摘要:

This paper discusses the preparation and characterization of sol‐gel produced thin films for barrier layer and high Tcsuperconductor applications. Hydrolyzed metal alkoxide solutions were spin coated on Si substrates with subsequent thermal processing. Barrier layers of alkaline earth oxides, perovskites, Y2O3, ZrO2and others were produced. Characterization was performed via SEM, XRD, ESCA, Auger depth profiling and four point probe resistivity measurements for 1‐2‐3 thin films. On Si wafers, barrier layer films were fairly smooth with some cracking and pitting present. Si migration was severe for alkaline earth thin films on Si wafers. Some perovskite films on Si showed formation of Ba‐Si‐O phases at the Si interface.Thin films of 1‐2‐3 on sol‐gel produced barrier layers of SrTiO3or ZrO2on Si showed Ba pileup at the Si interface and were not superconducting. 1‐2‐3 layers deposited on single crystal ZrO2were superconducting and showed onset temperatures of 90 K with zero resistance reached at about 55 K. SEM morphology differences were observed for the different 1‐2‐3 preparations.

ISSN:0094-243X    

DOI:10.1063/1.39037

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Effects of focused Ar+laser irradiation on YBa2Cu3Oxin air, N2, and O2ambients were studied with a Raman microprobe. Drastic changes in the Cu‐O stretching mode (Ag) in our Raman spectra recorded in air ambient indicate an orthorhombic to tetragonal phase transformation in the YBa2Cu3Oxsample. Laser irradiation of the sample in oxygen ambient shows a reversed behavior compared to that in air. When irradiated in nitrogen ambient, it shows additional Raman lines compared wot the spectra for the starting material. This may be due to formation of BaCuO2and O(4) vacancies. The reversibility of oxygen exchange in nitrogen and oxygen ambient was also studied. The observed effects in the Raman spectra were interpreted as the chemically induced oxygen desorption and absorption assisted by Ar+laser irradiation.

ISSN:0094-243X    

DOI:10.1063/1.39038

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The microstructure of YBa2Cu3O7−xfilms on (001) and (014) SrTiO3is examined by x‐ray diffraction, optical microscopy and cross‐sectional transmission electron microscopy. The growth of YBa2Cu3O7−xin as‐deposited films is extremely anisotropic resulting in plate‐like grains with (001) surfaces. These types of YBa2Cu3O7−xgrains, 90°‐misoriented from each other, are observed to grow on both (001) and (014) surfaces of single crystal SrTiO3because in cubic SrTiO3there are three equivalent ⟨100⟩ directions that the c‐direction [001] of YBa2Cu3O7−xcan align with. On (001) SrTiO3, the (001) oriented platelets become dominant over the whole film due to the anisotropic growth‐rates, while on (014) none of the three orientations dominates resulting in smaller grain size and smoother surface. The 3‐fold degenerate epitaxy of YBa2Cu3O7−xon SrTiO3results in near‐&Sgr;1 grain boundaries with two 90°‐misoriented abutting crystals. These high angle yet crystallographically special boundaries are inevitably free of extraneous phases because of their low boundary energies and therefore, they provide intact connections between YBa2Cu3O7−xgrains throughout the film. While some extraneous phases do exist in these films, they are located away from the bounaries, and hence, are not damaging to superconducting transport. The argument agrees with the high critical current density measured (2×106A/cm2) at 77 K for a YBa2Cu3O7−xfilm on (014) SrTiO3where the measured supercurrent had to pass through at least 10 such boundaries.

ISSN:0094-243X    

DOI:10.1063/1.39040

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Wet chemical techniques are described for treatment of YBa2Cu3O7−xsurfaces, resulting in the formation of native compounds with little or no reactivity to water. Promising native compounds include CuI, BaSO4, CuS, Cu2S, YF3, and the oxalates. Formation of surface layers on chemically‐treated YBa2Cu3O7−xfilms in which these nonreactive native compounds are major constituents is verified with x‐ray photoelectron spectroscopy (XPS). No significant changes are observed in the spectra when the sulfide, sulfate, or oxalate films are dipped in water, while the iodide and fluoride films show evidence of reaction with water.

ISSN:0094-243X    

DOI:10.1063/1.39041

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

When exposed to air, YBa2Cu3O7−xreacts to form a nonsuperconducting surface layer which degrades the quality of devices formed by subsequent deposition of overlayers. Degradation may also be caused by contamination of the surface during prior processing. We have investigated the use of thermal oxidation treatments as a means of restoring these surfaces,in situ, prior to overlayer deposition. The oxidations were performed in a dual chamber UHV system equipped with x‐ray and ultraviolet photoelectron spectroscopies, ion scattering spectroscopy, and a four point probe forin situresistance measurements. Sputter‐deposited, annealed YBa2Cu3O7−xthin films exposed to air underwent treatments at 350–650 °C for 5–60 min in 10–300 Torr of oxygen. Carbonate contamination was removed at T≥460 °C. Chlorine contamination present on some samples was removed at T≥600 °C. Resistance measurements indicate that the oxygen content within the films can be maintained during these treatments if a suitable temperature profile is used. The resulting surfaces are not ideal—an insulating layer ∼1 nm thick and deficient in yttrium is present. Nevertheless, the treatments described here may prove to be useful steps in device fabrication.

ISSN:0094-243X    

DOI:10.1063/1.39042

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A multiple technique approach is used to study YBa2Cu3O7grown on SrTiO3as a function of post‐deposition annealing temperature. X‐ray diffraction data are used to determine the relative amounts of a‐axis and c‐axis oriented growth. These results are compared to the surface morphology of the films observed by SEM. Secondary ion mass spectrometry (SIMS) is used to study the diffusion of substrate elements into the YBCO films as a function of post‐deposition annealing temperature. The data obtained from all these techniques are correlated to determine an optimized temperature for post‐deposition annealing. The results of this study show that the desired c‐axis oriented growth can be obtained with minimal diffusion of substrate elements into the film at annealing temperatures of 750 °C.

ISSN:0094-243X    

DOI:10.1063/1.39043

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Results on electrical characterization of high Tcthin film surfaces modified by different surface treatments are presented. In particular, this work examines the effect of a Br/ethanol chemical etch, which has previously been shown to remove surface contamination while preserving the Cu(+2) oxidation state. Electrical measurements of YBa2Cu3O7−x/Au/Nb contact structures fabricated using polycrystalline, post‐annealed YBa2Cu3O7−xfilms with Br‐etched surfaces, show improvements of approximately one to two orders of magnitude in contact current densities and resistivities (resistance‐area products) relative to unetched contacts. The Br‐etch process has produced 10×10 &mgr;m2contacts with contact current densities greater than 400 A/cm2and RnA products as low as 4×10−7&OHgr;‐cm2.

ISSN:0094-243X    

DOI:10.1063/1.39044

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

X‐ray photoelectron spectroscopy (XPS) and electrical characterization have been used to investigate metal contacts to thin films of superconducting YBa2Cu3O7−x(YBCO). The metals studied are Au, Pt, Pd, Sn, and Ti, which cover a wide range of physical properties including reactivity with oxygen and bulk resistivity. XPS was also used to investigate the sputter cleaning and subsequent heating of the YBCO films prior to contact formation. Contacts for electrical study were defined by a shadow mask, while contacts studied by XPS were formed by sequential deposition of several A˚ of metal. XPS of the O 1s, Cu 2p, and the various metal core levels confirm that the more reactive metals cause more disruption at the metal/YBCO interface. These observations of interface chemistry are correlated with contact resistivity measured at room temperature and at 77 K. We find that although Au, Pt and Pd have similar contact resistivity at room temperature, only the Au contacts show a large decrease in contact resistance at 77 K. Ti and Sn have much higher contact resistivities at room temperature than the noble metals. These results are discussed in terms of interface chemistry and material properties.

ISSN:0094-243X    

DOI:10.1063/1.39045

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The low surface resistance (Rs) of high‐temperature superconductors (HTS) will lead to the development of passive microwave devices for application in radar, electronic warfare, and satellite systems with performance significantly better than achieved with normal conductors. In particular, delay line based devices such as phase shifters, convolvers, and correlators will have low lossses and multi‐GHz bandwidths. Low‐loss filters which presently occupy cubic feet in waveguide will be fabricated in compact microstrip or stripline, and ultra‐high Q resonators which currently require liquid helium refrigeration will be operated at around 77 K. Measurement of Rsof HTS is important both for device design and for optimization of the film growth process. Several approaches have been developed which provide data over a wide range of frequency and temperature, including stripline, cacity, and dielectric resonator techniques. HTS films for microwave applications should have at least Rs(HTS(<Rs(Cu)/10, at the same temperature and frequency, on both the top and bottom film surfaces. In order to achieve this the film should be epitaxial with their c‐axis normal to the film. YBCO films with c‐axis normal produced by laser ablation have the lowest reported surface resistance of 8 m&OHgr; at 86.7 GHz and 77 K, which is as good as Nb at the same reduced temperature.

ISSN:0094-243X    

DOI:10.1063/1.39046

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Films of Tl‐Ba‐Ca‐Cu were dc magnetron sputtered from a single multielement target. The films were deposited onto substrates of: (1) magnesium oxide, (2) a silver based alloy (Consil 995), (3) a nickel based alloy (Haynes 230), and (4) buffer layers of barium fluoride or copper oxide on Consil. To form superconducting phases, post‐deposition anneals were made on these films using an alumina crucible with an over pressure of thallium and flowing oxygen. After annealing, the film phases were determined using x‐ray diffraction. The film surface resistances (Rs) were measured at 22 GHz in a TE011cavity.

ISSN:0094-243X    

DOI:10.1063/1.39047

出版商:AIP    

年代:1990

数据来源: AIP