Electrical properties of a-Ge<Subscript>x</Subscript>Se<Subscript>100-<Emphasis Type="Italic">x</Emphasis></Subscript>

In general, the conductivity in chalcogenide glasses at higher temperatures is dominated by band conduction (DC conduction). But, at lower temperatures, hopping conduction dominates over band conduction. A study at lower temperature can, eventually, provide useful information about the conduction mechanism and the defect states in the material. Therefore, the study of electrical properties of Ge_xSe_100-x in the lower temperature region (room temperature) is interesting. Temperature and frequency dependence of Ge_xSe_{100-x (x} = 15, 20 and 25) have been studied over different range of temperatures and frequencies. An agreement between experimental and theoretical results suggested that the behaviour of germanium selenium system (Ge_xSe_100-x ) have been successfully explained by correlated barrier hopping (CBH) model.     

Electrical conduction of SnBi4Se7

Polycrystalline samples of Bi₂Se₃ and stoichiometric ternary compounds in the quasi-binary system SnSe-Bi₂Se₃ were characterized by measurements of temperature and field dependence of electrical conductivity. The current density–electric field characteristics were found to be non-linear, especially when the applied electric field exceeds a certain value which is dependent on the temperature T. Furthermore, the electrical conductivity can be enhanced by the applied electric field. The characteristic length a(T) seemed to be enhanced with increasing temperature. Electrical conductivity measurements elucidated the semiconducting behaviour of both compounds, especially when the temperature of measurement exceeds a certain value for SnBi₄Se₇, and hopping and band type conduction are dominant at low and high ranges of temperature, respectively. Below 200 K, the electrical conductivity of SnBi₄Se₇ decreases with increasing temperature. Meanwhile, additional scattering and hopping seemed to characterize the behaviour of SnBi₄Se₇ due to the Sn doping of Bi₂Se₃ resulting in additional states at the Fermi level. PACS 72.20.-i; 72.15.-v     

Structure formation and mechanisms of DC conduction in thermally evaporated nanocrystallite structure ZnIn2Se4 thin films

ZnIn₂Se₄ is of polycrystalline structure in as synthesized condition. It transforms to nanocrystallite structure of ZnIn₂Se₄ film upon thermal evaporation. Annealing temperatures influenced crystallite size, dislocation density and internal strain. The hot probe test showed that ZnIn₂Se₄ thin films are n-type semiconductor. The dark electrical resistivity versus reciprocal temperature for planar structure of Au/ZnIn₂Se₄/Au showed existence of two operating conduction mechanisms depending on temperature. At temperatures >365 K, intrinsic conduction operates with activation energy of 0.837 eV. At temperatures <365 K, extrinsic conduction takes place with activation energy of 0.18 eV. The operating conduction mechanism in extrinsic region is variable range hopping. The parameters such as density of states at Fermi level, hopping distance and average hopping energy have been determined and it was found that they depend on film thickness. The dark current–voltage characteristics of Au/n-ZnIn₂Se₄/p-Si/Al diode at different temperatures ranging from 293–353 K have been investigated. Results showed rectification behavior. At forward bias potential <0.2 V, thermionic emission of electrons from ZnIn₂Se₄ film over a potential barrier of 0.28 V takes place. At forward bias potential >0.2 V, single trap space charge limited current is operating. The trap concentration and trap energy level have been determined as 3.12×10¹⁹ cm⁻³ and 0.24 eV, respectively.     

Charge carriers and mobility of Cu-Ti ferrite

The concentration and drift mobility of charge carriers in Cu_1–x Ti _x Fe₂O₄ ferrite are calculated, over a wide range of temperatures (300–773 K), employing d.c. conductivity and thermoelectric power data. With increasing temperature the concentration of charge carriers decreases whilst the drift mobility exhibits an exponential increase. Over the above-mentioned temperature range, the obtained density of charge carriers varies between 10²¹ and 10²² cm^–3 whereas the drift mobility has values between 10^–8 and 10^–4 cm²/V s. The results are discussed on the basis of a small-polaron hopping conduction. The activation of the d.c. conductivity has been attributed to the thermal activation of the mobility.     

Electrical conduction and optical properties of amorphous (Sb2Se3)100−xSnx thin films

The variation of DC electrical conductivity and the optical properties of thermally evaporated a- (Sb₂Se₃)_100−xSn_x thin films with temperature have been studied. It is found that the thermal activation energy decreases, while the optical gap first increases (up to x=1) and then decreases, with the increase in Sn content. These results have been explained by taking into consideration the structural modifications induced by the incorporation of Sn into the parent alloy. The variation in the conductivity prefactor (σ_o) with Sn addition indicates a change in the dominant conduction transport mechanism from extended states to localized states. An experimental correlation between the activation energy and the pre-exponential factor has been observed, indicating the validity of Meyer–Neldel rule in the studied samples.     

Electrical properties of nickel-doped arsenic trisulphide

Results of temperature and frequency dependent a.c. conductivity of pure and nickel-doped a-As₂S₃ are reported. The a.c. conductivity of pure As₂S₃ obeys a well-known relationship: σ_ac ∝ω ^s. Frequency exponents is found to decrease with increasing temperature. Correlated barrier hopping (CBH) model successfully explains the entire behaviour of a.c. conductivity with respect to temperature and frequency for pure As₂S₃. But a different behaviour of a.c. conductivity has been observed for the nickel doped As₂S₃. At higher temperatures, distinct peaks have been observed in the plots of temperature dependence of a.c. conductivity. The frequency dependent behaviour of a.c. conductivity (σ_ac ∝ω ^s) for nickeldoped As₂S₃ is similar to pure As₂S₃ at lower temperatures. But at higher temperatures, ln σ_ac vs lnf curves have been found to deviate from linearity. Such a behaviour has been explained by assuming that nickel doping gives rise to some neutral defect states (D ^0′) in the band gap. Single polaron hopping is expected to occur between theseD ^0‘ andD ⁺ states. Furthermore, allD ⁺,D ^0′ pairs are assumed to be equivalent, having a fixed relaxation time at a given temperature. The contribution of this relaxation to a.c. conductivity is found to be responsible for the observed peak in the plots of temperature dependence of a.c. conductivity for nickel-doped As₂S₃. The entire behaviour of a.c. conductivity with respect to temperature and frequency has been explained by using CBH and “simple pair” models. Theoretical results obtained by using these models, have been found to be in agreement with the experimental results.     

Dielectric-spectroscopic and a.c. conductivity studies on layered Na2-XKXTi3O7 (X=0.2, 0.3, 0.4) ceramics

The dependence of loss tangent (tanδ) and relative permittivity (ε_r) on temperature and frequency has been reported for Na_2-XK_XTi₃O₇ (with X=0.2, 0.3, 0.4) ceramics. The losses are characteristic of dipole mechanism and electrical conduction. The peaks of ε_r at high temperature indicate a possible ferroelectric phase transition for all three compositions. The results of a.c. conductivity studies on the same samples have also been reported. The corresponding ln(σT) versus 1000/T plots have been divided into five regions namely I, II, III, IV and V. The various conduction mechanisms in the different regions have been stressed. Furthermore, the log(σ) versus frequency plots for all the above samples reveal that the electronic hopping (polaron) conduction, which diminishes with the rise in temperature, is dominant in the lower temperature region. The interlayer ionic conduction seems to play a major role in conduction towards higher temperature.     

Changes in electrical properties in α- and γ-exposed PADC track detector

The changes in the dielectric properties and temperature dependence of the d.c. conductivity of α-exposed poly allyl diglycol carbonate (PADC) have been studied. On α-irradiation the dielectric constant (′) as a function of frequency has been found to decrease significantly. The temperature dependence of resistivity in pristine and γ-irradiated samples is of the form ρ(T)=ρ_∝ exp(T₀/T) which can be attributed to conduction of thermally generated carriers. In case of (γ+α) irradiated samples the temperature dependence of resistivity is of the form ρ(T)=ρ_∝ exp(T₀/T)^1/2 which is due to one-dimensional hopping of carriers.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Transport properties in non-oxide perovskite ferroelectric relaxors

Electric transport in a Cu-doped Cd salt [(CH₂)₃(NH₃)₂Cd_{1? x} Cu _x Cl₄, x?=?0.0, 0.07, 0.395 and 0.69] was investigated in the frequency range 60?Hz–100?kHz and the temperature range 290–450?K. The conductivity increases with increasing copper doping. Samples with x?=?0.0 and 0.07 undergo phase transitions at 374?K and 369?K, respectively. Ferroelectric relaxor-like behaviour appears for x?=?0.395 and 0.69. The conduction mechanism of the samples with x?=?0.0 and 0.07 depends on the temperature region. Below the transition temperatures chlorine vacancy and proton hopping prevails, whereas above the transition temperatures mainly proton conduction dominates. Transport in the new non-oxide ferroelectric relaxors, where x?=?0.395 and 0.69, can be explained by the jump relaxation model where proton and ionic hopping contribute to the conductivity throughout the whole temperature range.     

Manifestation of MnCl2 fillers incorporated into the polymer matrices in their dielectric properties

Polyethyl methacrylate (PEMA) films filled with different mass fractions of MnCl₂ were prepared using a casting method. The structural and electrical properties were studied. The filling content dependence of certain IR absorption bands was correlated with the obtained physical parameter characterizing the other properties. DC electrical resistivity (ρ) was measured in the temperature range 340-420 K for PEMA films filled with MnCl₂ fillers. An intrachain one-dimensional interpolaron hopping mechanism was assumed to interpret the electrical conduction. AC conductivity behavior of all the prepared samples was investigated over the frequency range (42-5M) Hz and under different isothermal stablilization in the temperature range 300-423 K. It suggested that the hopping mechanism might be playing an important role in the conduction process, in low temperature regime. The values of σ₀, A, and S satisfying the suitable fit of the conductivity data, as well as the corresponding (σ_DC).     

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.     

Effect of impurity (Sb and Ag) incorporation on the a.c. conductivity and dielectric properties of a-Se70Te30 glassy alloy

The effect of additives (Sb and Ag) on a.c. conductivity and dielectric properties of Se₇₀Te₃₀ glassy alloy at temperature range 300-350 K and frequency range 1 kHz-5 MHz has been studied. Experimental results indicate that a.c. conductivity and dielectric parameters depend on temperature, frequency and the impurity incorporated in Se-Te glassy system. The a.c. conductivity in the aforesaid frequency range is found to obey the ω^s law. A strong dependence of a.c. conductivity and exponent s in the entire temperature and frequency range contradicts quantum-mechanical tunneling (QMT) model and can be interpreted in terms of the correlated barrier hopping (CBH) model. The temperature and frequency dependence of the dielectric parameters are also studied and it is found that the results agrees by the theory of hopping of charge carriers over potential barrier as suggested by Elliott in chalcogenide glasses. The change in the dielectric parameters with the opposite influence of the replacement of Te by Sb on the one hand, and by Ag, on the other hand is being correlated by the nature of covalent character of the studied composition and with the change in density of defect states.     

Effect of iron doping on the characterization and transport properties of calcium phosphate glassy semiconductors

X-ray diffraction (XRD), differential scanning calorimeter (DSC), density (d) and dc conductivity (σ) of the glasses in Fe₂O₃-CaO-P₂O₅ system were reported. The dc conductivity in the temperature range 303-453 K was measured. The overall features of these XRD curves confirm the amorphous nature of the present samples. The density of glasses increases from 2.750 to 2.892 g/cm³ with increasing Fe₂O₃ content as a result of a strengthening of cross-linking within glass network. The glass temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The glasses had conductivities ranging from 10⁻⁹ to 10⁻⁵ Sm⁻¹ at temperatures from 303 to 453 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.     

Magnetic and electric properties of antimony selenide (Sb2Se3) crystals

The magnetic susceptibility in the three principal crystallographic directions, and the electrical conductivity and thermoelectric power parallel and perpendicular to the basal plane of Sb₂Se₃ single crystals, have been measured over the temperature range 100–550 K. The different results obtained from the magnetic and electric studies have been accounted for on the basis of non-stoichiometry of the compound and localized lone electrons. A hopping mechanism in the higher temperature region simultaneously with impurity excitation is suggested.     

Deconvolution of temperature dependence of conductivity,its reduced activation energy,and Hall-effect data for analysing impurity conduction in n-ZnSe

ABSTRACT

The temperature dependence of the reduced activation energy w?=?ε/k_BT of the conductivity σ has been utilised for determining the impurity conduction mechanism in doped semiconductors in many studies. Herein, the formula for deconvoluting w when plural conduction mechanisms appear is used to confirm the analysis of the data of the Hall-effect measurements on Al-doped n-ZnSe samples. The analysis is performed on the basis of an impurity-Hubbard-band model which includes ε ₂ conduction in the top Hubbard band as well as ε ₃ and Efros-Shklovskii (ES) variable-range hopping (VRH) conduction processes in the bottom Hubbard band. As the result of the analysis, transitions among the three hopping conduction mechanisms of ε ₂, ε ₃, and ES VRH are clearly shown in the temperature dependence of w as well as in that of the Hall mobility, which are hardly noticed in the temperature dependence of σ. In addition, the power-law exponent of the prefactor of ES VRH conductivity is determined through the fit to the temperature dependence of w to show that it decreases from ～ 1.5 to ～ 0 with increasing net donor concentration.     

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe₂ single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe₂ single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T?210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, N _F=1.3×10¹⁹eV·cm³)^?1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe₂ single crystals increases from 130 Å to 170 Å as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe₂ single crystals is investigated.     

Structural and electrical characterization of MxNb3Se4 (M = Cu, H)

The compounds Cu_xNb₃Se₄ (0≤x≤0,45) and H_xNb₃Se₄ (0≤x≤2·10⁻³) were prepared by electrochemical titration from Nb₃Se₄. The samples were characterized by X-ray analysis and q-probe conductivity measurements as a function of temperature. The Cu-compound is isostructural with Nb₃Se₄ for 0≤x≤0.2 and shows new phases for 0,2≤x≤0,45. The H-compound shows an impurity controlled conductivity in the temperature range from 20 to 200 °C and an intrinsic type conductivity in the temperature range from 330 to 450 °C. The activation engines are 0 and 0.15 eV, respectively. Rapid proton conduction in H_xNb₃Se₄ makes it difficult to control the composition as demonstrated by exposure of the samples to different atmospheres. An increasing H-concentration decreases drastically the conductivity by several orders of magnitude.     

Non-adiabatic polaron hopping conduction in CaMn1−xCrxO3 (0?x?0.3)

DC electrical conductivity (σ_dc) of electron-doped antiferromagnetic CaMn_1−xCr_xO₃ (0?x?0.3) has been discussed elaborately in the light of polaron hopping conduction. The increase in Cr doping concentration increases the conductivity and decreases the activation energy. Non-adiabatic polaron hopping conduction is observed in all the manganites at high temperatures. The analysis of σ_dc data shows that small polarons are formed at lower concentrations (?5%) of Cr doping and undoped samples. However, large polarons are materialized at higher doping (?10%) concentrations. This is consistent with the fact that doped Cr³⁺ has larger ionic size compared to that of Mn⁴⁺. Again, strong electron-phonon (e-ph) interaction is perceived in undoped and 5% Cr-doped samples but not in manganites with larger doping concentration. This also confirms the formation of larger polarons with the increase of x. Mott's variable range hopping (VRH) model can elucidate the dc conductivity at very low temperatures. It has been detected that single phonon-assisted hopping is responsible for the dc conduction in the Cr-doped CaMnO₃ manganites.     

Conductivity and dielectric studies of Li2SnO3

Lithium stannate (Li₂SnO₃) has been prepared by solution evaporation method. The precursor obtained is sintered at 800°C for 5, 6, and 7?h, respectively. X-ray diffractogram confirmed that the sample obtained after sintering is Li₂SnO₃. The pelletized Li₂SnO₃ after heating at 500?°C for 3?h is used for electrochemical impedance spectroscopy characterization. Impedance measurements have been carried out over frequency range from 50?Hz to 1?MHz and temperature range from 563 to 633?K. The conductivity?Ctemperature relationship is Arrhenian. Several important parameters such as activation energy, ionic hopping frequency and its rate, carrier concentration term, mobile ion number density, ionic mobility, and diffusion coefficient have been determined. The characteristics of log conductivity and log ionic hopping rate against temperature for the system suggest that the conduction and ionic hopping processes are thermally activated. The values of activation energy for conduction and relaxation processes as well as activation enthalpy for ionic hopping are about the same.