Semiconduction in xFe2O3.(1-x) [3B2O3.PbO] glasses

The electrical resistivity of iron lead borate glasses was measured over the temperature range 300–700 K. The resistivity increases with the iron content and is a function of the Fe²⁺/Fe_tot ratio, but the minimum does not appear for 0.28 ≤ c ≤ 0.50. The samples with x > 15 mol % Fe₂O₃ show the presence of two activation energies for conduction. A change in the activation energy can be explained by charge transfer between iron ions in similar positions at low temperatures, and between iron ions in different positions at higher temperatures. In order to analyse the conductivity data, we have considered in all the glasses a polaronic model for conduction.     

Electrical investigations in the normal and incommensurate phases of [N(CH3)4]2ZnCl4 single crystals

The dielectric permittivity (?) of TMA-ZC single crystals was measured along the mean crystallographic axes a, b and c, in a temperature range from 273 to 340 K. The ?-T relationship exhibited peak values at T_i=296 K and T_c1=279 K for the three axes. These peaks are attributed to the contribution of discommensurations. The d.c. and a.c. electrical conductivity showed anomalous variation at the same transition temperatures, with a remarkable change in the value of the activation energy around the transition temperatures. The j-E characteristic indicates different types of electrical conduction. The mechanism of the phase transition and the electrical process were discussed on the basis of Shottky and Frenkel conduction mechanisms.     

Thermopower in the region of hopping conduction in TlNiS<Subscript>2</Subscript>

Samples of the composition TlNiS₂ in the hexagonal system with the unit cell parameters a=12.28 Å, c=19.32 Å, and ρ=6.90 g/cm³ are synthesized. The results of the investigation into the electrical and thermoelectrical properties of TlNiS₂ samples in the temperature range 80–300 K indicate that TlNiS₂ is a p-type semiconductor. It is found that, at temperatures ranging from 110 to 240 K, TlNiS₂ samples in a dc electric field possess variable-range-hopping conduction at the states localized in the vicinity of the Fermi level. The density of localized states near the Fermi level is determined to be N_F=9×10²⁰ eV^?1 cm^?3, and the scatter of the states is estimated as J≈2×10^?2 eV. In the temperature range 80–110 K, TlNiS₂ exhibits activationless hopping conduction. At low temperatures (80–240 K), the thermopower of TlNiS₂ is adequately described by the relationship α(T)=A+BT, which is characteristic of the hopping mechanism of charge transfer. In the case when the temperature increases to the temperature of the onset of intrinsic conduction with the activation energy ΔE=1.0 eV, there arise majority intrinsic charge carriers of both signs. This leads to an increase in the electrical conductivity σ and, at the same time, to a drastic decrease in the thermopower α; in this case, the thermopower is virtually independent of the temperature.     

The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.     

Calorimetric studies of the crystallization process in a-Se75S25−xAgx chalcogenide glasses

Calorimetric studies of amorphous Se₇₅S_25−xAg_x (x = 2, 4, 6 and 8) chalcogenide glasses are made at different heating rates (5, 10, 15 and 20 K/min) under non-isothermal condition using Differential scanning calorimetry. The values of glass transition temperature and crystallization temperature are observed to be composition and heating rate dependence. From the heating rate dependence of glass transition temperature and crystallization temperature, the activation energy for structural relaxation (ΔE_t), the activation energy of crystallization (ΔE_c) and the order parameter (n) have been calculated. It is observed that Se₇₅S₁₉Ag₆ has a minimum value of activation energy for structural relaxation (ΔE_t), which indicates that this particular glass has a larger probability to jump to a state of lower configurational energy and higher stability in the glassy region. On the basis of the obtained experimental data the temperature difference (T_c − T_g) is found to be maximum for Se₇₅S₁₉Ag₆, which further indicate that this glass is the thermally most stable in the entire composition range of investigation.     

On the Electrical and Optical Properties of Some Poly(Azomethine Sulfone)s in Thin Films

Poly(azomethine sulfone)s were synthesized by reacting 4,4′-sulfonyl bis(4-chlorophenyl) with 2,2-bis(4-hydroxyphenyl)propane and azomethine bisphenol in different molar ratios. Thin films were deposited from solution onto glass substrates. Study of the temperature dependences of the electrical conductivity, σ, and Seebeck coefficient, S, were performed in the temperature range 300 K–500 K. Thermal activation energies of electrical conduction, E_a , calculated from these dependences, ranged between 1.50 eV and 1.85 eV. The values of E_a were smaller for polymers with extended conjugation systems. The possibility to use the polymers in thermistor technology is discussed. The aspect of the temperature dependences of σ and S shows that a model based on the energy band-gap representation can be successfully used for explaining the electronic transport mechanism in the higher temperature range. In the lower temperature range, the mechanism of the electrical conduction is discussed in terms of the Mott variable range hopping conduction. The values of some optical parameters (absorption coefficient, optical band gap, etc.) were determined from transmission spectra.     

Differential scanning calorimetric study of Ga5Se95 glass

Results of differential scanning calorimetry, at different heating rates, α, on Ga₅Se₉₅ glass are reported and discussed. From the heating rates dependence of values of T_g and T_p, the glass activation energy, E_g and the crystallization activation energy, E_c, are derived. The crystallization results are interpreted in terms of recent analyses developed for non-isothermal crystallization and also for the evaluation of E_c. The crystallization mechanism is then characterized. From the obtained results, the glassy Ga₅Se₉₅ has two-dimensional growth, the average value of the order of crystallization mechanism, n is 3. The average value of the glass activation energy, E_g and crystallization activation energy, E_c, for Ga₅Se₉₅ glass are 189±4 and 69±5 kJ/mol, respectively.     

Electrical conduction and 1ƒ noise in Li-DOPED MnO

Electrical resistivity, thermoelectric power and current noise were measured on Li-doped MnO single crystals in the temperature range from 300 to 1000 K. Below 700 K the crystals are p-type and the activation energy of the resistivity is 0.75 eV. Around 700 K the activation energy changes from 0.75 to 1.25 eV owing to a change from p- to n-type conduction. The depth of the Li acceptor is found to be 0.65 eV. From resistivity and thermoelectric power data it is concluded that the bandgap in first approximation can be written as E_s(T) = E_o ? γT between 750 and 1000 K, with E_o = 1.9 eV and γ = 6 × 10^?4 eV/K. The current noise spectra show $\text{1}\text{?}$ noise. The magnitude of the $\text{1}\text{?}$ noise is strongly temperature dependent. From the noise data it is deduced that E_o = 2.2 eV and γ = 10^?3 eV/K in the temperature range 430–700 K.     

Electrical transport characteristics of ZnO–Bi2O3–B2O3 glasses

Optically clear glasses in the ZnO–Bi₂O₃–B₂O₃ (ZBBO) system were fabricated via the conventional melt-quenching technique. Dielectric constant and loss measurements carried out on ZBBO glasses unraveled nearly frequency (1 kHz–10 MHz)-independent dielectric characteristics associated with significantly low loss (D?=?0.004). However, weak temperature response was found with temperature coefficient of dielectric constant 18?±?4 ppm °C^?1 in the 35–250 °C temperature range. The conduction and relaxation phenomena were rationalized using universal AC conductivity power law and modulus formalism respectively. The activation energy for relaxation determined using imaginary parts of modulus peaks was 2.54 eV which was close to that of the DC conduction implying the involvement of similar energy barriers in both the processes. Stretched and power exponents were temperature dependent. The relaxation and conduction in these glasses were attributed to the hoping and migration of Bi³⁺ cations in their own and different local environment.     

DC conductivity of new single and mixed alkali oxyfluorovanadate glasses

New glasses have been prepared according to these formulas (70-x)V₂O₅-30BaF₂-xAF, where AF=LiF or NaF and (60-x)V₂O₅-30BaF₂-10LiF-xAF, where AF=NaF and x=10, 15, 20, 25 and 30 mol%. Density of the glasses was measured and molar volume calculated and they correlated with the AF content. The dc conductivity has been measured in the temperature range from 302 to 453 K. The dc conductivity increases with temperature and V₂O₅ content, while it decreases with the alkali fluoride content. Conductivity has been correlated with the calculated polaron distance, R, and glass transition temperature, T_g. The activation energy, W, increases with the increase in the alkali fluoride, while it decreases with the V₂O₅ content. Some parameters like polaron distance, R, polaron radius, r_p, ion concentrations, n(V), n(Li) or n(Na), hopping energy, W_H, density of localized states at Fermi level, N(E_F), polaron coupling constant, γ_p, polaron band width, J, hopping mobility, μ, and carrier density for electronic conduction, N_c, were calculated to explain the conduction mechanism and behavior of the present glasses.     

The temperature dependence of optical gap and photoconductivity threshold in undoped hydrogenated amorphous silicon films

We report on the temperature dependences of the optical gap E_o and the photoconductivity threshold (?ω)_o for undoped hydrogenated amorphous silicon films. When increasing the temperature, both E_o and (?ω)_o are seen to linearly decrease at respective rates β= 3.5 10^?4 eV K^?1 (temperature range 290 K–460 K) and γ= 5.2 10^?4 eV K^?1 (temperature range 220 K – 360 K). At higher temperatures E_o decreases at the rate β = 14.3 10^?4 ev K^?1. Our results are discussed in terms of conduction in extended states. We show there is no physical reason in relating the temperature dependence of the activation energy and that of the gap as generally assumed. From optical absorption we deduce a minimum metallic conductivity σ_min the value of which agrees with Mott's predictions. On the contrary, σ_min measured from dark conductivity is nearly two orders of magnitude lower. A discussion is proposed infering band bending at the film substrate interface.     

Effect of composition and thermal annealing on the absorption and electrical conduction of Se85−xTe15Sbx glasses

The optical absorption of the as-prepared and thermally annealed Se_85−xTe₁₅Sb_x (0≤x≤9) thin films was measured. The mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap (E₀) decreased from 1.12 to 0.84 eV with increasing Sb content of the as-prepared films from 0 to 9 at.%. The as-prepared Se₇₆Te₁₅Sb₉ films showed an increase in (E₀) with increasing the temperature of annealing in the range above T_g (363 K). The electrical conductivity of the as-prepared and annealed films was found to be of Arrhenius type with temperature in the range 300-360 K. The activation energy for conduction was found to decrease with increasing both the Sb content and temperature of annealing. The results were discussed on the basis of the lone-pair electron effect and of amorphous crystalline transformation.     

Memory switching of germanium tellurium amorphous semiconductor

The dc conductivity and switching properties of amorphous GeTe thin film of thickness 262 nm are investigated in the temperature range 303-373 K. The activation energy ΔE_σ, the room temperature electrical conductivity σ_RT and the pre-exponential factor σ₀ were measured and validated for the tested sample. The conduction activation energy ΔE_σ is calculated. The I-V characteristic curves of the thin film samples showing a memory switching at the turnover point (TOP) from high resistance state (OFF state) to the negative differential resistance state (NDRS) (ON state). It is found that the mean values of the threshold electrical field E_th decreased exponentially with increasing temperatures in the investigated range. The switching activation energy ΔE_th is calculated. Measurements of the dissipated threshold power P_th and the threshold resistance R_th were carried out at TOP point at different temperatures of the samples. The activation energies ΔE_R and ΔE_P caused by resistance and power respectively are deduced. The results obtained support thermal model for initiating switching process in this system.     

Transport parameters of n-type GaSb

The conduction band parameters of GaSb have been investigated in the temperature region 70–280 K. Hall and magnetoresistance data have been analysed according to the two band model to obtain information about the energy separation ΔE₁₂ of the conduction band minima and its temperature coefficient α₂.     

Thermal kinetics and short range order parameters of Se80X20 (X = Te,Sb) binary glasses

Bulk Se₈₀Te₂₀ and Se₈₀Sb₂₀ glasses were prepared using the melt–quench technique. Differential scanning calorimetry (DSC) curves measured at different heating rates (5 K/min≤α≤50 K/min) and X-ray diffraction (XRD) are used to characterize the as-quenched specimens. Based on the obtained results, the activation energy of glass transition and the activation energy of crystallization (E _g, E _c) of the Se₈₀Te₂₀ glass are (137.5, 105.1 kJ/mol) higher than the corresponding values of the Se₈₀Sb₂₀ glass (106.8, 71.2 kJ/mol). An integer n value (n=2) of the Se₈₀Te₂₀ glass indicates that only one crystallization mechanism is occurring while a non-integer exponent (n=1.79) in the Se₈₀Sb₂₀ glass means that two mechanisms are working simultaneously during the amorphous–crystalline transformations. The total structure factor, S(K), indicates the presence of the short-range order (SRO) and the absence of the medium-range order (MRO) inside the as-quenched alloys. In an opposite way to the activation energies, the values of the first peak position and the total coordination number (r ₁, η ₁), obtained from a Gaussian fit of the radial distribution function, of the Se₈₀Te₂₀ glass are (2.42 nm, 1.99 atom) lower than the corresponding values (2.55 nm, 2.36 atom) of the Se₈₀Sb₂₀ specimens.     

Quantum theory of nonresonant electromagnetic and ultrasonic absorption in glasses

We develop quantum theory of nonresonant ultrasonic and electromagnetic absorption in glasses at low temperatures. In the quantum region where ?ω?kT the nonresonant absorption coefficients are proportional to ω³ which seems to be in agreement with the existing experimental data.The existence of characteristic temperature T_c (or characteristic energy E_c = kT_c) of the order of 10 + 20 K is established. At higher interlevel spacing E the concept of two-level systems in their conventional form is not applicable because of their strong coupling to the phonons. Neither the perturbation theory is applicable for calculation of absorption in the frequency interval ?ω?_c or at temperature interval T?T_c = E_c/k.     

Electrical studies on bulk Se96Sn4 semiconducting glass before and after gamma irradiation

Bulk Se₉₆Sn₄ chalcogenide glass was prepared by melt quenching technique and irradiated by different doses of 4, 8, 12, 24 and 33 kGy using ⁶⁰Co gamma emitter. I-V characteristics were obtained for this glass, before and after gamma irradiation, in the temperature range 200-300 K. Ohmic behavior was observed at low electric fields (≤1×10⁴ V/m), while at higher fields, a deviation from ohmic towards non-ohmic behavior was observed. The plots of ln(I/V) vs. V were found to be straight lines and the slopes of these lines decrease linearly with temperature indicating the presence of SCLC. In the temperature range of measurements, the dependence of DC conductivity on temperature at low electric field shows two types of conduction channels, one in high temperature range 270-300 K and the other at low temperature range 200-270 K. Analysis of the experimental data shows that the conductivity at room temperature decreases with increase in irradiation dose. This is attributed to rupturing of SnSe_4/2 structural units, upon irradiation, and rebuilt of Se atoms between Se chains. This redistribution of bonds, induced by gamma irradiation, is responsible for the corresponding increase in the activation energy. The obtained values of the activation energy indicate that the conduction occurs due to thermally assisted charge carriers movement in the band tail of localized states. However, in the low temperature range, results obtained from Mott’s variable range hopping (VRH) model reveal that the density of localized states has its maximum value at a gamma dose of 12 kGy, while the disorder parameter T_o, hopping distance R_hop and hopping energy W have their minimum value at this particular dose.     

Diffusion of gallium into cadmium sulphide

Measurements of the Ga diffusion into CdS, in the presence of exces Ga metal, using optical and mixroprobe analyser techniques are reported. A reaction layer of CdGa₂S₄ forms on the CdS crystals below 1240±20 K. Above this temperature the reaction layer is liquid. The Ga diffusion is concentration dependent and also orientation dependent with the faster diffusion perpendicular to the hexagonal c-axis of CdS. The anisotropy of the activation energy was calculated to be 0.20±0.06 eV. In the temperature range 940–1240K the linearly concentration dependent diffusion yielded activation energy values for defect motion of 2.39±0.13 eV perpendicular to the c-direction and 2.21 ±0.13 eV parallel to the c-direction.     

Microwave electrophysical properties of superionic crystals Ag2HgI4

The paper concerns itself with the investigation results of temperature dependencies of electric conduction and dielectric properties of Ag₂Hgl₄ crystals in the frequency range of 10⁷–7,8·10¹⁰ Hz. The obtained data have shown that in α-phase at T=326 K, the electric conductivity σ is proportional v^0,28 in the frequency range of 10⁷–10⁹ Hz and σ is proporti onal v^0,5 in the range of (1,1–78)·10⁹ Hz. The dependence σ(v) in the range of (1,1–78)·10⁹ Hz may be conditioned by the jumping mechanism of the conductivity and low frequency oscillations of the crystal lattice. It is believed, tha in the σ-phase of Ag₂Hgl₄ a condition of the existence of the ionic polaron is satisfied. The activation energy of the polaron is ΔE_p=0,09 eV.     

Optical properties of the ternary compound Tl Sb S2

We present an investigation of the near band-gap optical properties of TlSbS₂ between 2 and 300 K. We use both transmission and reflectivity measurements. The resolution of the first exciton line permits to obtain an accurate determination of the temperature coefficients of both the direct band-gap E₀ and the second threshold E₁. The absorption curves have been fitted according to the Toyozawa's model. We find a strong interaction with a phonon mode of energy 22 meV for both the E₀ and E₁ thresholds. The low temperature reflectivity spectra reveal clearly several direct transitions in the range 1.5–5.5 eV. All these structures have been identified as transitions between the highest valence band and the lowest conduction band.