the 2201 and 2212 grains embedded in the nonmetallic matrix.
The ac conductivity and dielectric properties of Ge_{15}Se_{60} X_{25} (X = As or Sn) thin films are reported in this paper. The thin films were deposited by thermal evaporation at 10^{-5} Torr pressure. The films were well characterized by X-ray diffraction, differential thermal analysis and energy dispersive X-ray spectroscopy. The ac conductivity was measured over temperature range 303-413 K and frequency range 10^2-10^5 Hz. The frequency dependence of the ac conductivity was found to be linear with slope which lies very close to unity and is independent of temperature. This behavior can be explained in terms of the correlated barrier hopping between centers forming intimate valence alternation pairs. Values of the dielectric constant ε_1 were found to decrease with frequency and increase with temperature. The maximum barrier height W_{m} for each sample, which was calculated from the dielectric measurements according to the Guinitin equation, agrees with the theory of hopping of charge carriers over potential barrier as suggested by Elliott in case of chalcogenide glasses. The density of localized state near the Fermi level was estimated for the studied films.
The effect of annealing at different temperatures between T_{g} and T_{c} on the AC conductivity and dielectric properties was studied for Se_{70}Te_{15}Bi_{15} films grown by thermal evaporation technique. The films were characterized by X-ray diffraction, differential thermal analysis, and energy dispersive X-ray spectroscopy. X-ray diffraction analysis shows the occurrence of amorphous to polycrystalline transformation for films annealed at annealing temperature T_{a} ≥ 473 K. AC conductivity σ_{AC}(ω) was studied as a function of T_{a}, frequencies (0.1-100 kHz) and working temperatures (303-393 K). It was found that σ_{AC}(ω) obeyed Aω^{s} law. According to the values of s and its temperature dependence, the AC conduction mechanism was determined in terms of the correlated barrier hopping and quantum mechanical tunneling models for the as deposited and annealed films, respectively. The DC and AC activation energies were determined as a function of T_{a}. Values of dielectric constant ε_1 and dielectric loss ε_2 were found to increase with increasing T_{a}. A Debye-like relaxation of dielectric behavior was observed for polycrystalline films, and was found to be a thermally activated process.
The temperature dependence of the DC and AC electrical conductivity were measured for Ge_{1}Se_{1.35}Tl_{0.1} films. The value of DC electrical conduction energy ΔE_{σ} does not depend on film thickness in the investigated range with mean value of 0.72eV. The AC conductivity σ_{AC} is related to frequency by the expression σ_{AC}=Aω^{S}, where S is the frequency exponent which decreases linearly with increasing temperature. This can be explained in terms of the pair (bipolaron) correlated barrier hopping model suggested by Elliott. The frequency and temperature dependence of real and imaginary parts of the dielectric constant were studied for Ge_{1}Se_{1.35}Tl_{0.1} films. The dielectric constant (real part) and the dielectric loss (imaginary part) increase with increasing temperature and decrease with increasing frequency in the investigated range of frequency and temperature. The maximum barrier height W_{M} can be calculated according to the Giuntini equation at different temperatures. The obtained value of W_{M} is in good agreement with the theory of hopping of charge carriers over a potential barrier as suggested by Elliott in case of chalcogenide glasses.
Thin films of Se_{90}Sb_{10-x}Ag_x (x=0, 2, 4, 6, 8) glasses have been prepared by vacuum evaporation technique. Present study reports the quantitative estimation of light induced defects in aforesaid thin films by using thermally stimulated current technique. Measurements have been made in a vacuum ≈10¯³ Torr before and after exposing amorphous films to white light for different exposure times (0 to 6 h). Results indicate that light induced defects are created due to prolonged exposure of light and this is explained by a microscopic model proposed by Shimakawa and co-workers. It is also found that fractional increase in light induced defect density decreases as Ag concentration increases. A discontinuity has, however, been observed at 4 at.% of Ag which is explained in terms of average coordination number.
The transport properties of CuO-Bi_{2}O_{3} pellets (pressed at room temperature 303 K) like dc electrical conductivity etc. are measured. The activation energy, dielectric constant etc. are reported. The hopping conduction is examined. Non-adiabatic hopping conduction is observed. The plot of -log σ versus 1/T is found to be linear. Activation energy of pellets containing CuO (80, 70, 60 mol%) shows electronic conduction while the pellet containing CuO (50 mol%) shows ionic conduction. The effect of content of CuO (mol%) and frequency on dielectric constant is also studied. The variation of dielectric constant with CuO (mol%) shows zigzag nature and may be due to the relaxation effects.
In this work we study the main differences between the superconducting properties of the Bi_4Sr_3Ca_3Cu_4O_x and (Bi_{0.8}Pb_{0.2})_4Sr_3Ca_3Cu_4O_x glass-ceramics. The Bi_4Sr_3Ca_3Cu_4O_x and (Bi_{0.8}Pb_{0.2})_4Sr_3Ca_3Cu_4O_x glass-ceramics prepared in the same conditions contain similar amounts of the superconducting 2212 phase, however their superconducting and normal-state electrical properties differ significantly. The main reason which makes BiSrCaCuO glass-ceramics worse superconductor than BiPbSrCaCuO is its microstructure.
SrBi_2Nb_2O_9 (SBN) is a bismuth layered perovskite compound, due to its relatively high Curie temperature, has potential application as high ceramic transducer. Also, it is an attractive ferroelectric material that is being considered in non-volatile random access memory cells. Present article describes preparation, dielectric, impedance and modulus characteristics. Temperature and frequency dependence of dielectric permittivity, impedance and electric modulus of barium strontium bismuth niobate (Ba_{0.1}Sr_{0.9}Bi_2Nb_2O_9, BSBN) have been studied in the range of 35-590°C and 45 Hz-5 MHz, respectively. The structural analysis of compound revealed orthorhombic at room temperature. Complex impedance Cole-Cole plots are used to interpret the relaxation mechanism. These plots shows the relaxation behavior as non-Debye type. By using the Cole-Cole plots grain and grain boundary contributions towards conductivity have been estimated. From electrical modulus formalism polarization and conductivity relaxation behavior in BSBN have been discussed. DC and AC conductivity measurements have been performed on BSBN.
We present results of in situ temperature measurements of resistivity for some amorphous or partially crystalline Heusler alloy films: Co_{2}CrAl, Co_{2}MnGa and off-stoichiometric Ni_{2}Mn_{1+x}Sn_{x}, Ni_{2}Mn_{1-x}Ga_{x} that are known to exhibit half-metallic properties and martensitic transformations, respectively. From ρ vs. T characteristics we distinguish various stages of chemical and structural ordering in the films. They appear to be quite distinct in both systems investigated. The resistivity results are compared with magnetic characteristics for Co_{2}MnGa with a high Curie temperature.
The I(V) characteristics of amorphous chalcogenides usually show a negative differential conductance region, which makes the material switch from a high-resistive to a high-conductive state. This feature is of the utmost importance for adopting these materials in the manufacturing of solid-state memory devices. We propose here two complementary models for the interpretation of the switching mechanism, both stemming from and updating the literature analysis. The former is based on macroscopic equations that can be solved analytically; the latter is a current-driven three-dimensional Monte Carlo simulation of the device. A critical analysis of the two models is also performed in order to identify the fundamental conditions accounting for the voltage snap-back of the I(V) curve.
Zn-O-N thin films fabricated by reactive radio frequency magnetron sputtering have been investigated for their compositional, structural, transport and optical properties. In contrast to processes in which the reaction for either the oxide or the nitride is dominant, the multireaction process yields a substantially amorphous films with the Hall mobility within the range from 15 to 80 cm²/(V s). In addition, it has been observed that the Hall mobility increases for Zn-O-N. Since it has a narrower bandgap than ZnO, it is put forward that the high mobility is due to the valence band maximum in this material lying above the trap states in the gap commonly observable in ZnO. These traps originate from oxygen vacancies and are localized at the bottom of the band gap influencing the carrier mobility.
Thin films of hydrogenated amorphous Si-Ge alloys were obtained by r.f. sputtering in Ar + H_{2} gas atmosphere using composite targets of Si and Ge. Dark conductivity and photoconductivity were measured in the temperature range of 300-500 K for films with x varying from 0.11 to 0.63. Both dark conductivity and photoconductivity exibit activation type dependences in the temperature range studied. Heterogeneity two-phase model and a model based on Fermi level shift with temperature were invoked to discuss the conduction mechanism.
The parameters of potential well, which arises around inhomogeneities of technological origin in n-InP, have been analyzed using electrical measurements data. Model of spherical space-charge regions surrounding disordered regions was applied for explanation of results and found to be in fair agreement with experimental data. Comparison of experimental data with theoretical computations displays scattering of current carriers due to the disordered regions in n-InP additional to lattice and impurity ions scattering.
In-Ga-Zn-O thin films were fabricated by means of reactive RF magnetron sputtering. Mechanism of free electrons generation via oxygen vacancies formation is proposed to determine the relationship between oxygen content in the deposition atmosphere and the transport properties of IGZO thin films. The depletion-mode a-IGZO thin film transistor with field-effect mobility of 12 cm^2/(Vs) has been demonstrated.
Zinc oxide-based extrinsic composite was investigated. The sample was selected from a series of components of one production batch, prepared by standard sintering technology. The content of extrinsic elements in ZnO base was determined by SEM. Van der Pauw method with four-point electrode fixture was used for study of conducting phenomena in square-shaped sample. It is normaly preferred to assume the symmetric uniformity of the electrical properties of sample, for which sheet resistance, bulk resistivity and Hall mobility, sheet carrier density and carrier concentration can be calculated. When the uniformity of measured parameters is breached, the anisotropy in the arrangement of the internal structure may be the cause. There remains the question of whether the extrinsic ZnO material can be isotropic, regarding the electrical conductivity. Although the Hall effect has been measured, preliminary measurements indicate the presence of anisotropy in the measured samples. Before measurement the following phenomena should be taken into account: magneto-electric effect, photo-electric effect and the isothermal condition should be preserved. Paper discusses the uniformity deviations for the defined setup configurations for positive and negative magnetic field directions. Bulk resistivity has been calculated by numerical solution of van der Pauw equation. Large offset voltage during the measurement is discussed.
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