Thermoelectric Power of Sb2Te3 Films

The thermoelectromotive force of vacuum deposited Sb₂Te₃ films on glass substrates has been measured by the integral method in the temperature range 30–250 °C. The film thickness varied between 1200–9600 Å. Thick antimony pads evaporated onto the sample at both ends served as contacts. The thermoelectric power (α) is found to be positive and increases with temperature for thinner samples and at higher temperatures a steady value is achieved. Further increase in the temperature decreases the thermoelectric power. When the film thickness is increased, α decreases. This is explained as due to the reduction in intercrystalline barrier at elevated temperatures and at higher film thicknesses.     

Enhancement of thermoelectric efficiency in vapor deposited Sb2Te3 and Sb1.8In0.2Te3 crystals

Pure and indium doped antimony telluride (Sb₂Te₃) crystals find applications in high performance room temperature thermoelectric devices. Owing to the meagre physical properties exhibited on the cleavage faces of melt grown samples, an attempt was made to explore the thermoelectric parameters of p‐type crystals grown by the physical vapor deposition (PVD) method. The crystal structure of the grown platelets (9 mm× 8 mm× 2 mm) was identified as rhombohedral by x‐ray powder diffraction method. The energy dispersive analysis confirmed the elemental composition of the crystals. The electron microscopic and scanning probe image studies revealed that the crystals were grown by layer growth mechanism with low surface roughness. At room temperature (300 K), the values of Seebeck coefficient S (⊥ c) and power factor were observed to be higher for Sb_1.8In_0.2Te₃ crystals (155 μVK⁻¹, 2.669 × 10⁻³ W/mK²) than those of pure ones. Upon doping, the thermal conductivity κ (⊥ c) was decreased by 37.14% and thus thermoelectric efficiency was improved. The increased figure of merit, Z = 1.23 × 10⁻³ K⁻¹ for vapour grown Sb_1.8In_0.2Te₃ platelets indicates that it could be used as a potential thermoelectric candidate.     

Capacitance-voltage characteristics of In0.3Ge2Sb2Te5 thin films

The influence of indium doping on the capacitance variation with temperature and applied bias voltage of Ge₂Sb₂Te₅ is investigated. The capacitance-voltage (C-V) measurements of In_0.3Ge₂Sb₂Te₅ and Ge₂Sb₂Te₅ thin films were performed for a sweep of voltages from −20 to +20 V at different temperatures. The results show different capacitance behavior of In_0.3Ge₂Sb₂Te₅ and Ge₂Sb₂Te₅ films. As the temperature increases the capacitance of the indium-doped sample decreases and becomes negative. The negative capacitance effect might be attributed to a significant increase of the film’s conductivity due to temperature and applied bias voltage. The nonlinearity in the capacitance and conductivity could be related to the nucleation mechanism as the temperature becomes close to the amorphous-crystalline transition temperature.     

Effects of Ge addition on the optical and electrical properties of eutectic Sb70Te30 films

The aim of this work is to study the influence of Ge addition on the optical and electrical properties in eutectic SbTe thin films (with the compositions: Sb₇₀Te₃₀, Ge₂Sb₇₀Te₂₈, Ge₅Sb₇₀Te₂₅ and Ge₁₀Sb₆₅Te₂₅) using visible optical reflectance, ellipsometry measurements, near infrared transmittance spectra, and four probe electrical resistivity measurements. From near infrared transmittance measurements the optical band gap was determined using Tauc’s expression for amorphous materials, a value of about 0.47 eV was obtained without any clear dependence on the Ge content. All amorphous films have approximately the same reflectance value, however the contrast ratio between the crystalline and amorphous phases decrease with increase of Ge. Using in situ four probe measurements versus temperature the dependence of the activation energy of conductance and the onset of the crystallization temperature have been determined for different materials. Four probe measurements have shown that the resistivity of amorphous films increases with increase of Ge. The results obtained have shown that optical and electrical properties of SbTe films with near eutectic composition change with the Ge content and depending on the application, in either optical or electrical memory devices, the most suitable composition needs to be determined.     

The amorphous Sb1−xTex system

Amorphous films of Sb_1?xTe_x, where 0.11 ? x ? 0.86, have been prepared by coevaporation. Crystallization temperatures occur near room temperature for all compositions and appear to depend on film thickness. The optical and transport properties have been investigated as a function of temperature. Optical band gaps at room temperature, E_g, ranged between 0.3 and 0.7 eV and decreased non-linearly with temperature. For the composition Sb₂Te₃, E_g ≈ 0.7 eV, in contrast to the semimetallic character of the crystalline form. All compositions were p-type with the Fermi level close to the middle of the gap. The results have been interpreted in terms of the chemical bonding.     

Structure, electrical, optical and thermal properties of Ge4Sb4Tex (x = 8, 9 and 10) thin films

The crystalline samples of Ge₄Sb₄Te₁₀, Ge₄Sb₄Te₉, and Ge₄Sb₄Te₈ were prepared and their amorphous semiconducting thin films obtained by flash evaporation. Their sheet resistance decreased slowly with temperature up to 147–160 °C with activation energy of electrical conductivity ΔE = 0.40–0.44 eV. Above these temperatures, the sheet resistance drops abruptly by several orders due to crystallization. The drop of resistivity proceeds in two steps. Two steps of phase change were also found on curves of DSC and on the temperature dependence of index of refraction. It pays for slow heating rates, crystallization induced by short (≈30 ns) laser pulses proceeds probably in one step only for all studied samples (as it follows from X-ray diffraction), not only for Ge₂Sb₂Te₅ in which a single phase formation was confirmed. The crystallization temperatures are increasing slightly with decreasing Te content in the series Ge₄Sb₄Te₁₀–Ge₄Sb₄Te₉–Ge₄Sb₄Te₈ from 147 to 160 °C. The X-ray diffractograms revealed that in laser crystallized samples can be found only cubic modification of Ge₂Sb₂Te₅ type (a = 0.6 nm), while the samples annealed (230 °C, 2 h) or annealed after the crystallization with laser pulse, contain also small amounts of hexagonal phase.     

Smallest (∼10 nm) phase-change marks in amorphous and crystalline Ge2Sb2Te5 films

Electrical nano-scale crystallization and amorphization in amorphous and crystalline Ge₂Sb₂Te₅ films have been studied using scanning probe microscopes. In scanning tunneling microscopes, the phase changes can be induced, not by tunneling currents, but by conducting currents flowing through contacted probes. In an atomic force microscope, metallic cantilevers can produce phase-change marks with minimal sizes of ∼10 nm. The crystallization and amorphization processes show different dependences upon thickness of Ge₂Sb₂Te₅ films. These features are discussed from thermo-dynamical and microscopic structural points-of-view.     

Structural and electrical properties of Germanium-doped Sb70Te30 eutectic thin films

Ge-doped Sb₇₀Te₃₀ eutectic films show high potential for high transfer rate recording and non-volatile memory applications. Their potential applications are based on the difference in optical and electrical properties between the crystalline and amorphous phases. However, the structure and crystallization mechanism of such films is not yet well understood.The aim of this work is to study the amorphous-to-crystalline phase transformation mechanism in eutectic thin films of pure Sb₇₀Te₃₀ and doped with 2, 5 and 10 at% of Ge. Results of isokinetics and isothermal annealing were carried out using various techniques: four-probe electrical, optical reflectance and X-ray measurements.The results of optical and electrical measurements of Ge doped Sb₇₀Te₃₀ eutectic films showed that the addition of Ge increases both, the crystallization temperature and the effective activation energy of crystallization and that the mechanism of crystallization does not depend on Ge contents.In amorphous films, Ge acts as impurity center and does not affect the optical band gap value, but decreases the pre-exponential factor in the thermally activated conductivity. In crystalline films, Ge is probably incorporated into the Sb₇₀Te₃₀ structure and Ge vacancy are responsible for p-type metallic conduction, which increases with increasing the Ge content.     

Effect of annealing temperature on the structural and optical properties of amorphous Sb2Te2Se thin films

Thin films of Sb₂Te₂Se were prepared by conventional thermal evaporation of the presynthesized material on Corning glass substrates. The chemical composition of the samples was determined by means of energy‐dispersive X‐ray spectrometry. X‐ray diffraction studies on the as‐deposited and annealed films revealed an amorphous‐to‐crystalline phase transition. The as‐deposited and annealed films at T _a = 323 and 373 K are amorphous, while those annealed at T _a= 423 and 473 K are crystalline with a single‐phase of a rhombohedral crystalline structure as that of the source material. The unit‐cell lattice parameters were determined and compared with the reported data. The optical constants (n , k ) of the investigated films were determined from the transmittance and reflectance data at normal incidence in the spectral range 400–2500 nm. The analysis of the absorption spectra revealed non‐direct energy gaps, characterizing the amorphous films, while the crystalline films exhibited direct energy gaps. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impurity effects of some metals on electrical properties of amorphous As2Se1Te2 films

Amorphous As₂Se₁Te₂ (a-As₂Se₁Te₂) films, into which Cu or Cd ions were doped by thermal diffusion below the glass transition temperature exhibited the increase of conductivity by several orders of magnitude. In addition, these samples exhibited enhanced photoconductivity and the activation type conduction. From the measurements of the thermoelectric power, it was found that Cu doped samples were p-type and Cd doped samples were n-type. The impurities was also studied by SIMS.     

Thickness dependence of crystallization and melting kinetics of eutectic Sb70Te30 phase change recording film

As the thickness of eutectic Sb₇₀Te₃₀ phase change recording film was reduced, the crystallization temperature and activation energy for crystallization would increase, while the melting temperature and activation energy for melting would decrease. Accordingly, the archival stability and recording sensitivity can be improved. However, the recording speed for direct over write will be slowed down. Based upon the Johnson-Mehl-Avrami equation, it was found that the increase of film thickness would increase the nucleation rate of eutectic Sb₇₀Te₃₀ recording film and make the crystallization process become more nucleation-dominated.     

Thermoelectric power of AsSeTe glasses

The result of the measurement of the thermoelectric power of glasses in the As₂Se₃As₂Te₃ system are reported. The results indicate that towards the As₂Te₃ end of the composition, there is an anomalous increase in thermoelectric power, the origin of which is not clear. Polaron hopping seems to contribute to conductivity in tellurium rich glasses. Structural restrictions on polaron hopping, such as the availability of AsTeAsTe chains seems to be important in discussing transport behaviour.     

Electrical conductivity and thermoelectric power of liquid AgSb Te2

The electrical conductivity and thermoelectric power of liquid AgSb Te₂ have been investigated as a function of temperature. Experimental data are analyzed in terms of a recent model proposed by Mott. The activation energy for electrical conductivity and thermoelectric power is found to be approximately 0.50 eV with a large temperature coefficient γ ～ 7 × 10^?4 eV/deg K. The gradual transition from a semiconducting to a metallic behaviour has been observed at high temperature.     

The field effect in amorphous chalcogenides: An investigation of localized states and electronic transport

The temperature dependence of the field effect response permits an unambiguous determination of the identity of those states responsible for electrostatic screening in the amorphous chalcogenides. We observe (1) in As₂Te₃, field effect screening by localized states at the Fermi level at low temperatures (～ 10¹⁹ cm^?3 eV^?1) and by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) at high temperatures, and a transition from p-type to two-carrier (primarily n-type) conductivity as the temperature is raised above ～320 K; (2) in As₂SeTe₂, screening by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) with strongly type conductivity; (3) in As₂Se₂Te, screening by localized states at the Fermi level (～ 10¹⁹ cm^?3 eV^?1) with strongly p-type conductivity; and (4) in Sb₂Te₃, a very high density of localized states at the Fermi level (～ 2 × 10²⁰ cm^?3 eV^?1) with both electron and hole contributions to the conductivity. Correlation with thermoelectric power results suggests that the p-type conductivity in As₂Te₃ is due to near-equal contributions from two processes: hopping in localized states plus extended state conduction. Aging and annealing behavior is described with the aid of a “chaotic potential model” that appears to be able to account for large changes in mobile carrier density that leave the conductivity unaltered.     

Influence of temperature on the microcrystalline structure of thermally evaporated Sb2S3 thin films

Thin films of antimony trisulfide (Sb₂S₃) were prepared by thermal evaporation under vacuum (p=5×10^–5 torr) on glass substrates maintained at various temperatures between 293 K and 523 K. Their microstructural properties have obtained by transmission electron microscopy (TEM). The electron diffraction analysis showed the occurrence of amorphous to polycrystalline transition in the films deposited at higher temperature of substrates (523 K). The polycrystalline thin films were found to have an orthorhombic structure. The interplanar distances and unit‐cell parameters were determined by high‐resolution transmission electron microscopy (HRTEM) and compared with the standard values for Sb₂S₃. The surface morphology of Sb₂S₃ thin films was investigated by scanning electron microscopy (SEM). The optical transmission spectra at normal incidence of Sb₂S₃ thin films have been measured in the spectral range of 400–1400 nm. The analysis of the absorption spectra revealed indirect energy gaps, characterizing of amorphous films, while the polycrystalline films exhibited direct energy gap. From the photon energy dependence of absorption coefficient, the optical band gap energy, E_g, were calculated for each thin films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of thermoannealing on crystallinity and optical properties of Sb2S3 thin films

Sb₂S₃ thin films are obtained by evaporating of Sb₂S₃ powder onto glass substrates maintained at room temperature under pressure of 2×10^‐5 torr. The composition of the thin films was determined by energy dispersive analysis of X‐ray (EDAX). The effect of thermal annealing in vacuum on the structural properties was studied using X‐ray diffraction (XRD) technique and scanning electron microscopy (SEM). The as‐deposition films were amorphous, while the annealed films have an orthorhombic polycrystalline structure. The optical constants of as‐deposited and annealed Sb₂S₃ thin films were obtained from the analysis of the experimental recorded transmission spectral data over the wavelength range 400‐1400 nm. The transmittance analysis allowed the determination of refractive index as function of wavelength. It was found that the refractive dispersion data obeyed the single oscillator model, from which the dispersion parameters (oscillator energy, E₀, dispersion energy, E_d) were determined. The static refractive index n(0), static dielectric constant, ε_∞, and optical band gap energy, E_g, were also calculated using the values of dispersion parameters. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Facile solvothermal synthesis,growth mechanism and thermoelectric property of flower‐like Bi2Te3

Hierarchical flower‐like Bi₂Te₃ was synthesized through a facile solvothermal method. The crystal structure and morphology of the as‐prepared samples were characterized by X‐ray diffraction (XRD), filed emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high resolution TEM. The reaction parameters such as reaction time, the amount of glucose, concentration of NaOH and the reaction temperature were systematically investigated. Based on the FESEM observations, a possible mechanism defined as a self‐assembly process accompanied by anisotropic growth mechanism was proposed. Moreover, the thermoelectric properties were measured at the temperature range of 300–600 K. The hierarchical flower‐like Bi₂Te₃ presented good thermoelectrical properties. The maximum ZT value reached up to 0.6 at 600 K, which was higher than that of Bi₂Te₃ nanoparticles.     

Sb2Te3 Single Crystals Doped with Chromium Atoms

Chromium-doped Sb₂Te₃ single crystals were prepared from elements of semiconductor purity using a modified Bridgman method. The samples of these crystals were characterized by means of X-ray diffraction analysis, measurements of the reflectivity in the plasma resonance frequency range ω_p, of the Hall constant R_H and electrical conductivity σ. It was found that the incorporation of Cr atoms into the Sb₂Te₃ crystal lattice reduces the volume of the unit cell, whereas the values of ω_p and R_H (i.e. the concentration of holes) remain unaltered and the values of σ decrease with increasing chromium content. This effect is qualitatively explained by an interaction of incorporated Cr atoms with antisite defects of Sb₂Te₃ crystal lattice. The In (R_Hσ) vs. In T dependences show that the dominant mechanism of scattering of free charge carriers in the studied crystals is the scattering by acoustic phonons; the presence of chromium atoms increases the scattering rate by ionized impurities.     

Glass formation in the GexTe100−x binary system: Synthesis by twin roller quenching and co-thermal evaporation techniques

The Ge–Te system exhibits one main composition domain where glasses can be easily prepared by melt quenching technique; this domain is centered on the eutectic composition Ge₁₅Te₈₅. In this work, bulk flakes and films of composition Ge_xTe_100−x with x  50 at.% were prepared by two different quenching techniques: (i) the twin roller quenching for bulk flakes and, (ii) the co-thermal evaporation for films (with thickness comprised between 2 and 14 μm). Electron Probe Micro-Analysis was used to check the composition of the materials while X-ray diffraction allowed identifying the amorphous state and/or the crystalline phases present in the Ge_xTe_100−x samples. Thermal properties for both types of materials were investigated by differential scanning calorimetry. The glass-forming regions were: 11.7–22.0 at.% Ge for bulk flakes and 10.2–35.9 at.% Ge for films. A similar thermal behavior of bulk flakes and thick films was highlighted by Differential Scanning Calorimetry.     

Influence of Thermal Processing Parameters and Material Properties on Velocity Configurations in Semiconductor Melts During the Vertical Bridgman Growth Technique

The paper deals with the influence of thermal boundary conditions and thermophysical material properties on velocity configurations in Bridgman arrangements. Numerical simulations are presented for (Bi_1—xSb_x)₂Te₃ melts as a representative for semiconductor melts of low Prandtl numbers. Based on two characteristic temperature profiles, results have been calculated for 2D-axisymmetric and 3D Bridgman configurations applying the FIDAP^TM FEM code using pseudo-steady-state conditions with a constant growth rate. For simulation close to real growth conditions the model used includes real geometry parameters as well as the experimentally measured temperature distribution at the outer ampoule surface and temperature depending material properties. The calculations show significant variations in the flow configuration and the resulting radial inhomogeneity of the grown crystal depending on the thermal processing parameters used.