摘要:

We have used high resolution photoemission spectroscopy to probe the electronic structure of a wide variety of gold/high temperature superconductor interfaces, the majority of which were formed by low‐temperature (20 K) gold evaporations on cleaved high quality single crystals. Forc‐axis interfaces formed on the 123 family of superconductors, we find that the gold deposition essentially destroys the metallicity of the superconducting substrate in the near surface region (∼5 Å), while the near surface region of Bi2Sr2CaCu2O8remains metallic. We have also used photoemission spectroscopy to search for a proximity‐effect induced superconducting gap in gold overlayers onc‐axis single crystals anda‐axis thin films, though no such effect was found.

ISSN:0734-2101    

DOI:10.1116/1.577418

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Low resistivity normal metal contacts to YBa2Cu3O7−x(YBCO) films have been investigated. It has previously shown that the contact resistivity of Au contact exhibits a strong temperature dependence, decreasing 2–3 orders of magnitude at a transition temperature nearTcof YBCO film. Other metal contacts, including Pd and Nb, do not show this effect. The contact resistivity of metal contacts has been correlated with interfacial reactions and disruption studied by x‐ray photoelectron spectroscopy (XPS). In this work we demonstrate that a thin interlayer, specifically 10 Å of Au, between the YBCO and a metal contact such as Nb can allow the formation of a low resistance contact. XPS of the effect of the interlayer is presented, and the implications for carrier coupling are discussed. These results indicate a methodology for low contact resistance bilayer‐metal structures for supercondcuting device applications.

ISSN:0734-2101    

DOI:10.1116/1.577419

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The properties of the YBa2Cu3O7−x(‘‘123’’) superconducting systems are generally determined by metal ratios, crystal structure and oxygen concentration. In order to obtain the correct oxygen content and crystal structure, it is normally necessary to subject the material to a high‐temperature annealing process in which the temperature and oxygen pressure must be controlled in order to avoid oxygen loss and prevent the formation of second phase material, while maintaining the correct cation stoichiometry. For materials with the optimal cation stoichiometry, poor performance as a superconductor is generally assumed to be the result of improper annealing. Recently, we have performed a detailed study of the resistivity of an optimal ‘‘123’’ ceramic system, as measured by a four‐point probe in the presence of controlled atmospheres of oxygen and other gases ranging in pressure (at room temperature) from ∼1 mTorr to 70 Torr. These studies were conducted in a closed cycle He refrigerator, during both cooling and heating cycles.Cooling the ‘‘123’’ material, in the presence of an ambient oxygen background equivalent to several Torr at room temperature, produced a metallicRversusTbehavior withT0(onset) of ∼103 K andTcof ∼91 K. As long as this atmosphere was maintained above the ‘‘123’’ material, only minor differences were detected in the resistance versus temperature characteristic upon reheating and repeated temperature cycling. Lowering the oxygen pressure, followed by repeated temperature cycling however, produced a continuous reduction inTcto an eventual value of ∼60 K at 1 mTorr. Reintroduction of various doses of air or O2(at any point in the cycle) immediately increasedTc, with apparent total restoration to the optimal resistance values at ∼5 to 12 Torr. Atmospherically controlled changes of oxygen concentration or reordering of oxygen vacancy sites are suspected to be responsible for these variations inTc, which occur with out annealing and with no apparent changes in structure.A variety of other reproducible alterations in theRversusTbehavior of this material (including unique humps and shifts) have been induced (and removed) through controlled changes in the ambient atmosphere. There are reasons to suspect that these surface‐oriented effects originate in the grain boundaries of these materials.

ISSN:0734-2101    

DOI:10.1116/1.577420

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Metalorganic chemical vapor deposition (MOCVD) has the potential of emerging as a viable technique to fabricate ribbons, tapes, coated wires, and the deposition of films of high temperature superconductors, and related materials. As a reduced thermal budget processing technique, rapid isothermal processing (RIP) based on incoherent radiation as the source of energy can be usefully coupled to conventional MOCVD. In this paper we report on the deposition and characterization of high quality superconducting thin films of Y–Ba–Cu–O (YBCO) on MgO and SrTiO3substrates by RIP assisted MOCVD. By using a mixture of N2O and O2as the oxygen source films deposited initially at 600 °C for 1 min and then at 740 °C for 30 min are primarilyc‐axis oriented and with zero resistance being observed at 84 and 89 K for MgO and SrTiO3substrates, respectively. The zero magnetic field current densities at 77 K for MgO and SrTiO3substrates are 1.2×106and 1.5×106A/cm2, respectively. It is envisaged that high energy photons from the incoherent light source and the use of a mixture of N2O and O2as the oxygen source, assist chemical reactions and lower overall thermal budget for processing of these films.

ISSN:0734-2101    

DOI:10.1116/1.577421

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

ErBa2Cu3O7−xfilms have been produced by simultaneous deposition of Er, Ba, and Cu from three ionized cluster beam (ICB) sources at acceleration voltages of 0.3–0.5 kV. Combining ozone oxidation with ICB deposition at 650 °C eliminated any need of post anneal processing. The substates were rotated at 10 rotations per minute during the deposition which took place at a rate of about 3 to 4 nm. ErBa2Cu3O7−xfilms with areas up to 70 mm in diameter have been made by ICB deposition. These films, 100‐nm thick, were deposited on SrTiO3(100) substrates at 650 °C in a mixture of 6 at. % O3in O2at a total pressure of 4×10−4Torr. They hadTcranging from 84.3 to 86.8 K over a 70 mm diameter andJcabove 106A/cm2at 77 K. Another set of three samples, deposited within a 50 mm diameter, was examined by magnetization measurements. These samples hadJcranging from 8.2×106to 1.1×107A/cm2at 4.2 K and from 2.4×105to 5.1×105A/cm2at 70 K. X‐ray diffraction measurements of the three samples showed preferentialc‐axis orientation normal to the substrate surface. Rocking curves showed small variation in grain misorientation with sample position relative to the center of the substrate holder. Scanning electron micrographs (SEM) of the three samples also show some texture dependence on sample position. For the three samples, there is a correlation between SEM texture, full width at half‐maximum of rocking curves andJcversus temperature curves.

ISSN:0734-2101    

DOI:10.1116/1.577422

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Experimental data are presented that confirm ferroelectric thin films of BaMgF4can be grown by ultrahigh vacuum (UHV) sublimation on semiconductor and metallized substrates at temperatures below 500 °C. Hysteresis properties of the films are shown to be qualitatively consistent with those measured on thinned single‐crystal samples cut perpendicular to the ferroelectric (orthorhombic)a axis. Loops from weakly oriented films on Si(100) display aPr/Psratio of approximately 0.84, withPs=1.6 μC/cm2andEC=150 kV/cm. These values are explained on the basis of the film orientation and the measured properties for the bulk crystals. Metal–insulator–semiconductor structures formed with fluoride layers grown on lightly doped Si substrates yieldedC–Vswitching properties indicating their suitability for use in high‐performance ferroelectric field‐effect transistor memory structures. In particular, switching attributable to polarization charge reversalinsidethe ferroelectric layer, rather than by tunnel injection of carriers from the Si into the ferroelectric, has been achieved for the first time with this material.

ISSN:0734-2101    

DOI:10.1116/1.577423

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

This is a review paper on the history of ferroelectric thin films, current practices in their fabrication, characteristics of interest which relate to their use, and the application of these films to memory and electrooptic devices.

ISSN:0734-2101    

DOI:10.1116/1.577424

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The saturation magnetostriction λsof thin Permalloy films, used in magnetoresistive reproduce heads, has to be strictly controlled to be negative and less than 3×10−7. The value of λsfor NiFe is a strong function of film composition. [E. Klokholm and J. A. Aboaf, J. Appl. Phys.52, 2474, (1981).] [R. M. Borzorth and J. G. Walker, Phys. Rev.89, 624 (1951).] Since the composition of the target material could be controlled to only ±0.2% of Ni, the potential changes in λscould have been as large as 6×10−7, which exceeded the material specification. Therefore, the effect of substrate bias (0 to −60 V), sputtering pressure (2 to 20 mTorr), and target voltage (200 to 1000 V) on magnetostriction has been investigated to identify the best process control parameter. Sputtering has been performed in an argon atmosphere using a triode source. The target was vacuum cast NiFe. Thermal oxide‐coated Si wafers were used as a substrate. It was found that the substrate bias has a strong effect on λs; however, the thickness uniformity becomes unacceptable. The target voltage has been found to have no effect on magnetostriction. The value of λshowever, can easily be modified by varying the sputtering pressure. Varying the pressure from 4 to 10 mTorr can change λsas much as 4×10−7without degradation of the other film magnetic properties, such as anisotropy, coercivity, and magnetoresistance. Therefore, this method has been successfully used in control of λsin our thin‐film magnetoresistive heads.

ISSN:0734-2101    

DOI:10.1116/1.577425

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Recent advances in heteroepitaxy have produced novel structures allowing the detailed investigation of material properties of the heterostructure components. We examine transport phenomena in 120 Å thick films of the rare earth silicide TbSi2−x. These films are epitaxial on Si(111) with channeling minimum yields below 10% as determined by Rutherford backscattering spectrometry. Electrical resistivity decreases linearly as the temperature is lowered to 38 K, where a rapid drop is observed due to magnetic ordering near the antiferromagnetic‐paramagnetic transition. Hall constant data show that the majority carriers are electrons, with anomalies at 38 and 16 K. We measure for the first time the transverse and longitudinal magnetoresistance down to 4 K in magnetic fields up to 10 Tesla. At temperatures below 38 K the longitudinal magnetoresistance has a cusplike anomaly near 8 Tesla resulting from the magnetic field‐induced suppression of the antiferromagnetic component of the structure. This anomalous behavior does not occur when the magnetic field is applied perpendicular to thecaxis of the epitaxial layers, indicating that the antiferromagnetic structure is along in‐plane directions.

ISSN:0734-2101    

DOI:10.1116/1.577426

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Fe/Cu multilayers have been deposited using the ion assisted/ion beam sputtering technique. X‐ray diffraction and selected area transmission electron diffraction show that Fe is in the body‐centered‐cobic phase. These results were in agreement with extended x‐ray absorption fine structure measured at the FeKedge using synchrotron radiation. Magnetic measurements using vibrating sample magnetometer and ferromagnetic resonance show nominal moment of the Fe, but moderate out of plane anisotropy.

ISSN:0734-2101    

DOI:10.1116/1.577427

出版商:American Vacuum Society    

年代:1991

数据来源: AIP