Thickness and temperature effects on thermoelectric power and electrical resistivity of (Bi0.25Sb0.75)2Te3 thin films

Thin films (Bi_0.25Sb_0.75)₂Te₃ alloy of thickness in the range 400–2200 Å have been deposited on clean glass substrates by the flash evaporation technique in a vacuum of 1 × 10⁻⁵ Torr. Investigation by X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that all the films were polycrystalline and the grain size increased with increasing film thickness. Annealing treatment leads to grain growth. Electrical resistivity and thermoelectric power measurements were carried out for different thicknesses of the films in the temperature range 300–450 K. The dependences of resistivity and thermoelectric power on the temperature show that (Bi_0.25Sb_0.75)₂Te₃ films are semiconducting. Least square fit of electrical resistivity and thermoelectric power data with reciprocal thickness indicates that the results on (Bi_0.25Sb_0.75)₂Te₃ alloy films agree with the prediction of size effect theories. The effective mean free path model of size effect with perfect diffuse scattering is used for the analysis of the data. From the fit, the important physical parameters i.e., mean free path (l_g), bulk resistivity (ρ_g), bulk thermoelectric power (S_g) and energy dependent mean free path relation power index (U_g) have been evaluated.     

Synthesis and thermoelectric properties of bismuth–antimony–tellurium-based nanopowders

Bismuth–antimony–tellurium-based nanopowders were fabricated by a chemical process in which dissolved Bi, Te and Sb salts were directly reduced in each element via surfactant-assistant polyol reducing agents. XRD patterns of the synthesized nanopowders showed that the formed phases correspond mainly to (Bi_0.5Sb_0.5)₂Te₃ and Bi_0.5Sb_1.5Te₃, respectively. The phases revealed that the three different elements were stably alloyed as ternary composition in one powder via the simple chemical route. The nanopowders were consolidated into bismuth telluride-based bulk materials that exhibited electrical resistivity above 5.6 × 10^?5 Ωm, 150 μ V/K of the Seebeck coefficient and 0.7 W/mK of thermal conductivity at room temperature. These results showed that p-type thermoelectric nanopowders obtained from a simplified chemical process could be used in making thermoelectric materials towards high performances.     

Enhanced thermoelectric properties in Co4Sb12 − xTex alloys prepared by HPHT

Skutterudite compounds Co₄Sb_12 ? xTe_x with bcc crystal structure were prepared by high pressure and high temperature (HTHP) method. The study explored chemical doping with Te at the Sb site in an attempt to optimize the thermoelectric figure of merit ZT in the system Co₄Sb_12 ? xTe_x. The electrical resistivities, Seebeck coefficients and thermal conductivities of the samples were measured in the temperature range of 300–710 K. We found that the presence of Te substantially decreased the electrical resistivity without any detrimental effect on the Seebeck coefficients, which improved the power factor. Among all the samples, Co₄Sb_11.5Te_0.5 shows the highest power factor of 35.3 µw/(cmK²) at 710 K, and the maximum ZT value reaches 0.67 at 710 K.     

Characterization of Agx(Ge2Sb2Te5)1 − xthin film by RF magnetron sputtering

We reported the Ag adding effects on the crystallization behavior in Ge₂Sb₂Te₅ film. Ag_x(Ge₂Sb₂Te₅)_1 − x films (where x = 0–0.2) were deposited on SiO₂ wafer and glass substrate by RF magnetron co-sputtering and annealed by RTA (rapid thermal annealing) at various temperature to crystallize. The effects of Ag adding on the structural, thermal and electrical properties were measured by X-ray diffraction, X-ray reflectivity, AFM, SEM, DSC and 4-point probe analysis. It was found that the crystallization temperature increased by Ag adding in Ge₂Sb₂Te₅ films. However, the surface of Ag_x(Ge₂Sb₂Te₅)_1 − x films got rough when annealing temperature and Ag contents increased. According to the Kissinger method, the activation energy for crystallization increased as the Ag content increased. It is thought that Ag atoms in Ge₂Sb₂Te₅ act as an amorphous stabilizer and they make it hard to switch from amorphous to crystalline phase. From this study, we would show the Ag_0.06(Ge₂Sb₂Te₅)_0.94 film is suitable for phase change memory material because of its higher crystallization temperature and structural stability.     

Thermoelectric properties of the flash-evaporated n-type Bi2Te2.4Se0.6 thin films

The electronic transport properties of the flash-evaporated n-type Bi₂Te_2.4Se_0.6 thin films have been investigated. The thickness dependence of Seebeck coefficient and electrical resistivity has been analyzed by the effective mean free path model. It was found that both the Seebeck coefficient and the electrical resistivity have a linear relation with the reciprocal of film thickness. The data analyzed by the effective mean free path model have been combined to evaluate important material parameters such as mean free path, its energy dependence, and Fermi energy. Annealing effects on the electronic transport properties have been also examined in conjunction with the antistructure defects. It was found that the annealing treatment is necessary for Bi₂Te_2.4Se_0.6 thin films to reduce the antistructure defects and to improve their thermoelectric properties.     

Synthesis and thermoelectric/electrical characterization of electrodeposited SbxTey thin films

Sb_xTe_y films were potentiostatically electrodeposited from acidic nitric baths at room temperature by controlling the applied potential. Near-stoichiometric Sb₂Te₃ thin films were obtained at applied potentials between ?0.15 and ?0.30 V vs. saturated calomel electrode (SCE). Post-annealing in a reducing environment resulted in an improvement in the crystal structure without the evaporation of the Te element. This result was indicated by a significant reduction in the electrical resistance and decrease in the FWHM of the main diffraction peaks. The power factor (σS²) increased from 44.2 to 372.1 μW/m K² after annealing at 473 K.     

Structural and thermoelectric characterizations of high pressure sintered nanocrystalline Bi2Te3 bulks

Using Bi₂Te₃ nanopowders prepared from ball milling of elemental Bi and Te, nanocrystalline Bi₂Te₃ bulks were fabricated with high pressure sintering technique under variable pressures. The structural and thermoelectric properties were characterized, revealing a strong correlation with the sintering pressure. The nanocrystalline Bi₂Te₃ bulk fabricated under 2 GPa exhibits good thermoelectric properties with ZT over 0.8 from 300 to 460 K and a peak ZT of 1.03 occurring at 403 K, which can be attributed to the small thermal conductivity from enhanced phonon scattering by grain boundaries and defects, as well as to the good electrical property comparable to that of the zone melting material.     

Physical properties of Bi2 (Te,Se)3 and Bi2Se1.2Te1.8 prepared using solid-state microwave synthesis

Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 bulk products were synthesised using standard solid-state microwave synthesis. The Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 were then deposited thermally onto glass substrates at a pressure of 10^? 6 Torr. The structure of the samples was analysed using X-ray diffraction (XRD), and the powders and thin films were observed to be polycrystalline and rhombohedral in structure. The surface morphology of the samples was determined using scanning electron microscopy (SEM). From the measurements of optical properties, the energy gap values for the Bi₂Te₃, Bi₂Se₃, and Bi₂Se_1.2Te_1.8 thin films were 0.43, 0.73, and 0.65 eV, respectively.     

Spray pyrolysis deposition of lanthanum telluride thin films and their characterizations

Spray pyrolysis method is used to deposit lanthanum telluride (La₂Te₃) thin films on glass substrates. The films are deposited by pyrolysis of sprayed solutions of LaCl₃ and Te metal dissolved in concentrated HCl and HNO₃ along with hydrazine hydrate as a reducing agent. X-ray diffraction analyses show that the films are polycrystalline with La₂Te₃ phase. The films have a direct optical band gap of 2.2 eV. The films are p-type semiconductors with an electrical resistivity of the order of 10⁴ Ω cm at ambient temperature (27 °C).     

Enhancement of photosensitivity by annealing in Bi2S3 thin films grown using SILAR method

Bi₂S₃ thin films were grown by successive ionic layer adsorption and reaction method (SILAR) onto the glass substrates at room temperature. The as prepared thin film were annealed at 250 °C in air for 30 min. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrical measurement systems. The X-ray diffraction patterns reveal that Bi₂S₃ thin film have orthorhombic crystal structure. SEM images showed uniform deposition of the material over the entire glass substrate. The optical energy band gap observed to be decreased from 1.69 to 1.62 eV for as deposited and annealed films respectively. The I–V measurement under dark and illumination condition (100 W) show annealed Bi₂S₃ thin film gives good photoresponse as compared to as deposited thin film and Bi₂S₃ thin film exhibits photoconductivity phenomena suggesting its useful in sensors device. The thermo-emf measurements of Bi₂S₃ thin films revealed n-type electrical conductivity.     

Structure and electrical properties of Er2O3 doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 lead-free piezoelectric ceramics

Er₂O₃ (0–0.8 wt.%)-doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (BNKT18) lead-free piezoelectric ceramics were synthesized by a conventional solid-state reaction method. The effects of Er₂O₃ on the microstructure and electrical properties were investigated. X-ray diffraction (XRD) data shows that Er₂O₃ in an amount of 0.2–0.8 wt.% can diffuse into the lattice of the BNKT18 ceramics and form the pure perovskite phase. Scanning electron microscope (SEM) images indicate that the grain sizes of BNKT18 ceramics decrease with the increase of Er₂O₃ content; in addition, the modified ceramics have the clear grain boundary and a uniformly distributed grain size. At room temperature, the electrical properties of the BNKT18 ceramics have been improved with the addition of Er₂O₃, and the BNKT18 ceramics doped with 0.6 wt.% Er₂O₃ have the highest piezoelectric constant (d₃₃ = 138 pC/N), the highest planar coupling factor (k_p = 0.2382), the highest remnant polarization (P_r = 25.2 μC/cm²), the higher relative dielectric constant (ε_r = 936) and lower dissipation factor (tanδ = 0.047) at a frequency of 10 kHz. Moreover, the T_m and T_d of the samples increase with the addition of Er₂O₃.     

Spray pyrolysed bismuth oxide thin films and their characterization

Uniform, adherent and reproducible bismuth oxide thin films have been deposited on glass substrates from aqueous Bi(NO₃)₃ solution, using the solution spray technique. Their structural, surface morphological, optical, and electrical properties were investigated by XRD, AFM, optical absorption, electrical resistivity and thermo-emf measurements. The structural analysis from XRD pattern showed the formation of mixed phases of monoclinic Bi₂O₃ (predominant), tetragonal β-Bi₂O₃ and nonstiochiometric Bi₂O_2.33. The surface morphological studies on atomic force micrographs revealed round grain morphology of bismuth oxide crystallites. The optical studies showed a direct band gap of 2.90 eV for as-prepared bismuth oxide films. The electrical resistivity measurements of bismuth oxide films indicated a semiconducting behavior with the room temperature electrical resistivity of the order of 10⁷ Ω cm. From thermo-emf measurements, the electrical conductivity was found to be of n-type.     

Optical constants of thermally evaporated Se–Sb–Te films using only their transmission spectra

Amorphous Se_82 ? xTe₁₈Sb_x thin films with different compositions (x = 0, 3, 6 and 9 at.%) were deposited onto glass substrates by thermal evaporation. The transmission spectra, T(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Swanepoel has been applied to derive the optical constants and the film thickness. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing antimony content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 1.62 to 1.26 eV with increasing antimony content from 0 to 9 at.%. The chemical-bond approach has been applied successfully to interpret the decrease of the optical gap with increasing antimony content.     

The thermoelectric properties of InxCo4Sb12 alloys prepared by HPHT

In_xCo₄Sb₁₂ (0.1 ≤ x ≤ 0.5) skutterudite compounds with bcc crystal structure have been prepared by high-pressure and high-temperature (HPHT) method. Through this method, the processing time can be reduced from a few days to half an hour. The constituent phases of all resultant samples were determined by X-ray diffraction. The Seebeck coefficient, electrical resistivity, power factor and thermal conductivity of In_xCo₄Sb₁₂ (0.1 ≤ x ≤ 0.5) were all measured in the temperature range of 302–665 K. Among all the samples, In_0.5Co₄Sb₁₂ showed the highest power factor of 31.3 μWcm^? 1 K^? 2 at 616 K and the lowest thermal conductivity of 2.193 Wm^? 1 K^? 1 at 568 K. As a result, the maximum dimensionless figure of merit (ZT) value of In_0.5Co₄Sb₁₂ reached 0.88 at 665 K, which can be attributed to its low thermal conductivity and high power factor.     

Growth and characterization of indium oxide thin films prepared by spray pyrolysis

Highly transparent and conducting indium oxide thin films are prepared on glass substrates from precursor solution of indium chloride. These films are characterized by X-ray diffraction, scanning electron microscopy and optical transmission. The preferential orientation of these films is found to be sensitive to deposition parameters. A comparative study has been made on the dependence on the thickness of the film on substrate temperatures with aqueous solution and 1:1 C₂H₅OH and H₂O as precursors. Films deposited at optimum conditions have 167 nm thickness and exhibited a resistivity of 2.94 × 10⁻⁴ Ω m along with transmittance better than 82% at 550 nm. The analytical expressions enabling the derivation of the optical constants of these films from their transmission spectrum only have successfully been applied. Finally, the refractive index dispersion is discussed in terms of the single-oscillator Wemple and Didomenico model.     

Preparation and characterization of the bismuth sodium titanate (Na0.5Bi0.5TiO3) ceramic doped with ZnO

This study investigates effects of the zinc oxide (ZnO) addition and the sintering temperature on the microstructure and the electrical properties (such as dielectric constant and loss tangent) of the lead-free piezoelectric ceramic of bismuth sodium titanate (Na_0.5Bi_0.5TiO₃), NBT, which was prepared using the mixed oxide method. Three kinds of starting powders (such as Bi₂O₃, Na₂CO₃ and TiO₂) were mixed and calcined. This calcined NBT powder and a certain weight percentage of ZnO were mixed and compressed into a green compact of NBT–ZnO. Then, this green compact of NBT–ZnO was sintered to be a disk doped with ZnO, and its characteristics were measured. In this study, the calcining temperature was 800 °C, the sintering temperatures ranged from 1000 to 1150 °C, and the weight percentages of ZnO doping included 0.0, 0.5, 1.0, and 2.0 wt%. At a fixed wt% ZnO, the grain size increases with increase in the sintering temperature. The largest relative density of the NBT disk obtained in this study is 98.3% at the calcining temperature of 800 °C, the sintering temperature of 1050 °C, and 0.5 wt% ZnO addition. Its corresponding dielectric constant and loss tangent are 216.55 and 0.133, respectively.     

Influence of Ar/O2 ratio on the properties of transparent conductive ZnO:Ga films prepared by DC reactive magnetron sputtering

Ga-doped zinc oxide (ZnO:Ga) transparent conductive films with highly (002)-preferred orientation were deposited on glass substrates by DC reactive magnetron sputtering method in Ar + O₂ ambience with different Ar/O₂ ratios. The structural, electrical, and optical properties were investigated by X-ray diffraction, Hall measurement, and optical transmission spectroscopy. The resistivity and optical transmittance of the ZnO:Ga thin films are of the order of 10^− 4 Ω cm and over 85%, respectively. The lowest electrical resistivity of the film is found to be about 3.58 × 10^− 4 Ω cm. The influences of Ar/O₂ gas ratios on the resistivity, Hall mobility, and carrier concentration were analyzed.     

Gd2O3 doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 lead-free piezoelectric ceramics

Gd₂O₃ (0–0.8 wt.%)-doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (BNKT18) lead-free piezoelectric ceramics were synthesized by a conventional solid-state process. The effects of Gd₂O₃ on the microstructure, the dielectric, ferroelectric and piezoelectric properties were investigated. X-ray diffraction (XRD) data shows that Gd₂O₃ in an amount of 0.2–0.8 wt.% can diffuse into the lattice of BNKT18 ceramics and form a pure perovskite phase. Scanning electron microscope (SEM) images indicate that the grain size of BNKT18 ceramics decreases with the increase of Gd₂O₃ content; in addition, all the modified ceramics have a clear grain boundary and a uniformly distributed grain size. At room temperature, the ferroelectric and piezoelectric properties of the BNKT18 ceramics have been improved with the addition of Gd₂O₃, and the BNKT18 ceramics doped with 0.4 wt.% Gd₂O₃ have the highest piezoelectric constant (d₃₃ = 137 pC/N), highest relative dielectric constant (ε_r = 1023) and lower dissipation factor (tan δ = 0.044) at a frequency of 10 kHz. The BNKT18 ceramics doped with 0.2 wt.% Gd₂O₃ have the highest planar coupling factor (k_p = 0.2463).     

AlCu alloy films prepared by the thermal diffusion technique

100-nm thick films of Al_1 ? xCu_x alloys were prepared on glass substrates by thermal diffusion technique. The Cu atomic concentration was varied from 10% to 90%. Alloys were prepared at different temperatures into a vacuum oven with Argon atmosphere. Two thermal processes were used: i) heating the film at 400 °C in a single step, and ii) heating the films in sequential steps at 100, 200, 300 and 400 °C. Morphology, electrical resistivity, and crystalline orientation of the alloys were studied. The electrical resistivity and surface roughness of the alloys were found to depend strongly on the atomic composition and the diffusion temperature. However, we did not find differences between samples prepared under the two thermal processes. Alloys prepared with x = 0.6 and x = 0.1–0.3 as Cu at concentration exhibited values on electrical resistivity and surface roughness lower than pure Al. Different phases of the Al_1 ? xCu_x films were observed as a function of Cu concentration showing a good agreement with the AlCu phase diagram.     

Optical and thermal band gap energy of chemically deposited bismuth(III) selenide thin films

The band gap energy of bismuth(III) selenide in thin-film form was determined using the optical and thermal methods. The optical band gap energy of 0.35 eV was calculated on the basis of the recorded optical spectra in the near-infrared region, within the framework of a parabolic approximation for the dispersion relation, using the equations which arise from Fermi’s golden rule for electronic transitions from valence to conduction band. From the temperature dependence of the dark electrical resistance of the bismuth(III) selenide thin films in the region of intrinsic and extrinsic conduction, a thermal band gap energy of 0.37 eV and an ionization energy of the donor impurity level of 0.13 eV were calculated. The thermal, as well as the optical band gap energy are in excellent agreement with a literature value for bulk bismuth(III) selenide. On the basis of these data, several conclusions on the film microstructure (nanocrystalline versus glassy) are derived and also an estimation of the higher bound to the Bohr’s excitonic radius for bulk Bi₂Se₃ is given.