Effect of thallium doping on the mobility of electrons in Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> and holes in Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>

The Shubnikov–de Haas effect and the Hall effect in n-Bi_2–xTl_xSe₃ (x = 0, 0.01, 0.02, 0.04) and p-Sb_2–xTl_xTe₃ (x = 0, 0.005, 0.015, 0.05) single crystals are studied. The carrier mobilities and their changes upon Tl doping are calculated by the Fourier spectra of oscillations. It is found shown that Tl doping decreases the electron concentration in n-Bi_2–xTl_xSe₃ and increases the electron mobility. In p-Sb_2–xTl_xTe₃, both the hole concentration and mobility decrease upon Tl doping. The change in the crystal defect concentration, which leads to these effects, is discussed.     

Effect of the magnetic phase transition on the charge transport in layered semiconductor ferromagnets TlCrS<Subscript>2</Subscript> and TlCrSe<Subscript>2</Subscript>

TlCrS₂ and TlCrSe₂ crystals were synthesized by solid-state reaction. X-ray diffraction analysis showed that TlCrS₂ and TlCrSe₂ compounds crystallize in the hexagonal crystal system with lattice parameters a = 3.538 Å, c = 21.962 Å, c/a ≈ 6.207, z = 3; a = 3.6999 Å, c = 22.6901 Å, c/a ≈ 6.133, z = 3; and X-ray densities ρ _x = 6.705 and 6.209 g/cm³, respectively. Magnetic and electric studies in a temperature range of 77–400 K showed that TlCrS₂ and TlCrSe₂ are semiconductor ferromagnets. Rather large deviations of the experimental effective magnetic moment of TlCrS₂ (3.26 μ_B) and TlCrSe₂ (3.05 μ_B) from the theoretical one (3.85 μ_B) are attributed to two-dimensional magnetic ordering in the paramagnetic region of strongly layered ferromagnets TlCrS₂ and TlCrSe₂. The effect of the magnetic phase’s transition on the charge transport in TlCrS₂ and TlCrSe₂ is detected.     

Effect of copper doping on kinetic phenomena in <Emphasis Type="Italic">n</Emphasis>-Bi<Subscript>2</Subscript>Te<Subscript>2.85</Subscript>Se<Subscript>0.15</Subscript>

In single crystals of copper-doped and undoped Bi₂Te_2.85Se_0.15 solid solutions with an electron concentration close to 1 × 10¹⁹ cm^?3, the temperature dependences are investigated for the Hall (R ₁₂₃, R ₃₂₁) and Seebeck (S ₁₁) kinetic coefficients, the electrical-conductivity (σ ₁₁), Nernst-Ettingshausen (Q ₁₂₃), and thermalconductivity (k ₁₁) coefficients in the temperature range of 77–400 K. The absence of noticeable anomalies in the temperature dependences of the kinetic coefficients makes it possible to use the one-band model when analyzing the experimental results. Within the framework of the one-band model, the effective mass of density of states (m _d ≈ 0.8m ₀), the energy gap (ε_g ≈ 0.2 eV), and the effective scattering parameter (r _eff ≈ 0.2) are estimated. The obtained value of the parameter r _eff is indicative of the mixed electron-scattering mechanism with the dominant scattering by acoustic phonons. Data on the thermal conductivity and the lattice resistivity obtained by subtracting the electron contribution according to the Wiedemann-Franz law are presented.     

Electronic Processes in CdIn<Subscript>2</Subscript>Te<Subscript>4</Subscript> Crystals

The results of investigations of electrical, optical, and photoelectric properties of CdIn₂Te⁴ crystals, which were grown by the Bridgman method are presented. It is shown that electrical conductivity is determined mainly by electrons with the effective mass m_n = 0.44m₀ and the mobility 120–140 cm²/(V s), which weakly depends on temperature. CdIn₂Te₄ behaves as a partially compensated semiconductor with the donor-center ionization energy E_d = 0.38 eV and the compensation level K = N_a/N_d = 0.36. The absorption-coefficient spectra at the energy hν < E_g = 1.27 eV are subject to the Urbach rule with a typical energy of 18–25 meV. The photoconductivity depends on the sample thickness. The diffusion length, the charge-carrier lifetime, and the surface-recombination rate are determined from the photoconductivity spectra.     

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.     

Hysteresis in Ag<Subscript>2</Subscript>Te near and within the phase transition region

Electrical conductivity, the Hall coefficient, and thermoelectric power were studied and differential thermal analysis was carried out in Ag₂Te crystals near and within the range of phase transitions, in the directions of heating and cooling. A large hysteresis loop was observed. The results are discussed in terms of the theory of smeared phase transitions. Agreement between experimental data and theory is achieved in a second approximation of the inclusion function L₂(T) and its derivative with respect to temperature, dL₂/dT.     

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.     

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.     

<Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-Si-<Emphasis Type="Italic">n</Emphasis>-CdF<Subscript>2</Subscript> heterojunctions

Boron diffusion and the vapor-phase deposition of silicon layers are used to prepare ultrashallow p⁺-n junctions and p⁺-Si-n-CdF₂ heterostructures on an n-CdF₂ crystal surface. Forward portions of the I–V characteristics of the p⁺-n junctions and p⁺-Si-n-CdF₂ heterojunctions reveal the CdF₂ band gap (7.8 eV), as well as allow the identification of the valence-band structure of cadmium fluoride crystals. Under conditions in which forward bias is applied to the p⁺-Si-n-CdF₂ heterojunctions, electroluminescence spectra are measured for the first time in the visible spectral region.     

Magnetic susceptibility of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> alloys

The magnetic susceptibility of Czochralski-grown single crystals of Bi₂Te₃-Sb₂Te₃ alloys containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, or 100 mol % Sb₂Te₃ has been investigated. The magnetic susceptibility of these crystals was determined at the temperature T = 291 K and the magnetic field H oriented parallel (χ_‖) and perpendicularly (χ_⊥) to the trigonal crystallographic axis C ₃. A complicated concentration dependence of the anisotropy of magnetic susceptibility χ_‖/χ_⊥ has been revealed. The crystals with the free carrier concentration p ≈ 5 × 10¹⁹ cm^?3 do not exhibit anisotropy of magnetic susceptibility. The transition to the isotropic magnetic state occurs for the compositions characterized by a significantly increased (from 200 to 300 meV) optical bandgap.     

Sectoral transition between modes <Emphasis Type="Italic">H</Emphasis><Subscript>10</Subscript> and <Emphasis Type="Italic">H</Emphasis><Subscript>01</Subscript>

The excitation of main parasitic modes E ₁₁, H ₁₁, and H ₂₁, which have cutoff sections in a sectoral transition between modes H ₁₀ and H ₀₁, is investigated. It is shown that, for magnetic modes, including modes H ₁₁ and H ₂₁, it is unnecessary to use the Airy equation and that this circumstance simplifies the design relationships. The energies of these modes are calculated as functions of the transition parameters.     

Electrical and photoelectric properties of <Emphasis Type="Italic">n</Emphasis>-TiN/<Emphasis Type="Italic">p</Emphasis>-Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> heterostructures

n-TiN/p-Hg₃In₂Te₆ heterostructures are fabricated by depositing a thin n-type titanium nitride (TiN) film onto prepared p-type Hg₃In₂Te₆ plates using reactive magnetron sputtering. Their electrical and photoelectric properties are studied. Dominant charge-transport mechanisms under forward bias are analyzed within tunneling-recombination and tunneling models. The fabricated n-TiN/p-Hg₃In₂Te₆ structures have the following photoelectric parameters at an illumination intensity of 80 mW/cm²: the open-circuit voltage is V_OC = 0.52 V, the short-circuit current is I_SC = 0.265 mA/cm², and the fill factor is FF = 0.39.     

Optical Properties of K<Subscript>2</Subscript>O-Li<Subscript>2</Subscript>O-WO<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Glasses: Evidence of Mixed Alkali Effect

Glass with compositions xK₂O-(30 ? x)Li₂O-10WO₃-60B₂O₃ for 0 ≤ x ≤ 30 mol.% have been prepared using the normal melt quenching technique. The optical reflection and absorption spectra were recorded at room temperature in the wavelength range 300–800 nm. From the absorption edge studies, the values of the optical band gap (E _opt) and Urbach energy (ΔE) have been evaluated. The values of E _opt and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple Di-Domenico model.     

First-Principles Study of Structural,Electronic, Optical,and Thermal Properties of BeSiSb<Subscript>2</Subscript> and MgSiSb<Subscript>2</Subscript>

Structural, electronic, optical, and thermal properties of ternary II–IV–V₂ (BeSiSb₂ and MgSiSb₂) chalcopyrite semiconductors have been calculated using the full-potential linearized augmented plane wave scheme?in the generalized gradient approximation. The optimized equilibrium structural parameters (a, c, and u) are in good agreement with theoretical results obtained using other methods. The band structure and density of states reveal that BeSiSb₂ has an indirect (Γ–Z) bandgap of about 0.61 eV, whereas MgSiSb₂ has a direct (Γ–Γ) bandgap of 0.80 eV. The dielectric function, refractive index, and extinction coefficient were calculated to investigate the optical properties, revealing that BeSiSb₂ and MgSiSb₂ present very weak birefringence. The temperature dependence of the volume, bulk modulus, Debye temperature, and heat capacities (C _v and C _p) was predicted using the quasiharmonic Debye model at different pressures. Significant differences in properties are observed at high pressure and high temperature. We predict that, at 300 K and 0 GPa, the heat capacity at constant volume C _v, heat capacity at constant pressure C _P, Debye temperature θ _D, and Grüneisen parameter γ will be about 94.91 J/mol K, 98.52 J/mol K, 301.30 K, and 2.11 for BeSiSb₂ and about 96.08 J/mol K, 100.47 J/mol K, 261.38 K, and 2.20 for MgSiSb₂, respectively.     

AC Electrical Conductivity of FeIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> Single Crystals

The results obtained in a study of the frequency and temperature dependences of the ac electrical conductivity of FeIn₂Se₄ single crystals are presented. It is found that the law σ ~ f ^S (0.1 ≤ S ≤ 1.0) is obeyed for electrical conductivity in the 295–375 K temperature range at frequencies of 2 × 10⁴–10⁶ Hz. It shown that the frequency dependence of the conductivity in an FeIn₂Se₄ single crystal can be accounted for in terms of the multiplet model, and, consequently, the conductivity in these single crystals is characterized by the band-hopping mechanism.     

Room-Temperature Mid-, and Far-Infrared Absorption and Electrical Properties of Intercalated GaSe and TlInS<Subscript>2</Subscript> Crystals

We report room-temperature measurements of the mid- and far-IR absorption throughout the 400 – 4000 cm^?1 and 10 – 700 cm^?1 spectral ranges and the resistivity of layered p-GaSe and p-TlInS2 intercalated with Li⁺. Intercalation was performed by immersing Bridgman grown crystals in 0.5 M solutions of LiCl in distilled water at ambient conditions. The crystal structure and the stoichiometry of the grown crystals were determined by X-ray diffraction and XRF methods. It is shown that intercalation does not change the frequency of the IR-, and Raman active low-frequency“rigid layer” mode (GaSe), the space symmetry group or the lattice parameters of the crystals. It was found that for both crystals, the resistivity versus time dependencies are nearly the same. Three ranges in the resistivity-intercalation time dependencies were explained qualitatively. The resistivity increase due to intercalation was explained by assuming that the intercalated lithium ions act as ionized donors and compensate the host p-type crystal. The highest degree of compensation for GaSe and TlInS₂ crystals was achieved after intercalation during 12 and 10 days, respectively.     

Optical and photoelectrical properties of AgCd<Subscript>2</Subscript>GaS<Subscript>4</Subscript> single-crystal compounds

The poorly known AgCd₂GaS₄ single-crystal compounds that crystallize in a rhombic structure (space group Pmn2₁) are studied. Deviations from the stoichiometric composition of the samples and random occupation of the cation sublattice sites by Ag and Ga ions result in violation of long-range order in the atomic arrangement and make the AgCd₂GaS₄ compounds structurally closer to disordered systems. In this case, it is found that the fundamental optical-absorption edge is smeared and shifted to longer wavelengths, and is adequately described by the Urbach rule. In addition, a broadening of the spectral peaks of photoconductivity and luminescence is observed. The concentration of charged point defects responsible for the smearing of the absorption edge is calculated. It is found to be 1.2×10²⁰ cm^?3. The AgCd₂GaS₄ single crystals are photosensitive semiconductors. From the position of the absorption edge, the optical band gap of the compound is estimated (E _g0 = 2.28 eV at T = 297 K). The photoluminescence spectra of the AgCd₂GaS₄ single crystals are similar to the spectra of defect-containing CdS single crystals; for the AgCd₂GaS₄ crystals, the emission peaks are shifted to longer wavelengths with respect to the peaks for CdS crystals by Δλ = 0.06?0.1 μm. From the analysis of the experimental data, some conclusions on the nature of photoactive centers in AgCd₂GaS₄ compounds are drawn.     

Structural transformations and optical properties of As<Subscript>2</Subscript>S<Subscript>3</Subscript> chalcogenide glasses

The effects of melt temperature T _i and quenching rate V _i on the structure and optical properties of As₂S₃ glasses is studied. It is found that the glass band gap increases with T _i and V _i , whereas a decrease is observed in the glass density, refractive index (from 2.71 to 2.48), and two-photon absorption coefficient (from 0.37 to 0.15 cm/MW), which is accompanied by an increase in the optical-breakdown damage threshold.     

Fabrication and photosensitivity of heterojunctions based on CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> crystals

Heterojunctions based on p-CuIn₃Se₅ crystals are fabricated by magnetron sputtering of an n-ZnO:Al target and by putting naturally cleaved n-GaSe thin wafers onto polished surfaces of p-CuIn₃Se₅ wafers. The current-voltage characteristics and mechanisms of current flow in the diodes under study are analyzed. The photovoltaic effect revealed in the fabricated structures is discussed. It is shown that the fabricated photosensitive heterojunctions are promising for the development of selective analyzers of linearly polarized radiation.     

Specific thermoelectric properties of lightly doped Bi<Subscript>2</Subscript>(TeSe)<Subscript>3</Subscript> solid solutions

Electrical and thermoelectric properties of a lightly doped n-Bi₂Te_2.7Se_0.3 solid solution have been studied in the temperature range 77–300 K. The results are compared with data for the compound PbTe_0.9Se_0.1 with a similar magnitude of the Seebeck coefficient S at 84 K. Along with lower thermal conductivity, Bi₂Te_2.7Se_0.3 has a higher electrical conductivity σ and a much weaker temperature dependence. As a result, the power coefficient S ²σ in optimal samples begins to decrease only when the density of minority carriers becomes significant. In this case, |S| considerably exceeds the standard value of 200 μV/K. The reduction of the electron density reduces the thermoelectric figure of merit Z at its maximum and slightly lowers the temperature of the maximum; therefore, the expected effect on the average value of Z in the range 77–300 K is absent. Similar behavior is observed in Bi₂Te_2.88Se_0.12, although the effect is less pronounced. The experimental results are discussed taking into account possible changes in the dominant scattering mechanisms, carrier density, and electron energy spectrum. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 7, 2004, pp. 811–815. Original Russian Text Copyright ? 2004 by Konstantinov, Prokof’eva, Ravich, Fedorov, Kompaniets.