Superconducting MgB2 Thin Films Research Articles

Hybrid Physical Chemical Vapour Deposition (HPCVD) of Superconducting MgB2 Thin Films on three Dimensional Copper Substrates

The present work reports the design, development and application of a novel Hybrid Physical Chemical Vapour Deposition (HPCVD) technique for depositing MgB2 thin films, with potential superconductivity, directly on three dimensional (3D) surfaces. A novel solenoid magnetron based set up was used for depositing MgB2 thin films on 3D surfaces of Cu tube. Mg rod was used as the sputter target and source of Mg while high purity BBr3 was used as a novel boron precursor, which was injected into the system using Argon as carrier gas. The plasma mediated decomposition of BBr3 in presence of H2 gas was followed by chemical reaction between Mg and B atoms to deposit MgB2 film on the substrate. Samples were characterized by SEM, EDX, XRD and SQUID techniques. SEM-EDX confirmed deposition of a homogeneous, pore free and dense MgB2 film, while XRD analysis revealed the film to be polycrystalline and multiphasic rather than being purely c-axis oriented. Superconductivity analysis carried out using SQUID measurements indicated a sharp transition with Tc value of 39 K. From the M-H hysteresis loop, the lower critical field Hc1 and critical current density Jc at 4.2 K were calculated to be 700 Oe and 3.5 x 10 7 A/cm 2 , respectively.

Fabrication and characterization of superconducting MgB2 thin film on graphene

The building of superconductor/graphene hybrid structure has attracted a lot of attention in recent years as it provides an exciting platform to study the novel superconducting proximity effect in graphene and has promise in potential device applications. Here we report an experimental investigation on the fabrication of MgB2/graphene bilayer films. The fabrication process consists of two major steps: the transfer of monolayer graphene grown on copper foil to the target substrate, and then the deposition of MgB2 thin film on transferred graphene with a hybrid physical-chemical vapor deposition technique. The obtained MgB2 film on graphene shows a continuous film surface with prevailing c-axis orientation, and displays a narrow superconducting transition with high transition temperature Tc at 36 K, close to the value of 39 K in bulk MgB2. The decent crystalline property and high Tc of the film demonstrate the effectiveness of the present method in producing the MgB2/graphene hybrid structure, which lays a foundation for further exploring the proximity effect in graphene and developing related superconducting devices such as the Josephson junctions to work at relatively high temperatures.

Structure of superconducting MgB2 thin films prepared by vacuum evaporation and ex-situ annealing in Ar and O2 atmospheres

Superconducting MgB2 thin films were fabricated on c-cut sapphire substrates by vacuum co-evaporation of magnesium and boron followed by an ex situ post-annealing. We show that annealing in oxygen atmosphere can significantly improve the superconducting properties of the MgB2 thin films compared to the typical annealing in argon atmosphere. In this work, influence of the ex-situ annealing atmosphere on the structure, texture and morphology of the superconducting MgB2 thin films has been studied by transmission electron microscopy, X-ray diffraction and pole figure measurements. Samples annealed at 800 °C in Ar, at 800 °C in O2 and at 500 °C in O2 have been compared. The annealing in O2 at 800 °C produces MgB2 thin films with the highest superconducting transition temperature and critical current density. We show that this is thanks to the thickest MgO layer at the surface produced in this case, which acts as a protecting barrier against out-diffusion of Mg during the annealing and leads to better stoichiometry and larger MgB2 grains compared to the samples annealed in Ar. Our method can be alternative to the customary ex-situ post annealing of Mg-B precursor in sealed vapor cell. In all samples, the MgB2 phase showed single axis texture with the (0 0 0 1) planes slightly inclined by 0° to 15° with respect to the Al2O3(0 0 0 1) substrate surface incurred with the initial precursor evaporation geometry. During the annealing, the excess Mg also reacts with the Al2O3 substrate and minor MgAl2O4 and MgO phases are produced at the substrate-layer interface.

Enhanced critical current density of MgB2 thin films deposited at low temperatures by ZnO seed impurity

We report a facile method to enhance the critical current density (Jc) of superconducting MgB2 thin films. MgB2 thin films were deposited on zinc acetate dehydrate (Zn(CH3COO)22H2O) spin-coated Al2O3 (000l) substrates by using a hybrid physical-chemical vapor deposition system at low temperatures. Synthesis of MgB2 at low temperatures can reduce the substitution of Zn into the Mg site, hence avoiding the reduction of superconducting critical temperature. MgB2 thin films grown on ZnO-buffered layers showed a significant enhancement of Jc in the magnetic field due to the creation of additional pinning sources, namely point defects and grain boundaries. Broad peaks were observed in the magnetic field dependence of the flux pinning force density, indicating competition of different pinning sources.

Epitaxial growth of superconducting MgB2 thin films with a Mg buffer layer at 110 °C

Since the discovery of MgB2, its application to superconducting electronics has been limited by the absent of proper microfabrication techniques. In this study, we grew crystalline MgB2 thin films using molecular beam epitaxy at a low substrate temperature of 110 °C under ultra-high vacuum of about 10−6 Pa. MgB2 thin films were deposited with an epitaxial Mg buffer layer on c-plane 4H-SiC or sapphire substrates. In spite of the low growth temperature, superior crystallinity and surface flatness were confirmed by in situ reflection high-energy electron diffraction and X-ray diffraction measurements. Moreover, we successfully confirmed the occurrence of a sharp superconducting transition at 27 K.The present growth temperature was lower than any in prior reports on superconducting MgB2 thin films, and is lower than the applicable temperature of an organic-based lift-off resist. Our new MgB2 thin film growth process is promising for the development of an alternative nanofabrication technique for MgB2 thin films by means of a standard lift-off process with an organic resist.

The Fabrication of MgB<sub>2</sub> Film and Nano-Bridge by EB and FIB

100nm thin Mg/B precursor films were prepared on SiC substrates in ZZSX-500 vacuum coating machine. They were annealed by electron-beam(EB) which only took fractions of a second. In this paper the best annealing duration to fabricate the superconducting MgB2 thin films was investigated. Under the optimized annealing condition(accelerating voltage 15kV, electron beam current 5mA, annealing duration 0.7s), the superconducting MgB2 thin film with critical temperature Tconset~35.3 K and transition width ∆Tc~1K was fabricated. Besides that, a nano-bridge (about 100×200nm2) was etched on the superconducting MgB2 thin film by Focused Ion Beam (FIB). It’s a relative simple and efficient method. The nano-bridge exhibited the effect of Josephson junction with RSJ characteristics. At the same time a little loss of superconducting property was detected.

Ambient temperature epitaxial growth of MgB2 thin films with a Mg buffer layer

We grew crystalline MgB2 thin films using molecular beam epitaxy at a low substrate temperature of 110 °C under an ultrahigh vacuum of about 10−6 Pa. MgB2 thin films were deposited on the (001) surface of a 4H-SiC substrate with an epitaxial Mg buffer layer. Superior crystallinity and surface flatness were confirmed by in situ reflection high-energy electron diffraction and X-ray diffraction measurements. The growth temperature was lower than any in prior reports on superconducting MgB2 thin films. Moreover, we successfully confirmed the occurrence of a sharp superconducting transition at 27.2 K.

Superconducting MgB2 film prepared by chemical vapor deposition at atmospheric pressure of N2

A simple and effective chemical vapor deposition equipment was developed for deposition of superconducting MgB2 thin films. The pure precursor Boron films were prepared in base pressure of low vacuum and deposited in atmospheric pressure. After the precursor film annealed in Mg vapor, the superconducting MgB2 film was fabricated. During the precursor Boron films preparation, N2 and Ar were used as carrier gas. Compared to Ar gas, the films show better crystallization, surface morphology and superconducting performance when N2 is adopted as carrier gas. With flow rate of 200 sccm of N2 gas, the fabricated MgB2 films exhibit the highest superconducting transition temperature of 39.5K, which is among the best results of MgB2 thin films. This method provides a suitable method to realize high quality MgB2 Josephson junctions and industrial manufacture of MgB2 superconducting thin films on a large scale.

MgB2 thin film fabrication with excess Mg by sequential e-beam evaporation and transport properties under magnetic fields

Superconducting MgB2 thin films with different thicknesses were fabricated by excess Mg using sequential electron beam evaporation technique. The flux rate of Mg and B were chosen as 3 nm/sn and 0.5 nm/sn respectively. The thickness of Mg/B layer was determined as 3/1 which causes an excess Mg for MgB2 formation. After two-step preparation process, the superconducting transition temperatures were found in the range of 32–37 K with a sharp transition to zero resistance state. The irreversibility fields, upper critical fields and activation energies were derived from magneto resistivity curves. The transport critical current densities of films were determined from I–V characteristics under different applied fields and the data was extrapolated for determination of value of the 0 K.

Fabrication of Superconducting MgB2 Thin Films by Magnetron co-Sputtering on (001) MgO Substrates

We fabricated superconducting MgB2 thin films on (001) MgO substrates by magnetron rf and dc co-sputtering on heated substrates. We annealed the samples ex-situ and in-situ at temperatures between 450 ∘C and 750 ∘C. The substrate temperature during the sputtering process and the post annealing temperatures play a crucial role in forming MgB2 superconducting thin films. We achieved a critical onset temperature of up to 27.1 K for the ex-situ and 25.6 K for the in-situ annealed samples at a film thickness of 30 nm. The samples shows an out of plane (0002)-Peak which was determined by x-ray diffraction.

The Effect of Nano-Particles in Superconducting MgB<sub>2</sub> Thin Films on Stainless Steel Substrates

We have fabricated several superconducting MgB2thin films on stainless steel substrates by using hybrid physical-chemical vapor deposition (HPCVD) in pure argon atmosphere. These films were observed by scanning electron microscopes (SEM) and used the energy dispersive X-ray spectroscopy (EDX) to make elements analyses. The film thickness is about 800~1000 nm. There were some cracks on the film surface when the film is bent by different angle. The number of cracks and their width increased with the increasing bending angle. Nevertheless, the films were attached to the substrates firmly. It concludes that the superconducting MgB2thin films have great ductility and adhesion to the stainless steel substrates. We found in these films many granules about tens of nanometers in size. These nano-granules can balance both the inner structure and the surface activity of the MgB2crystal. This might be an important reason for the ductility observed with the superconducting thin films. The exact explanation depends on further research.

Transport measurements on lateral MgB2/Fe/MgB2 junctions

The magnetic anisotropy and transport properties of superconducting MgB2 thin films on MgO (100) substrates were studied. The films were prepared by rf/dc-magnetron cosputtering and with in situ annealing temperatures of 650 °C. The film orientation was measured by X-ray diffractometry, which revealed a c-axis orientation of the MgB2 films. The critical onset temperature without field cooling is 15.5 K. We found a critical field of 14.73 T parallel to the film plane and 10.79 T perpendicular to the film plane from transport measurements of the dependence of the applied magnetic field. Differential conductance measurements of a lateral MgB2/Fe/MgB2 junction show the Δπ gap and the Δσ gap.

As-grown superconducting MgB2 thin films prepared at extreme deposition conditions

Superconducting (SC) MgB2 thin films were prepared in situ in one step by vacuum co-deposition at extremely low growth temperature(Ts ∼ 110–150 °C) and at very low Mg deposition rate, far off from theMgB2 stability regime of the thermodynamic phase diagram. As-grown films are nanocrystallineand exhibit (002) texture, moderate surface roughness and SC onset up toTcon = 18 K. This is the highest critical temperature ever observed inlow-Ts as-grownMgB2 films. Our preparationmethod at very low Ts is favorable for the fabrication of layered SC hybrid electronic devices, wheredeposition and post-annealing at high temperature are detrimental to devicefunctioning. Simple post-annealing of our films in ultrahigh vacuum enhancesTcon up to 22 K due to the improvement in structural properties.

Effects of In-situ and Ex-situ Heat-Treatment Procedures on the Transport Properties of the MgB2 Superconducting Thin Films Fabricated by Ultrasonic Spray Pyrolysis (USP) System

Superconducting MgB2 thin films have been prepared using Ultrasonic Spray Pyrolysis (USP) system. 2.4 MHz USP system and various solutions which contain different Mg, B and de-ionized water and LAPSA concentrations and gas atmospheres were used to obtain 500 nm to 1.0 μm thick MgB2 films. Some of the films produced were heat treated in-situ in the spraying chamber during deposition and some of them were annealed ex-situ in the tube furnace under Mg vapor. Tc and Tzero of the samples were obtained to be 39.5 K and 37.4, 39.5 and 37.0 for ex-situ and in-situ prepared films, respectively. The highest critical current density was obtained to be 4.12×106 A cm−2 for ex-situ annealed films and 4.01×106 A cm−2 for in-situ produced films. The result obtained indicated that the ex-situ preparation method provides improvement in the transport and magnetic properties.

Impact of substitutional and interstitial carbon defects on lattice parameters in MgB2

Carbon (C)-doping has been found to increase the upper critical field HC2 in superconducting MgB2 thin-film and bulk samples. However, the C effects on both HC2 and lattice parameters are very different between thin films and bulk, suggesting C may incorporate differently in the two cases. This paper combines ab initio calculations and thin-film lattice parameter measurements to explore the connection between substitutional and interstitial C in MgB2 and experimental bulk and thin-film lattice parameters.

Etching of ${\rm MgB}_{2}$ Thin Films for Transition Edge Superconducting Bolometer Fabrication

Membrane-structured superconducting MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films are potential candidates for the development of moderately cooled bolometers sensitive to far infra-red radiation. On the path to developing such devices, we present a comparison of three etching techniques for MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films namely, chlorine plasma etch, aqueous hydrochloric acid etch and an aqueous nitric acid etch. Out of the three etch techniques, the aqueous 50% nitric acid solution etch, using standard photolithography, proved to have a high MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> etch rate (> 51 nm/s), with better sidewall delineation and selectivity to the underlying SiN-film-coated Si substrate, moreover the etched film structure showed good superconductivity transition characteristics, namely, a superconducting critical transition at 38.57 plusmn 0.6 K, a transition width of 0.09 K and a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RRR</i> of 2.22.

Growth of ${\rm MgB}_{2}$ Thin Films In Situ by RF Magnetron Sputtering With a Pocket Heater

We have grown MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films using RF magnetron sputtering combined with a pocket heater. This technique relies on a low-pressure environment for sputter deposition of boron and a high-pressure environment for thermal evaporation of Mg. We have obtained superconducting MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films using substrate temperatures of 480-540degC and Mg furnace temperatures of 730-750degC. The T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> onset of the thin films increased from 21.6 K to 35 K with increasing substrate temperature due to better crystallization. Higher boron deposition rates also increase T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . The highest J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the films at 5 K and near zero magnetic field is 1.5 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> which is comparatively lower than the films grown using a pocket heater with boron deposition by chemical vapor deposition or electron beam evaporation. The chemical composition analysis by WDS exhibits a high concentration of oxygen and carbon in the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films, which is due to a high background base pressure and an impure B sputtering target. These results suggest that the T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , and resistivity of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films are mainly determined by an inhomogeneous microstructure and superconducting percolation paths through impurity phases such as MgO. By comparison with carbon or oxygen doped films, the high impurity content in the sputtered MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films might act as method to achieve high H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> .

Nano-bridges based on the superconducting MgB2 thin films

Precursor MgB thin films were prepared on sapphire substrates by magnetron sputtering. Influence of the ex situ annealing process on the roughness of the superconducting MgB2 thin films is discussed. Optimized annealing process of MgB precursor thin films in vacuum results in smooth superconducting MgB2 thin films with roughness below 10nm, critical temperature Tcon=31K and transition width ΔTc less than 1K. Nano-bridges based on the superconducting MgB2 thin films using optical and focused ion beam lithography were prepared. Critical current density jc (4.2K) measured on 50nm wide strip was 7.3×106A/cm2 and no significant loss of superconducting properties was detected.

Fabrication and characterization of High-Q NMR antenna coil using superconducting MgB2 thin films

A superconducting antenna coil for NMR measurement was fabricated by using MgB2 thin films, and its quality factor was evaluated at 4.2 K. The antenna coil was made of two MgB2 thin film coils formed on Al2O3 substrates. From the measurement results, the quality factors Q=6550–10640 were obtained in the frequency range of 300–500 MHz. These values were 2–11 times higher than that of normal metal Cu antenna coils under the same condition. Present experimental results indicate that the superconducting MgB2 antenna coil has a great advantage to normal metal antenna coils for high-sensitivity NMR measurement.

In-situ growth of superconducting MgB2 thin films by HPCVD method at low temperature

An improved hybrid physical chemical vapor deposition (HPCVD) method for the preparation of magnesium diboride (MgB2) films is proposed. In this experiment, the temperature for the evaporation of magnesium is set to be ∼700°C, while the substrate is kept below 500°C. Inside the vacuum chamber, double heaters control the temperature for the evaporation of Mg and of the substrate independently. The deposition of MgB2 films on a polycrystalline Al2O3 substrates is obtained by the reaction between evaporated magnesium atoms and boron atoms that decompose from diborane near the heaters. The superconducting MgB2 films show a transition temperature at 38.5K and a zero-resistance temperature at 38K. The critical current density of the films is 105A/cm2.