Correlation Between the Morphology of Ag and the Contact Resistivity of the Ag/YBa2Cu3O7?δ Thin Film Contact

The morphology of the silver films deposited and annealed on laser ablated YBa₂Cu₃O_7– thin films and the corresponding contact resistivity have been systematically investigated. A minimum contact resistivity of 6 × 10^–8 cm² was reached at 77 K by annealing Ag/YBa₂Cu₃O_7– contact at the optimum temperature. The effect of the annealing temperature on the contact resistivity was explained by considering the morphology of the silver films and the diffusion of silver into YBa₂Cu₃O_7– film, etc. The difference of the contact resistivity for Ag contact to polycrystalline, single crystal and thin film of YBa₂Cu₃O_7– were also explained.     

Electrical properties of stacked CuInSe<Subscript>2</Subscript> thin films

CuInSe₂ thin films have been prepared by the processing of stacked elemental layer (SEL) on glass substrates followed by annealing in air for different times. Films produced at 300 and 350 ^∘C showed n-type semiconductor due to indium interstitial donors. Electrical properties (resistivity) of the CuInSe₂ films have been systematically studied in terms of annealing temperatures and times. The resistivity was in the interval 10¹–10⁴ Ω cm and influenced by the heating time and decreased with temperature. The activation energy ranged from 0.046 to 0.154 eV in the range 300–600 K. The scattering process due to the energy barriers that exist between adjacent grains was considered.     

Anomalous High-Temperature Properties of BaPbO3-Based Solid Solutions

The thermal expansion, thermal stability, and electrical resistivity of the Ba_{1 – x} M _x Pb_{1 + y} O_{3 +} (M = Sr, Ca; 0 x 1.0, 0 y 0.2) and Ba_{1 – x} M" _x Pb_{1 – y} M" _y O_{3 +} (M" = K, La; M" = Sc, Sb; x, y= 0.01) ceramic materials were studied between 293 and 1073 K in air. The linear thermal expansion coefficient of the ceramics was found to increase abruptly at 700 K, from (10–14) × 10^–6K^–1in the range 300–600 K to (13–18) × 10^–6K^–1in the range 800–1000 K. The electrical resistivity of the ceramics passes through a sharp maximum near 750 K, with the largest jump in resistivity at the compositions Ba_0.6Sr_0.4PbO₃and Ba_0.9Ca_0.1PbO₃. The anomaly in thermal expansion is likely associated with the rearrangement of the lead–oxygen polyhedra in the structure of the solid solutions, and the jump in resistivity is attributable to changes in the average oxidation state of Pb ions in the surface layer of the ceramics.     

Crystal Growth, Thermal Stability, and Electrical Properties of LiCu2O2

LiCu₂O₂ single crystals up to 6 × 10 × 10 mm in dimensions are grown by slow melt cooling and are characterized by thermal analysis, dc and ac (0.1–100 kHz) resistivity measurements from 20 to 300 K, and thermoelectric power measurements in the range 130–300 K. The temperature stability range of LiCu₂O₂ is 890–1050°C, and its cation composition may experience deviations from stoichiometry. LiCu₂O₂ is shown to be a p-type semiconductor. Between 80 and 260 K, its dc resistivity follows Mott's law, = Aexp(T ₀/T)^1/4, and charge transport is dominated by hopping conduction between localized states near the Fermi level.     

The Transport Properties of Bi-Riched Bi2Sr2CuO6+δ Single Crystal

As-grown superconducting Bi-riched Bi₂Sr₂CuO₆₊ single crystals have been grown by the traveling solvent floating zone technique. The superconducting transition temperature T _c was about 6 K and the room temperature resistivity was about 2×10^–3 Ohm-cm. Transport properties, such as resistivity, magnetoresistance and Hall effect were measured from overdoped to underdoped samples annealed in inert atmosphere at 650°C. The transition temperature can be raised to 12 K after post annealing. The Hall measurement shows that the hole carrier density decrease after annealing. The temperature dependence of Hall angle is T ^1.5, not quadratic as observed for most high-T _c superconducting oxides such as YBa₂Cu₃O₇. The variation of onset T _c with different external magnetic field is very different from high-T _c superconductors. The in-plane conductivity shows the dependence of ln T and can be explained by weak localization theory.     

Recrystallization of YBa2Cu3Ox superconductors in vacuum

YBa₂Cu₃O _x ceramics were recrystallized in vacuum at high temperatures. Recrystallized layers consisting of small grains were observed near the surfaces of the original large-grain 1 2 3 ceramics. The small grains consisted of transformation twins and were identified to orthorhombic 1 2 3 using X-ray diffraction. As vacuum annealing time increased, the thickness of the recrystallized layer increased. The relationship between the thickness and the annealing time showed a linear relationship and an effective diffusion coefficient of 6.25 × 10^–10cm²s^–1. The recrystallized layer showed a critical temperature of 90 K.     

Engineering of luminescence from Gd2O3:Eu nanophosphors by pulsed laser deposition

Nanostructured Gd₂O₃:Eu³⁺ thin films were prepared by pulsed laser ablation technique. The dependence of structural, morphological and optical properties of these films on photoluminescence was systematically studied by varying the annealing temperature, Eu³⁺ incorporation concentration and laser fluence. The intensity of the XRD peak from (2 2 2) crystal plane was found to increase with annealing temperature in the range 973–1173 K. Films annealed at 1173 K show a preferential growth along (2 2 2) crystal plane of the cubic Gd₂O₃ and enhanced photoluminescence at 612 nm. XRD and Micro-Raman spectra and lattice strain investigations suggest that Eu³⁺ incorporation introduce a strong lattice distortion in Gd₂O₃ matrix. Morphological investigations using atomic force microscopy indicate a strong influence of the annealing process on the surface roughness and particle size. This kind of transparent thin film phosphors may promise for applications in flat-panel displays and X-ray imaging systems.     

Sintering studies on submicrometre-sized Y-Ba-Cu-oxide powder

The isothermal sintering behaviour of submicrometre-sized (<50 nm) powders of single-phase YBa₂Cu₃O _x (123) and unreacted stoichiometric mixture of submicrometre-sized (<50 nm) powders of BaCO₃, Y₂O₃ and CuO (which on calcination at 1173 K gives YBa₂Cu₃O _x ) was investigated through dilatometry under different sintering atmospheres. The sintering rate of the powder compacts was impeded by the presence of oxygen. The activation energies,Q, of sintering were determined to be 1218 kJ mol^–1 in argon, 1593 kJ mor^–1 in air and 2142 kJ mol^–1 in oxygen. A decrease in the apparent sintered density with increasing oxygen partial pressure was also observed. X-ray diffraction and thermal analyses (thermogravimetry and differential thermal analysis) showed no reaction during sintering of the single-phase product. Pellets fabricated from uncalcined powder exhibit two stages of sintering, one between 1073 and 1173 K having an activation energyQ=627kJ mol^–1, and a second one above 1173 K withQ=383.7 kJ mol^–1. A.c. susceptibility, resistivity and critical current density were determined as a function of the temperature of the sintered samples.     

HighT c of liquid-quenched Bi1.6Pb0.4Sr2Ga2Cu3Ox

AlthoughT _c cannot be found for a liquid-quenched Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x glassy sample, a highT _c is found after annealing for 24 h at 1100 K. The maximum offset temperature of the superconducting transition is 113.3 K at 2.2 × 10^–2mAmm^–2. The maximumT _{c off} is larger than that (the maximumT _{c off} is 103.4 K at 2.0 × 10^–2 mAmm^–2) of sintered specimens before liquid quenching.     

Effect of M and M-Ca Substitution on the Structure and Superconductivity of GdBa2Cu3O7?δ [M = Mo, Hf ]

The structural and superconducting properties of (Gd_1–x–y Ca _y M _x )Ba₂Cu₃O _z with M = Mo, Hf are investigated using X-ray diffraction, electrical resistivity, and oxygen content measurements. The effect of increasing the Mo concentration in (Gd_1–x Mo _x )Ba₂Cu₃O _z changes the structure from orthorhombic to tetragonal accompanied by a large increase in resistivity and a fast decrease in T _c at the rate of 1.9 K per at.% of Mo, unlike that of Hf substitution in (Gd_1–x Hf _x )Ba₂Cu₃O _z , which maintains the orthorhombic structure and decreases T _c very slowly at the rate of 0.6 K per atm.% of Hf with nearly no change in resistivity. The suppression of T _c by M = Mo, Hf can be counterbalanced by hole doping by Ca which increases T _c with increasing Ca content showing maximum compensation for Mo. A comparative study of M = Mo, Hf doped samples in (Gd_1–x–y Ca _y M _x )Ba₂Cu₃O _z indicates that the valence of the dopant M = Mo^4+,6+, Hf⁴⁺ and its ionic radius play an important role in controlling the structural and superconducting properties of the systems.     

Properties of indium oxide/tin oxide multilayered films prepared by ion-beam sputtering

The preparation and characterization of indium oxide (InO _x )/tin oxide (SnO _y ) multilayered films deposited by ion-beam sputtering are described and compared with indium tin oxide (ITO) films. The structure and the optoelectrical properties of the films are studied in relation to the layered structures and the post-deposition annealing. Low-angle X-ray diffraction analysis showed that most films retained the regular layered structures even after annealing at 500° C for 16 h. As an example, we obtained a resistivity of 6×10^–4 cm and a transparency of about 85% in the visible range at a thickness of 110 nm in a multilayered film of InO _x (2.0 nm)/SnO _y (0.2 nm)×50 pairs when annealed at 300° C for 0.5 h in air. Hall coefficient measurements showed that this film had a mobility of 17 cm² V^–1 sec^–1 and a carrier concentration (electron density) of 5×10²⁰ cm^–3.     

Effects of pulling speed and hot zone temperature on the directional growth of YBa2Cu3Ox superconductor

The effects of pulling speed and hot zone temperature on the microstructure and the superconducting properties of YBa₂Cu₃O_x sample textured using directional growth of Y₂BaCuO₅, BaCuO₂, and CuO powder mixture were studied. The grain size, the alignment, and the critical current density of the sample were increased as the pulling speed decreased. The sample grown directionally at 1.5 mm h^–1 pulling speed consisted of a single grain. The zero resistivity temperature and the critical current density of the sample increased as the hot zone temperature increased up to 1120°C beyond which the sample consisted of Y₂O₃ and impurities and showed resistivity at 77 K. The sample grown directionally at 1.5 mm h^–1 pulling speed and at 1120 °C hot zone temperature showed sharp resistivity transition of 91 K zero resistivity and over 6000 Acm^–2. The sample showed well aligned microstructure. Compared to data from another study, the hot zone temperature required to produce maximum critical current density is lower due to low liquid forming temperature.     

Ion irradiation effects in EuBa2Cu3Oy thin film

The effect of He ion irradiation on the pinning potential in EuBa₂Cu₃O _y , thin film was investigated by measuring the temperature dependence of resistivity in magnetic fields. The pinning potential decreased as the ion fluence increased. A slower decrease of pinning potential was observed in higher magnetic field in the fluence region <3.5×10¹⁵ cm^–2.     

Sintering condition and PTCR characteristics of porous (Ba,Sr)(Ti,Sb)O3

We fabricated porous (Ba,Sr)(Ti,Sb)O₃ ceramics by adding potato-starch (1–20 wt %) and investigated the effects of sintering temperature (1300–1450 °C) and time (0.5–10 h) on the positive temperature coefficient of resistivity characteristics of the porous ceramics. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics decreased with increasing sintering temperature, while that of the ceramics increased with increasing sintering time. For example, the room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics for the samples sintered at 1300 °C and 1450 °C for 1 h is 6.8×10³ and 5.7×10² cm, respectively, while that of the ceramics is 6.5×10² and 1.3×10⁷ cm, respectively, for the samples sintered at 1350 °C for 0.5 h and 10 h. In order to investigate the reason for the decrease and increase of room-temperature electrical resistivity of the samples with increasing sintering temperature and time, the average grain size, porosity, donor concentration of grains (N _d), and electrical barrier height of grain boundaries () of the samples are discussed.     

Structural,electrical and optical properties of copper selenide thin films deposited by chemical bath deposition technique

A low cost chemical bath deposition (CBD) technique has been used for the preparation of Cu_2–xSe thin films on glass substrates. Structural, electrical and optical properties of these films were investigated. X-ray diffraction (XRD) study of the Cu_2–xSe films annealed at 523 K suggests a cubic structure with a lattice constant of 5.697 Å. Chemical composition was investigated by X-ray photoelectron spectroscopy (XPS). It reveals that absorbed oxygen in the film decreases remarkably on annealing above 423 K. The Cu/Se ratio was observed to be the same in as-deposited and annealed films. Both as-deposited and annealed films show very low resistivity in the range of (0.04–0.15) × 10^–5 -m. Transmittance and Reflectance were found in the range of 5–50% and 2–20% respectively. Optical absorption of the films results from free carrier absorption in the near infrared region with absorption coefficient of 10⁸ m^–1. The band gap for direct transition, E_g.dir varies in the range of 2.0–2.3 eV and that for indirect transition E_g.indir is in the range of 1.25–1.5 eV.     

DC resistivity of Ni–Zn ferrites prepared by oxalate precipitation method

Polycrystalline ferrites with general formula Ni_1−xZn_xFe₂O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by oxalate precipitation method. The samples were characterized by X-ray diffraction (XRD), IR and scanning electron microscope (SEM) techniques. All compositions show cubic Spinel structure. Lattice constant increases with increase in zinc content, obeying Vegard's law. The physical densities are about 98.14% of their X-ray density. Average crystallite size lies in the range 27.59–31.49 nm. Infrared studies show two absorption bands near about 400 cm⁻¹ and 600 cm⁻¹ for octahedral and tetrahedral sites, respectively. The resistivity of all the samples was studied. It is observed that the resistivity of nickel–zinc ferrites prepared by oxalate precipitation method is higher than that prepared by ceramic and citrate precursor method. It is attributed to greater homogeneity and smaller grain size. Activation energy in paramagnetic region is higher than that of ferrimagnetic region.     

Thermal Conductivity of Chalcogenide As<Subscript>2</Subscript>S<Subscript>3</Subscript> Thin Films

In order to investigate the photo-induced thermal property changes in chalcogenide thin films, amorphous As ₂ S ₃ thin film samples, whose thicknesses are 0.5, 1.0, 2.0, and 4.0 m, were prepared on silicon wafers by thermal evaporation. Their thermal conductivity was measured by the 3 method between room temperature and 100 °C. These measurements were repeated after the illumination of an Ar⁺ laser beam whose photon energy is consistent with the bandgap energy of As ₂ S ₃, and repeated again for annealed films at 180 °C for 1 h. The result shows that the thermal conductivities of fresh films were 0.14 to 0.27 W·m ^–1·K ^–1; however, the values increase to 0.28–0.47 W·m ^–1·K ^–1 after illumination of the sample and decrease to 0.19–0.42 W·m ^–1·K ^–1 after annealing of the sample. These changes can be explained by the change in microstructure produced from the photo-darkening and thermal annealing.     

Zr-Hf interdiffusion in polycrystalline Y2O3-(Zr+Hf)O2

Lattice and grain-boundary interdiffusion coefficients were calculated from the concentration distributions determined for Zr-Hf interdiffusion in polycrystalline 16Y₂O₃·84(Zr_1–x Hf _x )O₂ withx=0.020 and 0.100. The lattice interdiffusion coefficients were described byD=0.031 exp [–391 (kJ mol^–1)/RT] cm² sec^–1 and the grain-boundary diffusion parameters byD=1.5×10^–6exp [–309(kJ mol^–1)/RT] cm³ sec^–1 in the temperature range 1584–2116° C. Comparison of the results with those for the systems CaO-(Zr+Hf)O₂ and MgO-(Zr+Hf)O₂ indicated that the Zr self-diffusion coefficient was insensitive to the dopants in the fluorite-cubic ZrO₂ solid solutions.     

Chemical deposition of Al2O3 thin films on Si substrates

Uniform Al₂O₃ films were deposited on silicon substrates by the sol–gel process from stable coating solutions. The technological procedure includes spin coating deposition and investigating the influence of the annealing temperature on the dielectric properties. The layers were studied by Fourier transform infrared spectroscopy and Scanning Electron Spectroscopy. The electrical measurements have been carried out on metal–insulator–semiconductor (MIS) structures. The C–V curves show a negative fixed charge at the interface and density of the interface state, Dit, 3.7 × 10¹¹ eV^− 1cm^− 2 for annealing temperature at 750 °C.