Electrodeposition and Thermoelectric Characteristics of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films for Thermopile Sensor Applications

A thermopile sensor was processed on a glass substrate by electrodeposition of n-type bismuth telluride (Bi-Te) and p-type antimony telluride (Sb-Te) films. The n-type Bi-Te film electrodeposited at −50 mV in a 50 mM electrolyte with a Bi/(Bi + Te) mole ratio of 0.5 exhibited a Seebeck coefficient of −51.6 μV/K and a power factor of 7.1 × 10⁻⁴ W/K² · m. The p-type Sb-Te film electroplated at 20 mV in a 70 mM solution with an Sb/(Sb + Te) mole ratio of 0.9 exhibited a Seebeck coefficient of 52.1 μV/K and a power factor of 1.7 × 10⁻⁴ W/K² · m. A thermopile sensor composed of 196 pairs of the p-type Sb-Te and the n-type Bi-Te thin-film legs exhibited sensitivity of 7.3 mV/K.     

Thermoelectric Properties of Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript> and Ni<Subscript>0.06</Subscript>Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript>

Mo₃Sb₇, crystallizing in the Ir₃Ge₇ type structure, has poor thermoelectric (TE) properties due to its metallic behavior. However, by a partial Sb-Te exchange, it becomes semiconducting without noticeable structure changes and so achieves a significant enhancement in the thermopower with the composition of Mo₃Sb₅Te₂. Meanwhile, large cubic voids in the Mo₃Sb₅Te₂ crystal structure provide the possibility of filling the voids with small cations to decrease the thermal conductivity by the so-called rattling effect. As part of the effort to verify this idea, we report herein the growth as well as measurements of the thermal and electrical transport properties of Mo₃Sb_5.4Te_1.6 and Ni_0.06Mo₃Sb_5.4Te_1.6.     

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.     

Switching effects in the films based on Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript>

Experimental results on the switching effects related to the phase transitions in Ge₂Sb₂Te₅ in the presence of external voltage or laser irradiation are presented. An electron model of the reversible switching is discussed.     

Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Superlattices Grown by Nanoalloying

In this work, Bi₂Te₃-Sb₂Te₃ superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V₂VI₃ compounds. Nanoalloyed super- lattices consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb₂Te₃ layers were p-type as expected; however, it was impossible to synthesize p-type Bi₂Te₃ layers. Hence the Bi₂Te₃-Sb₂Te₃ superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.     

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.     

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.     

Characterization of Ge<Subscript>22</Subscript>Sb<Subscript>22</Subscript>Te<Subscript>56</Subscript> and Sb-Excess Ge<Subscript>15</Subscript>Sb<Subscript>47</Subscript>Te<Subscript>38</Subscript> Chalcogenide Thin Films for Phase-Change Memory Applications

A phase-change memory device that utilizes an antimony (Sb)-excess Ge₁₅Sb₄₇Te₃₈ chalcogenide thin film was fabricated and its electrical properties were measured and compared with a similar device that uses Ge₂₂Sb₂₂Te₅₆. The resulting electrical characteristics exhibited I _reset values of 14 mA for Ge₂₂Sb₂₂Te₅₆ and 10.6 mA for Ge₁₅Sb₄₇Te₃₈. Also, the set operation time (t _set) for the device using Ge₁₅Sb₄₇Te₃₈ films was 140 ns, which was more than twice as fast as the Ge₂₂Sb₂₂Te₅₆ device. The relationship between the microstructure and the improved electrical performance of the device was examined by means of transmission electron microscopy (TEM).     

Thermal Stability of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> in Contact with Ti and TiN

The thermal stability of a Ge₂Sb₂Te₅ chalcogenide layer in contact with titanium and titanium nitride metallic thin films has been investigated mainly using x-ray diffraction and elastic nuclear backscattering techniques. Without breaking vacuum, Ti and TiN have been deposited on Ge₂Sb₂Te₅ material using magnetron sputtering. Thermal treatments have been performed in a 10⁻⁷ mbar vacuum furnace. On annealing up to 450°C, the TiN metallic film does not interact with the chalcogenide film, but at the same time adhesion problems and instabilities in contact resistance arise. To improve the adhesion and eventually stabilize the contact resistance, an interfacial Ti layer has been considered. At 300°C, a TiTe₂ compound is formed by interacting with Te segregated from the Ge₂Sb₂Te₅ layer. At higher temperatures, the Ti layer decomposes the chalcogenide film, forming several compounds tentatively identified as GeTe, Ge₃Ti₅, Ge₅Ti₆, TiTe_2,, and Sb₂Te₃. It has been found that the properties of the Ge₂Sb₂Te₅ film can be retained by controlling the decomposition rate of the chalcogenide layer, which is achieved by providing a limited supply of Ti and/or by depositing a Te-rich Ge₂Sb₂Te₅ film.     

Thermoelectric Properties of Cu-doped Bi<Subscript>0.4</Subscript>Sb<Subscript>1.6</Subscript>Te<Subscript>3</Subscript> Prepared by Hot Extrusion

Cu_0.003Bi_0.4Sb_1.6Te₃ alloys were prepared by using encapsulated melting and hot extrusion (HE). The hot-extruded specimens had the relative average density of 98%. The (00l) planes were preferentially oriented parallel to the extrusion direction, but the specimens showed low crystallographic anisotropy with low orientation factors. The specimens were hot-extruded at 698 K, and they showed excellent mechanical properties with a Vickers hardness of 76 Hv and a bending strength of 59 MPa. However, as the HE temperature increased, the mechanical properties degraded due to grain growth. The hot-extruded specimens showed positive Seebeck coefficients, indicating that the specimens have p-type conduction. These specimens exhibited negative temperature dependences of electrical conductivity, and thus behaved as degenerate semiconductors. The Seebeck coefficient reached the maximum value at 373 K and then decreased with increasing temperature due to intrinsic conduction. Cu-doped specimens exhibited high power factors due to relatively higher electrical conductivities and Seebeck coefficients than those of undoped specimens. A thermal conductivity of 1.00 Wm^?1 K^?1 was obtained at 373 K for Cu_0.003Bi_0.4Sb_1.6Te₃ hot-extruded at 723 K. A maximum dimensionless figure of merit, ZT _max = 1.05, and an average dimensionless figure of merit, ZT _ave = 0.98, were achieved at 373 K.     

Thermoelectric properties of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> ribbons prepared by melt spinning

The results of studying the thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ alloy samples prepared by melt spinning quenching are presented. The material after melt spinning is shaped as thin ribbons and has a quasi-amorphous structure. The thermoelectric properties (thermoelectric power and electrical resistance) and crystallization processes of as-prepared melt-spun ribbons are studied at 300–800 K for the first time. The stability range of the initial state, the crystallization-onset temperature, and the effect of thermal annealing on the thermoelectric-power factor of the alloy are determined.     

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.     

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}.     

Electrical and Thermal Conductivity and Conduction Mechanism of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> Alloy

Ge₂Sb₂Te₅ alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge₂Sb₂Te₅ alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge₂Sb₂Te₅ alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge₂Sb₂Te₅ alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge₂Sb₂Te₅ alloy accounts for the thermal conduction mechanism.     

Phase transitions in thin Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> chalcogenide films according to Raman spectroscopy data

Data on the Raman spectra of thin Ge₂Sb₂Te₅ chalcogenide semiconductor films are reported. The study is performed with the purpose of determining the temperatures of phase transitions initiated by laser radiation.     

Response of thermoelectric parameters of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> films to secondary recrystallization

Flash evaporation is used to grow Bi_0.5Sb_1.5Te₃ films 1200 nm thick on mica substrates. The average lateral crystallite sizes in the as-grown films are ~800 nm. The (0001) plane in the crystallites is preferentially parallel to the substrate plane. After heat treatment in an argon atmosphere, the effective lateral size of crystallites in which the third-order axis is perpendicular to the substrate plane increased by a factor of 3–5. The crystallites were preferentially oriented in the substrate plane as well. The thermoelectric-power parameter of Bi_0.5Sb_1.5Te₃ films after their heat treatment in an inert environment increased approximately twofold to values close to that of the corresponding single crystals.     

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.     

Thermal Conductivity of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Affected by Grain Size and Pores

A fine measurement system for measuring thermal conductivity was constructed. An accuracy of 1% was determined for the reference quartz with a value of 1.411 W/m K. Bi_0.5Sb_1.5Te₃ samples were prepared by mechanical alloying followed by hot-pressing. Grain sizes were varied in the range from 1 μm to 10 μm by controlling the sintering temperature in the temperature range from 623 K to 773 K. The thermal conductivity was 0.89 W/m K for the sample sintered at 623 K, while a grain size of 1.75 μm was measured by optical microscopy and scanning electron microscopy. The thermal conductivity increased on the sample sintered at 673 K because of grain growth and decreased on those sintered at the temperatures from 673 K to 773 K because the increase of pore size caused to decrease thermal conductivity. The increase of thermal conductivity for the samples sintered at temperatures above 773 K was affected by the increase of carrier concentration.     

Electrical and Structural Real-Time Changes in Thin Thermoelectric (Bi<Subscript>0.15</Subscript>Sb<Subscript>0.85</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films by Dynamic Thermal Treatment

A recent trend in thermoelectrics is miniaturization of generators or Peltier coolers using the broad spectrum of thin-film and nanotechnologies. Power supplies for energy self-sufficient micro and sensor systems are a wide application field for such generators. It is well known that thermal treatment of as-deposited p-type (Bi_0.15Sb_0.85)₂Te₃ films leads to enhancement of their power factors. Whereas up to now only the start (as-deposited) and the end (after annealing) film stages were investigated, herein for the first time, the dynamical changes of sputter-deposited film properties have been observed by real-time measurements. The electrical conductivity shows a distinct, irreversible increase during a thermal cycle of heating to about 320°C followed by cooling to room temperature. The interpretation of the Seebeck and Hall coefficients points to an enhancement in Hall mobility after annealing. In situ x-ray diffractometry shows the generation of an additional Te phase depending on temperature. This is also confirmed by energy-dispersive x-ray microanalysis and the corresponding mapping by scanning electron microscopy. It is presumed that the Te enrichment in a separate, locally well-defined phase is the reason for the improvement in the integral film transport properties.