Electrical properties of crystalline PbxSn1−xTe0.5Se0.5 thin films

Electrical conductivity in the dark, σ, and thermoelectric power, S, of Pb_xSn_1−xTe_0.5Se_0.5 films with x = 0.4, 0.6, 0.8, and 1 were studied for films annealed at 473 K in the temperature range 300-473 K, while the Hall voltage was investigated at room temperature. The temperature dependence of σ revealed an intrinsic conduction mechanism above 370 K, while for temperatures less than 370 K an extrinsic conduction is dominant. Both activation energy, ΔE₁, and the energy gap, E_g, were found to decrease with increasing Sn content. This decrease of E_g with increasing Sn content revealed that band inversion exists. The variation of S with temperature revealed that the investigated samples are non-degenerate semiconductors with p-type conduction. Also, the Fermi energy, E_F, was determined from the linear variation of S with 1/T in the intrinsic range. The compositional dependence of the room temperature Hall constant, R_H (0.21-0.38 cm³/Coul.), hole carrier's concentration, p (2.9-1.6 × 10¹⁹ cm⁻³), Hall mobility, μ_H (0.88-0.03 cm²/V s), and effective mass, m^∗/m_e (0.28-0.78) are given.     

Structural, electrical and optical properties of Bi2Se3 and Bi2Se(3−x)Tex thin films

Thin films of Bi₂Se₃, Bi₂Se_2.9Te_0.1, Bi₂Se_2.7Te_0.3 and Bi₂Se_2.6Te_0.4 are prepared by compound evaporation. Micro structural, optical and electrical measurements are carried out on these films. X-ray diffraction pattern indicates that the as-prepared films are polycrystalline in nature with exact matching of standard pattern. The composition and morphology are determined using energy dispersive X-ray analysis and scanning electron microscopy (SEM). The optical band gap, which is direct allowed, is 0.67 eV for Bi₂Se₃ thin films and the activation energy is 53 meV. Tellurium doped thin films also show strong optical absorption corresponding to a band gap of 0.70-0.78 eV. Absolute value of electrical conductivity in the case of tellurium doped thin film shows a decreasing trend with respect to parent structure.     

Hydrothermal synthesis and thermoelectric properties of nanostructured Bi0.5Sb1.5Te3 compounds

Single-phase Bi_0.5Sb_1.5Te₃ compounds have been prepared by hydrothermal synthesis at 150 °C for 24 h using SbCl₃, BiCl₃ and tellurium powder as precursors. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) have been applied to analyze the phase distributions, microstructures and grain sizes of the as-grown Bi_0.5Sb_1.5Te₃ products. It is found that the hydrothermally synthesized Bi_0.5Sb_1.5Te₃ nanopowders have a morphology dominated by irregular hexagonal sheets due to the anisotropic growth of the crystals. The Bi_0.5Sb_1.5Te₃ nanosheets are parallelly stacked in certain direction to form sheet-agglomerates attribute to the temperature gradients in the solution.     

The substrate effect on the optical band gap of the Mg0.53Zn0.47O thin films

MgZnO thin films were deposited on c-plane sapphire and fused quartz substrates with the same growth parameters by rf-reactive magnetron sputtering. Both the MgZnO thin films on the two substrates are single hexagonal phase and show similar Mg content. However, the absorption edge of MgZnO thin film on sapphire substrate shows a blue shift compared with that on fused quartz substrate. Similar shift also appears in the photoresponse of the detectors based on them. These phenomena were attributed to the more Mg atoms in grain boundary caused by the smaller grain size in MgZnO film on fused quartz substrate.     

Z-scan measurement for the nonlinear absorption of Bi2.55La0.45TiNbO9 thin films

Bi_2.55La_0.45TiNbO₉ (BLTN-0.45) thin films with layered aurivillius structure were fabricated on fused silica substrates by pulsed laser deposition technique. Their structure, fundamental optical constants, and nonlinear absorption characteristics have been studied. The film exhibits a high transmittance (> 60%) in visible-infrared region. The optical band gap energy was found to be 3.44 eV. The optical constant and thickness of the films were characterized using spectroscopic ellipsometric (SE) method. The nonlinear optical absorption properties of the films were investigated by the single-beam Z-scan method at a wavelength of 800 nm laser with a duration of 80 fs. We obtained the nonlinear absorption coefficient β = 4.64 × 10^− 8 m/W. The results show that the BLTN-0.45 thin film is a promising material for applications in absorbing-type optical device.     

Piezoelectric properties of Bi4Ti3O12 thin films annealed in different atmospheres

Bismuth titanate (Bi₄Ti₃O₁₂—BIT) films were evaluated for use as lead-free piezoelectric thin-films in micro-electromechanical systems. The films were grown by the polymeric precursor method on Pt/Ti/SiO₂/Si (1 0 0) (Pt) bottom electrodes at 700 °C for 2 h in static air and oxygen atmospheres. The domain structure was investigated by piezoresponse force microscopy (PFM). Annealing in static air leads to better ferroelectric properties, higher remanent polarization, lower drive voltages and higher piezoelectric coefficient. On the other hand, oxygen atmosphere favors the imprint phenomenon and reduces the piezoelectric coefficient dramatically. Impedance data, represented by means of Nyquist diagrams, show a dramatic increase in the resistivity for the films annealed in static air atmopshere.     

Growth paths for the sulfurization of Cu-rich Cu/In thin films

In-situ energy-dispersive X-ray diffraction performed at the BESSY and HASY-LAB synchrotron facilities was used to observe sulfur pressure dependent growth paths for the formation of CuInS₂ thin films from Cu-rich metallic precursors. CuInS₂ can form directly from the intermetallic phases, through binary sulfides or via CuIn₅S₈. Particular attention is given to the latter reaction sequence, typical of rapid thermal processing (RTP), with complementary EDS and SEM analysis.     

Optical and electrical properties of hydrided palladium thin films studied by an inversion approach from transmittance measurements

Palladium (Pd) thin films have been deposited by electron beam evaporation, and exposed to increasing hydrogen pressures. Transmittance spectra in the range of visible light have been measured to obtain from them, by means of a spectral projected gradient method, the wavelength dependence of the dielectric function. The decreasing metallic character of Pd with hydrogen absorption is displayed. This effect is more pronounced when Pd is deposited on metallic substrates, and there is a correlation with an increase in the effective polarization of the core electrons determining the optical dielectric constant value. Another optimization approach is devised to separate the contribution of the free carriers and of the interband transitions to the optical conductivity and to the dielectric function. Very good agreement is found between the optimized parameters characterizing the free carrier contribution and the corresponding values reported in the literature and obtained by independent experimental methods.     

Effect of deposition conditions on the characteristics of ZnO-SnO2 thin films deposited by filtered vacuum arc

ZnO-SnO₂ thin films were deposited on microscope glass substrates by filtered vacuum arc deposition system. The effects of deposition conditions on film characteristics were studied using cathodes prepared with three different ratios of atomic concentrations of Zn to Sn. The micro and the macro properties of the films were investigated as a function of cathode composition, arc current, background oxygen deposition pressure, and deposition time. X-ray diffraction analysis indicated that deposited films were amorphous, independent of the cathode composition. The atomic concentration ratio of Zn to Sn in the film as determined by XPS analysis were 33.9%: 10.6%, 43.9%: 3.8%, 44.7%: 4.7% for 50%: 50%, 70%: 30% and 90%: 10% Zn-Sn alloy cathodes, respectively. Film transmission in the visible was 70 to 90%, affected by interference effects. The maximal and minimal values of the refractive index n and the absorption coefficient k in the visible were 2.11 to 1.94 and 0.07 to 0.001, respectively. The optical band gap was in the range of 3.13 to 3.59 eV. All films were highly resistive independent of deposition conditions used.     

Preparation of Li2B4O7 thin films by chemical solution decomposition method

Li₂B₄O₇ polycrystalline films on silica glass and Si(111) substrates were prepared by chemical solution decomposition(CSD) method. After spin coating, the wet film was dried at 200 °C, and then annealed at different temperatures to form polycrystalline Li₂B₄O₇ film. These annealed films were characterized by using X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), transmission electron microscope (TEM) and selective area electron diffraction (SAED). All these results show that the main component of the film is Li₂B₄O₇ crystalline phase and the average crystalline size of these films is in the range of 20-50 nm.     

The effect of argon pressure on the structural and photocatalytic characteristics of TiO2 thin films deposited by d.c. magnetron sputtering

TiO₂ thin films of 200-300 nm thickness were deposited by d.c. magnetron sputtering onto glass substrates from a semiconducting TiO_2−x target in pure Ar using pressures between 0.1 and 1.0 Pa. The obtained TiO₂ coatings are transparent and have refractive indices between 2.5 and 1.9. Post deposition heat treatment at different temperatures was performed to achieve crystallization of anatase TiO₂. The as-deposited and heat treated films were examined with UV-VIS (transmission), SEM and XRD to investigate the influence of the argon pressure during deposition on the structural development during heat treatment. Additionally, the photocatalytic activity of the films was tested by measuring the decomposition rate of ethanol in a controlled gas atmosphere simulating air, and was related to their respective microstructures.     

Optical properties of ZnO thin film on γ-LiAlO2 substrate grown by pulsed laser deposition

Optical properties were investigated of ZnO thin films grown on (100) γ-LiAlO₂ (LAO) substrates by pulsed laser deposition method. C-axis oriented ZnO film was grown on (100) LAO substrate at the substrate temperature of 550 °C. The transmittances of the films were over 85%. Peaks attributed to excitons were seen in the absorption spectra, indicating that the thin films have high crystallinity. Photoluminescence spectra were observed at room temperature; the peak at 550 nm is ascribed to oxygen vacancies in the ZnO films caused by the diffusion of Li from the substrate into the film during deposition.     

Study of scattering of charge carriers in thin films of (Bi0.25Sb0.75)2Te3 alloy with 2% excess Te

Thin films of (Bi_0.25Sb_0.75)₂Te₃ with 2% excess Te of different thicknesses were prepared by the flash evaporation technique. Electrical resistivity and thermoelectric power were measured for different thickness films and at different temperatures. Applying Jain-Verma theory of carrier energy dependent relaxation time, thermoelectric data of thin films were analysed to understand the nature of scattering mechanisms in this thermoelectric material. The scattering parameter b was calculated from the thermoelectric and resistivity data. This gives us an indication about the nature of scattering processes in the before mentioned composition thin films. It is found from such an analysis of the thermoelectric data of thin films of the alloy with 2% excess Te that the scattering index parameter b varies in this material thin films from −0.2 to −0.1 and to positive values at higher temperatures.     

Fabrication and characterization of n-CdSe0.7Te0.3/p-CdSe0.15Te0.85 solar cell

Hot wall deposited CdSe_xTe_1−x where 0 ≤ x ≤ 1 thin films for solar cell applications have been prepared from a compound synthesized by direct reaction of high purity Cd, Se and Te elements. Crystal structure and composition of the films were analyzed by X-ray diffraction, scanning electron microscope and EDAX. X-ray diffraction studies carried out on pseudo-binary system revealed that the films are polycrystalline in nature with CdSe_0.7Te_0.3 film exhibiting hexagonal structure and CdSe_0.15Te_0.85 film exhibiting cubic zinc blende structure. The type of conduction was determined by Hall studies. A novel solar cell with structure n-CdSe_0.7Te_0.3/p-CdSe_0.15Te_0.85 has been fabricated and the efficiency was found to be 3.13%.     

Effect of oxidation on thickness dependencies of thermoelectric properties in PbTe/mica thin films

The dependencies of the Hall coefficient R_H and Seebeck coefficient S at room temperature on the thickness (d=10-550 nm) of thin PbTe films prepared by the thermal evaporation in vacuum of n-type PbTe crystals with various charge carrier concentrations (10¹⁷-10¹⁹ cm⁻³) and their deposition on mica substrates were obtained. It was established that, with decreasing thickness of PbTe films, a transition from an electron to a hole conductivity occurs, and the inversion point shifts to smaller d values as the electron concentration in the target material increases. The experimental R_H(d) and S(d) dependencies are interpreted in terms of the acceptor states created by oxygen on the film surface. These dependencies were also calculated theoretically, taking into account the existence of two types of charge carriers (electrons and holes). The theoretical curves are found to be in good agreement with the experimental data.     

Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal co-evaporation

The optimization of the thermal co-evaporation deposition process for n-type bismuth telluride (Bi₂Te₃) thin films deposited onto polyimide substrates and intended for thermoelectric applications is reported. The influence of deposition parameters (evaporation rate and substrate temperature) on film composition and thermoelectric properties was studied for optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the formation of Bi₂Te₃ thin films. Seebeck coefficient (up to 250 μV K^− 1), in-plane electrical resistivity (≈10 μΩ m), carrier concentration (3×10¹⁹-20×10¹⁹ cm^− 3) and Hall mobility (80-170 cm² V⁻¹ s^− 1) were measured at room temperature for selected Bi₂Te₃ samples.     

Effect of processing on the properties of Bi3.15Nd0.85Ti3O12 thin films

Various crystallization parameters were studied during the fabrication of Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) thin films on Pt/Ti/SiO₂/Si (100) by metal organic solution decomposition method. The effect of crystallization processes, crystallization ambients on the properties of BNdT thin films such as orientation, ferroelectric properties were examined. By adopting different fabrication processes, it is possible to get both highly c-axis oriented as well as randomly oriented thin films. Highly c-axis oriented BNdT thin film showed a large remnant polarization (2P_r) of 70 μC/cm² at an applied voltage of 10 V and exhibited a fatigue free behavior unto 2 × 10⁹ switching cycles. The improved ferroelectric properties of BNdT thin films suggest their suitability for high density ferroelectric random access memory applications.     

Size effects of polycrystalline lanthanum modified Bi4Ti3O12 thin films

The film thickness dependence on the ferroelectric properties of lanthanum modified bismuth titanate Bi_3.25La_0.75Ti₃O₁₂ was investigated. Films with thicknesses ranging from 230 to 404 nm were grown on platinum-coated silicon substrates by the polymeric precursor method. The internal strain is strongly influenced by the film thickness. The morphology of the film changes as the number of layers increases indicating a thickness dependent grain size. The leakage current, remanent polarization and drive voltage were also affected by the film thickness.     

Band gap and intergrain barrier activation energies in Bi90Sb10 thin films

The electrical resistivity and Hall effect measurements have been made on vacuum evaporated Bi₉₀ Sb₁₀ alloy films of various thickness (350 A to 4500 A), in the temperature range of 77 to 510 K. As observed earlier, the alloy system behaves like a semiconductor, but with a band gap quite higher than previously reported for bulk single crystals. Also a kind of intergrain barrier is found in these films. The activation energy of these barriers is found to decrease with increasing film thickness and substrate temperature. This trend agrees with the earlier observations in other materials and also in the same alloy system. The higher band gaps in these films are attributed to quantum size effect and high dislocation density in these films. The decrease in the inter-grain barrier activation energy with increasing thickness and substrate temperature has been attributed to increased grain size of the films.