Dark electrical conductivity and photoconductivity of Ga4Se3S layered single crystals

Ga₄Se₃S layered crystals were studied through the dark electrical conductivity, and illumination- and temperature-dependent photoconductivity in the temperature region of 100-350 K. The dark electrical conductivity reflected the existence of two energy states located at 310 and 60 meV being dominant above and below 170 K, respectively. The photoconductivity measurements revealed the existence of another two energy levels located at 209 and 91 meV above and below 230 K. The photoconductivity was observed to increase with increasing temperature. The illumination dependence of photoconductivity was found to exhibit linear and supralinear recombination above and below 280 K, respectively. The change in recombination mechanism was attributed to the exchange in the behavior of sensitizing and recombination centers.     

Study of trapping and recombination centres in Tl2InGaTe4 chain crystals by dark electrical conductivity and photoconductivity measurements

Dark electrical conductivity and photoconductivity of Tl₂InGaTe₄ single crystals have been measured and analyzed in the temperature region 100–300 K. The dark electrical conductivity measurements revealed an intrinsic- or extrinsic-type of conductivity above or below 210 K, respectively. From intrinsic conductivity data analysis, the energy band gap of Tl₂InGaTe₄ crystals was determined as 0.85 eV. In the extrinsic region, the dark conductivity arises from a donor energy level located at 0.30 eV below the conduction band. The photocurrent increases with increasing illumination intensity. The recombination mechanism in the crystal changes as temperature decreases due to the effect of exponential trapping centres. Two trapping and/or recombination centres located at 89 and 27 meV were determined from the temperature dependence of the photocurrent, which decreased or increased with increasing temperature in the regions above or below 180 K, respectively.     

Space-charge-limited currents and photoconductive properties of Tl2InGaSe4 layered crystals

The extrinsic electronic parameters of Tl₂InGaSe₄ layered crystals were investigated through measurement of the temperature-dependent dark conductivity, space-charge-limited currents and photoconductivity. Analysis of the dark conductivity reveals the existence of two extrinsic energy levels at 0.40 and 0.51 eV below the conduction band edge, which are dominant above and below 260 K, respectively. Current–voltage characteristics show that the one at 0.51 eV is a trapping energy level with a concentration of (4.8–7.7) × 10¹⁰ cm^?3. Photoconductivity measurements reveal the existence of another energy level located at 0.16 eV. In the studied temperature range, the photocurrent increases with increasing temperature. The dependence of the photoconductivity on the incident light intensity exhibits a linear recombination character near room temperature and a supralinear character as the temperature decreases. The change in recombination mechanism is attributed to an exchange in the behavior of sensitizing and recombination centres.     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Anisotropic properties of self-flux grown LiB3O5 single crystals

In this paper, we report the existence of anisotropic behavior along the crystallographic axes in optical, electrical and thermal properties of lithium tri borate, a recently developed vacuum UV-NLO material. The variation of refractive index with the wavelength along the crystallographic axes was investigated by prism coupling method. The results of impedance spectroscopy measurement reveal the presence of a strong anisotropy in ionic conductivity and dielectric constant along the axes and also show the super-ionic conduction behavior along the c-axis with the activation energy of Δ∼0.20 eV. A thermo-mechanical study in the temperature range of 300-900 K indicates the existence of a strong variation in the linear thermal expansion coefficient (positive value along the a-axis, and negative value along the c-axis) of LiB₃O₅ crystals.     

Synthesis, electrical resistivity, thermo-electric power and magnetization of cubic ZnMnO3

Cubic ZnMnO₃ powder in the form of well-crystalline nanoflakes have been synthesized at low temperatures from a nitrate precursor. The electrical properties of cubic ZnMnO₃ samples have been established by DC resistivity (ρ) and thermo-electric power (Seebeck coefficient) measurements on a pressed pellet. The material exhibits insulator behavior with 0.7 eV acceptor ionization energy in the measured temperature range of 170-300 K. The thermo-electric power indicates a positive sign of the charge carriers. The obtained material exhibits a superparamagnetic signature with a blocking temperature of 9 K and the ZFC-FC splitting temperature of 15 K.     

Thermoelectric properties of SnzCo8Sb24 skutterudites

Sn-filled CoSb₃ skutterudite compounds were synthesized by the induction melting process. Formation of a single δ-phase of the synthesized materials was confirmed by X-ray diffraction analysis. The temperature dependences of the Seebeck coefficient, electrical resistivity and thermal conductivity were examined in the temperature range of 300-700 K. Positive Seebeck and Hall coefficients confirmed p-type conductivity. Electrical resistivity increased with increasing temperature, which shows that the Sn-filled CoSb₃ skutterudite is a degenerate semiconductor. The thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. The lowest thermal conductivity was achieved in the composition of Sn_0.25Co₈Sb₂₄.     

Heat treatment effects on the structural and electrical properties of thermally deposited AgIn5S8 thin films

The heat treatment effects on structural and electrical properties of thermally deposited AgIn₅S₈ thin films have been investigated. By increasing the annealing temperature of the sample from 450 to 500 K, we observed a change in the crystallization direction from (420) to (311). Further annealing of the AgIn₅S₈ films at 550, 600 and 650 K resulted in larger grain size in the (311) preferred direction. The room temperature electrical resistivity, Hall coefficient and Hall mobility were significantly influenced by higher annealing temperatures. Three impurity levels at 230, 150, and 78 meV were detected for samples annealed at 350 K. The electrical resistivity decreased by four orders of magnitude when the sample annealing temperature was raised from 350 to 450 K. The temperature dependent electrical resistivity and carrier concentration of the thin film samples were studied in the temperature ranges of 25-300 K and 140-300 K, respectively. A degenerate-nondegenerate semiconductor transition at approximately 180 was observed for samples annealed at 450 and 500 K. Similar type of transition was observed at 240 K for samples annealed at 600 and 650 K.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

Temperature dependence of AC-photoconductivity of HgI2 crystal

An ac impedance method has been used to study the electrical properties of an illuminated HgI₂ crystal as a function of temperature [10–350 K] and frequency [1–10₄ Hz]. The complex impedance plane plots enabled us to determine the bulk resistance of the crystal as a function of temperature. Activation energies of [0.08±0.005 eV] and [0.25 ±0.01eV] are then found; they are attributed to acceptor and donor trapping levels, respectively. At temperatures lower than 230 K, a weak temperature dependence of the bulk resistance is observed. This weak dependence is supposed to be due to photoconductivity.     

AgGaSe2 : A highly photoconductive material

High resistivity single crystals of AgGaSe₂ were grown by the horizontal Bridgman technique. The near band edge photoconductivity of the grown crystal at room temperature was found to be up to 2×10⁴ times higher than the dark conductivity, under the illumination of 10⁻³ W/cm². The photoconductivity spectrum consists primarily of three peaks, which are attributed to the transitions from Γ₇(A), Γ₆(B) and Γ₇(C) states of valence band to the conduction band Γ₆. The crystal field splitting and the spin-orbit splitting were determined from these peak energy positions of the photoconductivity spectrum.     

Effect of Zn substitution for Co on the transport properties of Y BaCo4O7

Electrical resistivity and Seebeck coefficients of Y BaCo_4−xZn_xO₇ (x=0.0,0.5,1.0,2.0) were investigated in the temperature range 350-1000 K. It was found that the electrical resistivity and activation energy increase with increasing Zn concentration, while Seebeck coefficients do not increase but decrease when electrical resistivity increases. We explained the increase of electrical resistivity and the drop of Seebeck coefficients for Zn-substituted samples by the decrease of carrier mobility, rather than of carrier concentration. The effect of oxygen absorption and desorption on the electrical resistivity and Seebeck coefficients was also investigated. An abrupt change of transport properties happens at about 650 K for x=0.0 and 0.5 samples measured in oxygen. For x=1.0 and 2.0 samples, however, such change disappears and the transport behavior in oxygen is almost same as that in nitrogen due to the significant suppression of oxygen diffusion caused by the higher Zn concentration in these samples.     

Energy band gap and oscillator parameters of Ga4Se3S single crystals

The optical properties of the Bridgman method grown Ga₄Se₃S crystals have been investigated by means of room temperature, transmittance and reflectance spectral analysis. The optical data have revealed an indirect allowed transition band gap of 2.08 eV. The room temperature refractive index, which was calculated from the reflectance and transmittance data, allowed the identification of the dispersion and oscillator energies, static dielectric constant and static refractive index as 21.08 and 3.85 eV, 6.48 and 2.55, respectively.     

Electrical conductivities of the Ag6PS5X and the Cu6PSe5X (X=Br, I) argyrodites

The Ag₆PS₅Br and Ag₆PS₅I argyrodites crystallize in a face-centered-cubic lattice at room temperature. Both compounds exhibit purely Arrhenius behavior throughout the temperature range 150-400 K with similar activation energies of about 0.23 eV. Cu₆PSe₅Br and Cu₆PSe₅I also crystallize in a face-centered-cubic structure at room temperature. Cu₆PSe₅Br exhibits a distinctive anomaly in electrical conductivity near 286 K while Cu₆PSe₅I undergoes a first-order electrical phase transition near 265 K. Their activation energies above room temperature are 0.13 and 0.30 eV, respectively.     

Electrical conductivity and thermoelectric power of AgInSe2 in the solid and liquid states

The electrical conductivity and thermoelectric power of AgInSe₂ have been investigated as a function of temperature from 420°C to 950°C. Experimental data are analyzed in terms of a model developed for the density of states and electrical transport in solid amorphous semiconductors [1]. The activation energy calculated from electrical conductivity data is found to be 0.06 eV for the solid and 0.37 eV for the liquid phase. Moreover, the coefficient of the linear decrease of the energy gap with temperature was found to be 1.96×10^–4 eV/K.     

SUPERCONDUCTING PROPERTIES AND STRUCTURAL PERFECTION OF EPITAXIAL YBa2Cu3O7-x THIN FILMS

The photoconductivity spectra of semimagnetic semiconductor Cd_0.7Mn_0.3Se have been measured at 50-300 K and in the wavelength range of 5500? to 8000?. It is found that the energy gap of Cd_0.7Mn_0.3Se changes with the temperature linearly, and the temperature coefficient (dE_g/dT) is about -7×10^-4 eV/K. A photoconductivity peak which is related to Mn²⁺ is also found. This peak is located around 1.85 eV, nearly unshifted with variation of the temperature. The possible transition mechanisms in Cd_1-xMn_xSe have been discussed in the light of group theory and crystal field theory, and the results are in good agreement with experiments.     

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.     

Structural,magnetic and electrical properties of Co1−xZnxFe2O4 synthesized by co-precipitation method

Nanoparticles of Co_1−xZn_xFe₂O₄ with stoichiometric proportion (x) varying from 0.0 to 0.6 were prepared by the chemical co-precipitation method. The samples were sintered at 600 °C for 2 h and were characterized by X-ray diffraction (XRD), low field AC magnetic susceptibility, DC electrical resistivity and dielectric constant measurements. From the analysis of XRD patterns, the nanocrystalline ferrite had been obtained at pH=12.5–13 and reaction time of 45 min. The particle size was calculated from the most intense peak (3 1 1) using the Scherrer formula. The size of precipitated particles lies within the range 12–16 nm, obtained at reaction temperature of 70 °C. The Curie temperature was obtained from AC magnetic susceptibility measurements in the range 77–850 K. It is observed that Curie temperature decreases with the increase of Zn concentration. DC electrical resistivity measurements were carried out by two-probe method from 370 to 580 K. Temperature-dependent DC electrical resistivity decreases with increase in temperature ensuring the semiconductor nature of the samples. DC electrical resistivity results are discussed in terms of polaron hopping model. Activation energy calculated from the DC electrical resistivity versus temperature for all the samples ranges from 0.658 to 0.849 eV. The drift mobility increases by increasing temperature due to decrease in DC electrical resisitivity. The dielectric constants are studied as a function of frequency in the range 100 Hz–1 MHz at room temperature. The dielectric constant decreases with increasing frequency for all the samples and follow the Maxwell–Wagner's interfacial polarization.     

Optical and electrical properties of thermally evaporated In49Se48Sn3 films

Nearly stoichiometric thin films of In₄₉Se₄₈Sn₃ were deposited at room temperature, by conventional thermal evaporation of the presynthesized materials, onto precleaned glass substrates. The microstructural studies on the as-deposited and annealed films, using transmission electron microscopy and diffraction (TEMD), revealed that the as-deposited films are amorphous in nature, while those annealed at 498 K are crystalline. The optical properties of the investigated films were determined from the transmittance and reflectance data, in the spectral range 650-2500 nm. An analysis of the optical absorption spectra revealed a non-direct energy gap characterizing the amorphous films, while both allowed and forbidden direct energy gaps characterized the crystalline films. The electrical resistance of the deposited films was carried out during heating and cooling cycles in the temperature range 300-600 K. The results show an irreproducible behavior, while after crystallization the results become reproducible. The analysis of the temperature dependence of the resistance (ln(R) vs. 1000/T) for crystalline films shows two straight lines corresponding to both extrinsic and intrinsic conduction. The room temperature I-V characteristics of the as-deposited films sandwiched between similar Ag metal electrodes shows an ohmic behavior, while non-ohmic behavior attributed to space charge limited conduction has been observed when the films are sandwiched between dissimilar Ag/Al metal electrodes.     

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La_0.5Bi_0.5MnO₃ ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn³⁺ and Mn⁴⁺ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.