Thermoelectric Power of Polycrystalline Sb2Te3 Films

Polycrystalline stochiometric films of Sb₂Te₃ with different thickness were prepared on glass substrates by a flash evaporation technique at constant substrate temperature of 423 K. The thermoelectric power of these films was determined by measuring integrally the developed thermo — e.m.f. at different temperature differences between the hot and cold ends. The thermoelectric power of Sb₂Te₃ films was determined as a function of the temperature and thickness of the films. It was found that the Fermi level becomes pinned at higher temperatures. The values of γ, E₀, and A parameters were determined as functions of the thickness of the films. The dependence of the thermoelectric power on the reciprocal thickness of the films was explained on the basis of the grain size effect.     

Transport properties of amorphous germanium prepared by the glow discharge technique

The paper deals with conductivity, thermoelectric power and field effect measurements on amorphous Ge specimens prepared by the decomposition of germane gas in a rf glow discharge. Substrate temperatures T_d of 300, 400 and 500 K were used during deposition. The sign of the thermoelectric power S is negative throughout the temperature range investigated (200–500 K). Above 300 K, the conductivity activation energy in specimens prepared at T_d = 500 K lies between 0.40 and 0.43 eV; it is equal to the gradient of the S versus 1/T curves, suggesting transport in the extended electron states. Below room temperature there is an increasing contribution in all specimens from electron hopping transport in localized states lying about 0.25 eV below ?_C. Both conductivity and thermoelectric power results can be interpreted satisfactorily in terms of these two current paths. Hopping at the Fermi level has not been observed. The preliminary field effect measurements indicate that, as in amorphous Si, ?_f lies near a density of state minimum. The density of states at ?_f is appreciably higher than that in similarly prepared Si specimens.     

Electrical resistivity,magnetic susceptibility and thermoelectric power of amorphous niobium-nickel alloys synthesized by rf-sputtering

Thin film samples (10–20 μ thick) of niobium-nickel alloys in the composition range Nb-5 to 95% Ni were vapour quenched by rf sputtering onto fused quartz substrates held at a temperature of 450 K. At room temperature, the electrical resistivity of these alloys lies between 176–210 μΩ cm, and the absolute thermoelectric power S between 2.20–2.52 μV/K. Magnetic susceptibility for Ni_0.5Nb_0.5 and Ni_0.4NB_.6 amorphous alloys show a Pauli magnetic behaviour with values of x of about 1.5 × 10^?4 and 1.8 × 10^?4 emu g^?1, respectively.     

Electrical properties of Al-CeO2-Al thin film capacitor structures

Cerium Oxide films were prepared by vacuum thermal evaporation from tantalum boat in a conventional vacuum coating unit. Current-voltage characteristics were studied for different film thicknesses. The breakdown voltage (V_B) and dielectric field strength (E_B) were calculated. It is found that the breakdown voltage increases and dielectric field strength decreases as the thickness of the film increases. The applicability of Forlani-Minnaja relation is discussed. Current-voltage characteristics were also drawn at different temperatures and breakdown voltages were calculated. The breakdown voltage decreases as the temperature of the structure increases but the variation is nonlinear. The variation of current density with temperature was studied and the activation energy for the migration of charge carriers was calculated and it is about 0.52 ev. The results were discussed.     

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.     

Electrical properties of amorphous and crystalline InSb and InAs thin films

Amorphous films of indium antimonide (0.02–0.26 μm) and indium arsenide (0.03–0.3 μm) were formed on goldseal glass, freshly cleaved mica and NaCl substrates by using a “flash evaporation” technique. The post-deposition heat treatment was carried out on these films when the amorphous → crystalline transformation was observed. The transformation was characterized by a sudden and large fall in the resistance of the film at a particular temperature depending on the thickness. This transformation was confirmed by transmission electron micrographs and diffraction patterns obtained on the films before and after heat treatment. The transformation temperatures lie between 495–525 K for indium antimonide and 550–575 K for indium aresenide, for the thickness range involved in our investigations. The electrical conductivity measurements showed a temperature dependent activation in the high temperature region and hopping conduction in the low temperature region (Mott's theory). The activation energies, at different temperatures for various thicknesses were calculated and presented. While no Hall mobility could be observed in as-deposited films, very low mobilities were observed in annealed thick films (t > 2000 Å). Thermoelectric power for InSb films was found to vary from 0.075–0.17 mV/K for films of thickness ranging from 1000–2300 Å, whereas for InAs films, its value varied from 0.09–0.27 mV/K for the thickness range, 1250–2500 Å. These measurements indicated the conductivity to be n-type and supported the hopping conduction mechanism observed in low temperature conductivity measurements.     

Photobleaching in a g-Ge0.27Sn0.03Se0.7 thin film

Photobleaching was observed by optical transmission measurements performed using constant 655 nm (sub-band gap) illumination of a g-Ge_0.27Sn_0.03Se_0.7 thin (<2 μm) film sample deposited by thermal evaporation of bulk starting materials on silica. The threshold power density for destroying the film was 2.7 kW cm⁻² for a film thickness of 1.75 μm. The effective thermal conductivity was calculated and the temperature increase in the illuminated spots was estimated. A little below the destruction threshold photobleaching may be attributed to band gap increase with thermal annealing. But photobleaching is also observed at low power densities inducing spot temperatures well below the glass transition. No noticeable crystallization of the bleached film region was observed either optically or by X-ray diffraction.     

Ferroelectric PbTiO3 Powders and Thin Films Derived from Sol-Gel Processing

Lead titanate powders and thin films were prepared by the sol-gel process of metal alkoxide solutions and solvents. From DSC measurements, phase transition temperature of crystallized PbTiO₃ powders was obtained at about 484 °C. From XRD investigation, it was confirmed that the tetragonal phase of polycrystalline PbTiO₃ thin films is formed by coating of concentrated solution on all of the substrates we used after heat treatment above 500 °C. It was found by SEM and ellipsometric analysis that the thin film coated with 0.25 M concentrated solutions once had an average thickness of about 720 Å. Surfaces of thin films were crack-free, uniform, and its average grain size investigated by SEM was 0.6–0.8 μm. Band gap energy of PbTiO₃ thin film coated on the Al₂O₃ (2243) substrate was 3.45 eV, which is assumed to be due to the direct band to band transition. Dielectric constant (ϵ) and dielectric loss (tan δ) of PbTiO₃ thin film amounted to 60–70 and 0.01–0.02 in the region of 10 kHz ∼ 1 MHz at room temperature, respectively, and transition temperature was 486 °C at 1 MHz. From the hysteresis loop of PbTiO₃ thin film, spontaneous polarization of 12 μC/cm² and coercive filed of 45 kV/cm were obtained.     

Structure and thermal stability of (Zr0.6Al0.4)O1.8 thin film on strained SiGe layer

Zr_0.6Al_0.4O_1.8 dielectric films were deposited directly on strained SiGe substrates at room temperature by ultra-high vacuum electron-beam evaporation (UHV-EBE) and then annealed in N₂ under various temperatures. X-ray diffraction (XRD) reveals that the onset crystallization temperature of the Zr_0.6Al_0.4O_1.8 film is about 900 °C, 400 °C higher than that of pure ZrO₂. The amorphous Zr_0.6Al_0.4O_1.8 film with a physical thickness of ∼12 nm and an amorphous interfacial layer (IL) with a physical thickness of ∼3 nm have been observed by high-resolution transmission electron microscopy (HRTEM). In addition, it is demonstrated there is no undesirable amorphous phase separation during annealing at temperatures below and equal to 800 °C in the Zr_0.6Al_0.4O_1.8 film. The chemical composition of the Zr_0.6Al_0.4O_1.8 film has been studied using secondary ion mass spectroscopy (SIMS).     

Photoelectrical properties of crystalline titanium dioxide thin films after thermo‐annealing

This paper reports the photoelectrical properties of sol gel derived titanium dioxide (TiO₂) thin films annealed at different temperatures (425‐900°C). The structure of the as‐grown film was found to be amorphous and it transforms to crystalline upon annealing. The trap levels are studied by thermally stimulated current (TSC) measurements. A single trap level with activation energy of 1.5 eV was identified. The steady state and transient photocurrent was measured and the results are discussed on the basis of structural transformation. The photocurrent was found to be maximum for the films annealed at 425°C and further it decreases with annealing at higher temperatures. The photoconduction parameters such as carrier lifetime, lifetime decay constant and photosensitivity were calculated and the results are discussed as a function of annealing temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of thermal and thermoelectric treatments on structure and electric properties of B2O3–Li2O–Nb2O5 glasses

A transparent glass with the composition 60B₂O₃–30Li₂O–10Nb₂O₅ (mol%) was prepared by the melt quenching technique. The glass was heat-treated with and without the application of an external electric field. The as-prepared sample was heat-treated (HT) at 450, 500 and 550 °C and thermoelectric treated (TET) at 500 °C. The following electric fields were used: 50 kV/m and 100 kV/m. Differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman, dc and ac conductivity, as a function of temperature, were used to investigate the glass and glass-ceramics properties. LiNbO₃ crystals were detected, by XRD, in the 500 °C HT, 550 °C HT and 500 °C TET samples. The presence of an external electric field, during the heat-treatment process, improves the formation of LiNbO₃ nanocrystals at lower temperatures. However, in the 550 °C HT and in the TET samples, Li₂B₄O₇ was also detected. The value of the σ_dc decreases with the rise of the applied field, during the heat-treatment. This behavior can indicate an increase in the fraction of the LiNbO₃ crystallites present in these glass samples. The dc and ac conduction processes show dependence on the number of the ions inserted in the glass as network modifiers.The Raman analysis suggests that the niobium ions are, probably, inserted in the glass matrix as network formers.These results reflect the decisive effect of temperature and electric field applied during the thermoelectric treatment in the structure and electric properties of glass-ceramics.     

Herstellung dünner Schichten von Titandioxid

TiO₂ is evaporated in high vacuum on air cleaved and vacuum cleaved NaF-, NaCl- and NaBr-substrates. It is investigated in dependence on substrate temperature and film thickness. TiO₂ is amorphous at low temperatures, but with high substrate temperatures there exist simultaneously rutile and Ti₄O₇.     

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.     

Characteristics of p-type nanocrystalline silicon thin films developed for window layer of solar cells

Different rf-power and chamber pressures have been used to deposit boron doped hydrogenated silicon films by the PECVD method. The optoelectronic and structural properties of the silicon films have been investigated. With the increase of power and pressure the crystallinity of the films increases while the absorption decreases. As a very thin p-layer is needed in p–i–n thin film solar cells the variation of properties with film thickness has been studied. The fraction of crystallinity and thus dark conductivity vary also with the thickness of the film. Conductivity as high as 2.46 S cm⁻¹ has been achieved for 400 Å thin film while for 3000 Å thick film it is 21 S cm⁻¹. Characterization of these films by XRD, Raman Spectroscopy, TEM and SEM indicate that the grain size, crystalline volume fraction as well as the surface morphology of p-layers depend on the deposition conditions as well as on the thickness of the film. Optical band gap varies from 2.19 eV to 2.63 eV. The thin p-type crystalline silicon film with high conductivity and wide band gap prepared under high power and pressure is suitable for application as window layer for Silicon thin film solar cells.     

Fabrication and ionic conductivity of amorphous Li–Al–Ti–P–O thin film

Li⁺ ion conducting Li–Al–Ti–P–O thin films were fabricated on ITO-glass substrates at various temperatures from 25 to 400 °C by RF magnetron sputtering method. When the substrate temperature is higher than 300 °C, severe destruction of ITO films were confirmed by XRD (X-ray diffraction) and the abrupt transformation of one semi-circle into two semi-circles on the impedance spectra. These as-deposited Li–Al–Ti–P–O solid state electrolyte thin films have an amorphous structure confirmed by XRD and a single semicircle on the impedance spectra. Good transmission higher than 80% in the visible light range of these electrolyte thin films can fulfill the demand of electro-chromic devices. Field emission scanning electron microscopy and atomic force microscopy showed the denser, smoother and more uniform film structure with the enhanced substrate temperature. Measurements of impedance spectra indicate that the gradual increased conductivity of these Li–Al–Ti–P–O thin films with the elevation of substrate temperature from room temperature to 300 °C is originated from the increase of the pre-exponential factor (σ₀). The largest Li-ion conductivity can come to 2.46 × 10^? 5 S cm^? 1. This inorganic solid lithium ion conductor film will have a potential application as an electrolyte layer in the field such as lithium batteries or all-solid-state EC devices.     

Resistive and Magnetic Susceptibility Transitions in Superconducting (TMTSF)2C104

We report measurements of the ac magnetic susceptibility and dc resistive superconducting transitions in the organic superconductor (TMTSF)₂C10₄. Inductive measurements show complete diamagnetic shielding below a broad transition and initial flux penetration at very low fields [H_c1(0) < 1 Oe]. The resistive transition is also broad, but occurs at a significantly higher temperature than the inductive transition, T_c = 1.0 K and 0.65 K respectively. Resistance measurements also show evidence of a phase transition in the vicinity of 24 K. Magnetic field induced transitions, measured both inductively and resistively, show marked anistropy both in magnitude and in breadth of the transition. Results suggest that (TMTSF)₂C10₄ is a quasi ID or 2D superconductor at high temperatures and high magnetic fields and an anisotropic bulk superconductor at low temperatures and fields. Associated thermoelectric power measurements suggest that spin density waves coexist with the superconducting state.     

Temperature dependence of photoluminescence from ordered GaInP2 epitaxial layers

The temperature behavior of the integrated intensity of photoluminescence (PL) emission from ordered GaInP₂ epitaxial layer was measured at temperatures of 10 ‐ 300 K. Within this temperature range the PL emission is dominated by band‐to‐band radiative recombination. The PL intensity temperature dependence has two regions: at low temperatures it quenches rapidly as the temperature increases, and above 100 K it reduces slowly. This temperature behavior is compared with that of disordered GaInP₂ layer. The specter of the PL emission of the disordered layer has two peaks, which are identified as due to donor‐accepter (D‐A) and band‐to‐band recombination. The PL intensity quenching of these spectral bands is very different: With increasing temperature, the D‐A peak intensity remains almost unchanged at low temperatures and then decreases at a higher rate. The intensity of the band‐to‐band recombination peak decays gradually, having a higher rate at low temperatures than at higher temperatures. Comparing these temperature dependencies of these PL peaks of ordered and disordered alloys and the temperature behavior of their full width at half maximum (FWHM), we conclude that the different morphology of these alloys causes their different temperature behavior. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of ZnO nanostructures composed of nanosheets with controllable morphologies

ZnO nanostructures composed of nanosheets have been synthesized by a facile low temperature reaction of Zn(OH)₂ and NaOH without the aid of any organic molecular templates. The influences of the reaction parameters, such as the concentrations of Zn(NO₃)₂, reaction temperatures, and reaction time on the morphologies of ZnO have been investigated. The thickness of ZnO nanosheets can be adjusted from 10–20 nm to 30–40 nm by altering the reaction temperatures from 80 °C to 180 °C. ZnO nanosheets are single crystals and the growth direction is perpendicular to [1100]. A possible gradual nucleation – rapid growth formation mechanism of ZnO nanosheets is proposed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of substrate temperature on the properties of vacuum evaporated InSb films

Indium Antimonide (InSb) thin films were grown onto well cleaned glass substrates at different substrate temperatures (303, 373 and 473 K) by vacuum evaporation. The elemental composition of the deposited InSb film was found to be 52.9% (In) and 47.1% (Sb). X‐ray diffraction studies confirm the polycrystallinity of the films and the films show preferential orientation along the (111) plane. The particle size (D), dislocation density (δ) and strain (ε) were evaluated. The particle size increases with the increase of substrate temperature, which was found to be in the range from 22.36 to 32.59 nm. In Laser Raman study, the presence of longitudinal mode (LO) confirms that the deposited films were having the crystalline nature. Raman peak located at 191.26 cm^–1 shift towards the lower frequencies and narrows with increase in deposition temperature. This indicates that the crystallinity is improved in the films deposited at higher substrate temperatures. Hall measurements indicate that the films were p‐type, having carrier concentration ≅10¹⁶ cm^–3 and mobility (4–7.7) ×10³ cm²/Vs. It is observed that the carrier concentration (N) decreases and the Hall mobility (μ) increases with the increase of substrate temperature. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Amorphous to crystalline transition of selenium thin films of different thicknesses

Thin films of amorphous selenium obtained by vacuum evaporation display an increase of “optical gap” E_g^opt with an increase of thickness of the film. From the observed dependence of E_g^opt on the thickness of the film, the influence of the thickness on the gap states is interpreted in terms of the density of states model proposed by Mott and Davis. The amorphous to crystalline transition obtained by heat treatment of the specimen is also investigated. The minimum temperature for an appreciable change in crystallisation determined by the transmission of light through selenium films is also a function of the thickness and binding energy of the films. The crystalline structures resulting from heat treatment at different temperatures have been identified by scanning electron microscopy. The generation of different crystalline structures is reported in terms of the thickness and preparation conditions of the amorphous films.