Tl2Ba2CaCu2O8 thin films on 2 in. LaAlO3(0 0 1) substrates

High quality Tl₂Ba₂CaCu₂O₈ (Tl-2212) superconducting thin films are prepared on both sides of 2 in. LaAlO₃(0 0 1) substrates by off-axis magnetron sputtering and post-annealing process. XRD measurements show that these films possess pure Tl-2212 phase with C-axis perpendicular to the substrate surface. The thickness unhomogeneity of the whole film on the 2 in. wafer is less than 5%. The superconducting transition temperatures T_cs of the films are around 105 K. At zero applied magnetic field, the critical current densities J_cs of the films on both sides of the wafer were measured to be above 2 × 10⁶ A/cm² at 77 K. The microwave surface resistance R_s of film was as low as 350 μΩ at 10 GHz and 77 K. In order to test the suitability of Tl-2212 thin films for passive microwave devices, 3-pole bandpass filters have been fabricated from double-sided Tl-2212 films on LaAlO₃ substrates.     

Transport properties of textured Bi2Sr2CaCu2O8+y, thin films

Superconducting films of the high-T _c compound Bi₂Sr₂CaCu₂O_8+y , have been grown on (111)-oriented gadolinium gallium garnet substrates by a liquid-phase technique. The films show a very high degree of preferential orientation with thec-axis perpendicular to the substrates. The onset of the resistive transition was 85 K while zero resistance was obtained at 78 K. Results concerning the critical current properties of the films are described. Measurements of the paraconductivity effects on the electrical resistivity above the superconducting transition due to thermodynamic fluctuations are also reported.Supported by Ansaldo S.p.A Divisione Ansaldo Ricerche, Via Corso Perrone 25, I-16100 Genova, Italy.     

Surface resistance of large-area Tl2Ba2CaCu2O8 thin films at microwave and millimeter wave frequencies measured by three noncavity techniques

The surface resistanceR _s of Tl₂Ba₂CaCu₂O₈ films fabricated on LaAlO₃ wafers up to 3 inches (7.6 cm) in diameter through a post-deposition anneal process was measured over the frequency range 5.55–94.1 GHz by the following techniques: 5.55 and 27.5 GHz high-temperature superconductor (HTS)-sapphire resonators, 10 GHz parallel plate resonator, and 94.1 GHz scanning confocal resonator.R _s was found to exhibit a quadratic dependence on frequencyf at 77 K:R _s f ^2.0±0.1. The highest-quality films yieldR _s =145±15 at 10 GHz and 77 K. Scanning confocal resonator mapping ofR _s across a 2-inch (5.1 cm) diameter wafer yielded a base value forR _s of 16±1 m at 77 K and 94.1 GHz (equivalent to 180±10 at 10 GHz) and good uniformity inR _s across the wafer. HTS-sapphire resonator measurements ofR _s for fifteen 1.2 cm square parts cut from a 3-inch diameter wafer yieldedR _s values scaled to 10 GHz of 196±10 at 80 K. Similar values were measured for Tl₂Ba₂CaCu₂O₈ films prepared on both sides of a 2-inch diameter wafer.Rs values at 10 GHz and 80 K of 147–214 were maintained over the course of 40 independent and successive deposition runs and corresponding anneals under nominally identical film fabrication conditions. Surface resistance at 5.55 GHz remained below 80 for maximum rf magnetic fields up to 85 Oe at 4.2 K and 7 Oe at 80 K, respectively. Results are compared with predictions of the two-fluid model. The relative advantages and disadvantages of the different techniques for measuring surface resistance are discussed.     

Flux Dynamics in Tl2Ba2CaCu2O8 Thin Films

Investigations on flux dynamics of ring-shaped T1₂Ba₂CaCu₂O₈ superconducting thin films have been carried out by measuring the temperature dependent magnetization M(T) during field-cooling (FC) and zero-field-cooling (ZFC) processes. For a given magnetic field, from the magnetization behavior two distinct temperatures, T_kink and T_irr, can be defined. Below T_kink, a clear hysteretic behavior of M(T) is observed leading to a large irreversible signal M = M_FC – M_ZFC. Above T_kink, this irreversible signal, though being very small, is still non-zero until it eventually vanishes at a higher temperature T_irr. Above T_irr, both curves M_FC(T) and M_ZFC(T) merge together and become temperature independent. We attribute the first region ( T< T_kink) to a 3D vortex-glass phase, the second region (T_kink < T < T_irr) to a vortex line liquid state and the third region (T_irr < T < T_c) to a pancake liquid state.     

The Influence of Mobile Holes in Hg1 ? x PbxBa2Ca2Cu3O8 + δ Superconductors

By means of X-ray diffraction (XRD), neutron diffraction spectra (NDS), electron probe microanalysis (EMPA), and X-ray photo electron spectroscopy (XPS) etc., we studied the superconductivity of doped-Pb Hg-1223 compound. The crystal parameters a and c and the unit cell volume of the Pb-doped sample do not change considerably, but the intercalated O(4) of the sample increases more than that of the pure-phase Hg-1223. As for the valence number of Hg is still 2 and that of Pb is either 2 or 4, the bond length of Cu(2)–O(3) increases, Raman frequency of the A_Ig mode of the apical O(3) increases, the bond angle of O(2)–Cu(2)–O(2) is smaller and the hole density is approximately uniformly shared between the three CuO₂ planes. The above have been affirmed by our experiment data and calculations based on Bond Valence Sums (BVS). Thus, the enhancement in T _c of the Pb doping effect is due to the variation of the electronic state and its distribution.     

Study and fabrication of ZnNb2O6 thin films by sol-gel method

Zinc niobium oxide (ZnNb₂O₆) thin films were grown on ITO/glass substrate by sol-gel process. Microstructure and surface morphology of the ZnNb₂O₆ thin films have been studied by X-ray diffraction and scanning electron microscopy. Optical properties of the ZnNb₂O₆ thin films were obtained by UV-visible recording spectrophotometer. The dependence of the microstructure, optical transmittance spectra, optical band gap on annealing temperature was also investigated.     

Is Tl2Ba2CuO6 a superconductor-metal array?

From structural analysis we have shown in a previous investigation that the series TlBa₂Ca _n Cu _n+1O_2n+5 and Tl₂Ba₂Ca _n Cu _n+1O_2n+6 are composed of superconductor-semiconductor arrays. In this paper, we demonstrate that the unit cell of the Tl₂Ba₂Ca _n Cu _n+1O_2n+6 series can further be viewed as composed of a composite block of superconductor-metal-semiconductor, where TlBa₂CuO₅ is the fundamental superconductor, TlO the metallic unit, and CaCuO₂ the semiconductor block. It is indicated that theT _c of Tl(2201) can be deduced from that of Tl(1201) based on Kresin's model of composite systems.     

Quantitative analysis of lattice disorder and superconducting transition in epitaxial YBa2Cu3O6.9 films

We show that the average lattice disorder in YBa₂Cu₃O_6.9 films grown by ion-beam sputtering is homogeneous and can be quantified by introducing the lattice coherence lengthr _c that is extracted from the width of X-ray diffraction rocking curves. The superconducting properties of the films are correlated withr _c T _c decreases with increasing disorder forT _c 10 nm, while the width of the resistive transition and the normal-state resistivity increase.     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.     

Structural and magnetic properties of epitaxial SnFe2O4 thin films

Epitaxial thin films of SnFe₂O₄ are deposited on sapphire substrate by ablating the sintered SnFe₂O₄ target with a KrF excimer laser (λ = 248 nm and pulsed duration of 20 ns). X-ray diffraction study reveals that SnFe₂O₄ films are epitaxial along (222) direction. The optical bandgap of SnFe₂O₄ film is estimated using transmittance vs. wavelength data and is observed to be 2.71 eV. The presence of hysteresis loop at room temperature in magnetization vs. field plot indicates the ferromagnetic behavior of the film. It is observed that the coercive field and remnant magnetization decrease with increase in temperature.     

Growth and structure of electron beam evaporated V2O5 thin films

The growth, structure and room temperature electrical conductivity of electron beam evaporated V₂O₅ thin films were studied in detail as a function of deposition temperature. The films deposited at T_s≈553 K and subsequently annealed in oxygen atmosphere at 693 K exhibited orthorhombic layered structure.     

Dielectric properties of sol-gel derived Ta2O5 thin films

The dielectric constant and the dielectric loss of tantalum pentoxide (Ta₂O₅) thin films, produced by sol-gel spin-coated process on Corning glass substrates, have been investigated in the frequency range of 20-10⁵ Hz and the temperature range of 183-403 K, using ohmic Al electrodes. The frequency and temperature dependence of relaxation time has also been determined. The capacitance and loss factor were found to decrease with increasing frequency and increase with increasing temperature. The activation energy values were evaluated and a good agreement between the activation energy values obtained from capacitance and dielectric loss factor measurements were observed.     

Pinning potential transport measurements of Bi2Sr2CaCu2Ox thin films

Flux creep characteristics ofc-axis-oriented Bi₂Sr₂CaCu₂O _x thin films grown by the method of electron beam evaporation of stoichiometric target and subsequent annealing have been investigated in the absence of external magnetic field. A decrease of the pinning potential with decreasing temperature is obtained fromI–V curves. This result is explained by the presence of the spatially nonlinear pinning potential.     

Preparation and second harmonic generation of Ba2TiSi2O8 films

The synthesis and characterization of the Ba₂TiSi₂O₈ films are described. The Ba₂TiSi₂O₈ crystal was obtained after heat treatment at above 630 °C of a sol-gel derived glassy material which has a chemical composition (mole ratio) 2BaO, TiO₂, 2SiO₂, and then the Ba₂TiSi₂O₈ films were formed from the hydration of CaO-P₂O₅ glass powders. Heat treatment conditions and crystallization of the synthesized materials were studied by DSC-TG, XRD, and FT-IR. Second order nonlinear optical property has been verified by second harmonic generation test at 1064 nm. These results showed that the hydration process has a potential in rendering shape-comfortable optical materials.     

Photocatalytic properties of porous TiO2/Ag thin films

In this study, nanocrystalline TiO₂/Ag composite thin films were prepared by a sol-gel spin-coating technique. By introducing polystyrene (PS) spheres into the precursor solution, porous TiO₂/Ag thin films were prepared after calcination at a temperature of 500 °C for 4 h. Three different sizes (50, 200, and 400 nm) of PS spheres were used to prepare porous TiO₂ films. The as-prepared TiO₂ and TiO₂/Ag thin films were characterized by X-ray diffractometry (XRD) and by scanning electron microscopy to reveal structural and morphological differences. In addition, the photocatalytic properties of these films were investigated by degrading methylene blue under UV irradiation.When PS spheres of different sizes were introduced after calcination, the as-prepared TiO₂ films exhibited different porous structures. XRD results showed that all TiO₂/Ag films exhibited a major anatase phase. The photodegradation of porous TiO₂ thin films prepared with 200 nm PS spheres and doped with 1 mol% Ag exhibited the best photocatalytic efficiency where ∼ 100% methylene blue was decomposed within 8 h under UV exposure.     

纳米晶ZnFe2O4薄膜的制备及其丙酮气体敏感性能研究

以尖晶石结构ZnFe2O4材料为研究对象,以可溶性无机盐为原料,利用溶胶-凝胶技术在Al2O3基片上制备了ZnFe2O4薄膜,研究其对低浓度丙酮气体的敏感特性.通过DTA,TG,XRD及SEM分析手段对制膜过程及薄膜形态进行了表征.研究发现,采用柠檬酸作为络合剂的无机盐原料溶液-溶胶-凝胶(ISG)工艺,在700℃烧结温度下,可以得到覆盖良好、结构均匀、晶粒尺寸约在100nm的尖晶石结构ZnFe2O4薄膜.通过该薄膜对丙酮气敏特性测试表明,ZnFe2O4材料对丙酮具有较好的敏感性,在550℃的操作温度下,材料对丙酮气体敏灵敏度为8,响应与恢复时间小于5s.     

Microstructure and electrical properties of RuO2-CeO2 composite thin films

RuO₂-CeO₂ composite thin films are deposited on various Si substrates by a radiofrequency magnetron sputtering technique. Compacted polycrystalline pellets of the nanostructured CeO₂-RuO₂ composite system are used as standard samples for comparative electrical analyses. All films and composite samples are analyzed by X-ray diffraction and transmission electron microscopy. Electrical measurements of radiofrequency sputtering of thin films are performed as a function of the RuO₂ fraction and of the temperature (between 25 and 400 °C). A nonlinear variation in the electrical conductivity of the RuO₂-CeO₂ composite thin films as a function of the RuO₂ volume fraction (Φ) is observed and discussed. It is interpreted in terms of a power law (in (Φ − Φc)^m ), where m and Φc are parameters characteristic of the distribution of the conducting phase in a composite medium.     

Point contact investigation on Bi2Sr2CaCu2O8+y thin films

We have measured point contacts with a gold tip on Bi₂Sr₂CaCu₂O_8+y thin films and Bi₂Sr₂CaCu₂O_8+y/SrTiO₃ double layers. The results show tunneling or direct conductivity behaviour depending on the junction parameters and can be fitted by corresponding theoretical models. From fitting procedure of differential characteristics by modified Blonder-Tinkham-Klapwijk (BTK) theory the Fermi velocity v_F=5×10⁵ m/s, ratio 2(4.2K)/kT_c=7.9 and coherence length _ab=3.5 nm were obtained. The changing of the interface transmission by additional layers of different thickness corresponding to metallic or insulating behaviour is shown. In the frame of the inelastic scattering of quasiparticles the linear background of differential conductance is discussed.This work was supported by German BMFT under Contract No.l3N5924A and Slovak Grant Agency for Sciences (Grants Nos. 2/990125/93 and 2/999185/92).     

Structure, morphology and optical properties of nanocrystalline yttrium oxide (Y2O3) thin films

Yttrium oxide (Y₂O₃) thin films were grown onto Si(1 0 0) substrates using reactive magnetron sputter-deposition at temperatures ranging from room temperature (RT) to 500 °C. The effect of growth temperature (T_s) on the growth behavior, microstructure and optical properties of Y₂O₃ films was investigated. The structural studies employing reflection high-energy electron diffraction RHEED indicate that the films grown at room temperature (RT) are amorphous while the films grown at T_s = 300-500 °C are nanocrystalline and crystallize in cubic structure. Grain-size (L) increases from ∼15 to 40 nm with increasing T_s. Spectroscopic ellipsometry measurements indicate that the size-effects and ultra-microstructure were significant on the optical constants and their dispersion profiles of Y₂O₃ films. A significant enhancement in the index of refraction (n) (from 2.03 to 2.25) is observed in well-defined Y₂O₃ nanocrystalline films compared to that of amorphous Y₂O₃. The observed changes in the optical constants were explained on the basis of increased packing density and crystallinity of the films with increasing T_s. The spectrophotometry analysis indicates the direct nature of the band gap (E_g) in Y₂O₃ films. E_g values vary in the range of 5.91-6.15 eV for Y₂O₃ films grown in the range of RT-500 °C, where the lower E_g values for films grown at lower temperature is attributed to incomplete oxidation and formation of chemical defects. A direct, linear relationship between microstructure and optical parameters found for Y₂O₃ films suggest that tuning optical properties for desired applications can be achieved by controlling the size and structure at the nanoscale dimensions.     

AC conductivity and dielectric properties of Sb2Te3 thin films

Sb₂Te₃ films of different thicknesses, in the thickness range 300-620 nm, were prepared by thermal evaporation. X-ray analysis showed that the as-deposited Sb₂Te₃ films are amorphous while the source powder and annealed films showed a polycrystalline nature. The AC conductivity and dielectric properties of Sb₂Te₃ films have been investigated in the frequency range 0.4-100 kHz and temperature range 303-373 K. The AC conductivity σ_AC(ω) was found to obey the power law ω^s where s?1 independent of film thickness. The temperature dependence of both AC conductivity and the exponent s can be reasonably well interpreted by the correlated barrier hopping (CBH) model. The experimental results of the dielectric constant ε₁ and the dielectric loss ε₂ are frequency and temperature dependent and thickness independent. The maximum barrier height W_M calculated from dielectric measurements according to the Guintini equation agrees with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott for chalcogenide glasses. The effect of annealing at different temperatures on the AC conductivity and dielectric properties was also investigated. Values of σ_AC, ε₁ and ε₂ were found to increase with annealing treatment due to the increase of the degree of ordering of the investigated films. The Cole-Cole plots for the as-deposited and annealed Sb₂Te₃ films have been used to determined the molecular relaxation time τ. The temperature dependence of τ indicates a thermally activated process.