Magnesium Diboride Thin Films Research Articles

MgB2 thin films fabricated on Fe tape and effects of annealing on their properties

Magnesium diboride (MgB2) thin films were fabricated on Fe tapes by an electron-beam evaporation method and post-annealed at 650 °C for 1–5 h. Appropriate post- annealing (1 h) resulted in a critical temperature (T c) of 34.4 K and infield critical current density (J c) of 0.20 MA cm−2 at 20 K under 6 T. Characterization suggests that annealing improves the crystallinity of the MgB2 thin film; however, Fe diffuses into the MgB2 layer when annealing for longer durations, which deteriorates the superconductivity. MgB2 thin films on Fe tape can be utilized in diverse superconducting applications under external magnetic fields.

MgB2 Thin Films Fabricated by Pulsed Laser Deposition Using Nd:YAG Laser in an In Situ Two-Step Process

Magnesium diboride (MgB2) thin films on r-cut sapphire (r-Al2O3) single crystals were fabricated by a precursor, which was obtained at room temperature via a pulsed laser deposition (PLD) method using a Nd:YAG laser, and an in situ postannealing process. The onset superconducting transition, Tconset, and zero-resistivity transition, Tczero, were observed at 33.6 and 31.7 K, respectively, in the MgB2 thin films prepared by a Mg-rich target with a ratio of Mg:B = 3:2. The critical current density, Jc, calculated from magnetization measurements reached up to 0.9 × 106 A cm−2 at 20 K and 0 T. The broad angular Jc peak was found at 28 K when the magnetic fields were applied in a direction parallel to the film surface (θ = 90°). This could be indicative of the granular structure with randomly oriented grains. Our results demonstrate that this process is a promising candidate for the fabrication of MgB2 superconducting devices.

Growth of magnesium diboride thin films on boron buffered Si and silicon-on-insulator substrates by hybrid physical chemical vapor deposition

We report on the growth of high quality MgB2 thin films on silicon and silicon-on-insulator substrates by hybrid physical chemical vapor deposition. A boron buffer layer was deposited on all sides of the Si substrate to prevent the reaction of Mg vapor and Si. Ar ion milling at a low angle of 1° was used to reduce the roughness of the boron buffer layer before the MgB2 growth. An Ar ion milling at low angle of 1° was also applied to the MgB2 surface to reduce its roughness. The resultant MgB2 films showed excellent superconducting properties and a smooth surface. The process produces thin MgB2 films suitable for waveguide-based superconducting hot electron bolometers and other MgB2-based electronic devices.

Large scale two-dimensional arrays of magnesium diboride superconducting quantum interference devices

Magnetic field sensors based on two-dimensional arrays of superconducting quantum interference devices were constructed from magnesium diboride thin films. Each array contained over 30 000 Josephson junctions fabricated by ion damage of 30 nm weak links through an implant mask defined by nano-lithography. Current-biased devices exhibited very large voltage modulation as a function of magnetic field, with amplitudes as high as 8 mV.

Large-Area <jats:formula formulatype="inline"> <img src="/images/tex/812.gif" alt="\hbox {MgB}_{2}"> </jats:formula> Films Fabricated by Scaled-Up Hybrid Physical–Chemical Vapor Deposition

A scaled-up hybrid physical-chemical vapor deposition (HPCVD) method has been developed to fabricate large-area high-quality magnesium diboride (MgB2) thin films. The 2-in-diameter MgB2thin films on sapphire (0001) show uniform superconducting properties, includingTc~ 39 K, residual-resistivity ratio ~ 14, andJc(4.2 K) ~ 107A/cm2. X-ray diffraction showed that the films are epitaxial withc-axis normal to the film surface. The film properties are uniform across the 2-in substrate.

Fabrication of superconducting magnesium diboride thin films by electron beamannealing

The technique of electron beam annealing for fabricatingMgB2 thin films hasbeen explored. MgB2 thin films were prepared by e-beam evaporation of M–B precursor films on 6H–SiC(0001) substrates followed by ex situ electron beam annealing without any extraMg vapor or argon gas protection. The very short annealing duration, about 1 s,effectively prevented volatilization and oxidation of Mg and decomposition ofMgB2. In comparisonwith the MgB2 thin films grown by other techniques, our films show mediumqualities including a superconducting transition temperature ofTconset ∼ 35.3 K, a transition widthΔTc ∼ 0.2 K, and a criticalcurrent density of Jc(5 K, 0 T) ∼ 3.2 × 106 A cm − 2. Such an electron beam annealing technique has potential for the fabrication of large-scaleMgB2 thin filmsas well as MgB2 wires and tapes.

Magnesium Diboride Flexible Flat Cables for Cryogenic Electronics

Magnesium diboride (MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) thin films are a potential alternative to low-temperature superconductors (LTS) due to a higher critical temperature ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> ) of approximately 39 K. The reactive evaporation deposition technique also affords relatively simple growth of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films on flexible substrates compared to high-temperature superconductors (HTS). We have designed and fabricated a cable architecture consisting of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> traces on flexible yttria-stabilized zirconia (YSZ) compatible with commercially available connectors or direct wirebonds. Key performance metrics such as critical current density ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Jc</i> ) and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> are measured and compared. We discuss thermal conductivity and passivation schemes for these cables.

Local thermal bistability in MgB2 microwave coplanar resonators: Opposite jumpwise response to weak-link switching and to vortex avalanches

We report on specific nonlinear features in the response of microwave coplanar resonators obtained from magnesium diboride thin films. Different jumpwise behaviors observed in the resonance curves are found to be generated by either weak-link switching or vortex avalanches. Both mechanisms can be explained in terms of local-heating-induced thermal bistability, but they result in opposite effects, i.e., opposite signs in the abrupt variations of the quality factor and of the resonance frequency, due to the rf-current distribution. The weak-link-switching response shows promising characteristics (jumpwise signal, bistability, and hysteresis) for applications of MgB2 films in nonlinear devices.

Sputtered magnesium diboride thin films: Growth conditions and surface morphology

Magnesium diboride (MgB 2) thin films were deposited on C-plane sapphire substrates by sputtering pure B and Mg targets at different substrate temperatures, and were followed by in situ annealing. A systematic study about the effects of the various growth and annealing parameters on the physical properties of MgB 2 thin films showed that the substrate temperature is the most critical factor that determines the superconducting transition temperature ( T c ), while annealing plays a minor role. There was no superconducting transition in the thin films grown at room temperature without post-annealing. The highest T c of the samples grown at room temperature after the optimized annealing was 22 K. As the temperature of the substrate ( T s ) increased, T c rose. However, the maximum T s was limited due to the low magnesium sticking coefficient and thus the T c value was limited as well. The highest T c , 29 K, was obtained for the sample deposited at 180 °C, annealed at 620 °C, and was subsequently annealed a second time at 800 °C. Three-dimensional (3D) AFM images clearly demonstrated that the thin films with no transition, or very low T c , did not have the well-developed MgB 2 grains while the films with higher T c displayed the well-developed grains and smooth surface. Although the T c of sputtered MgB 2 films in the current work is lower than that for the bulk and ex situ annealed thin films, this work presents an important step towards the fabrication of MgB 2 heterostructures using rather simple physical vapor deposition method such as sputtering.

A novel technique for synthesizingMgB2 thin films with high upper critical fields

We report a new method for synthesizing magnesium diboride(MgB2) thin films which exhibit a steep increase in the slope of the upper critical fieldHc2 versus decreasingtemperature (i.e. |dHc2/dT|Tc|≈2.5 T K−1). An Hc2 of 38.4 T at 4.2 K and an extrapolated value ofHc2(0) above 43 T at 0 K are reported. Films of magnesium plus boron were deposited on unheatedc-plane sapphire substrates by co-evaporation, and were subsequentlyannealed in a reducing atmosphere at temperatures below600 °C. The use of a combination of a magnesium-rich stoichiometry(Mg/B>1/2) forthe as-deposited film and a two-step annealing process was found to be critical in obtaining these highvalues of Hc2 and |dHc2/dT|Tc|.

THE MICROWAVE PROPERTIES OF THE DOUBLE-SIDED THIN FILM MgB2 RESONATOR

Double-sided magnesium diboride ( MgB 2) thin films have been grown on double-sided polished sapphire substrates by the technique of hybrid physical chemical vapor deposition (HPCVD). The zero resistance temperature ( Tc ) of the films is higher than 37 K. The microwave characteristic of the double-sided MgB 2 thin films at 8.7GHz is studied systematically by the microstrip resonator technique. Surface resistance of the thin films is extracted by analyzing the resonance curves at different temperatures, and it is 1.2 mΩ at 11K.

Effects of Neutron Irradiation on Magnesium Diboride Thin Films

The effect of disorder on normal state and superconducting properties is studied in a series of neutron irradiated magnesium diboride thin films. Irradiation increases residual resistivity by more than two orders of magnitude and progressively decreases critical temperature down to the complete suppression of superconductivity. In this disorder-controlled set of samples, we analyze magnetoresistivity, extracting the scattering times in each band, and upper critical field. H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> is extremely high and only weakly depends on resistivity at low irradiation level. We propose a simple explanation for this puzzling topic, and we discuss the importance of reaching high magnetic field to correctly extrapolate H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> (0).

Systematic study of disorder induced by neutron irradiation in MgB2 thin films

The effects of neutron irradiation on normal state and superconducting properties of epitaxial magnesium diboride thin films are studied up to fluences of 1020cm−2. All the properties of the films change systematically upon irradiation. Critical temperature is suppressed and, at the highest fluence, no superconducting transition is observed down to 1.8K. Residual resistivity progressively increases from 1to190μΩcm; c axis expands and then saturates at the highest damage level. We discuss the mechanism of damage through the comparison with other damage procedures. The normal state magnetoresistivity of selected samples measured up to high fields (28 and 45T) allows to determine unambiguously the scattering rates in each band; the crossover between the clean and dirty limit in each sample can be monitored. This set of samples, with controlled amount of disorder, is suitable to study the puzzling problem of critical field in magnesium diboride thin films. The measured critical field values are extremely high (of the order of 50T in the parallel direction at low fluences) and turns out to be rather independent of the experimental resistivity, at least at low fluences. A simple model to explain this phenomenology is presented.

Growth of diborides thin films on different substrates by pulsed laser ablation

The growth of scandium, titanium and zirconium diborides thin films by pulsed laser ablation technique on different substrates has been studied. In situ reflection high energy electron diffraction and ex situ X-ray diffraction analyses indicate that the films are strongly c-axis oriented on all the substrates and also epitaxial, apart from Si(111), where the in plane orientation is poor. Atomic force microscopy imaging reveals a flat surface in all the epitaxial samples, with roughness lower than 1 nm. The results on silicon carbide and sapphire are very promising for using these materials as buffer layers in magnesium diboride thin films growth, especially to improve epitaxy and to prevent oxygen diffusion from the substrate, and also to study the influence of lattice strain on MgB2 critical temperature.

Upper Critical Fields Up to 60 T in Dirty Magnesium Diboride Thin Films

Upper critical fields of several magnesium diboride thin films were measured up to 28 T at the Grenoble High Magnetic Field Laboratory (GHMFL) in Grenoble and up to 60 T at the Laboratoire National des Champs Magnetiques Pulses (LNCMP) in Toulouse. The samples were prepared both by pulsed laser deposition (PLD) and hybrid physical chemical vapour deposition (HPCVD) technique; they have critical temperatures between 29 and 39 K and normal state resistivities between 5 and 250 &#956;ohmcm; one of them has been intentionally doped with carbon. The measured critical fields were exceptionally high; we obtained the record value of 52 T at 4.2 K in the parallel orientation. In contrast with the BCS predictions, no saturation in Hc2 at low temperature was observed. Furthermore, films with a wide range of resistivity values showed similar critical fields, suggesting that in a two band system resistivity and Hc2 are not trivially linked. The high Hc2 values seem to be related with the expanded c-axis. The structure of one of the samples was carefully investigated with X-ray diffraction at European Synchrotron Radiation Facility (ESRF) in Grenoble.

Epitaxial MgB2 thin films on ZrB2 buffer layers: structural characterization by synchrotron radiation

Structural and superconducting properties of magnesium diboride thin films grown by pulsedlaser deposition on zirconium diboride buffer layers were studied. We demonstrate that theZrB2 layer is compatiblewith the MgB2 two step deposition process. Synchrotron radiation measurements, inparticular anomalous diffraction measurements, allowed us to separateMgB2 peaksfrom ZrB2 ones and revealed that both layers have a single in plane orientation with a sharp interfacebetween them. Moreover, the buffer layer avoids oxygen contamination from the sapphiresubstrate. The critical temperature of this film is near 37.6 K and the upper critical fieldmeasured at the Grenoble High Magnetic Field Laboratory up to 20.3 T is comparable withthe highest ones reported in literature.

Novel preparation method for superconducting magnesium-diboride thin films with high- Tc

We report new fabrication techniques of growing superconducting MgB 2 thin films making use of a melting and/or sublimating process using the pulsed laser deposition method. These are called “cap and melt (CM) method” and “metal-plate cap and melt (MCM) method” which is the former improved. The concept of these techniques is to raise up Mg vapor pressure locally in the capped MgB 2 sample. The fabricated MgB 2 thin films showed the highest T c0 (Δ T c) values of 20.4 K (0.5 K) and 32.2 K (0.3 K) for the CM and MCM methods, respectively. Moreover, to our surprise, the MgB 2 thin films prepared under O 2 gas of 20 Torr showed superconductivity with T c0=30.6 K for the MCM method.

Josephson effects in magnesium diboride based Josephson junctions

We report on Josephson effects in Josephson junctions fabricated from magnesium diboride(MgB2) thin films using a focused ion beam (FIB) milling technique. The films were deposited onSrTiO3(100) and substrates at room temperature using e-beam evaporation ofMgB2 with a post-annealing stage in Mg vapour. Conventional photolithographyand Ar-ion-beam milling were used to pattern the films into 4 and8 µm wide microbridges. A focused ion beam was used to narrow the microbridges to2 µm. The narrowed microbridges were thinned by making 50 nm wide cuts across them. Thedepth of each cut was calibrated to remove 75% of the film thickness. A thinMgB2 layer remained on the substrate after this process. The current–voltage(I–V) characteristics of junctions made using this technique show that thejunctions carry excess current. The first Shapiro step was observed whenone of the junctions was irradiated with a microwave field of frequencyf = 8.92 GHz. The Shapiro step appeared at a voltage valueV = hf/2e = 18.445 µV.

Pulsed laser deposition of epitaxial titanium diboride thin films

Epitaxial titanium diboride thin films have been deposited on sapphire substrates by pulsed laser ablation technique. Structural properties of the films have been studied during the growth by reflection high energy electron diffraction (RHEED) and ex-situ by means of X-ray diffraction techniques; both kinds of measurements indicate a good crystallographic orientation of the TiB 2 film both in plane and along the c axis. A flat surface has been observed by atomic force microscopy imaging. Electrical resistivity at room temperature resulted to be five times higher than the value reported for single crystals. The films also resulted to be very stable at high temperature, which is very promising for using this material as a buffer layer in the growth of magnesium diboride thin films.

The road to magnesium diboride thin films, Josephson junctions and SQUIDs

The remarkably high critical temperature at which magnesium diboride (MgB2) undergoes transition to the superconducting state, Tc ≈ 40 K, has aroused great interest and has encouraged many groups to explore the properties and application potential of this novel superconductor. For many electronic applications and further basic studies, the availability of superconducting thin films is of great importance. Several groups have succeeded in fabricating superconducting MgB2 films. An overview of the deposition techniques for MgB2 thin film growth will be given, with a special focus on the in situ two-step process.Although, meanwhile, many problems to obtain suitable films have been solved, such as oxygen impurities and magnesium volatility, the question of how single-phase epitaxial films can be grown still remains. The possibility of growing single-crystalline epitaxial films will be discussed from the deposition conditions' point of view as well as substrate choice. Necessary conditions are discussed and possible routes are reviewed.The applicability of MgB2 in superconducting electronic devices depends on the possibility of making well-controlled, i.e., reproducible and stable, Josephson junctions. The first attempts to make MgB2–MgO–MgB2 ramp-type junctions and SQUIDs from MgB2 nanobridges are discussed.