Anomalies in the thermal and electrical conductivity of CuIn5Se8 crystals

Anisotropy, asymmetry, and other anomalies of electrical and thermal properties are revealed in CuIn₅Se₈ crystals. It is shown that the nature of these anomalies is associated with the natural coexistence of order and disorder in these ordered-defect compounds.     

Effect of deviations from stoichiometry on electrical conductivity and photoconductivity of CuInSe2 crystals

The results of studying the photoelectric properties of bulk CuInSe₂ crystals grown at various deviations from the stoichiometry are reported. The crystals were grown by the Bridgman method. The concentration and mobility of charge carriers were measured. The dependence of the properties of CuInSe₂ crystals with n-and p-type conductivities on the deviation of composition from the stoichiometry is discussed.     

Photoluminescence of CdTe crystals grown by physical-vapor transport

High-quality CdTe crystals with resistivities higher than 10⁸ Ω cm were grown by the physical-vapor transport (PVT) technique. Indium, aluminum, and the transition-metal scandium were introduced at the nominal level of about 6 ppm to the source material. Low-temperature photoluminescence (PL) has been employed to identify the origins of PL emissions of the crystals. The emission peaks at 1.584 eV and 1.581 eV were found only in the In-doped crystal. The result suggests that the luminescence line at 1.584 eV is associated with Cd-vacancy/In complex. The intensity of the broadband centered at 1.43 eV decreases strongly with introduction of Sc.     

X-ray conductivity of ZnSe single crystals

The experimental I–V and current–illuminance characteristics of the X-ray conductivity and X-ray luminescence of zinc-selenide single crystals feature a nonlinear shape. The performed theoretical analysis of the kinetics of the X-ray conductivity shows that even with the presence of shallow and deep traps for free charge carriers in a semiconductor sample, the integral characteristics of the X-ray conductivity (the current–illuminance and I–V dependences) should be linear. It is possible to assume that the nonlinearity experimentally obtained in the I–V and current–illuminance characteristics can be caused by features of the generation of free charge carriers upon X-ray irradiation, i.e., the generation of hundreds of thousands of free charge carriers of opposite sign in a local region with a diameter of <1 μm and Coulomb interaction between the free charge carriers of opposite signs.     

Precipitation in CdTe crystals studied through mie scattering

Below gap optical losses in as-grown n-type CdTe crystals were analyzed in terms of free carrier absorption and Mie extinction due to Te precipitates. Experimental absorption spectra measured between 2 to 20 μm exhibited the well-known free carrier absorption behavior α_FCA∼λ^x with x=3 due to scattering by polar optical phonons. In shorter wavelength regions below 6 μm, however, additional contributions to the light loss due to absorption and scattering by precipitates were also observed. Assuming a log-normal size distribution, the precipitate extinction spectra were calculated according to Mie theory within the electric and magnetic dipole and electric quadrupole approximation. A comparison with the experimental spectra identifies the precipitates and enables estimation of their sizes and total number density. In this investigation, both undoped and In-doped CdTe crystals grown from stoichiometric melts by vertical asymmetric Bridgman method were used. It was found that In doping, in general, suppresses Te precipitation. At high doping level (melt containing∼10¹⁹ In atoms cm⁻³), the formation of In₂Te₂ is also indicated. It is demonstrated that the Mie extinction analysis offers an, expedient method to rapidly analyze the precipitates in CdTe and in similar other wide gap materials in a nondestructive manner.     

Electrical properties and low-temperature photolumincesence of Si-doped CdTe crystals

The CdTe:Si single crystals with Si concentration in the range of C _Si ⁰ =2×10¹⁸–5×10¹⁹ cm^?3 are grown by the Bridgman-Stockbarger method. The samples were of the n-and p-type with electrical conductivity σ=2×10^?1–8×10^?9 Ω^?1 cm^?1. Being heated in the temperature range 300–440 K, the p-CdTe crystals were annealed, and their conductivity decreased. The shape of the low-temperature (5–20 K) photoluminescence spectra of the samples are indicative of their high structural quality. The specific feature of the emission of the CdTe:Si crystals is its decrease in the intensity of all lines induced by donors as the samples are cut progressively closer to the ingot top. The results obtained indicate that the Si impurity, in contrast with Ge, Sn, and Pb, does not exhibit the compensating and stabilizing effect in CdTe.     

Features of the dark conductivity of zinc selenide

The experimentally obtained I–V dependences of the dark conductivity of zinc-selenide single crystals have a nonlinear character. The contact phenomena, injection currents, and the Poole-Frenkel effect are considered among the possible causes of such behavior of the I–V characteristics. As a result of investigation, the following is established: (i) the contact between the n-ZnSe single crystal and metallic In is ohmic; (ii) in low fields, the I–V characteristic obeys Ohm’s law; (iii) in strong fields, the dark conductivity of the investigated samples is determined by the Poole-Frenkel effect. Also Poole-Frenkel theory is supplemented for different potentials of interaction of the local center with the charge carrier.     

Generalized character of the dielectric response of CdTe crystals grown from the melt

It was found that the specific features of the photodielectric response of CdTe crystals grown from the melt are determined by ensembles of macroscopic growth defects. The analysis of diagrams ?*(λ) and ?*(X), which characterize the dependences of the complex dielectric constant on the wavelength and coordinates, as graphic images of sequences of induced states of the crystal was carried out. It was demonstrated that such analysis allows for the identification of the ensembles of macroscopic defects, which are the sources of internal fields.     

Transport and noise properties of CdTe(Cl) crystals

Experimental studies of transport and noise characteristics of CdTe (Cl doped) crystals, prepared by travelling heater method, have been carried out. The basic material is of p-type with p=1.8×10¹⁴ m⁻³, μ_h=0.0065 m² V⁻¹ s⁻¹, μ_e=0.13 m² V⁻¹ s⁻¹. The current and noise spectral density was measured as a function of the sample illumination, voltages across the sample and incident light wavelengths. Two types of effective charge carrier mobility are assumed: namely, the effective transport mobility, which is 0.065 m² V⁻¹ s⁻¹ and the effective noise mobility, which reaches a value of 0.125 m² V⁻¹ s⁻¹, both for high illumination. Under the same conditions, the density of light generated charge carrier pairs is 1.7×10¹⁵ m⁻³. Experimental results are in a good agreement with the four-level recombination model. The values of 1/f noise parameter α range from 4×10⁻⁴ to 2.5×10⁻³. The α parameter grows with almost the photocurrent square root. The signal-to-noise ratio improves if the electric field strength in the CdTe detector is set to a higher value.     

A practical analysis of the electrical conductivity of blood

Recent developments in indicator-dilution measurement of pulmonary edema have generated new interest in the use of electrical-conductivity sensing for measurement of indicator concentrations in blood. This approach has always suffered from the lack of an appropriate and validated model of the dependence of electrical conductivity on blood composition and indicator concentration. Such a model is developed here, based in part on a review of earlier work on variation with hematocrit and on recognition of the profound effect on conductivity of even transient alterations of blood temperature by the indicator itself. Shifts of water into and out of erythrocytes, in response to osmotic pressures, are properly included. The model predicts approximately linear changes in blood conductivity'over a large range of indicator concentrations (e. g., independent nonlinearity of 2.6 percent for 3 percent saline over a 0-30 percent concentration in blood). Changes in resistivity can be nonlinear, particularly with hypertonic indicators (e. g., independent nonlinearity of 7.9 percent for 3 percent saline over a 0-15 percent concentration in blood). Overlooking the thermal and osmotic effects can lead to significant errors in signal interpretation; consequent errors in flow rate determinations can be greater than 10 percent. Model predictions are verified in vitro with various indicators and canine blood using a miniature tetrapolar conductivity cell of our own design. The multicomponent nature of most conductivity indicators and the associated implications are discussed.     

Temperature dependences of the electrical parameters of anisotype NiO/CdTe heterojunctions

The I–V characteristics of NiO/CdTe heterostructures fabricated by reactive magnetron sputtering are measured at different temperatures. It is established that current transport through the NiO/CdTe heterojunction is mainly controlled via generation–recombination and tunneling under forward bias and via tunneling under reverse bias. The investigated heterostructures generate an open-circuit voltage of V _oc = 0.26 V and a short-circuit current density of I _sc = 58.7 μA/cm² at an illumination intensity of 80 mW/cm².     

On the flow dependency of the electrical conductivity of blood

Experiments presented in the literature show that the electrical conductivity of flowing blood depends on flow velocity. The aim of this study is to extend the Maxwell-Fricke theory, developed for a dilute suspension of ellipsoidal particles in an electrolyte, to explain this flow dependency of the conductivity of blood for stationary laminar flow in a rigid cylindrical tube. Furthermore, these theoretical results are compared to earlier published measurement results. To develop the theory, we assumed that blood is a Newtonian fluid and that red blood cells can be represented by oblate ellipsoids. If blood flows through a cylindrical tube, shear stresses will deform and align the red blood cells with one of their long axes aligned parallel to the stream lines. The pathway of a low-frequency (< 1 MHz) alternating electrical current will be altered by this orientation and deformation of the red blood cells. Consequently, the electrical conductivity in the flow direction of blood increases. The theoretically predicted flow dependency of the conductivity of blood corresponds well with experimental results. This theoretical study shows that red blood cell orientation and deformation can explain quantitatively the flow dependency of blood conductivity.     

Experimental manifestations of correlated hopping in the temperature dependences of the conductivity of doped CdTe

The behavior of hopping transport in the region of the crossover from Mott conduction to conduction among Coulomb-gap states is investigated in doped CdTe crystals. Inconsistencies are discovered in individual parameters (the localization radius and the dielectric constant) estimated from the behavior of the conductivity on different sides of the crossover. These inconsistencies are explained within a simplified model that takes into account the role of assisting hops. Fiz. Tekh. Poluprovodn. 32, 703–707 (June 1998)     

Hall effect in CdTe crystals doped with Sn from the vapor phase

The Hall effect in semiinsulating CdTe crystals doped with a Sn impurity from the vapor phase in a closed volume is studied. It is found that the conductivity is due to a donor center with E _t ≈ 0.7 eV and the concentration of electrons and their mobility at 300 K are (4–8) × 10⁶ cm^?3 and 200–300 cm² V^?1 s^?1, respectively.     

Effect of chlorine impurities on the long-wavelength absorption edge of CdTe single crystals

The optical absorption spectra of the nominally undoped and Cl-doped CdTe single crystals in the region of the fundamental long-wavelength absorption edge are studied at room temperature. The crystals are grown by the modified physical vapor transport method. It is shown that the exponential dependence of the absorption coefficient of the undoped and lightly doped samples can be described by the Urbach rule. In the heavily doped crystals, the absorption edge is due to optical transitions involving density-of-state tails formed by fluctuations of the dopant concentration. The typical energy range of fluctuations of the extrinsic potential profile and the concentrations of charged centers are calculated for the heavily doped material.     

Dislocation electrical conductivity of synthetic diamond films

A relationship between the electric resistance of single-crystal homoepitaxial and polycrystalline diamond films and their internal structure has been investigated. It is established that the electrical conductivity of undoped homoepitaxial and polycrystalline diamond films is directly related to the dislocation density in them. A relation linking the resistivity ρ (～10¹³–10¹⁵ Ω cm) with the dislocation density Γ (～10¹⁴?4 × 10¹⁶ m^?2) is obtained. The character of this correlation is similar for both groups of homoepitaxial and polycrystalline diamond films. Thin (～1–8 μm) homoepitaxial and polycrystalline diamond films with small-angle dislocation boundaries between mosaic blocks exhibit dislocation conductivity. The activation energy of dislocation acceptor centers was calculated from the temperature dependence of the conductivity and was found to be ～0.3 eV. The conduction of thick diamond films (h > 10 μm) with the resistivity ρ ≈ 10⁸ Ω cm is determined by the conduction of intercrystallite boundaries, which have a nondiamond hydrogenated structure. The electronic properties of the diamond films are compared with those of natural semiconductor diamonds of types IIb and Ic, in which dislocation acceptor centers have activation energies in the range 0.2–0.35 eV and are responsible for hole conduction.     

Equilibrium characteristics and low-temperature photoluminescence of CdTe:Pb single crystals

Semiconductors - Using the Bridgman-Stockbarger method, CdTe:Pb single crystals with various dopant concentrations in the solution (C Pb 0 =5×1018, 1019, and 5×1019 cm−3) were...     

Electrical properties of CdTe crystals grown by vuvg from nonstoichiometric charges

High purity CdTe crystals were grown by the vertical unseeded vapor growth technique. The growth charge composition varied between CdTe +0.1 mol% Cd and CdTe + 0.1 mol% Te. At a deviation from stoichiometry of δTe > 0.05 mol% in the charge p-type crystals with an electrical resistivity of 10⁶ Ωcm were obtained, n-type crystals were obtained with charge compositions of δTe < 0.02 mol%, with the resistivity ρ increasing with δTe from 10³ to < 10⁹ Ωcm. Maximum resistivity ∿2.10⁹ Ωcm was found when δTe ∿0.02 mol%. In all cases, the crystal composition shows a higher Cd concentration of ∿0.02 mol% relative to the charge, suggesting noncongruent sublimation of CdTe.     

Electrical characteristics of CdTe:Pb single crystals at high temperatures

The electrical properties of CdTe:Pb single crystals at high temperatures (400–900°C) and under controlled Cd vapor pressure (0.001–3 atm) were investigated for the first time. The temperature and baric dependences of the conductivity and Hall coefficient were measured. Low (in comparison with undoped CdTe) electron concentration indicates an increase in the number of impurity point defects related to the Pb impurity. The results obtained are explained within the Kröger theory of quasi-chemical reactions of defect formation on the assumption that lead may exist in the isolated state (Pb _Cd ⁺ ) and as a component of (Pb _Cd ⁺ V _Cd ^2? )^? associates.     

Influence of annealing on the dislocation-related electrical conductivity of germanium

Germanium n-type single crystals with a donor concentration of 3×10¹² cm^?3 were deformed at 760°C to strains of δ≤71% with a rate of 6×10^?3 s^?1, cooled to room temperature, and then annealed for t≤20 h at 900°C. Low-temperature static electrical conductivity due to holes trapped by dislocations and transported along a branching dislocation network was measured before and after annealing of the deformed samples. It was found that annealing enhances the dislocation-related electrical conductivity in the samples with δ<50% and diminishes this conductivity in the samples with δ>60%. Selective etching and X-ray diffraction analysis showed that the main structural distinction of the samples with δ>60% is the presence of recrystallized regions. The influence of annealing on dislocation-related electrical conductivity is explained by an increase in connectedness of the dislocation network for δ<50% and by a decrease in this connectedness in the case of δ>60%.