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,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 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 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 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.     

Preparation and some properties of synthetic proustite single crystals

Conditions for the preparation of proustite single crystals are reported. For selecting these conditions temperature gradients in the crystallisation zone and in the melt, the rate of moving of the crystallisation front, the radial gradient, tube geometry, stabilisation of temperature, mechanisms of zone travelling, partial pressures were changed. – Differential-thermal, X-ray, and spectral-chemical analysis were applied to initial substances and final crystal bodies. To decrease the influence of spontaneous crystallisation an optimal temperature gradient was applied approaching maximum undercooling at the liquid-solid interface. – Optically transparent proustite crystals resulted. In the range from 1 to 12 μm they show 80% transparency. Microhardness was measured along the growth direction which deviated from the c-axis about 25°.     

Photodefinable carbon films: Electrical properties

Carbon images were prepared by the thermal decomposition of photodefined novalac resist (HPR-206) patterns. The electrical resistivity of the films decreased over 18 orders of magnitude as the pyrolysis temperature was increased to 1050°C. The electrical characteristics of the pyrolyzed photoresist patterns (carbon images) undergo a series of transitions over this temperature range; from insulating, to semiconducting, to semimetallic states. These properties may be understood in terms of the tunneling of charge carriers between isolated, conductive regions in the film where trigonal bonding dominates. This is the first example of the direct lithography of semiconducting and semimetallic features.     

Electrical properties of TiO2 thin films

The electrical properties of n-type titanium dioxide thin films deposited by magnetron-sputtering method have been investigated by temperature-dependent conductivity. We observed that the temperature dependence of the electrical conductivity of titanium dioxide films exhibits a crossover from T^?1/4 to T^?1/2 dependence in the temperature range between 80 and 110 K. Characteristic parameters describing conductivity, such as the characteristic temperature (T₀), hopping distance (R_hop), average hopping energy (Δ_hop), Coulomb gap (Δ_C), localization length (ξ) and density of states (N(E_F)), were determined, and their values were discussed within the models describing conductivity in TiO₂ thin films.     

Electrical and photoelectrical properties of chalcopyrite n-AgInS2 crystals

The chalcopyrite n-type crystals have been grown from AgInS₂ material having stoichiomtric Ag excess. The temperature dependence of the Hall effect in these crystals have been studied. The ionization energies of donors have been determined. The dependence of the photoconduction on the photon energy, the light intensity and the temperature in the n-AgInS₂ crystals have been measured. The recombination model of photoconductor with one class of recombination centres has been proposed for explanation of the photoelectrical results.     

Electrical properties of AgGaSe2 epitaxial layers

Epitaxial layers of AgGaSe₂ with thicknesses in the range from 50 to 70 nm were deposited onto (111)A-oriented semi-insulating GaAs substrates by flash-evaporation in the substrate temperature range T_S = 825 … 900 K. Films grown at T_S ≦ 850 K are n-type conducting whereas p-type conductivity was observed at T_S ≧ 875 K. In the p-type samples two acceptor states with ionization energies of 60 and 410 meV were found from an analysis of the electrical measurements.     

Growth and optical properties of high-density InN nanodots

High density InN/GaN nanodots were grown by pulsed mode (PM) metal–organic chemical vapor deposition (MOCVD). InN nanodots density of up to ∼5×10¹⁰ cm⁻² at a growth temperature of 550 °C was achieved. The high diffusion activation energy of 2.65 eV due to high NH₃ flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy (V_In) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E_a∼51 meV, indicating strong localization of carriers in the high density InN nanodots.