Superionic conductivity in TlGaTe<Subscript>2</Subscript> crystals

Temperature dependences of electrical conductivity σ(T) and permittivity ɛ(T) of one-dimensional (1D) TlGaTe₂ single crystals are investigated. At temperatures higher than 305 K, superionic conductivity of the TlGaTe₂ is observed and is related to diffusion of Tl⁺ ions via vacancies in the thallium sublattice between (Ga³⁺Te₂^{2− −} nanochains. A relaxation character of dielectric anomalies is established, which suggests the existence of electric charges weakly bound to the crystal lattice. Upon the transition to the superionic state, relaxors in the TlGaTe₂ crystals are Tl⁺ dipoles ((Ga³⁺Te₂²⁻)⁻ chains) that arise due to melting of the thallium sublattice and hops of Tl⁺ ions from one localized state to another. The effect of a field-induced transition of the TlGaTe₂ crystal to the superionic state is detected.     

On the photoconductivity of TlInSe<Subscript>2</Subscript>

The current–voltage (I–V) and photocurrent–light intensity (I _pc –Φ) characteristics and the photoconductivity relaxation kinetics of TlInSe₂ single crystals are investigated. Anomalously long relaxation times (τ ≈ 10³ s) and some other specific features of the photoconductivity are observed, which are explained within the barrier theory of inhomogeneous semiconductors. The heights of the drift and recombination barriers are found to be, respectively, E _dr ≈ 0.1 eV and E _r ≈ 0.45 eV.     

On a neutron detector based on TlInSe<Subscript>2</Subscript> crystals intercalated with a lithium isotope

The possibility of increasing the sensitivity of a neutron detector based on a TlInSe₂ crystal by introducing the ⁶Li isotope into this crystal is investigated. Introduction of the isotope is conducted by the method of electrochemical intercalation from aqueous and nonaqueous solutions of LiCl and also from the melt of the LiCl–KCl eutectic. It is shown that intercalation by the electrochemical method from a LiCl solution in propylene carbonate, performed along the “c” axis of the crystal (along strong bond chains) is efficient. The attained concentration of introduced lithium amounts to (1–1.2) × 10²¹ cm^–3, which increased the sensitivity of the detector by approximately four times. Good agreement between the calculated and experimental values of the detector sensitivity is demonstrated.     

The conductivity and Hall effect in CdF<Subscript>2</Subscript>:In and CdF<Subscript>2</Subscript>:Y

Temperature dependences of the static conductivity σ and Hall coefficient R_H in CdF₂ crystals doped with bistable indium and donor yttrium impurities have been measured. It is shown that this material contains different types of free carriers, i.e., electrons and polarons. A comparison of the calculated temperature dependences of σ and R_H with the experimental data also shows that the impurity-band conductivity makes a significant contribution (due to hops of bound polarons or holes) to σ and R_H.     

Effect of a high electric field on the conductivity of MnGa<Subscript>2</Subscript>S<Subscript>4</Subscript>, MnIn<Subscript>2</Subscript>S<Subscript>4</Subscript>, and MnGaInS<Subscript>4</Subscript> single crystals

The results of studying the effect of a high electric field on the conductivity of MnGa₂S₄, MnIn₂S₄, and MnGaInS₄ single crystals are reported. The activation energy is determined in high and low electric fields. It is established that the decrease in the activation energy with increasing the external voltage is associated with decreasing the depth of the potential well, in which the electron is located.     

Nitrogen-doped Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films for nonvolatile memory

Nitrogen-doped Ge₂Sb₂Te₅ (GST) films for nonvolatile memories were prepared by reactive sputtering with a GST alloy target. Doped nitrogen content was determined by using x-ray photoelectron spectroscopy (XPS). The crystallization behavior of the films was investigated by analyzing x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results show that nitrogen doping increases crystallization temperature, crystallization-activation energy, and phase transformation temperature from fcc to hexagonal (hex) structure. Doped nitrogen probably exists in the grain vacancies or grain boundaries and suppresses grain growth. The electrical properties of the films were studied by analyzing the optical band gap and the dependence of the resistivity on the annealing temperature. The optical band gap of the nitrogen-doped GST film is slightly larger than that of the pure GST film. Energy band theory is used to analyze the effect of doped nitrogen on electrical properties of GST films. Studies reveal that nitrogen doping increases resistivity and produces three relatively stable resistivity states in the plot of resistivity versus annealing temperature, which makes GST-based multilevel storage possible. Current-voltage (I-V) characteristics of the devices show that nitrogen doping increases the memory’s dynamic resistance, which reduces writing current from milliampere to microampere.     

An impurity band in Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals doped with silicon

The influence of silicon impurity on the energy-band spectrum in the Hg₃In₂Te₆ semiconductor compound, which incorporated a high concentration of stoichiometric vacancies, was studied on the basis of the results of electrical and optical measurements. It is shown that silicon impurity forms an impurity band of donor states whose density can be approximated by a Gaussian distribution with a peak at E_c-0.29 eV. The emergence of the impurity band is accompanied with the formation of a quasi-continuous spectrum of localized states in the band gap (E_g=0.74 eV); the density of these states is shown to increase as the doping level increases. All states merge into a continuous band if the impurity concentration N_Si>4.5×10¹⁷ cm^?3. Experimental data are explained on the basis of the effect of impurity self-compensation, in which case donor impurity states arise simultaneously with acceptor states of defects.     

Anomalous thermoelectric power in Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals

Based on data on the Hall coefficient, it is shown that the existence of potential barriers in the region of impurity conductivity of highly compensated Hg₃In₂Te₆ crystals is possible. The role of barriers in the anomalous behavior of transport phenomena is discussed qualitatively. Extremely large values of the thermoelectric power are related to the combination of thermoelectric powers of contact potentials for regions with different concentrations of electrons.     

Photosensitive structure on CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

Single crystals of the CdGa₂S₄ ternary compound were grown either from melt or by chemical-vapor deposition. The crystal-lattice parameters and some physical properties of homogeneous crystals having defect chalcopyrite structure with the point symmetry group \(I\bar 4(S_4^2 )\) are determined. A number of photosensitive structures—Schottky barriers, heterostructures, photoelectrochemical cells, and natural-protein-CdGa₂S₄ barriers—were formed for the first time on the basis of the single crystals under investigation. The photoelectric properties of the structures obtained were studied using natural and linearly polarized light at T=300 K. The main parameters of these structures are determined, and it is concluded that they can be used in photodetectors.     

Photosensitive structures based on In<Subscript>2</Subscript>S<Subscript>3</Subscript> crystals

Crystals of the compound In₂S₃ were grown by planar crystallization of the melt. The composition, structure, and electrical characteristics of the crystals obtained were determined. Photosensitive structures based on the grown In₂S₃ crystals were fabricated for the first time; spectral dependences of photoconversion quantum efficiency for H₂O/In₂S₃ cells were measured. The features of the band-to-band absorption are discussed; energies of the direct and indirect optical transitions for In₂S₃ crystals are estimated. It is stated that In₂S₃ crystals can be used in wide-range (1.5–3.5 eV) photoconverters of nonpolarized radiation (in particular, in solar cells).     

Electrical properties of FeIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> single crystals

The temperature dependence of electrical conductivity and current-voltage characteristics of FeIn₂Se₄ single crystals were studied. It is shown that the current in the nonlinear range of the current-voltage characteristic is caused by the field effect. The activation energy of charge carriers, the concentration of traps, and the shape of a potential well in the region of a trap are determined.     

Current-voltage characteristics of MnGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> single crystals

The current-voltage characteristics of MnGa₂Se₄ single crystals have been investigated. The measurements were performed in the range of electric fields from the level at which the Ohm law is satisfied to 10 V/cm, and in the temperature range 300–400 K. The data obtained are interpreted within the theory of injection-contact phenomena and the theory of field ionization of traps due to the Poole-Frenkel effect.     

Conductivity of Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals in high electric fields

The effect of electric field and temperature on the conductivity of bulk Hg₃In₂Te₆ crystals is investigated. It is shown that the I–V characteristics in high electric fields are of the S type with the effect of switching into a low-resistance state. The critical voltage of transition from the Ohm law to the exponential dependence of the current (I) on the voltage (U) and the threshold voltage of transition into the region of negative differential resistance dU/dI = s< 0 linearly depend on the sample thickness. The activation energies of conductivity in low and high electric fields are determined. It is established that the superlinear portion of the I–V characteristic with dU/dI > 0 is described by the dependence of the type I = I ₀ exp(U/U ₀) and caused by the electron transitions from the local centers with the energy level E _t = 0.19 eV.     

Special features of charge transport in PbGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals

The current-voltage (I-V) characteristics of PbGa₂Se₄ single crystals grown by the Bridgman-Stockbarger method with a resistivity of 10¹⁰–10¹² Ω cm were measured. The value of the majority carrier mobility μ=14 cm² V^?1 s^?1, calculated by the differential method of analysis of I-V characteristics, makes it possible to evaluate a number of parameters: the carrier concentration at the cathode (n_c0=2.48 cm^?3), the width of the contact barrier d_c=5.4×10^?8 cm, the cathode transparency D _c ^* =10^?5–10^?4 eV, and the quasi-Fermi level E_F=0.38 eV. It is found that a high electric field provides the charge transport through PbGa₂Se₄ crystals in accordance with the Pool-Frenkel effect. The value of the dielectric constant calculated from the Frenkel factor is found to be equal to 8.4.     

Magnetic properties of the FeIn<Subscript>2</Subscript>S<Subscript>4</Subscript> ternary-compound crystals

Crystals of the ternary compound FeIn₂S₄ are grown by directional crystallization of a melt (the horizontal Bridgman method). Composition of the crystals and their crystal structure are determined. Magnetic properties of the FeIn₂S₄ crystals are studied in the temperature range 4–310 K in magnetic fields of 0–140 kOe. It is shown that the crystals under study are paramagnets up to ∼12 K and their specific magnetic moment monotonically increases with decreasing temperature. The antiferromagnetic character of indirect interactions between Fe²⁺ cations is established. The most probable causes and the mechanism of the formation of the magnetic state in the FeIn₂S₄ crystals are discussed.     

Photosensitive structures based on ZnIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> single crystals

Photosensitive structures of surface-barrier and homojunction types have been fabricated for the first time on the basis of ZnIn₂Se₄ single crystals. The spectral dependence of the quantum efficiency of photoconversion has been studied and discussed. It is concluded that the structures are promising for commercial applications.     

Electrical properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals and photosensitivity of Al/In<Subscript>2</Subscript>Se<Subscript>3</Subscript> Schottky barriers

In₂Se₃ single crystals ∼40 mm long and 14 mm in diameter were grown by the Bridgman method. The composition of grown single crystals and their crystal structure were determined. The conductivity (σ) and Hall constant (R) of grown single crystals were measured and the first Schottky barriers Al/n-In₂Se₃ were fabricated. Rectification and photovoltaic effect were detected in the new structures. Based on the study of the photosensitivity spectra of Al/n-In₂Se₃ structures, the nature of the interband transitions and band gap of In₂Se₃ crystals were determined. It was concluded that the new structures can be applied to develop broadband photoconverters of optical radiation.     

Frenkel thermal-field effect in MnGaInS<Subscript>4</Subscript> layered single crystals

The I–V characteristics are investigated in the region of high electric fields in MnGaInS₄ single crystals. It is shown that the current in the nonlinear portion of the I–V characteristic is caused by the Frenkel thermal-field effect. The permittivity, trap concentration, and potential-well shape are determined.     

Photovoltaic effects in CdV<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals and structures based on them

Single crystals of the CdV₂S₄ ternary compound are grown, and their crystal structure, electrical properties, and optical absorption are studied. The substitution of vanadium for Group III element in A^IIB ₂ ^III C ₄ ^VI compounds results in the formation of crystals of n-type conduction with an electron density of ～10¹⁸ cm^?3 and a Hall mobility U_n≈150 cm²/(V s) at T=300 K, which is limited by scattering on lattice vibrations. Rectifying photosensitive structures based on CdV₂S₄ single crystals are fabricated for the first time, their photoelectric properties are studied, and a conclusion is made on their applicability in the design of wide-spectral-range photodetectors of unpolarized light.