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).     

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.     

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.     

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.     

Thermal stress effects of Ge2Sb2Te5 phase change material: Irreversible modification with Ti adhesion layers and segregation of Te

This paper reviews material properties of chalcogenide phase change material Ge₂Sb₂Te₅ under thermal anneal treatments. Stress evolutions of pure Ge₂Sb₂Te₅ films and stacks of Ge₂Sb₂Te₅ integrating with Ti adhesion layers are investigated. Segregation of Te atoms in the Ge₂Sb₂Te₅ film to the interface drives an interaction between Ti and Te atoms and formation of Ti-Te binary phases. The irreversible phase segregation and modification of Ge₂Sb₂Te₅ change the crystallization process, completely suppress the final transformation into otherwise stable hcp phase, and thus impact the ultimate life-cycle of such a phase change based memory cell. Since the adhesion layer is required in cell applications, the optimization of adhesion layer material and thickness may improve the life-cycles and reliability of devices.     

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.     

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.     

A high-selective positive-type developing technique for phase-change inorganic resist Ge2Sb2(1−x)Bi2xTe5

Recently chalcogenide phase-change resist Ge₂Sb_2(1−x)Bi_2xTe₅, which is compatible in next generation full-vacuum microelectronic manufacturing, has been paid much more attention due to the its excellent properties, such as high etching selectivity between Si and Ge₂Sb_2(1−x)Bi_2xTe₅ (about 500), wide spectral absorption and able to be prepared in vacuum. However, the very low developing selectivity (lower than 5) between its crystalline and amorphous phase limits its application in lithography. Here we developed a novel high-selective developing method to significantly improve the selectivity up to 22 (5 times than before), which enables the inorganic resist to be workability. Moreover, the developing mechanism is revealed, and this is helpful to dry developing technology of Ge₂Sb_2(1−x)Bi_2xTe₅.     

Characteristics of chalcogenide nonvolatile memory nano-cell-element based on Sb2Te3 material

Phase-change nonvolatile memory cell elements composed of Sb₂Te₃ chalcogenide have been fabricated by using the focused ion beam method. The contact size between the Sb₂Te₃ phase change film and electrode film in the cell element is 2826 nm² (diameter: 60 nm). The thickness of the Sb₂Te₃ chalcogenide film is 40 nm. The threshold switching current of about 0.1 mA was obtained. A RESET pulse width as short as 5 ns and the SET pulse width as short as 22 ns for Sb₂Te₃ chalcogenide can be obtained. At least 1000 cycle times with a RESET/SET resistance ratio >30 times is achieved for Sb₂Te₃ chalcogenide C-RAM cell element.     

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.     

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.     

Influence of Element Substitution on Corrosion Behavior of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Compounds

Atmospheric water may condense on the surface of Bi₂Te₃-based compounds constituting the Peltier module, depending on the operating environment used. In the stage of disposal, Bi₂Te₃-based compounds may come into contact with water in waste disposal sites. There are very few publications about the influence of condensed water on Peltier modules. Bi₂Te₃-Sb₂Te₃ or Bi₂Te₃-Bi₂Se₃ pseudo binary system compounds are used as p-type material or n-type material, respectively. The lattice distortion will be induced in the crystal of Bi₂Te₃-based compounds by element substitution due to the reduction in their thermal conductivity. However, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds remains unclear. In this study, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds with practical compositions has been investigated. Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 was prepared by the vertical Bridgman method. The electrochemical properties at room temperature were evaluated by cyclic voltammetry in a standard three-electrode cell. The working electrolyte was a naturally aerated 0.6 or 3.0 mass% NaCl solution. From the tendency for corrosion potential for all the samples, the corrosion sensitivity of ternary compounds was slightly higher than that of binary compounds. From the trend of current density, it was found that Bi_0.5Sb_1.5Te₃ had a corrosion resistance intermediate between Bi₂Te₃ and Sb₂Te₃. On the other hand, corrosion resistance was affected despite a small amount of Se substitution, and the corrosion resistance of Bi₂Te_2.85Se_0.15 was close to or lower than that of Bi₂Se₃. From the observation results of the corrosion products, the trends of morphology and composition of corrosion products for Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 were consistent with those of Sb₂Te₃ or Bi₂Se₃, respectively. From the results of x-ray photoelectron spectroscopy for the electrolyte after testing, the possibility that a corrosion product diffuses to the environment including the salt was suggested in Bi_0.5Sb_1.5Te₃. However, the amount of dissolved corrosion product was very low, and the chemical stability of the corrosion product was not changed or improved by element substitution.     

Electrodeposition of <Emphasis Type="Italic">p</Emphasis>-Type Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Thermoelectric Films

Thermoelectric Sb _x Te _y films were potentiostatically electrodeposited in aqueous nitric acid electrolyte solutions containing different concentrations of TeO₂. Stoichiometric Sb _x Te _y films were obtained by applying a voltage of −0.15 V versus saturated calomel electrode (SCE) using a solution consisting of 2.4 mM TeO₂, 0.8 mM Sb₂O₃, 33 mM tartaric acid, and 1 M HNO₃. The nearly stoichiometric Sb₂Te₃ films had a rhombohedral structure, R[`3]m R\bar{3}m , with a preferred orientation along the (015) direction. The films had hole concentration of 5.8 × 10¹⁸/cm³ and exhibited mobility of 54.8 cm²/Vs. A more negative potential resulted in higher Sb content in the deposited Sb _x Te _y films. Furthermore, it was observed that the hole concentration and mobility decreased with increasingly negative deposition potential, and eventually showed insulating properties, possibly due to increased defect formation. The absolute value of the Seebeck coefficient of the as-deposited Sb₂Te₃ thin film at room temperature was 118 μV/K.     

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.     

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.     

Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Compared with its Solvothermally Prepared Counterpart

We report on the successful hydrothermal synthesis of Bi_0.5Sb_1.5Te₃, using water as the solvent. The products of the hydrothermally prepared Bi_0.5 Sb_1.5Te₃ were hexagonal platelets with edges of 200–1500 nm and thicknesses of 30–50 nm. Both the Seebeck coefficient and electrical conductivity of the hydrothermally prepared Bi_0.5Sb_1.5Te₃ were larger than those of the solvothermally prepared counterpart. Hall measurements of Bi_0.5Sb_1.5Te₃ at room temperature indicated that the charge carrier was p-type, with a carrier concentration of 9.47 × 10¹⁸ cm⁻³ and 1.42 × 10¹⁹ cm⁻³ for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively. The thermoelectric power factor at 290 K was 10.4 μW/cm K² and 2.9 μW/cm K² for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively.     

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}.     

Thermoelectric Characteristics of <Emphasis Type="Italic">p</Emphasis>-Type (Bi,Sb)<Subscript>2</Subscript>Te<Subscript>3</Subscript>/(Pb,Sn)Te Functional Gradient Materials with Variation of the Segment Ratio

The p-type (Bi,Sb)₂Te₃/(Pb,Sn)Te functional gradient materials (FGMs) were fabricated by hot-pressing mechanically alloyed (Bi_0.2Sb_0.8)₂Te₃ and 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te powders together at 500°C for 1 h in vacuum. Segment ratios of (Bi,Sb)₂Te₃ to (Pb,Sn)Te were varied as 3:1, 1.3:1, and 1:1.6. A reaction layer of about 350-μm thickness was formed at the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGM interface. Under temperature differences larger than 340°C applied across a specimen, superior figures of merit were predicted for the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs to those of (Bi_0.2Sb_0.8)₂Te₃ and (Pb_0.7Sn_0.3)Te. With a temperature difference of 320°C applied across a specimen, the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs with segment ratios of 3:1 and 1.3:1 exhibited the maximum output powers of 72.1 mW and 72.6 mW, respectively, larger than the 63.9 mW of (Bi_0.2Sb_0.8)₂Te₃ and the 26 mW of 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te.     

Crystallization ability and optical and electrical properties of Ge<Subscript>10</Subscript>(Se-Te)<Subscript>90</Subscript> and Ge<Subscript>30</Subscript>(Se-Te)<Subscript>70</Subscript> chalcogenide glassy semiconductors

Chalcogenide glassy semiconductors of the ternary system Ge-Se-Te along the Ge₁₀(Se-Te)₉₀ and Ge₃₀(Se-Te)₇₀ joins have been synthesized. The crystallization ability, near-IR transmission spectra, and temperature dependence of the electrical conductivity of the alloys obtained have been studied. It is shown that chalcogenide glassy semiconductors along the Ge₁₀(Se-Te)₉₀ join have a lower softening and crystallization points compared with semiconductors belonging to the Ge₃₀(Se-Te)₇₀ join. A change in the electrical conductivity of samples by several orders of magnitude occurs upon a phase transition from the glassy to the crystalline state. Compositions of chalcogenide glassy semiconductors in the Ge-Se-Te system are found, which have α < 1 cm⁻¹ absorption coefficient at wavelengths of λ ≈ 1.5 μm and exhibit a thermally induced phase transition from the glassy to the crystalline state.     

A comparative study on the electronic and optical properties of Sb<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin film

The thin film of Sb₂Se₃ was deposited by thermal evaporation method and the film was annealed in N₂ flow in a three zone furnace at a temperature of 290°С for 30 min. The structural properties were characterized by scanning electron microscopy (SEM), transmission electron microscopy (ТЕМ), X-ray diffraction (XRD) and Raman spectroscopy, respectively. It is seen that the as-deposited film is amorphous and the annealed film is polycrystalline in nature. The surface of Sb₂Se₃ film is oxidized with a thickness of 1.15 nm investigated by X-ray photolecetron spectroscopy (XPS) measurement. Spectroscopic ellipsometry (SE) and UV–vis spectroscopy measurements were carried out to study the optical properties of Sb₂Se₃ film. In addition, the first principles calculations were applied to study the electronic and optical properties of Sb₂Se₃. From the theoretical calculation it is seen that Sb₂Se₃ is intrinsically an indirect band gap semiconductor. Importantly, the experimental band gap is in good agreement with the theoretical band gap. Furthermore, the experimental values of n, k, ε₁, and ε₂ are much closer to the theoretical results. However, the obtained large dielectric constants and refractive index values suggest that exciton binding energy in Sb₂Se₃ should be relatively small and an antireflective coating is recommended to enhance the light absorption of Sb₂Se₃ for thin film solar cells application.