S. Linzen

A morphological growth model for laser ablated Y1Ba2Cu3O7-x thin films

Abstract The aim of this work was to investigate the growth mechanism of thin Y1Ba2Cu3O7−x films ablated by LPVD. The Y1Ba2Cu3O7−x films were prepared by KrF-excimer laser ablation (248 nm) on SrTiO3 (100) substrates. To investigate film properties like surface morphology, epitaxy, c-axis length, critical current density jc (77 K) and critical temperature Toffc we varied the ablation parameters: target substrate distance dT-S, oxygen partial pressure pO2, substrate temperature Ts and laser energy density I. To analyze the films X-ray diffraction, Rutherford backscattering spectroscopy, scanning electron microscopy and electrical measurements were used. Three morphological images (zones) of the Y1Ba2Cu3O7−x film growth in dependence on pO2 and Ts can be distinguished and the growth model of Thornton can be applied as a general rule for optimization of Y1Ba2Cu3O7−x thin film growth. In a medium zone (zone T in the Thornton model) of pO2≈70Pa and Ts≈720°C with dT-S=4 cm and I=4.3 J/cm2 we get smooth, epitaxial films with Toffc > 90 K and jc (77 K) > 3x106 A/cm2. At high pO2, high dT-S and low I we found outgrowths on the films which are of the same stoichiometry as the surrounding film material.

On ageing and critical thickness of YBa2Cu3O7 films on Si with CeO2/YSZ buffer layers

Abstract The ageing behaviour of epitaxial YBa 2 Cu 3 O 7 (YBCO) films on Si (100) with CeO 2 /YSZ buffer layers is studied. For the YBCO/CeO 2 /YSZ/Si system, its ageing is weaker than the YBCO/YSZ/Si system. The experimental critical thickness of YBCO films for YBCO/CeO 2 /YSZ/Si systems is only 40 nm, otherwise cracks appear in the film. The formation of cracks is analyzed by means of fracture mechanics theory. The estimated thermal stress and elastic strain in crack-free YBCO layers reach 2.41 GPa and 1.5% at 77 K, respectively. Depositing another non-superconducting YBCO (YBCO*) film on top of YBCO as a passivation layer makes the critical thickness increase up to 70 nm. These films cannot bear surface etching and tempering temperatures higher than 110°C in lithography.

Large-area YBCO films for device fabrication

We have prepared YBCO thin films by a pulsed laser deposition (PLD) technique on substrates and on the naturally oxidized 2 in diameter silicon wafers with buffer layers. This large-area film PLD technique allows the preparation of homogeneous and reproducible YBCO films with high quality on classical and technical substrates. For 2 in silicon wafer, we obtained the critical temperature in the range from 87.5 K to 88.2 K and a critical current density at 77 K of from 0.53 to over the whole wafer.

Unusual crystal structure of non-superconducting Y1Ba2Cu3O7−x films on buffered silicon substrates

Abstract Superconducting common c -axis oriented YBCO films as well as non-superconducting films epitaxially grown at significantly reduced substrate temperatures were deposited by laser ablation on CeO 2 /YSZ buffered silicon substrates. The crystal structure of the non-superconducting films measured by XRD is collapsed from the original orthorhombic one, whereas the chemical composition detected using RBS and EDX remains identical. Especially the oxygen content in the non-superconducting material is as high as in the common 90 K superconductor. Thus, the change in the electrical behaviour is only due to a structural modification. The crystallographic model of a simple cubic YBCO perovskite unit cell does not represent completely our experimental results measured on the non-superconducting YBCO. A modified model based on detailed TEM and XRD measurements and known crystal defects of orthorhombic YBCO is proposed.

Recovery of superconductivity and recrystallization of ion-damaged YBa2Cu3O7−x films after thermal annealing treatment

Abstract We have systematically studied the annealing effects of high-quality epitaxial YBa 2 Cu 3 O 7− x (YBCO) films on (100) SrTiO 3 substrates implanted with 100 keV Ar ions at different fluences. The mechanisms for T c degradation induced by ion implantation have been discussed. After annealing in oxygen atmosphere in a temperature range from 450 to 950°C, R ( T ) curve measurements show a partial or complete recovery of superconductivity. The recrystallization of ion-implanted YBCO films have been investigated using X-ray diffraction (XRD) and Rutherford backscattering spectrometry with channeling. It is found that the annealing at 450°C indeed results in the movement of oxygen defects and ordering of the oxygen sublattice in the sample implanted with low to moderate fluences, however, only a small partial recovery of superconductivity is observed. The results suggest that ion implantation induced T c degradation is mainly caused by disordering of the cation sublattice not the oxygen sublattice. After annealing at 850°C, the recovery of superconductivity and recrystallization of implanted YBCO films are related to the fluence. For YBCO films implanted with low to moderate fluences (1 × 10 11 −1 × 10 14 /cm 2 ), the superconductivity can be nearly completely recovered, and the pure c -axis orientation of the samples is observed in the XRD spectra. However, recrystallization of amorphous YBCO films implanted with high fluences (1 × 10 15 −1 × 10 16 /cm 2 ) is quite different. The orientation of the samples depends on the annealing temperature. With the increasing of the annealing temperature from 750 to 950°C, the regrowth orientation gradually changes from a - to c -axis orientation. After annealing at temperatures of 750 and 800°C, the samples have the nearly pure a -axis orientation. At 850°C, the sample has a mixed a - and c -axis oriented structure but the a -axis orientation is still dominant. Above 900°C the structure of the annealed sample has the pure c -axis orientation. In the case of amorphous YBCO films caused by high-fluence Ar ion implantation, the T c0 is lower than 77 K in the whole annealing temperature range due to the polycrystalline structure. The experimental results indicate that the amorphous YBCO films formed by ion implantation regrow homogeneously rather than epitaxially and thus have the polycrystalline structure. This effect can be used to make a -axis oriented YBCO films and Josephson junctions or weak links by ion implantation with a narrow bridge mask or focused ion beam technique.

High-T/sub c/ Josephson junctions and DC SQUIDs

Step edge Josephson junctions and DC superconducting quantum interference devices (SQUIDs) made of laser ablated and sputtered YBaCuO (YBCO) films and grain boundary junctions in polycrystalline TlBaCaCuO (TBCCO) films have been investigated. Bridges on the micrometer scale have been structured by laser or ion beam etching as well as by the inhibit-layer technique. The Josephson junctions are characterized comparing I-V measurements under microwave irradiation to calculations within the resistively shunted junction (RSJ) model with finite capacitance and white noise. The dependence of the I/sub C/R/sub N/ product on temperature can be fitted to the microscopic theory of superconductor-normal insulator-normal superconductor (SNINS) or superconductor-normal conductor-superconductor (SNS) junctions with additional temperature-independent pair breaking. Flux modulation and noise properties of the DC SQUIDs at 77 K are shown.<<ETX>>

High-T/sub c/ DC-SQUID system for nondestructive evaluation

We developed different types of thin film high temperature superconductor (high-T/sub c/) DC-SQUID magnetometers and gradiometers for application in a nondestructive testing system. We used these sensors in a liquid nitrogen dewar mounted above a computer controlled x-y table within a compact /spl mu/-metal shielded chamber. The planar galvanically coupled gradiometers based on step-edge or bicrystal Josephson junctions were investigated in shielded and unshielded environment. We discuss the influence of junction type and SQUID parameters depending on the device geometry on the sensitivity of our complete testing system. Our interest is focused in particular on the noise properties and magnetic field resolution at 77 K. Measurements of ac and dc current distributions will be shown as well as measurements of magnetic moments of different samples.

Superconducting properties of ion beam modified YBCO microbridges

We prepared ion beam modified microbridges based on thin sputtered or laser ablated YBCO films on SrTiO3 substrates. The microbridges with a width of 10 μm were irradiated through slits in a 700 nm thick double layer resist mask. In our experiments we used O+ ions with an ion energy of 30 keV or 100 keV varying the dose between 1013 ions·cm−2 and 1014 ions·cm−2. We investigated the influence of film thickness and slit width on the superconducting properties of these junctions. We show the temperature dependence of the junctions properties on microwave radiation or external magnetic fields.

Cobalt silicide formation inside surface defects of a silicon substrate

The formation process of a CoSi2 layer after vacuum Co film deposition on an Si single crystal surface was investigated by means of scanning tunneling microscope (STM), SEM and RHEED. The pinholes which cause different kinds of film roughness show marks of melting processes. It is shown that in the limits of the theory of mechanochemical activity the activation energy of the silicide formation is lower inside the defect than in the bulk silicon. Therefore, the exothermic reaction of silicidation starts earlier which leads to a heating of the CoSi product. We calculated the time dependencies of the temperature inside the defect for different areas of its surface using the layer growth diffusion model and proved that the temperature increases up to the melting point at a certain critical area.

A thin film HTSC-Hall magnetometer––development and application

Abstract We have investigated a thin film Hall sensor coupled with a high- T C superconducting antenna structure. Based on theoretical calculations and on a well developed thin film technology, magnetometers with different layouts were realized on SrTiO 3 and buffered silicon substrates. The best sensors on SrTiO 3 show a sensitivity of 2.4 V/T and a white noise level of 2–3 nV/√Hz at 77 K. Further, we present eddy-current measurements with the hybrid HTSC-Hall magnetometer in a non-destructive evaluation system.