Charge carrier transport in Ge<Subscript>20</Subscript>As<Subscript>20</Subscript>S<Subscript>60</Subscript> chalcogenide semiconductor films

Charge-carrier transport in Ge₂₀As₂₀S₆₀ films has been studied using the transit time method under low-injection conditions at room temperature. It was found that drift mobilities of electrons and holes in Ge₂₀As₂₀S₆₀ films are close to each other, i.e., μ _e ≈ μ _h ≈ 2 × 10⁻³ cm² V⁻¹ s⁻¹ at T = 295 K and F = 5 × 10⁴ V/cm. It was shown that the time dependence of the photocurrent during carrier drift and the voltage dependence of the drift mobility allowed the use of the concept of anomalous dispersive transport. Experimental data were explained using the model of transport controlled by carrier trapping by localized states with energy distribution near conduction and valence band edges described by the exponential law with a characteristic energy of ∼0.05 eV.     

Doping of the Bi<Subscript>1.9</Subscript>Sb<Subscript>0.1</Subscript>Te<Subscript>3</Subscript> solid solution with Sn impurity

In (Bi_1.9Sb_0.1)_{1 − x} Sn _x Te₃ solid solution with different contents of Sn, the electrical conductivity (σ₁₁) and the Hall (R ₁₂₃ and R ₃₂₁), Seebeck (S ₁₁ and S ₃₃), and Nernst-Ettingshausen (Q ₁₂₃ and Q ₃₂₁) coefficients have been measured. It is shown that doping with tin strongly modifies temperature dependences of the kinetic coefficients. The effect of tin on electrical homogeneity of the samples has been studied: with increasing number of Sn atoms embedded, crystals become more homogeneous. These features indicate the presence of the quasi-local states of Sn in the valence band of Bi_1.9Sb_0.1Te₃. Within a one-band model, we estimated the effective mass of the density of hole states (m _d ), the energy gap extrapolated to 0 K (E _g0 = 0.20–0.25 eV), the energy of impurity states (E _Sn ≈ 40–45 meV), and the scattering parameter (r ≈ 0.1–0.4). Numerical values of the scattering parameter indicate a mixed mechanism of scattering in the samples under investigation with dominant scattering at acoustic phonons. With increasing content of tin in the samples, the contribution of impurity scattering increases.     

Thermoelectric Properties of Sb-Doped Mg<Subscript>2</Subscript>Si<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>

Mg₂(Si_0.3Sn_0.7)_1−y Sb _y (0 ≤ y ≤ 0.04) solid solutions were prepared by a two-step solid-state reaction method combined with the spark plasma sintering technique. Investigations indicate that the Sb doping amount has a significant impact on the thermoelectric properties of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y compounds. As the Sb fraction y increases, the electron concentration and electrical conductivity of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y first increase and then decrease, and both reach their highest value at y = 0.025. The sample with y = 0.025, possessing the highest electrical conductivity and one of the higher Seebeck coefficient values among all the samples, has the highest power factor, being 3.45 mW m⁻¹ K⁻² to 3.69 mW m⁻¹ K⁻² in the temperature range of 300 K to 660 K. Meanwhile, Sb doping can significantly reduce the lattice thermal conductivity (κ _ph) of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y due to increased point defect scattering, and κ _ph for Sb-doped samples is 10% to 20% lower than that of the nondoped sample for 300 K < T < 400 K. Mg₂(Si_0.3Sn_0.7)_0.975Sb_0.025 possesses the highest power factor and one of the lower κ _ph values among all the samples, and reaches the highest ZT value: 1.0 at 640 K.     

Superionic conductivity and switching effect with memory in TlInSe<Subscript>2</Subscript> and TlInTe<Subscript>2</Subscript> crystals

The temperature dependences of the conductivity σ(T) and the switching and memory effects in one-dimensional TlInSe₂ and TlInTe₂ single crystals have been studied. A specific feature is found in the dependence σ(T) above 333 K, which is related to the transition of crystals to the state with superionic conductivity. It is suggested that the ion conductivity is caused by the diffusion of Tl⁺ ions over vacancies in the thallium sublattice between (In³⁺Te₂²⁻)⁻ and (In³⁺Se₂²⁻)⁻ nanochains (nanorods). S-type switching and memory effects are revealed in TlInSe₂ and TlInTe₂ crystals, as well as voltage oscillations in the range of negative differential resistance. It is suggested that the switching effect and voltage oscillations are related to the transition of crystals to the superionic state, which is accompanied by “melting” of the Tl sublattice. The effect of electric-field-induced transition of TlInSe₂ and TlInTe₂ crystals to the superionic state is found.     

Phonon drag of electrons in Ag<Subscript>2</Subscript>S

The temperature dependences of the heat-conductivity coefficient χ and the thermopower 6h of Ag₂S are investigated in the range of 4.2–300 K. It is found that the value of 6h sharply increases (6h ∞ T^-3) with decreasing T at T < 100 K and passes through a maximum at 16–18 K. The heat-conductivity coefficient passes through a maximum at ≈30 K. The sharp increase in 6h is found to be caused by the effect of long-wavelength-phonon drag of electrons. It is shown that the shift of the 6h and χ peaks, as well as the temperature dependence of the phonon thermopower 6h_ph ∞ T^-3, agrees with the Herring theory.     

Nernst-ettingshausen tensor in Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystal

On one Sb₂Te₃ single crystal, the temperature dependences of all three independent components of the Nernst-Ettingshausen tensor (Q _ikl ) are measured in the temperature range of 85–450 K, all three components being negative. Alongside with the Nernst-Ettingshausen effect, the anisotropy of the Hall (R _ikl ) and Seebeck (S _ij ) coefficients and the conductivity (σ _ii ) is also investigated. The carried-out analysis of the experimental data on the Nernst-Ettingshausen and Seebeck effects indicates that there is the mixed scattering mechanism with the participation of acoustic phonons and impurity ions, the relative contributions of these mechanisms varying with temperature. In the relaxation-time-tensor approximation, the values of the effective scattering parameter (r) are determined. The obtained values point to the dominant scattering at acoustic phonons in the cleavage plane and to the substantial contribution of charged ions to the scattering along the trigonal axis c ₃. It is shown that it is possible to explain the major features of experimental data on the Nernst-Ettingshausen effect within the two-valence-band model with the participation of several groups of holes in the transport phenomena.     

The variance law and scattering mechanism of charge carriers in Zn-doped <Emphasis Type="Italic">p</Emphasis>-In<Subscript>0.5</Subscript>Ga<Subscript>0.5</Subscript>Sb

Temperature dependences of the electrical conductivity, Hall coefficient, and thermoelectric power of Zn-doped alloys of the equimolar composition In_0.5Ga_0.5Sb are studied. The concentration and temperature dependences of the effective mass of holes are determined. It is shown that, for all doped samples at T < 200 K, the charge carriers are scattered by impurity ions and, at T > 200 K, scattering by lattice vibrations also introduces a substantial contribution.     

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.     

Heat Capacity Study on Anharmonicity in Ae<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Ge<Subscript>30</Subscript> (Ae = Sr and Ba)

The heat capacity of single-crystalline samples of Sr₈Ga₁₆Ge₃₀ (SGG) and Ba₈Ga₁₆Ge₃₀ (BGG) clathrates was measured to investigate the anharmonicity of the encapsulated atoms. At low temperatures, BGG can be well described by a standard Debye model, and the C _p/T ³ versus T plot can be fitted with two Einstein temperatures: θ _E1 = 42 K and θ _E2 = 74 K. On the other hand, SGG shows deviation from the Debye model. Moreover, neither the Einstein model nor the soft potential model (SPM) alone can fit the peak in the C _p/T ³ versus T plot, and the peak should be fitted by employing both models. Our results indicate that the effective electron mass is enhanced by the anharmonic phonons.     

Features of reflection spectra of single crystals of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions in the region of plasma effects

Reflectance spectra of single crystals of Bi₂Te₃-Sb₂Te₃ solid solutions containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, and 100 mol % of Sb₂Te₃ have been studied in the range of 400–4000 cm⁻¹ at the temperature T = 291 K and with orientation of the vector of the electric-field strength E perpendicular to the trigonal axis of the crystal C ₃ (E ⊥ C ₃). The shape of the spectra is characteristic of plasma reflection; the spectra include the features in the range 1250–3000 cm⁻¹ corresponding to the optical band gap E _{g opt}. The features become more pronounced as the content of Bi₂Te₃ is increased to 80 mol % in the composition of the Bi₂Te₃-Sb₂Te₃ solid solution. A further increase in the content of Sb₂Te₃ is accompanied by discontinuities in the functional dependences of the parameters characterizing the plasma oscillations of free charge carriers on the solid-solution composition and also by a sharp increase in E _{g opt}.     

Current-voltage characteristics of ZnGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> compound polycrystals

Current-voltage characteristics of the In-ZnGa₂Se₄-In structure have been studied in the temperature range of 90–335 K. Based on the data calculated for the concentration of three trap types in ZnGa₂Se₄, the values N _t = 1.4 × 10¹³, 8.2 × 10¹², and 2.6 × 10¹² cm⁻³ are obtained. The contact region transparency D _k ^*= 10⁻⁵, surface recombination velocity S _k = 0.65 m/s, and carrier lifetime τ = 1.5 × 10⁻⁴ s were determined. It was found that the current transmission mechanism in electric fields weaker than 10³ V/cm is caused by monopolar carrier injection.     

Thermoelectric Properties of Zr<Subscript>3</Subscript>Mn<Subscript>4</Subscript>Si<Subscript>6</Subscript> and TiMnSi<Subscript>2</Subscript>

The Seebeck coefficient, electrical resistivity, and thermal conductivity of Zr₃Mn₄Si₆ and TiMnSi₂ were studied. The crystal lattices of these compounds contain relatively large open spaces, and, therefore, they have fairly low thermal conductivities (8.26 Wm⁻¹ K⁻¹ and 6.63 Wm⁻¹ K⁻¹, respectively) at room temperature. Their dimensionless figures of merit ZT were found to be 1.92 × 10⁻³ (at 1200 K) and 2.76 × 10⁻³ (at 900 K), respectively. The good electrical conductivities and low Seebeck coefficients might possibly be due to the fact that the distance between silicon atoms in these compounds is shorter than that in pure semiconductive silicon.     

Thermoelectric Properties of <Emphasis Type="Italic">In Situ</Emphasis> Formed Bi<Subscript>0.85</Subscript>Sb<Subscript>0.15</Subscript>/Bi-Rich Particles Composite

A Bi-15 at.%Sb alloy, homogenized by equal channel angular extrusion (ECAE) at T = 523 K, has been treated just above its solidus temperature, causing segregation of a secondary Bi-rich phase at the grain boundaries. This process results in an in situ composite. The thermoelectric properties of the composite have been measured in the range of 5 K < T < 300 K. The results are compared with those of the homogeneous alloy. The presence of a Bi-rich phase improves the Seebeck coefficient at T < 50 K, and enhances the electrical conductivity by a factor of 1.4 at T = 300 K up to a factor of 3.4 at T = 50 K; unfortunately, the thermal conductivity also increases by about 50% in the same temperature range. As a result, the figure of merit, Z, is slightly suppressed above T = 110 K, but increases at lower temperatures, reaching a peak value of 4.2 × 10⁻³ K⁻¹ at T = 90 K. The power factor considerably increases over the whole temperature range, rendering this material suitable as the n-type leg of a cryogenic thermoelectric generator for cold energy recovery in a liquefied natural gas plant.     

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.     

Conductivity of layers of a chalcogenide glassy semiconductor Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> in high electric fields

Effect of high electric fields on the conductivity of 0.5-1-μm-thick layers of a chalcogenide glassy semiconductor with a composition Ge₂Sb₂Te₅, used in phase memory cells, has been studied. It was found that two dependences are observed in high fields: dependence of the current I on the voltage U, of the type I ∝ U ⁿ , with the exponent (n ≈ 2) related to space-charge-limited currents, and a dependence of the conductivity σ on the field strength F of the type σ = σ₀exp(F/F ₀) (where F ₀ = 6 × 10⁴ V cm⁻¹), caused by ionization of localized states. A mobility of 10⁻³–10⁻² cm² V⁻¹ s⁻¹ was determined from the space-charge-limited currents.     

Influence of the defect structure of γ-La<Subscript>2(1 − <Emphasis Type="Italic">x</Emphasis>)</Subscript>Nd<Subscript>2<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> crystals on their spectroscopic properties

The photoluminescence and Raman scattering of undoped γ-Ln₂S₃ single crystals (Ln is a rare earth ion) and the decay kinetics of the ⁴ F _3/2 level of Nd ions in these crystals have been investigated. The distortion of the decay curve of the Nd ⁴ F _3/2 level upon excitation by light with λ = 0.53 μm is explained.     

Effect of Vacancy Distribution on the Thermal Conductivity of Ga<Subscript>2</Subscript>Te<Subscript>3</Subscript> and Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript>

Our group has focused attention on Ga₂Te₃ as a natural nanostructured thermoelectric material. Ga₂Te₃ has basically a zincblende structure, but one-third of the Ga sites are structural vacancies due to the valence mismatch between Ga and Te. It has been confirmed that (1) vacancies in Ga₂Te₃ exist as two-dimensional (2D) vacancy planes, and (2) Ga₂Te₃ exhibits an unexpectedly low thermal conductivity (κ), most likely due to highly effective phonon scattering by the 2D vacancy planes. However, the effect of the size and periodicity of the 2D vacancy planes on κ has been unclear. In addition, it has also been unclear whether only the 2D vacancy planes reduce κ or if point-type vacancies can also reduce κ. In the present study, we tried to prepare Ga₂Te₃ and Ga₂Se₃ with various vacancy distributions by controlling annealing conditions. The atomic structures of the samples were characterized by means of transmission electron microscopy, and κ was evaluated from the thermal diffusivity measured by the laser flash method. The effects of vacancy distributions on κ of Ga₂Te₃ and Ga₂Se₃ are discussed.     

Effect of Rapid Thermal Annealing on Sputtered CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> Thin Films

Calcium copper titanium oxide (CaCu₃Ti₄O₁₂, abbreviated to CCTO) films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature (RT) by radiofrequency magnetron sputtering. As-deposited CCTO films were treated by rapid thermal annealing (RTA) at various temperatures and in various atmospheres. X-ray diffraction patterns and scanning electron microscope (SEM) images demonstrated that the crystalline structures and surface morphologies of CCTO thin films were sensitive to the annealing temperature and ambient atmosphere. Polycrystalline CCTO films could be obtained when the annealing temperature was 700°C in air, and the grain size increased signifi- cantly with annealing in O₂. The 0.8-μm CCTO thin film that was deposited at RT for 2 h and then annealed at 700°C in O₂ exhibited a high dielectric constant (ε′) of 410, a dielectric loss (tan δ) of 0.17 (at 10 kHz), and a leakage current density (J) of 1.28 × 10⁻⁵ A/cm² (at 25 kV/cm).     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> Skutterudite Materials with Partial In Filling and Excess In Additions

The properties of Co₄Sb₁₂ with various In additions were studied. X-ray diffraction revealed the presence of the pure δ-phase of In_0.16Co₄Sb₁₂, whereas impurity phases (γ-CoSb₂ and InSb) appeared for x = 0.25, 0.40, 0.80, and 1.20. The homogeneity and morphology of the samples were observed by Seebeck microprobe and scanning electron microscopy, respectively. All the quenched ingots from which the studied samples were cut were inhomogeneous in the axial direction. The temperature dependence of the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) was measured from room temperature up to 673 K. The Seebeck coefficient of all In-added Co₄Sb₁₂ materials was negative. When the filler concentration increases, the Seebeck coefficient decreases. The samples with In additions above the filling limit (x = 0.22) show an even lower Seebeck coefficient due to the formation of secondary phases: InSb and CoSb₂. The temperature variation of the electrical conductivity is semiconductor-like. The thermal conductivity of all the samples decreases with temperature. The central region of the In_0.4Co₄Sb₁₂ ingot shows the lowest thermal conductivity, probably due to the combined effect of (a) rattling due to maximum filling and (b) the presence of a small amount of fine-dispersed secondary phases at the grain boundaries. Thus, regardless of the non-single-phase morphology, a promising ZT (S ² σT/κ) value of 0.96 at 673 K has been obtained with an In addition above the filling limit.     

Synthesis and Electronic Properties of the Misfit Layer Compound [(PbSe)<Subscript>1.00</Subscript>]<Subscript>1</Subscript>[MoSe<Subscript>2</Subscript>]<Subscript>1</Subscript>

An ultralow-thermal-conductivity compound with the ideal formula [(PbSe)_1.00]₁[MoSe₂]₁ has been successfully crystallized across a range of compositions. The lattice parameters varied from 1.246 nm to 1.275 nm, and the quality of the observed 00ℓ diffraction patterns varied through the composition region where the structure crystallized. Measured resistivity values ranged over an order of magnitude, from 0.03 Ω m to 0.65 Ω m, and Seebeck coefficients ranged from −181 μV K⁻¹ to 91 μV K⁻¹ in the samples after the initial annealing to form the basic structure. Annealing of samples under a controlled atmosphere of selenium resulted in low conductivities and large negative Seebeck coefficients, suggesting an n-doped semiconductor. Scanning transmission electron microscopy cross-sections confirmed the interleaving of bilayers of PbSe with Se-Mo-Se trilayers. High-angle annular dark-field images revealed an interesting volume defect, where PbSe grew through a region where a layer of MoSe₂ would be expected in the perfect structure. Further studies are required to correlate the density of these defects with the observed electrical properties.