In situ grown superconducting YBCO films on buffered silicon substrates for device applications

Thin films of YBa₂Cu₃O_7?δ (YBCO) have been grownin situ on silicon single-crystal (100) substrate by using SrTiO₃ as a buffer layer. The deposition has been carried out by on-axis rf magnetron sputtering method. The deposition condition have been optimized by studying the plasma characteristics and correlating them with the superconducting performance of the film. Films deposited at substrate temperature in the range of 680–700°C from stoichiometric YBCO targets in an argon + oxygen mixture (3∶1) are superconducting and showc-axis epitaxy. Compositional confirmation has been carried out using Rutherford backscattering. Scanning tunneling microscopy of the films reveal formation of well-defined layered structure with some defects in the initial stages ofin situ growth of the films. Films grown on SrTiO₃ substrates have excellent crystalline quality (XRD), transition temperatureT _c0=81 K and the critical current densityJ _c >2×10⁵ A/cm² for unpatterned films at 77 K. On silicon substrates using buffer layers thein situ deposited YBCO films shows a higher transition width andT _c0 is also slightly less (71 K).     

Oxygen incorporation duringin situ growth of YBCO films on both sides of substrates

Oxygen out-diffusion during cooling and heating ofin situ grown {ie685-1} (YBCO) films in low oxygen pressure used during growth by pulsed laser deposition was studied in the temperature range 700−450°C usingin situ resistance measurements. Results indicate that irrespective of the number of cooling and heating cycles seen by the films, full oxygenation of the films can be realized by the final cooling from the growth temperature in 500 torr oxygen pressure. This result has been successfully used to sequentially grow high quality YBCO films on both sides of LaAlO₃ substrates. These films have been used for the fabrication of X-band microstrip resonators with superconducting ground plane.     

In situ investigation of the magnetic domain wall in Permalloy thin film by Lorentz electron microscopy

We reported in situ investigation of the magnetic domains in Permalloy thin film with the out-of-focus methods of Lorentz microscopy. The perpendicular magnetic field to the foil is produced by the objective lens current. The component of the magnetic field, parallel to the foil plane, is applied in a novel way, simply by tilting the specimen. The circle Bloch line movements along the main wall between two cross-tie walls under the influence of a hard-axis field normal to the wall and a variation of the main wall curvature by buckling of located parts of the wall under the influence of an easy-axis field parallel to the wall has been in situ observed.     

In situ observation of tensile deformation of high-pressure die-cast specimens of AM50 alloy

In situ observation of HPDC AM50 alloy in the SEM chamber was performed to study the changes of the micro-voids and the β phase during tensile deformation. The results suggested that micro-voids had little change in the elastic region, opened linearly with increasing load in the plastic region and led to final fracture. The detachment of β phase from the interface was also observed in the plastic region and there was no evidence to suggest that the detachment led to the final fracture.     

Microwave Properties of In Situ Grown MgB2 Superconducting Thin Films

The electrodynamic response at 20 GHz of c-axis oriented MgB₂ superconducting thin films is reported. Mg-rich Mg-B precursor samples were grown on MgO and Al₂O₃ single crystal substrates by a d.c. planar magnetron sputtering technique, and subsequently annealed in situ at 800°C for 10 min in a In-sealed Nb box in the presence of saturated Mg vapor. The films were characterized by a variety of structural and electronic techniques including XRD, EDS, STM-AFM analyses, and transport measurements. The dependence of the surface impedance from temperature and radiofrequency (r.f.) field amplitude was measured via a dielectric resonator technique. Temperature data clearly confirm the s-wave nature of the newly discovered superconductor, even if the value of the energy gap is smaller than BCS prediction. An effective two-band model can be applied to quantitatively explain the experimental results. In spite of previous reports claiming the absence of weak link behavior in MgB₂, the power dependence show that granularity governs the performance of these films in the microwave region.     

Inorganic replication of human hair and in situ synthesis of gold nanoparticles

The structure of hair was replicated via a solgel process using human hair as a template. When using silicate and tetraethoxysilane (TEOS) as the precursor, the cell structure of the hair cuticle was not well replicated. When using Ti(OnBu)₄ as the precursor, however, titania microtubes were obtained, with nanopores in their walls and nanoporous platelets on their outer surfaces, which were derived from cuticle cells on the hair surfaces. The nanopores in the microtubes acted as an effective nanoreactor for in situ synthesis of Au nanoparticles. The microchannels, nanopores and noble metal nanoparticles may provide a unique combination that would be attractive in applications such as catalysis, adsorption, and separation. Translated from Journal of Inorganic Materials, 2006, 21(6): 1,313–1,318 (in Chinese)     

Buffer layers for high-Tc thin films on sapphire

Buffer layers of various oxides including CeO₂ and yttrium-stabilized zirconia (YSZ) have been deposited onR-plane sapphire. The orientation and crystallinity of the layers were optimized to promote epitaxial growth of YBa₂Cu₃O_7– (YBCO) thin films. An ion beam channeling minimum yield of 3% was obtained in the CeO₂ layer on sapphire, indicating excellent crystallinity of the buffer layer. Among the buffer materials used, CeO₂ was found to be the best one for YBCO thin films onR-plane sapphire. HighT _c andJ _c were obtained in YBCO thin films on sapphire with buffer layers. Surface resistances of the YBCO films were 4 m at 77 K and 25 GHz.     

Growth and applications of superconducting and nonsuperconducting oxide epitaxial films

Growth and characterization of high-temperature-superconducting YBa₂Cu₃O₇ and several metallic-oxide thin films by pulsed laser deposition is described here. An overview of substrates employed for epitaxial growth of perovskite-related oxides is presented. Ag-doped YBa₂Cu₃O₇ films grown on bare sapphire are shown to giveT _c=90 K, critical current >10⁶ A/cm² at 77 K and surface resistance =450μΩ. Application of epitaxial metallic LaNiO₃ thin films as an electrode for ferroelectric oxide and as a normal metal layer barrier in the superconductor-normal metal-superconductor (SNS) Josephson junction is presented. Observation of giant magnetoresistance (GMR) in the metallic La_0·6Pb_0·4MnO₃ thin films up to 50% is highlighted.     

Structure characterization of Pd/Co/Pd tri-layer films epitaxially grown on MgO single-crystal substrates

Pd/Co/Pd tri-layer films were prepared on MgO substrates of (001), (111), and (011) orientations at room temperature by ultra high vacuum rf magnetron sputtering. The detailed film structures around the Co/Pd and the Pd/Co interfaces are investigated by reflection high energy electron diffraction. Pd layers of (001)_fcc, (111)_fcc, and (011)_fcc orientations epitaxially grow on the respective MgO substrates. Strained fcc-Co(001) single-crystal layers are formed on the Pd(001)_fcc layers by accommodating the fairly large lattice mismatch between the Co and the Pd layers. On the Co layers,, Pd polycrystalline layers are formed. When Co films are formed on the Pd(111)_fcc and the Pd(011)_fcc layers, atomic mixing is observed around the Co/Pd interfaces and fcc-CoPd alloy phases are coexisting with Co crystals. The Co crystals formed on the Pd(111)_fcc layers consist of hcp(0001) + fcc(111) and Pd(111)_fcc epitaxial layers are formed on the Co layers. Co crystals epitaxially grow on the Pd(011)_fcc layers with two variants, hcp(11?00) and fcc(111). On the Co layers, Pd(011)_fcc epitaxial layers are formed.     

Synthesis of sequentially-deposited Cu/BaF2/Y films on SrTiO3 and buffered Si substrates

Sequentially-deposited layers of Cu/BaF₂/Y on buffered Si and SrTiO₃ substrates have been synthesised to produce YBa₂Cu₃O_7−δ thin film. An attempt has been made to see the effect of the ZrO₂ and Al buffer layers on Si of the annealing of the layered structure. X-ray diffraction studies reveal that on ZrO₂-buffered Si the annealing of the layered structure leads to formation of 211 phase while on Al-buffer layer 123 phase is obtained. Noticeable effect in annealed-layered structure of oblique deposition of BaF₂ from a Knudson Cell is seen in X-ray diffraction.     

1/f Noise properties of swift heavy ion irradiated epitaxial thin films of YBCO

Effect of 250 MeV¹⁰⁷Ag ion irradiation induced columnar defects on the noise properties of the YBCO superconductor in the normal and superconducting state have been investigated. Magnitude of the spectral density of the noise is found to scale inversely with the frequency and exhibit a quadratic dependence on the bias current confirming that the noise arises due to the resistance fluctuations. The magnitude ofS _v has been found to decrease with decrease in temperature and shows a noise peak in the transition region. The noise performance of these materials in the vicinity of the superconducting transition as well as in the normal state is found to improve by an order of magnitude after irradiation with 250 MeV¹⁰⁷Ag ions. The decrease in the magnitude of 1/f noise peak is due the irradiation induced enhanced flux pinning of the material which suppresses the flux motion induced noise in the vicinity ofT _c.     

Thermal model for the bolometric response of high Tc superconducting films to optical pulses

The voltage response of thin-film high T_c superconductors to electromagnetic radiation is the basis for highly promising optical detectors. Experiments with laser pulses have created a controversy over whether the observed responses are due to a thermal mechanism or caused by a non-equilibrium process. In part, this controversy was caused by inadequate thermal modelling of the bolometric response. The present study applies a rigorous thermal radiation and heat conduction analysis to a high T_c film irradiated by an optical pulse and compares the predicted bolometric voltage response to experimental data. Based on the assumption of thermal boundary resistance between film and substrate as predicted by acoustic mismatch theory, the calculated temperature increase for 200 ns pulses is not sufficient to account for the observed voltage response when the initial film temperature is well below the transition temperature.     

In situ study of short-beam shear tests for composite materials

The shear behaviour of a unidirectional E-glass/epoxy composite was studied with four-point short-beam bending tests carried out in the interior of a scanning electron microscope. The damage process of the composite was followed during the tests and the local matrix shear strain was measured. A relationship was established experimentally between the beam shear stress and the local interface shear strain. Finite element calculations gave the correct stress distribution in the beam.     

Hysteresis in theIc(H) characteristics of high-temperature superconducting ceramics and thin films

The experimental hysteretic behavior of the transport critical current observed in ceramic Y-Ba-Cu-O and (Bi-Pb)-Sr-Ca-Cu-O, as well as thin film Y-Ba-Cu-O, are presented. The data are analyzed semiqualitatively. The results show certain similarities among the ceramic samples and the films.     

Early Stage Morphology of Quench Condensed Ag, Pb and Pb/Ag Hybrid Films

In situ Scanning Tunneling Microscopy (STM) has been used to study the morphology of Ag, Pb and Pb/Ag bilayer films fabricated by quench condensation of the elements onto cold (T = 77 K) inert and atomically flat Highly Oriented Pyrolytic Graphite (HOPG) substrates. All films are thinner than 10 nm and show a granular structure that is consistent with earlier studies of Quench condensed (QC) films. The average lateral diameter, & of the Ag grains, however, depends on whether the Ag is deposited directly on HOPG (&=13 nm) or on a Pb film consisting of a single layer of Pb grains (&=26.8 nm). In addition, the critical thickness for electrical conduction (d_G) of Pb/Ag films on inert glass substrates is substantially larger than for pure Ag films. These results are evidence that the structure of the underlying substrate exerts an influence on the size of the grains in QC films. We propose a qualitative explanation for this phenomenon.     

Low-temperature hydrothermal-galvanic couple synthesis of BaTiO3 thin films on Ti-coated silicon substrates

Crystalline BaTiO₃ films were synthesized on Ti-coated silicon substrates at temperatures below 100 °C using a novel hydrothermal-galvanic couple method: Ti/Si was employed as the working electrode and platinum as the counter electrode without applying any external voltages or currents. Using this method, forming BaTiO₃ is possible at temperatures as low as 50 °C over a period of 2 h, which is not possible with the conventional hydrothermal method. The growth rate of BaTiO₃ is also much faster when prepared by such a novel method. The growth kinetics can be fitted rather well by a modified Johnson-Mehl-Avrami-Erofe'ev equation. The obtained activation energy for forming BaTiO₃ on Ti/Si is 97 ± 9 kJ mol^− 1.     

In situ ellipsometric study of copper growth on silicon

Thin copper films are of high interest for interconnect applications. However, optical studies, such as ellipsometry, of metallic thin films are still rare as the measurements are difficult to interpret due to the lack of a transparent range and often island-like growth at very low coverages. We investigated by in situ spectroscopic ellipsometry the growth of thermally evaporated thin copper films on silicon substrates from 0.5 nm to more than 100 nm, a thickness for which bulk-like response is observed. A strong change in the optical response was observed for films thinner than 10 nm as a result of plasmonic effects. The data at each thickness was modeled by employing the effective medium approximation theory. Besides the layer thickness and the void filling fraction, giving the film density, we obtained information about the mean free path of electrons in the thin films and compared it to single crystals. Cu oxidation was studied by deliberately introducing oxygen or air in the chamber, leading to a fast and pronounced change in the optical response. The data analysis provides detailed information on film thickness and quality.     

Characterization of superconducting thin films by Mo75Re25 target for rf cavity applications

superconducting thin films by Mo₇₅Re₂₅ single-target magnetron sputtering at various temperatures on sapphire substrates were prepared. Sharp superconductive transitions andT _c values above 10 K were achieved, and the deposited films showed a good metallic behavior. According to structural analyses (EDS and X-rays), all the samples have the same stoichiometry as the target and, even at low deposition temperatures, they exhibit theA15 phase. From measured residual resistivities and superconducting critical temperatures the theoretical BCS surface resistances were estimated and compared to those o Mo₆₀Re₄₀ films, presented in a previous work. Both these alloys, in fact, being characterized by low surface resistances and high critical fields, are good candidates as coating films in r.f. cavities.     

The Influence of Bi Doping on the In Situ Growth of TlBa2Ca2Cu3O9 High-Tc Films

The in situ process—laser ablation in combination with thermal evaporation of Tl₂O—has turned out to be a preparation method for single-phase and epitaxial TlBa₂Ca₂Cu₃O₉ (1223) thin films with T _c values up to 109 K. It was found by several groups that a partial substitution of Tl by Bi simplifies the phase development of the 1223 compound in the usual two-step process. We have investigated the influence of the Bi doping on the in situ growth. X-ray measurements show that the films consisted mainly of the 1223 compound. In 300-nm thin films there was no evidence of a Bi amount in the crystal structure, but thinner films (80 nm) show a small amount of Bi. We concluded that Bi doping supports the phase development of the 1223 compound only in an early stage of the film growth. The Bi-doped films have higher T _c values up to 114 K, higher j _c values up to 6 × 10⁵ A/cm² (77 K, 0 T), and lower surface resistances of 56 m (77 K, 87 GHz) than the undoped films.     

Mechanical properties of ZnO thin films deposited on polyester substrates used in flexible device applications

ZnO is growing in importance as a functional film in flexible devices because of the wide range of electrical properties that can be achieved through appropriate doping and the relative abundance of Zn. We have deposited ZnO films with various thicknesses by sputtering on polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) flexible substrates and measured their mechanical properties using compression and scratch tests coupled with in-situ optical microscopy. The cracking of ZnO, during compression, is thickness dependent and at lower thicknesses the films sputtered on PEN exhibit the highest crack onset strains, around 2%. During scratch testing, two major scratch failure mechanisms are observed, analyzed and discussed. It is also found that scratch resistance of ZnO is thickness dependent for both PET and PEN. At high scratch loads a secondary failure mechanism due to impregnation of film debris into the polymer substrates is observed.