Study of the correlation between the structure and critical temperatureT c of sputtered Nb-Ge films

Nb-Ge layers with continuous change of chemical and phase composition were prepared by the d.c. sputtering method. The dependence of critical temperatureT _c on phase composition, Ge-content, lattice imperfections and composition irregularities were studied. Films with highT _c contain beside the A-15 Nb₃Ge phase also the hexagonal and tetragonal modification of the Nb₅Ge₃ phase. Correlation betweenT _c and Nb₃Ge phase composition determined from the lattice parameter was found. In samples with highestT _c the lattice parametera ₀=0·5135 nm corresponding to 22–23 at.% of germanium was determined.     

Anomalous angular dependence of critical current in niobium films

An anomalous angular dependence of the critical current is observed in niobium films. This phenomenon manifests itself in the fact that, under small intensities of the external magnetic field, the critical current attains its maximum in a slightly tilted magnetic field. It is found that the position of the maximum depends on the external magnetic field, as well as on the initial conditions under which the samples were kept. A theoretical model is proposed to explain the results obtained. This model takes into account the effect of diamagnetic properties, pinning of vortices, and the initial conditions on the vortex system in Nb films.     

Mixed state and critical current in narrow semiconducting films

The mixed state of thin narrow superconducting films with an edge barrier placed in a transverse magnetic field is considered. The boundaries of the region for the existence of metastable mixed states with an assigned number of vortices N [H _min(N)⩽H⩽H _max(N)] are found. The magnetic-field dependence of the critical field is found for the films. The transition from the Meissner state to the static mixed state is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1773–1777 (October 1998)     

Magnetic microstructure of sputtered Co-Cr films

The magnetic microstructure of a thickness series of Co₈₁Cr₁₉ layers on Si₃N₄ membranes is investigated by modified differential phase contrast (MDPC) microscopy. The development from cross-rie wall structures for a thickness < 25 nm to more complicated structures for a thickness > 50 nm is related to the macroscopic VSM measurements and the crystallite orientation determined from electron diffraction experiments.     

高温超导薄膜临界电流密度的无损测量研究

临界电流密度Jc是超导薄膜的一个重要参量,它可以衡量超导薄膜的功率承载能力。大面积高温超导薄膜制成后,其Jc需要被无损精确测量。文中提出了一种新的交流磁场下的高温超导薄膜临界态模型:(1)基于此模型对薄膜的临界电流密度进行了精确无损测量;(2)并将实验测量的三次谐波电压曲线进行拟合研究。首先,根据麦克斯韦方程和伦敦方程,计算外加直流磁场超导薄膜Meissner态下电流和磁场在薄膜内的分布;然后分析薄膜进入临界态后内部电流的变化,在考虑顶扎力作用的情况下,提出了临界态电流和磁场非均匀分布模型;最后根据其模型,推导出三次谐波电压的表达式。为了验证该理论,分别对四片超导薄膜在不同频率下进行了三次谐波和临界电流密度测量。实验结果表明:三次谐波电压的理论与实验曲线一致;与四点传输法的测量结果相比较,该方法测量超导薄膜临界电流密度的误差在5%左右,具有高精度、无损伤、方便快捷等优点。     

Krypton incorporation in sputtered silicon films

The incorporation of Kr in sputtered a-Si films has been investigated in a systematic way by varying the Kr to Si flux, yielding Kr concentrations up to 5 at%. Compositions were determined with X-ray microanalysis. A model has been applied to describe the composition of the growing film. The layers were characterized by positron annihilation, Raman spectroscopy and Mössbauer spectroscopy. The present results clearly indicate that ion assisted growth leads to a strong reduction of open volume defects, and that the Kr resides in very small clusters.     

Temperature dependence of the critical current density in Y-Ba-Cu-O thin films

We present a study of the temperature dependence of the critical currentJ _c of several dc magnetron sputtered thin Y-Ba-Cu-O films on single crystalline SrTiO₃, ZrO₂ and Al₂O₃ substrates. Near the critical temperature T_c it is found thatJ _c(1–T/T_c)ⁿ withn=3 for the SrTiO₃ and ZrO₂ substrates, whilen=1·3 for the Al₂O₃ substrate. The temperature dependence in our samples approximately agrees with standard theories for weak links or with the Ambegaokar-Baratoff equation.     

RF sputtered thin films of highT cY-Ba-Cu-O

Y-Ba-Cu-O films are grown on strontium titanate substrates by RF sputtering from a single composite target. A barium-deficient annular ring with diameter equal to that of the target dimension is repeatedly observed. Films inside this ring are deposited exclusively at the close target-substrate distance. At larger distances the films grow more in exterior regions rather than in the interior region of the ring. Stoichiometry, structural morphology and superconducting transition temperatures are studied for various regions of the film. The films are superconducting when sintered in flowing oxygen at 950°C with onset at 100 K and a broad transition width.     

Heat treatment of bias sputtered ZnO films

The effect of a post-deposition heat treatment on the electrical and optical properties of RF sputtered ZnO films has been investigated in detail. The resistivity can be varied by several orders of magnitude and optimum values are obtained for films heated at ≈ 420°C for 5– 10 minutes in a hydrogen atmosphere. This treatment is also required in order to stabilize the as-sputtered films against Oxygen chemisorption. An optical transmission, larger than 80%, and a sharp absorption edge, (equivalent to an energy gap of 3.3 eV) independent of sample condition, were observed in all cases.     

Deposition and characterization of sputtered ZnO films

Zinc oxide thin films were deposited by radio frequency magnetron sputtering at room temperature using a metallic zinc target in a gas mixture of argon and oxygen. Plasma power, oxygen /argon gas ratio, gas pressure, and substrate temperature were varied, and an experimental design method was used to optimize these deposition parameters by considering their interdependence. Crystalline structures and film stresses were examined. Post-deposition rapid thermal annealing was also carried out to observe its effects on the film properties. Statistical analysis was then used to find the optimal sputtering conditions. Results indicated that plasma power and gas pressure have the largest effects on film crystallization and stress and that postdeposition annealing can be used to improve the quality of the film properties.     

Controlled doping of rf sputtered germanium films

Germanium films have been rf sputter deposited on a variety of substrates. A new techniques has been developed to control doping concentrations of the films at predetermined levels for bothp-type as well asn-type films. The hole concentrations of these films could be varied from 10¹⁵ to 2×10¹⁸/cm³ while the electron concentrations could be varied from 10¹⁵ to 5×10¹⁷/cm³ using this technique. Transmission electron microscope studies have been made to study the crystalline quality of the films.     

Electrochemical properties of sputtered iridium oxide films

Iridium oxide films previously used in association with WO₃ and a proton conducting electrolyte in electrochromic devices, have been directly sputtered on both sides of an anhydrous membrane of a new proton conducting polymer electrolyte (nylon 6–10, 2H₃PO₄). Charge-discharge experiments show that this symmetric cell works reversibly in the “rocking chair” mode owing to the presence of roughly equal amounts of Ir³⁺ and Ir⁴⁺ in the as-deposited films. Impedance spectroscopy has been applied to the bare and sputtered polymer in order to characterize the main elements of the equivalent circuits for the two systems. Paper presented at the 2nd Euroconference in Funchal, Madeira, Portugal. Sept. 10–16, 1995     

Superconductivity in sputtered thin films of Tl-Ba-Ca-Cu-O

Films of Tl-Ba-Ca-Cu-O have been made by multi-target magnetron sputtering. The best films show an onset of superconductivity at ≈ 110 K and zero resistance at 96 K. Preliminary X-ray diffraction analysis suggests the films to be predominantly oriented with the c-axis perpendicular to the film surface with the lowest multiple of lattice spacing along the c-axis being ∼ 2.94 nm, consistent with the Tl₂Ba₂CaCu₂O_x (2212) phase.     

Reaction of sputtered Pt films on GaAs

Pt/GaAs interface reaction has been investigated between 250–500°C. Schottky barrier Impatt diodes for microwave applications with this structure have an operating temperature of ～250°C and the solid state reaction at the interface can lead to degradation of the device. Using RF sputtered Pt films on (100) GaAs, the reaction has been investigated with the help of X-ray diffraction, Auger spectroscopy, Rutherford backscattering and electrical resistivity measurements.The reaction starts by a rapid diffusion and dissolution of Ga in Pt and is slowed down by the formation of PtAs₂ at the interface which acts as a barrier for further Ga diffusion and reaction. Five different Pt-Ga phases and PtAs₂ were identified at various stages of the reaction. The final reaction products are PtAs₂ and GaPt. The reaction becomes self limited (up to 500°C) for Pt films thicker than ～2000 Å, probably due to extremely slow diffusion of Ga through the interfacial PtAs₂. Electrical resistivity data seem to indicate that Pt₃Ga is more resistive than PtGa which is the final equilibrium Pt-Ga compound observed in this reaction.     

Secondary electron emission of sputtered alumina films

Secondary electron emission yieldδ was measured for thin films of alumina prepared byrf sputtering technique. Single pulse method was used along with 4-gridleed optics system to determineδ. Maximum value of 4·3 was obtained at primary energy of 350 eV. The Dionne’s theory was used to analyse the results and the emission probability escape depth and absorption coefficient of secondaries were also estimated. Fairly good correlation is observed between experimental and theoretical values ofδ for beam energies upto 1 keV.     

Lithium intercalation in sputtered MoO3 films

Molybdenum trioxide films have been prepared by RF-sputtering using an oxygen-argon mixture containing sputter gas and a molybdenum target. Samples were deposited at different oxygen flow rates in the range from 1.5 to 8 cm³/min onto ITO-covered thick glass plate or nickel foil substrates. Lithium intercalation of RF-sputtered films has been tested in Li/1M LiClO₄-PC-PMMA/MoO₃ galvanic cells. We report the electrochemical properties of various lithium-intercalated MoO₃ films. Influence of both the growth conditions and the nature of the film substrate have been investigated on the discharge behavior of cells and on the kinetics of intercalation. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997.