Effect of copper doping on kinetic coefficients and their anisotropy in PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript>

In anisotropic PbSb₂Te₄ and PbSb₂Te₄:Cu single crystals, nine main independent components of the Hall, electrical-conductivity, thermopower, and Nernst-Ettingshausen effects and their anisotropy in the range 77–450 K have been studied. PbSb₂Te₄ single crystals exhibit a high hole concentration (p ≈ 3 × 10²⁰ cm⁻³). Copper exhibits a donor effect and significantly (approximately by a factor of 2) reduces the hole concentration in PbSb₂Te₄. The temperature dependences of the kinetic coefficients, except for the Hall effect, have a form typical of the one-band model. The significant anisotropy of the Hall coefficient R ₁₂₃/R ₃₂₁ ≈ 2 at low temperatures corresponds to the multi-ellipsoid model of the energy spectrum of holes in PbSb₂Te₄. An important feature of the data on transport phenomena is the high thermopower anisotropy (ΔS ≈ 60–75 μV/K) in the mixed conductivity region caused by the mixed scattering mechanism. Data on the anisotropy of the transverse Nernst-Ettingshausen effect confirm the mixed mechanism of hole scattering; in the cleavage plane, scattering at acoustic phonons dominates, while in the trigonal axis direction, impurity scattering appears significant. Doping with copper enhances the role of impurity scattering in the direction of the trigonal axis c ₃; as a result, two components of the Nernst-Ettingshausen tensor Q ₃₂₁ and Q ₁₃₂ in the PbSb₂Te₄:Cu single crystal are positive at low temperatures, whereas, in the undoped crystal, only the Q ₃₂₁ component is positive.     

The temperature dependence of the thermopower of the InSb Corbino disc in a quantizing magnetic field

Thermopower of the Corbino disc made of InSb with n ₇₇ = 2 × 10¹⁴ cm^?3 in a transverse magnetic field as high as 30 kOe at temperatures of 60, 67, and 80 K is studied. It is established that the diffusion fraction of thermopower in a quantizing magnetic field rises according to the power law H ^2.2 at all mentioned temperatures. By the magnitude of saturation thermopower α_xx(∞) in a high field, the scattering mechanism of charge carriers is determined. It is established that in a temperature region of 60–80 K, the electrons are scattered by acoustic phonons.     

Effect of interband scattering on transport phenomena in p-PbSb2Te4

Experimental data on transport phenomena in p-PbSb₂Te₄ are qualitatively and quantitatively interpreted within the context of the two-band model with interband scattering in the temperature range 77–300 K. The parameters of the two-band model are determined. Specifically, it is found that the density-of-state effective masses of light and heavy holes are, correspondingly, m _d1 ≈ 0.5m ₀ and m _d2 ≈ 0.9m ₀ (m ₀ is the free electron mass) and the energy gap between nonequivalent extrema are ΔE _v(T) ≈ 0.23–4.5 × 10^?2(T/100 ? 1) eV.     

Electron-phonon processes in semiconductors at low temperatures

The thermal conductivity and thermopower are simultaneously studied in the semiconductors HgSe, HgTe, InP, GaAs, Cd _x H_{1 ? x} Te, and Ag₂S in which the effect of electron drag by phonons was detected at low temperatures T. It is found that x = 1–3 in the dependences K _ph ∝ T ^?x . It is shown that the underestimated values of x are caused by phonon scattering at certain defects. It is also found that the maxima of K _ph(T) and α_ph(T) coincide in all studied crystals, except for Ag₂S. For the first time, it is found that the strong effect α_ph ∝ T ^?3 is observed in Ag₂S, the α_ph maximum is at T = 16 K, and the K _ph maximum is at 27 K. The results obtained are in agreement with Callaway and Herring theory. The effect of magnetic field on the drag thermopower α_ph(T) in n-Cd _x H_1?x Te (in which the strong dependence α_ph ∝ T ^?3 and the strong effect of the magnetic field on it are detected) is considered for the first time. The results are compared with the Askerov theory. The force parameter A _ph(?) of the drag effect and its dependence on magnetic field are determined.     

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.     

Electron scattering by acceptor centers in <Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>2</Subscript>Te at low temperatures

Resonant electron scattering in p-Ag₂Te at acceptor concentrations N _a < 4.2 × 10¹⁶ cm⁻³ has been observed in the temperature range of 50–80 K. The contribution of the resonant scattering to the temperature dependences of the conductivity σ(T) and thermopower α₀(T) has been calculated. It is shown that this contribution exceeds that of charge carrier scattering by acoustic phonons.     

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.     

Cyano‐Substituted Oligo(p‐phenylene vinylene) Single Crystals: A Promising Laser Material

Organic crystals that combine high charge‐carrier mobility and excellent light‐emission characteristics are expected to be of interest for light‐emitting transistors and diodes, and may offer renewed hope for electrically pumped laser action. High‐luminescence‐efficiency cyano‐substituted oligo(p‐ phenylene vinylene) (CN‐DPDSB) crystals (η ≈ 95%) grown by the physical vapor transport method is reported here, with high mobilities (at ≈10^?2 cm² V^?1 s^?1 order of magnitude) as measured by time‐of‐flight. The CN‐DPDSB crystals have well‐balanced bipolar carrier‐transport characteristics (μ_hole≈ 2.5–5.5 × 10^?2 cm² V^?1 s^?1; μ_electron ≈ 0.9–1.3 × 10^?2 cm² V^?1 s^?1) and excellent optically pumped laser properties. The threshold for amplified spontaneous emission (ASE) is about 4.6 μJ per pulse (23 KW cm^?2), while the gain coefficient at the peak wavelength of ASE and the loss coefficient caused by scattering are ≈35 and ≈1.7 cm^?1, respectively. This indicates that CN‐DPDSB crystals are promising candidates for organic laser diodes.     

Electrical properties of proton-irradiated CdSnAs2

Electrical properties and isochronal annealing of proton-irradiated (5 MeV, 2 × 10¹⁶ cm^–2) n-and p-type CdSnAs₂ crystals have been studied. The limiting electrical parameters of the irradiated material were determined: Hall constant 〈R _H〉 ≈ ?1.2 cm³/C, electrical conductivity 〈σ〉 ≈ 1350 Ω^?1 cm^?1, Hall mobility 〈|R _H|〉〈σ〉 ≈ 1500 cm²/(V s), and Fermi level position F _lim ≈ 0.43–0.45 eV above the valence-band top. The energy position of the “neutral” point for the CdSnAs₂ compound was calculated.     

Capture cross sections of the gold donor and acceptor states in n-type Czochralski silicon

The hole and electron capture cross sections of the gold donor and acceptor have been measured directly in n-type silicon. The samples have been grown by the Czochralski technique and have originated from several different suppliers. They have been diffused with gold so that N_T ? 0.1 (N_D-N_A). Measurements have been made on both Schottky diodes and diffused junctions and similar results obtained from all samples. The electron cross section of the acceptor level was found to be (0.85±0.2) × 10^?16cm² and the hole cross section of the donor (3.5±0.8) × 10^?15cm², both were essentially temperature independent. The hole cross section of the acceptor was (0.9±0.2) × 10^?14cm² at 300 K and showed a T^?1.3 temperature dependence. The electron cross section of the donor was (0.9±0.2) × 10^?15cm² at 180 K with a T^?2 dependence.     

The dopant density and temperature dependence of hole mobility and resistivity in boron doped silicon

Theoretical expressions for computing resistivity and conductivity mobility of holes as functions of dopant density and temperature have been derived for boron-doped silicon. The model is applicable for dopant densities from 10¹³ to 3 × 10¹⁸ cm^?3 and temperatures between 100 and 400 K.Using a 3-band [i.e. heavy-hole, light-hole and the spin-orbit splitting (SO) band] model, the hole mobility was calculated by properly combining the contributions from scattering by lattice phonons, ionized impurities and neutral impurities. In addition, the effects of hole-hole (h-h) scattering and nonparabolicity of valence bands were taken into account in the mobility formulation.To verify our theoretical calculations, resistivity measurements on nine boron-doped silicon slices with dopant densities from 4.5 × 10¹⁴ to 3.2 × 10¹⁸ cm^?3 were performed for 100 ≤ T ≤ 400 K, using planar square-array test structure. Agreement between our calculated and measured resistivity values was within 6 percent over the entire range of dopant density and temperature studied here. Excellent agreement (within ±5%) between our calculated hole mobility values and those of Wagner [9] was obtained for N_A ≤ 10¹⁷ cm^?3 for boron-doped silicon, while discrepancies were found for boron densities greater than 10¹⁷ cm^?3. This discrepancy is attributed to neglecting the effect of deionization of boron impurities at higher dopant densities by Wagner (i.e. assuming hole density is equal to the total boron density).     

Electrophysical properties of solid solutions of silver in PbTe

The thermopower coefficient α₀ and the electrical conductivity σ of Pb_{1 − x} Ag _x Te solid solutions, where x = (0–0.007), are measured at T = 300 K. The hole concentration p is calculated. All samples are of the p type. With increasing silver content, α₀ decreases, while p and σ increase. For undoped crystals, α₀ = 251.0 μV/K, p = 1.1 × 10¹⁸ cm⁻³, and σ = 165 Ω⁻¹ cm⁻¹. At the silver-solubility limit for x = 0.007, α₀ = 193.8 μV/K, p = 2.3 × 10¹⁸ cm⁻³, and σ = 216 Ω⁻¹ cm⁻¹. The hole concentration in all samples is much lower than the concentration of introduced silver atoms. The hole gas in Pb_{1 − x} Ag _x Te solid solutions is weakly degenerate in the entire silver-concentration range.     

The mechanisms of hole scattering in p-Hg0.8Cd0.2Te crystals at low temperatures

The resistivity and Hall effect were investigated in p-Hg_0.8Cd_0.2Te crystals that contained from 1.5×10¹⁵ to 1.7×10¹⁸ cm^?3 Cu atoms. The measurements were carried out in the temperature range of 4.2–100 K. It is demonstrated that, in order to correctly determine the Hall mobility of holes at low temperatures, one should exclude the contribution of hopping charge transfer. It was found that heavy holes are scattered at 77 K by each other, by impurity ions, by composition fluctuations, and by lattice vibrations. In compensated crystals, holes are scattered only by lattice vibrations at low temperatures. For uncompensated crystals, when calculating the mobility, it is necessary to make allowance for the hole scattering by positively charged centers formed due to trapping of excess holes by acceptors.     

Reflectance of a PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript> crystal in a wide spectral range

For the anisotropic PbSb₂Te₄ single crystal with a high hole concentration (p ≈ 3 × 10²⁰ cm^–3) and high conductivity (σ ≈ 2500 Ω^–1 cm^–1), the reflectance spectrum for a cleavage plane orthogonal to the trigonal axis C₃ is recorded in a wide spectral range from 50 to 50000 cm^–1. It is shown that, in the long-wavelength and mid-infrared regions, the reflectance can be described with consideration for the contribution of plasma vibrations and two crystal-lattice vibrations. The quantitative characteristics of these vibrations are determined. The characteristics are in satisfactory agreement with the electrical parameters. The discrepancies between the values of the hole effective mass calculated by different methods are attributed to the complex structure of the crystal’s valence band.     

Structural changes induced by neutron irradiation and heat treatments in InSb single crystals

Structural transformations in InSb crystals exposed to fast neutrons (with energies E > 0.1 MeV) and to full-spectrum reactor neutrons with the ratio between the flux densities of slow and fast neutrons ?_sn/?_fn ≈ 1 are studied. It is shown that, in regard to the effect of fast neutrons on the lattice spacing, two portions can be distinguished in its dependence on the irradiation dose. At small fluences of fast neutrons (F _fn < 2.5 × 10¹⁷ cm^?2), no increase in the lattice spacing is observed. As follows from the diffuse X-ray scattering data, in this region of fluences, the clusters of vacancies dissociate and the number of small-sized interstitial-type clusters increases. At F _fn > 2.5 × 10¹⁷ cm^?2, the lattice spacing increases linearly with increasing fluences of neutrons, and numerous small-sized vacancy clusters and interstitial clusters capable of trapping the Sn dopant atoms are formed. Heat treatment of the exposed samples at temperatures up to 400°C results in complete restoration of the lattice spacing.     

Anisotropy of the thermopower in higher silicide of transitions metals

A new technique for measuring the thermopower anisotropy using polycrystals on the basis of repeated measurements of an array of randomly oriented microcrystals and statistical analysis of the data obtained is proposed. The thermopower anisotropy of a series of higher silicides of transition metals (β-FeSi₂, doped MnSi_1.75, ReSi_1.75, and CrSi₂) is measured using polycrystals. It is found that, in β-FeSi₂ crystals, a very high thermopower anisotropy can be observed; in ReSi_1.75 crystals, the thermopower can have different signs; and, in CrSi₂, the absolute value of the thermopower depends heavily on the heat-treatment mode. The temperature dependences of the electrical conductivity and thermopower of CrSi₂ needles are measured. The thermopower anisotropy in CrSi₂ needles, as in bulk single crystals, is retained in a wide temperature range.     

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.     

On the density-of-states effective mass and charge-carrier mobility in heteroepitaxial films of bismuth telluride and Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> solid solution

It is shown that the Seebeck coefficient α, the power factor α²σ, and the density-of-states effective mass m/m ₀ in heteroepitaxial films of Bi0.5Sb1.5Te3 solid solution are higher than the corresponding characteristics of bulk thermoelectric materials. The elevated values and weak temperature dependences of these parameters lead to a rise in the parameter proportional to the effective mass, the charge-carrier mobility, and the figure of merit. The character of change in α, α²σ, and m/m ₀ is determined by the peculiarities of the mechanism of charge-carrier scattering, the anisotropy of the constant-energy surface, and the possible influence of topological surface states of Dirac fermions in the films.     

Effect of a phase transition on the electron energy spectrum in Ag<Subscript>2</Subscript>S

Temperature dependences of electrical conductivity σ, Hall coefficient R, and thermopower α₀ in Ag₂S are reported. It is established that at T ≈ 435 ± 5 K, all kinetic parameters vary drastically, which is associated with a change in parameters of the conduction band. It is shown that the dispersion law of electron energy in β-Ag₂S corresponds to the Kane model.     

Thermal emission rates and capture cross sections of majority carriers at vanadium centers in silicon

The thermal emission rates and capture cross sections of majority carriers on the vandium associated centers in the depletion region of reverse biased silicon p-n junctions have been measured by the dark capacitance transient method. The three vanduim associated levels observed, two donor levels and a deep acceptor level, belong to the same vandium center. Least square fits of the emission data give the following emission rates; e_nl^t = 1.047 × 10⁶T² exp [?0.179±0.004 eV/kT], e_n0^t = 3.55 × 10⁷T² exp [?0.426±0.004 eV/kT] and e_p-2^t = 1.514 × 10⁶T² exp [?0.450±0.003 eV/kT]. The activation energy of the hole emission rate at the lower donor level is about 0.1 eV larger than the equilibrium thermal activation energy. The capture cross sections are σ_n0 = 3 × 10^?17cm² and σ_p0 = 8 × 10^?16cm² for the electron capture process at the deep acceptor level and the hole capture process at the upper donor level, respectively. The hole capture cross section on the lower donor level (σ_p-1) depends significantly on temperature. The large temperature dependence of the hole capture cross section can be expected due to the nonradiative multiphonon emission process.