Measurements of n-InGaAs with MBE layers: Relevance of negative magnetoresistance in the investigated samples

Measurements of n_H were performed. n_H values showed a distinct increase at temperatures below ~90 K (1.1 μm n-InGaAs samples) and a decrease at temperatures below ~30 K (7 μm n-InGaAs samples), depending on the doping level. These trends might be related to the magnetoresistance (MR) of the n-InGaAs samples. The MR behavior of the n-InGaAs samples with respect to magnetic field and temperature was apparently dependent on the doping level. Two n-InGaAs samples, one of which had a thin InGaAs epilayer (1.1 μm) and the other with a thicker (7 μm) epilayer, showed interesting behavior at low temperature. Their behavior at magnetic fields of approximately −15000 to +15,000 G were determined. The resistivity ((ρ_G – ρ₀)/ρ₀) of the 1.1 μm sample was negative at temperatures lower than 30 K.     

Fabrication and characterization of Pb1−xYbxTe-based alloy thin-film thermoelectric generators grown by thermal evaporation technique

In this study p-Pb_0.925Yb_0.075Te:Te and n-Pb_0.94Yb_0.06Te powders synthesized by solid-state microwave technique were used to fabricate thermally evaporated thin films. The nanostructure and composition of the films were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX). Electrical characterizations of the as-deposited films in terms of the Seebeck coefficient and electrical conductivity and power factor were conducted at a range of 298 K to 523 K. The microthermoelectric devices were composed of 20-pair and 10-pair p-Pb_0.925Yb_0.075Te:Te and n-Pb_0.94Yb_0.06Te thin films on glass substrates. The dimensions of the thin-film thermoelectric generators, which consisted of 20-pair and 10-pair legs connected by aluminum electrodes, were 23 mm×20 mm and 12 mm×10 mm, respectively. The 20-pair p–n thermocouples in series generated a maximum open-circuit voltage output (V_oc) of 0.581 V and a maximum output power of 25.87×10^?8 W at a temperature difference ΔT=164 K, whereas the 10-pair p–n thermocouples generated 0.311 V and 13.71×10^?8 W maximum V_oc and maximum output power, respectively, at ΔT=164 K.     

Structural and electrical properties of Pr1−xSrxMnO3 (x=0.25, 0.3, 0.35 and 0.4) manganites

In this work, we have studied the structural features and electrical conductivity of the polycrystalline Pr_1−xSr_xMnO₃ (x=0.25, 0.3, 0.35, 0.4) manganites. All the compounds have orthorhombic structure with space group Pbnm. The electrical resistivity of all the samples exhibits metal–insulator transition T_MI for both H=0 T and H=8 T. At H=8 T, ρ (T) drops down considerably which indicates the existence of the CMR effect. The electron–phonon, electron–electron and electron–spin fluctuation interactions are effective to describe the resistivity behavior for temperature less than the metal–insulator transition (T_MI). For temperatures, T>T_MI, the insulating nature is discussed with a small polaron conduction (SPC) model. The resistivity of all the samples shows a field-dependent minimum at low temperature. The above is due to the Coulomb interaction between carriers strongly enhanced by disorder and Kondo-like spin dependent scattering.     

Detection of H2S,SO2, and NO2 using electrostatic sprayed tungsten oxide films

This paper presents the electrostatic spray deposition of tungsten oxide (WO₃) films for the detection of different pollutant gases. The influence of several types of precursors on the structure and morphology of the films was studied by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. This preliminary study allowed to select the proper precursor for the preparation of pure and porous WO₃ films which offer high gas response (R_air/R_gas=1200) to low concentrations of H₂S (10 ppm) at low operating temperature (200 °C). The gas response to NO₂ and SO₂ is low at this temperature suggesting no possible interference with these two gases during the H₂S detection. Furthermore, the films are able to detect very low concentrations of NO₂ (less than 1 ppm) at 150 °C.     

Quantum size effects and transport phenomena in thin Bi layers

For thin Bi films with thicknesses d=10–60 nm the dependences of the Hall coefficient, Seebeck coefficient, electrical conductivity, and Hall carrier mobility on d have been obtained at room temperature. Distinct oscillations of the transport properties with period Δd=(5±1) nm have been observed in the thickness range d=25–60 nm and attributed to quantization of the energy spectrum of holes. It has been suggested that a deep minimum observed in the thickness dependences of the kinetic coefficients at d～25 nm is connected with the manifestation of the electronic spectrum quantization and/or manifestation of a semimetal–semiconductor transition. The experimental data are in good agreement with the results of theoretical calculations.     

Characterization of vacuum evaporated CdTe thin films prepared at ambient temperature

Polycrystalline cadmium telluride (CdTe) thin films were prepared by vacuum evaporation on glass substrates at ambient temperature. X-ray diffraction pattern (XRD) showed that the films are polycrystalline with predominant zinc blend structure. A strong reflection from the (111) plane of the cubic phase was seen beside two weak reflections from the (220) and (311) planes of the same phase. Three very weak lines that are characteristic of elemental tellurium were also observed. The average grain size was estimated by using Sherrer's formula and found to be 24±1 nm. The scanning electron microscope (SEM) image showed a uniform surface with submicron grain size. The difference between grain size obtained from Sherrer's formula and that observed in the SEM micrograph means that each grain consists of a large number of smaller crystallites. The composition of the films was explored by using energy dispersive spectroscopy (EDS), which revealed that the surfaces of the films have excess tellurium. The transmittance was measured in the wavelength range λ=400–1100 nm and used to estimate the optical bandgap energy E_g which was found to be E_g=1.48±0.01 eV. The absorption coefficient was calculated and plotted against the photon's energy and tailing in the bandgap was observed. This tailing was found to follow the empirical Urbach rule. The width of the tail was estimated and related to localized states. The linear current–voltage (I–V) plots were used to find the resistivity ρ, where a value of ρ=2.10×10⁶ Ω cm was obtained.     

Properties of fluorine-doped tin oxide films prepared by an improved sol-gel process

Fluorine-doped tin oxide (FTO) films were prepared by an improved sol-gel process, in which FTO films were deposited on glass substrates using evaporation method, with the precursors prepared by the conventional sol-gel method. The coating and sintering processes were combined in the evaporation method, with the advantage of reduced probability of films cracking and simplified preparation process. The effects of F-doping contents and structure of films on properties of films were analyzed. The results showed the performance index (Φ_TC=3.535×10⁻³ Ω⁻¹ cm) of the film was maximum with surface resistance (R_sh) of 14.7 Ω cm⁻¹, average transmittance (T) of 74.4% when F/Sn=14 mol%, the reaction temperature of the sol was 50 °C, and the evaporation temperature was 600 °C in muffle furnace, and the film has densification pyramid morphology and SnO_2−xF_x polycrystalline structure with tetragonal rutile phase. Compared with the commercial FTO films (Φ_TC=3.9×10⁻³ Ω⁻¹ cm, R_sh=27.4 Ω cm⁻¹, T=80%) produced by chemical vapor deposition (CVD) method, the Φ_TC value of FTO films prepared by an improved sol-gel process is close to them, the electrical properties are higher, and the optical properties are lower.     

Effect of zinc doping and temperature on the properties of sprayed CuInS2 thin films

Copper indium disulfide (CuInS₂) is an efficient absorber material for photovoltaic and solar cell applications. The structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified by suitably doping CuInS₂ thin films with dopants such as Zn, Sn, Bi, Cd, Na, N, O, P and As. In this work Zn (0.01 M) doped CuInS₂ thin films are (Cu/In=1.25) deposited on to glass substrates in the temperature range 300–400 °C. It is observed that the film growth temperature, ion ratio (Cu/In=1.25) and Zn-doping affect structural, optical, photoluminescence and electrical properties of sprayed CuInS₂ thin films. As the XRD patterns depict, Zn-doping facilitates the growth of CuInS₂ thin films along (112) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Zn in the films. The optical studies show, about 90% of light transmission occurs in the IR regions; hence Zn-doped CuInS₂ can be used as an IR transmitter. The absorption coefficient (α) in the UV–visible region is found to be in the order of 10⁴–10⁵ cm⁻¹ which is the optimum value for an efficient absorber. The optical band gap energies increase with increase of temperatures (1.66–1.78 eV). SEM photographs reveal crystalline and amorphous nature of the films at various temperature ranges. Photoluminescence study shows that well defined broad Blue and Green band emissions are exhibited by Zn-doped CuInS₂ thin films. All the films present low resistivity (ρ) values and exhibit semiconducting nature. An evolution of p-type to n-type conductivity is obtained in the temperature range 325–350 °C. Hence, Zn species can be used as a donor and acceptor impurity in CuInS₂ thin films to fabricate efficient solar cells, photovoltaic devices and good IR Transmitters.     

MISFET structures with barium titanate as a dielectric layer for application in memory cells

This work shows investigations of La₂O₃ containing BaTiO₃ thin films deposited on Si substrates by Radio Frequency Plasma Sputtering (RF PS) of sintered BaTiO₃ + La₂O₃ (2 wt.%) target. Round, aluminum (Al) electrodes were evaporated on top of the deposited layers. Thus, metal–insulator–semiconductor (MIS) structures were created with barium titanate thin films playing the role of an insulator. The MIS structures enabled a subsequent electrical characterization of the studied film by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements. Several electronic parameters, i.e., ε_ri, ρ, V_FB, ΔV_H were extracted from the obtained characteristics. Moreover, the paper describes technology process of MISFETs fabrication and possibility of their application as memory cells. The influence of voltage stress on transfer and output I–V characteristics of the transistors are presented and discussed.     

High photosensitivity with enhanced photoelectrical contribution in hybrid nanocomposite flexible UV photodetector

To devise a reliable strategy to develop an ultraviolet (UV) sensitive hybrid photodetector, plasma process is utilized as a single step method for production of large area nanocomposite films based on plasma polymerized aniline–titanium dioxide (PPani–TiO₂). The synthesis of PPani–TiO₂ nanocomposite films are made using reactive magnetron sputtering in combination with plasma polymerization. The deposited PPani–TiO₂ nanocomposite films are characterized and discussed in terms of structural, optical and electrochemical properties. A hybrid flexible nanostructured UV photodetector is constructed from PPani–TiO₂ nanocomposite and its optoelectronic properties are evaluated which exhibits a greatly enhanced photosensitivity resulting in high photoconductive gain (G = 4.56 × 10⁴) and high responsivity (R = 9.36 × 10³ AW⁻¹) under UV illumination of 254 nm. The flexible devices are successfully operated under bending up to 170° (bending radius, R = 8 mm) and showed a good folding strength and stability. The proposed plasma based method provides a green technology where the self-assembly of molecules, that is, the spontaneous association of atomic or molecular building blocks under plasma environment, emerge as a successful strategy to form well-defined structural and morphological units of nanometer dimensions.     

Long lengths of silver-clad Bi2223 superconducting tapes with high current-carrying capacity

Long lengths of silver-clad (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) high-T_c multifilamentary tapes were produced using the powder-in-tube (PIT) technique followed by a thermomechanical process. The relationships between microstructure and electrical, magnetic and mechanical properties of the heat treated tape were evaluated from the critical current density measurements, irreversibility magnetic field determination and mechanical bending tests. Emphasis was stressed on the J_c behavior in magnetic fields at different temperatures. A J_c of 10,000 A/cm² at 77 K in a zero field for a 10 m tape and 75,000 A/cm² at 23 K in a field of 3 T for a short tape was achieved. The results obtained showed that Bi2223/Ag high-T_c composite tapes are a potential alternative to conventional low-T_c superconductors in magnetic levitation (MAGLEV) applications.     

Dielectrical,conduction mechanism and thermal properties of rhodanine azodyes

In this paper, we report on the differential scanning calorimetry analysis (DSC) and thermogravimetric analysis (TGA) performed for 5-(4'-derivatives phenylazo)-2-thioxothiazolidin-4-one (HL_n) (n=1, R=OCH₃; n=2, R=CH₃; n=3, R=H; and n=4, R=NO₂) in the temperature range 46–800 °C. The values of the thermal activation energies of decomposition of HL₁, HL₃ and HL₄ are found in the range 59.10–299.72 kJ/mol. The molecular and electronic structures of the investigated compounds (HL_n) were also studied using quantum chemical calculations. The alternating current conductivity (σ_ac) and dielectrical properties of HL_n were investigated in the frequency range 0.1–100 kHz and temperature range 303–500 K. The temperature and frequency dependence of the real and the imaginary dielectrical constants are studied. The values of the thermal activation energy for derivatives under investigation were calculated at different frequencies. The values of thermal activation energies of electrical conductivity ΔE₁ and ΔE₂ for all ligands decrease with increasing the test frequency. The activation energies, ΔE₁ and ΔE₂, increase according to the following order p-(NO₂>H>CH₃>OCH₃). This is in accordance with that expected from Hammett's substituent coefficients (σ^R). The conductivities are found to be dependent on the structure of the compounds. The values of σ_ac are related to the frequency as σ_ac α ω^S where the behavior of the exponent S determines the operating conduction mechanism. The correlated barrier hopping (CBH) is the dominant conduction mechanism for HL_n. The values of maximum barrier height (W_m) were calculated.     

Aggregation,crystallization, and resistance properties of poly(3-hexylthiophene-2,5-diyl) solid films gel-cast from CHCl3/p-xylene mixed solvents

In this study we found that the gelation time and crystallinity of P3HT solid films are adjustable when aging and casting from CHCl₃/p-xylene mixed solvents. After aging for 36 h in pure p-xylene, CHCl₃, or various mixtures of the two as cosolvents, we found that the solid P3HT film gel-cast from 20 vol% CHCl₃ had the highest degree of crystallinity of its main chain (ϕ_m = 0.54), highest melting point of its main chain (T_m = 232.7 °C), fastest gelation time (30 min), largest melting enthalpy of its main chain (ΔH_m = 19.81 J g⁻¹), and lowest resistance (R_P = 0.76 MΩ); the latter value was three orders and one order of magnitude lower than those of the films cast from pure CHCl₃ (ca. 110 MΩ) and pure p-xylene (ca. 4.4 MΩ), respectively. In differential scanning calorimetry scans, we attribute the presence of melting peaks near 75 °C to the solid-to-solid phase transition of the side chain crystallites of P3HT, thereby affecting the aggregation of the P3HT main chain and resulting in the changes in resistance, crystallinity, melting enthalpy, and melting point of the gel-cast P3HT solid films.     

Fabrication of a CoSb3-based thermoelectric module

A CoSb₃-based thermoelectric module was fabricated using Ce_0.45Co_2.5Fe_1.5Sb₁₂ p-type leg and Yb_0.25Co₄Sb₁₂/Yb₂O₃ n-type leg. Ag–Cu foil was used to construct the junction of hot side legs. With two p–n couples, the module generated a maximum output power (P_max) of 140 mW and a maximum open-circuit voltage (V_o) of 210 mV under the thermal condition of hot side temperature T_h=810 K and a temperature difference ΔT=490 K. No deterioration in output power in vacuum was seen when thermal cycle of five times for the module was carried out under T_h=810 K and ΔT=490 K with natural cooling to room temperature, which shows the module has high durability.     

Chalcogen-based thermoelectric power generation device using p-type Bi0.4Sb1.6Se2.4Te0.6 and n-type Bi2Se0.6Te2.4 prepared by solid-state microwave synthesis

This study reports on the fabrication of a chalcogen-based thermoelectric power generation (TEG) device using p-type Bi_0.4Sb_1.6Se_2.4Te_0.6 and n-type Bi₂Se_0.6Te_2.4 legs. Electrical power generation characteristics were monitored by changing both the temperature conditions and the number of p–n couples required to generate maximum power. The significance of the resistances including the internal resistance and contact resistance between legs and electrodes, are discussed. The maximum output power obtained with the 18 p–n couples device was 273.2 mW under the thermal condition of T_H=523 K hot-side temperature and ΔT=184 K temperature difference.     

Chemical synthesis and charge transport mechanism in solution processed flexible polypyrrole films

Polypyrrole (PPy) has been synthesized by a chemical oxidation method using ammonium persulfate to obtain a solution processable PPy powder. The resultant PPy powder was then solution processed to deposit flexible thin films of PPy over flexible substrates. PPy film samples were then characterized using UV–vis spectroscopy, FTIR spectroscopy and X-ray diffraction. It was found that conductivity of PPy (σ=2.4×10⁻² S/cm) reduces by an order of magnitude after solution processing in the form of films. The temperature dependent conductivity of PPy pellet and flexible films of PPy were measured in the temperature range of 80–300 K. It was observed that PPy films show stronger temperature dependence than pelletized samples. Charge transport in PPy samples has been investigated using Kivelson׳s and Mott׳s variable range hopping models. Mott׳s parameters such as density of states at the Fermi level N(E_F), average hopping distance (R) and average hopping energy (W) have been estimated for PPy samples. The results showed that at room temperature average hopping distance for PPy film was about 22.3 Å and average hopping energy was 128.6 meV.     

Investigations on phosphorus doped amorphous/nanocrystalline silicon films deposited by a filtered cathodic vacuum arc technique in the presence of hydrogen gas

Phosphorus doped amorphous/nanocrystalline silicon (a-Si:H/nc-Si:H) thin films have been deposited by a filtered cathodic vacuum arc (FCVA) technique in the presence of hydrogen gas at different substrate temperatures (T_s) ranging from room temperature (RT) to 350 °C. The films have been characterized by using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dark conductivity (σ_D), activation energy (ΔE), optical band gap (E_g) and secondary ion mass spectroscopy. The XRD patterns show that RT grown film is amorphous in nature but high temperature (225 and 350 °C) deposited films exhibit nanocrystalline structure with (111) and (220) crystal orientations. The crystallite size of higher temperature grown silicon film evaluated was between 13 and 25 nm. Raman spectra reveal the amorphous nature of the film deposited at RT, whereas higher temperature deposited films show crystalline nature. The crystalline volume fraction of the silicon film deposited at higher temperatures (225 and 350 °C) was estimated to be 58 and 72%. With the increase of T_s, the bonding configuration changes from mono-hydride to di-hydride as revealed by the FTIR spectra. The values of σ_D, ΔE and E_g of silicon films deposited at different T_s were found to be in the range of 5.37×10⁻⁴–1.04 Ω⁻¹ cm⁻¹, 0.05–0.45 eV and 1.42–1.83 eV, respectively. Photoconduction of 3.5% has also been observed in n-type nc-Si:H films with the response and recovery times of 9 and 12 s, respectively. A n-type nc-Si:H/p-type c-Si heterojunction diode was fabricated which showed the diode quality factor between 1.6 and 1.8.     

Study on mobility enhancement in MOVPE-grown AlGaN/AlN/GaN HEMT structures using a thin AlN interfacial layer

Al_0.26Ga_0.74N/AlN/GaN high-electron-mobility transistor (HEMT) structures with AlN interfacial layers of various thicknesses were grown on 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy, and their structural and electrical properties were characterized. A sample with an optimum AlN layer thickness of 1.0 nm showed a highly enhanced Hall mobility (μ_Hall) of 1770 cm²/Vs with a low sheet resistance (ρ_s) of 365 Ω/sq. (2DEG density n_s = 1.0 × 10¹³/cm²) at room temperature compared with those of a sample without the AlN interfacial layer (μ_Hall = 1287 cm²/Vs, ρ_s = 539 Ω/sq., and n_s = 0.9 × 10¹³/cm²). Electron transport properties in AlGaN/AlN/GaN structures were theoretically studied, and the calculated results indicated that the insertion of an AlN layer into the AlGaN/GaN heterointerface can significantly enhance the 2DEG mobility due to the reduction of alloy disorder scattering. HEMTs were successfully fabricated and characterized. It was confirmed that AlGaN/AlN/GaN HEMTs with the optimum AlN layer thickness show superior DC properties compared with conventional AlGaN/GaN HEMTs.     

Magnetic and electric properties of BFO–NFO nanocomposites

Multiferroic nanocomposites of (1−x)BiFeO₃–xNiFe₂O₄ for x=0.2, 0.4, and 0.6 were prepared by a sol gel technique. The synthesized nanocomposites were characterized by X-ray diffraction (XRD). XRD confirmed, they being nanocomposites having desired phase with crystallite size ranging from 14.0 nm to 3.6 nm. The morphological analysis was done with the help of Transmission electron microscopy (TEM), which revealed the particle size to be in the range of 10–7 nm. Polarization–electric field (P–E) loop tracer was used to determine the ferroelectric properties of the nanocomposites. The dielectric constant at room temperature was analyzed upto 1 MHz frequency and was found to increase with increasing concentration. In order to investigate the magnetic behavior, a superconducting quantum interference device (SQUID) was used. The nanocomposites were analyzed by increasing the magnetic field up to 25 kOe and the magnetization was found to increase from 6 emu/g for x=0.2–10 emu/g for x=0.6, which was found to be optimum for the technological applications. The appropriate combination of two phases gave rise to higher magnetization.     

Pulse electrodeposited copper indium sulfide films

Copper indium sulfide (CISu) films were deposited by the pulse galvanostatic deposition technique at different duty cycles. The films are polycrystalline with peaks corresponding to the chalcopyrite phase of CISu. The grain size and surface roughness increased from 10 to 25 nm and 0.85 to 2.50 nm respectively with increase of duty cycle. Optical band gap in the range of 1.30–1.51 eV was observed for the films deposited at different duty cycles. Room temperature resistivity of the films is in the range of 0.1–3.67 Ω cm. Photoconductivity measurements were made at room temperature. Photocurrent spectra exhibited maximum corresponding to the band gap of copper indium sulphide. CdS/CuInS₂ fabricated with CISu films deposited at 50% duty cycle have exhibited a V_oc of 0.62 V, J_sc of 16.30 mA cm^?2, FF of 0.71 and efficiency of 7.16%.