Electrical and optical properties of CuGaTe2

The electrical and optical properties of CuGaTe₂ single crystals were investigated by resistivity and Hall effect measurements in the temperatur range T = 77… 300 K and optical transmission measurements in the temperature range T = 20… 300 K at photon energies hν = 1.15…1.50 eV. All samples were p-type conducting due to shallow acceptors with ionization energies E_A1 ≈︁ 10⁻³ eV and concentrations N_A1 ≈︁ p = (2…4). 10¹⁸ cm⁻³. The absorption spectra could be described by simultaneous consideration of acceptor - to - conduction band transitions with E_A2 = 360 meV and N_A2 ≈︁ 10²² cm⁻³ and valence band - to - conduction band transitions with E_G = 1.24 eV at room temperaure. The temperature coefficient of the fundamental edge is dE_G/dT = −4.0. 10⁻⁴ eV/K. The results are discussed with regard to some general trends found in the Cu-III–VI₂ compounds.     

Electrical and optical properties of CdS nanocrystalline semiconductors

II‐VI semiconductor nanoparticles have recently attracted a lot of attention due to the possibility of their application in various devices. In the present study, chemical method has been used in synthesis of CdS nanoparticles and thiophenol was used as capping agent. X‐ray diffraction studies of both samples were done. The dc conductivity of CdS increases at a lower rate or is approximately constant upto 500K and thereafter the conductivity increases at a rapid rate. Beyond T_c it is seen that the portion of the σ_dc versus 1000/T is almost a straight line showing an Arrehenious behaviour. The dielectric constant of nanoparticles of CdS is found to be larger than the corresponding values of CdS crystals. It is clearly observed that at lower wavelengths nanocrystalline samples show a blue‐shift. The three peaks of sample (S2), A, Band C can be ascribed to the transition from Cd‐O complex donor formed by adsorbed oxygen to the valance band, Cd ‐ excess acceptor and the surface states, respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical,structural and optical properties of amorphous carbon

The planar and transverse electrical resistivity of amorphous carbon (a-C) films getter-sputtered at low temperature (77–95 K) is well-fitted by the expression $\text{? = ?}{}_0\text{exp}\text{(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ The exponent T₀ being approximately the same in both cases (≈ 7 × 10⁷ K) suggests that the amorphous films are isotropic. Films thinner than 600 Å display a two-dimensional hopping conductivity from which one deduces a density of states N(E_F) at the Fermi level of 10¹⁸ eV^?1 cm^?3 and a radius of the localized wave functions (a) of 12 Å. Tunneling experiments and optical absorption measurements are consistent with a pseudogap of approximately 0.8 eV. Electron diffraction experiments indicate that a-C films consist of a mixture of diamond and graphite bonds; this fact taken in the light of the other experiments would suggest that the graphite bonds act as the localized conduction states.     

Electrical and optical properties of Co doped TlGaS2 crystals

In this study, Co doped TlGaS₂ single crystals which belongs to the class of A^IIIB^IIIX₂^VI have been investigated by means of XRD, temperature dependent dark and illuminated conductivity, Space Charge Limited Currents and absorption measurements. The room temperature conductivity and trap concentration values were about 10^‐8 (Ω‐cm)^‐1 and 7.5 × 10¹³ cm^‐3, respectively. From the temperature dependent conductivity measurements, two activation energies namely 271 and 12 meV have been determined in the high and low temperature regions, respectively. The trap level at 271 meV that was determined by the dark temperature dependent conductivity measurement has also been verified by Space Charge Limited Currents analysis. The absorption measurements have showed that the layered compound had indirect and direct band gaps and the values were determined to be 2.49 and 2.56 eV, respectively. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Linear optical properties and Raman spectroscopy of natural fluorapatite

Precision refractive indices and their dispersion in the visible and near IR range (430 – 2400 nm) and thermal expansion (including its anisotropy and temperature dependence between 144 K and 673 K) were determined for gem‐quality crystals of fluorapatite from Durango, Mexico. In addition, results of a polarized Raman spectroscopy study on these crystals are given. Fluorapatite crystals show moderate values of thermal expansion with small anisotropy. The linear optical properties of the crystals allow no phase matching for third harmonic generation, but signalize, together with the Raman characteristics, a potential suitability of fluorapatite as nonlinear optical material for χ⁽³⁾‐based stimulated Raman scattering.     

Electrical and optical properties of chalcogenide amorphous semiconductors modified with Ni

The electrical and optical properties of the chalcogenide semiconductor (Se₃₂Te₃₂As₄Ge₃₂)_100?xNixitx have been studied. As the Ni concentration is increased the electrical dc conductivity is drastically increased and variable range hopping conduction becomes dominant even above room temperature. The optical energy gap decreases with the Ni concentration from 1.18–0.95 eV. Ni-atoms in the chalcogenide semiconductor donate free electrons which occupy the gap state. This occupation causes the shift of the Fermi level toward the conduction band. It is an effect of this shift that the thermal activation energy is decreased. The decrease in optical energy gap is independent of the shift of the Fermi level and is ascribable to the appearance of the additional level located at 0.95 eV above the top of the valence band. This level originates from the 3d-level of the Ni-atom.     

Electrical and optical properties of bismuth telluride/gallium nitride heterojunction diodes

Semiconducting bismuth telluride has a band gap energy of about 0.15 eV at room temperature and is a good material for middle wavelength IR (MWIR) detection [S.K. Mishra, S. Satpathy, O. Jepsen, J. Phys. Condens. Matter, p. 461]. We have grown bismuth telluride thin films on gallium nitride (on sapphire) by pulsed laser deposition at a substrate temperature of 300 °C. The structural characteristics and surface morphology of the films were studied by X-ray diffraction and scanning electron microscopy, respectively. The chemical composition of the as-deposited bismuth telluride thin films was determined by X-ray photoelectron spectroscopy and was found to be different from that of the bulk target, changing from stoichiometric Bi₂Te₃ to bismuth rich. A bismuth rich p-Bi₂Te₃/n-GaN/Al₂O₃ heterojunction was fabricated for photovoltaic detection of low energy photons. The wide band gap semiconducting n-GaN layer and the Al₂O₃ substrate act as a window for IR transmission. A sensitive IR photoresponse of the heterojunction is obtained by back-side illumination. The photocurrent measured is 0.18 μA according to the irradiation change in the current–voltage characteristics. The current–voltage characteristics allow us to evaluate the series resistance (R_s), zero-bias resistance (R₀) and ideality factors (n) of the junctions. Our results have suggested that bismuth telluride is a potential candidate for photovoltaic mid-infrared detection.     

Electrical and optical properties of titanium nitride coatings prepared by atmospheric pressure chemical vapor deposition

In order to examine the possibility for TiN coatings to be low-E, TiN coatings were deposited on the glass substrates by atmospheric pressure chemical vapor deposition using titanium tetrachloride (TiCl₄) and ammonia (NH₃) as precursors. X-ray diffraction, sheet resistance measurement, optical transmittance spectroscopy and infrared reflectance spectroscopy were carried out to determine the relationships between the preparation parameters and the microstructure, electrical and optical properties of the coatings. The results showed that the concentration of crystals increased with increasing the substrate temperature and the flow of TiCl₄, resulting in a decrease of the electrical resistivity. The optical transmittance of TiN thin films was strongly dependent on the gas flow and the substrate temperature. Under optimum conditions, continuous polycrystalline TiN coatings with FCC structure were obtained with an electrical conductivity around 34.5 Ω/□, an optical transmittance around 50% in the visible range, and an infrared reflectance higher than 50% above 3000 nm. This indicates that TiN coated glasses may be possible candidates for high IR reflectance windows.     

Electrical and optical properties of binary glasses from the arsenictelluriumselenium system

An investigation of binary glasses derived from the arsenicseleniumtellurium system is described in which measurements were made of d.c. conductivity over a range of temperatures, space charge limited current, density and absorption in the near infrared region. For the higher temperature region the electrical results were consistent with conduction being due to carriers excited into extended states. While conduction at lower temperatures may in some cases be due to carriers hopping in localized states at the band edges, an alternative approach in which the localized states were considered to act as donors and acceptors gave satisfactory agreement with the experimentally determined conductivity versus temperature and activation energy versus temperature relationships and with the value of the pre-exponental constant in the conductivity expression. An anomalous field dependence of conductivity was found for annealed specimens of two glasses and it is tentatively suggested that this is associated with the creation of an impurity band. Measurements of space charge limited current showed that gold gave ohmic contacts to the glass, whereas silver and copper did not. The measurements also provided evidence for the localized states having an exponential distribution. Changes of electrical properties and density as a function of glass composition are discussed in the light of possible structural changes taking place in the glasses. The absorption edges of the glasses in the infrared region were found to be exponential in form and had slopes that did not very greatly for the range of compositions studied.     

Electrical and optical properties of light emitting devices with quantum dots dispersed in PVK matrix

ABSTRACT

In this study, we fabricated QD-LEDs with QD emitters dispersed in poly(9-vinylcarbazole) (PVK) matrix. The emission layer was prepared by spin-coating of the PVK solution dispersed with red QD nanoparticles. The concentration of QDs was varied to be 0.5, 2.0, 5.0, and 10.0 mg/ml in PVK solution of 25 mg/ml. As the QD concentration increases, the current efficiency of QD-LED gradually increases and the emission from PVK matrix decreases. In addition, we investigated energy transfer system with QD and organic dopant material using same structure in emissive layer. We obtained a maximum current efficiency of 4.1 cd/A and an external quantum efficiency of 3.0% were achieved at a QD concentration of 5.0 mg/ml in QD-LEDs.     

Electrical and infrared optical properties of CdIn2S4 single crystals grown by chemical transport

Single crystals of CdIn₂S₄ were grown by chemical transport with iodine as transporting agent using different transporter concentrations and temperature gradients. It is found that the electron concentration increases with increasing transporter concentration and decreases after annealing in a sulphur atmosphere. Infrared reflectivity and absorption spectra are measured at room temperature in the wavenumber range from 180 to 4000 cm⁻¹ in order to determine the optical mode parameters of the compound and to evaluate the scattering mechanism of the electrons from the free carrier absorption. Lattice scattering is found to be predominant at room temperature for electron concentrations below about 4 · 10¹⁷ cm⁻³.     

Electrical, structural and optical properties of SnO2 thin films prepared by spray pyrolysis

This study investigated the effect of the substrate temperature on the structural, optical, morphological, and electrical properties of undoped SnO₂ films prepared by a spray deposition method. The films were deposited at various substrate temperatures ranging from 300-500 °C in steps of 50 °C and characterized by different optical and structural techniques. X-ray diffraction studies showed that the crystallite size and preferential growth directions of the films were dependent on the substrate temperature. These studies also indicated that the films were amorphous at 300 °C and polycrystalline at the other substrate temperatures used. Infrared and visible spectroscopic studies revealed that a strong vibration band, characteristic of the SnO₂ stretching mode, was present around 630 cm⁻¹ and that the optical transmittance in the visible region varied over the range 75-95% with substrate temperature, respectively. The films deposited at 400 °C exhibited the highest electrical conductivity property.     

Electrical properties of phosphate glasses

The electrical properties of glasses in the Na₂OP₂O₅, Ag₂OP₂O₅ and (1?x)Na₂OxAg₂OP₂O₅ systems have been measured over a range of temperature and composition.The properties of the Na₂OP₂O₅ and Ag₂OP₂O₅ glasses have been compared within the phosphate system as well as with silicate glasses. The silver-containing glasses show higher conductivity and lower temperature coefficients when compared with the sodium-containing glasses. A maximum in the room temperature resistivity of the (1?x)Na₂O?xAg₂O?P₂O₅ system was found around the mole ratio of 0.16:0.84 Ag₂O:Na₂O, indicating a mixed-alkali effect. A similar effect was seen in the tan δ, but not in the T_g-against-composition plots. A linear relationship was noted for the tan δ-versus-log₁₀ (resistivity) plot, as has been seen in other glass-forming systems.     

Electrical properties of p-Type GaAs

Relations for the effective drift mobility, the Hall coefficient, and the Hall mobility in p-type group IV and III–V compounds are derived accounting for the degenerate valence band structure and acting scattering mechanisms. Improved deformation potentials are determined for holes in p-type GaAs. Zn-doped and Ge-doped p-type GaAs samples with hole concentrations in the range from 4 · 10¹⁷ to about 10²⁰ cm⁻³ were analysed with regard to the temperature dependence of the Hall mobility, and it was found that the Brooks-Herring formula is inadequate to describe ionized impurity scattering in p-type GaAs.     

Electrical properties of n-Type GaN

The electron mobility in GaN is calculated as function of the carrier and impurity concentrations. From a comparison of the theoretically calculated mobilities with electrical parameters measured experimentally it follows that native donor and acceptor concentrations of the same order of magnitude are present in undoped GaN.     

Tubular chains in the structures of natural and synthetic silicates

Tubular chains in the structures of natural and synthetic silicates have been described and systematized using the theory of graphs and by unfolding the tube topology onto a plane. Eight types of tubular chains have been considered: seven silicon-oxygen chains and one mixed titanosilicate chain; their topological and geometric characteristics are analyzed. It is shown that the presence of large cavities in tubes is generally related to their occupation by large low-valence cations (K⁺, Cs⁺, Ba²⁺, Sr²⁺, etc.) and water molecules. The overwhelming majority of minerals containing tubular silicate fragments were found in hydrothermal veins of alkaline massifs in Russia and abroad.     

Electrical and optical properties of RF-SP a-Ge:O:(H) deposited from GeO2

The electrical and optical properties of a-Ge:O:(H) prepared from sintered GeO₂ are investigated. Unlike SiO₂, GeO₂ can be reduced with H₂ gas. The properties of a-Ge:O:(H) thus prepared can be changed widely with the H₂ gas pressure, approaching those of a-Ge. It is found that, in addition to these changes, a configurational change from GeOGe to oxygen-vacancy complex occurs with increasing the H₂ gas pressure.     

Electrical properties of a-Ge-Se-In thin films

Steady state and transient photoconductivity has been measured on Ge₂₀Se_80−xIn_x (x = 0, 5, 10, 15, 20) vacuum evaporated thin films. Study of temperature dependent dark conductivity σ_d and photoconductivity σ_ph measurements in the temperature range 303-375 K, shows that the conduction in this glass is through an activated process having single activation energy. The activation energy value of photoconduction is smaller in comparison to activation energy in dark. The photosensitivity shows a maximum value at 10 at.% of In concentration. This is attributed to the decrease in the density of defect states of Ge-Se alloy with increase of In content. The results of intensity dependent steady state photoconductivity σ_ph follow a power law with intensity (F), i.e. σ_ph α F^γ where the value of power γ lies between 0.5 and 1.0, suggesting bimolecular recombination. Rise and decay of photocurrent for different concentration of In shows that photocurrent rises monotonically to the steady state value and the decay of photocurrent is also very fast. An attempt has been made to explain the results on the basis of defects and density of states.     

Electrical properties of bismuth nanocrystal-glass composite

Composites of bismuth nanocrystals and glass matrix have been obtained from Bi_0.33Ge_0.67O_1.84 and Bi_0.57Si_0.43O_1.72 glasses as a result of thermal treatment in hydrogen atmosphere. We have shown that the temperature dependence of the conductivity of bismuth germanate glass containing Bi nanocrystals, reduced in hydrogen exhibits crossover from T^−1/4 to T^−1/2 dependence in the temperature range between 170 and 190 K. Characteristic parameters describing conductivity were determined and their values were discussed within the models describing conductivity in granular metals. Conductivity in Bi-GeO₂ system may be described within the model of variable-range hopping between localized centers including Coulomb gap. On the other hand, in Bi-SiO₂ system the ES model is not a good explanation of conductivity. We suggest that Zvyagin’s model of conduction by hopping via virtual states (no Coulomb gap effects) may quite well describe conductivity in this system with high concentration of grains.     

Electrical properties of vanadate-glass threshold switches

The electrical properties of thick-film vanadate-glass switches, with both planar and sandwich geometries, have been investigated using a remote double-pulse technique. The effect on delay time of pulse separation, overvoltage and polarity were examined. No forming step was required for any of the devices. Photographic evidence of filament formation confirmed the thermal model of switching suggested by these experiments. Additional evidence, such as the effect of temperature on threshold voltage and electrical conductivity, established the importance of the semiconductor-metal transition in the VO₂ present in the material for the switching action. A computer simulation based on the discontinuous change in electrical conductivity, occurring at 68°C, and due to this transition, was shown to be in broad agreement with the experimental facts.