Hybrid Physical-chemical Vapor Deposition Research Articles

Significant enhancement of critical current density by effective carbon-doping in MgB2thin films

The pure and carbon (C)-doped <TEX>$MgB_2$</TEX> thin films were fabricated on <TEX>$Al_2O_3$</TEX> (0001) substrates at a temperature of <TEX>$650^{\circ}C$</TEX> by using hot-filament-assisted hybrid physical-chemical vapor deposition technique. The <TEX>$T_c$</TEX> value for pure <TEX>$MgB_2$</TEX> film is 38.5 K, while it is between 30 and 35 K for carbon-doped <TEX>$MgB_2$</TEX> films. Expansion in c-axis lattice parameter was observed with increase in carbon doping concentration which is in contrast to carbon-doped <TEX>$MgB_2$</TEX> single crystals. Significant enhancement in the critical current density was obtained for C-doped <TEX>$MgB_2$</TEX> films as compared to the undoped <TEX>$MgB_2$</TEX> film. This enhancement is most probably due to the incorporation of C into <TEX>$MgB_2$</TEX> and the high density of grain boundaries, both help in the pinning of vortices and result in improved superconducting performance.

Superconducting MgB2 rapid single flux quantum toggle flip flop circuit

Using self-shunted MgB2/MgO/MgB2 Josephson junctions, we have designed, fabricated, and tested a rapid single flux quantum toggle flip flop (TFF) circuit. The junctions used MgB2 films grown by hybrid physical-chemical vapor deposition with MgO barrier layer and insulating layer deposited by RF magnetron sputtering. The result showed the frequency-division function of the TFF circuit, evidenced by the output voltage being half the input voltage, up to 63 GHz at 20 K and 180 GHz at 3.2 K, which demonstrates the potential of high operating speeds and high working temperatures in MgB2 superconducting integrated circuits.

Hybrid Physical–Chemical Vapor Deposition of Ultrathin $\hbox{MgB}_{2}$ Films on MgO Substrate With High $T_{C}$ and $J_{C}$

MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> has been recognized as a candidate material for fabricating superconducting nanowire single photon detectors with the expectation of having an operating temperature of the detector at 10-20 K. Preparation of ultrathin MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films with high superconducting transition temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> is the first crucial step to achieve this goal. Here we present the hybrid physical-chemical vapor deposition of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconducting films with thicknesses around 10 nm. The films are epitaxially grown on (111) MgO substrates. The T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of the films, including for a 6-nm-thick film, is found to be above 34 K, close to the bulk value of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (39 K). The normal-state resistivity at 42 K of a 10-nm-thick film is shown to be 3.1 μΩ cm, indicating that the film is relatively clean. The superconducting critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of the films has also been measured on strip lines patterned in the films. The J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of the 10-nm-thick film reaches 7.7 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 2 K, so far the highest value reported for MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films of the same thickness. The high T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , low residual resistivity, and high J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of the ultrathin MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films on MgO substrates suggest that these films have the potential in developing MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based superconducting nanowire single photon detectors or other related superconducting devices.

High Quality MgB&lt;sub&gt;2&lt;/sub&gt;/Nb Deposited by HPCVD Method

Niobium with the transition temperature of 9 K is the usual material for TESLA superconducting accelerate cavity. The cavity must work at the temperature range of of 2~1.8 K, which would consume large energy. In addition, the low upper critical field is one of reasons causing the problem of accelerate cavities Q-slop. Comparing with Nb material, MgB2 has several advantages in this application, such as higher Tc of ~39 K, higher upper critical field and the possible lower microwave loss, which would help increasing accelerating gradient and saving operation cost. Because of these benefit, more and more focus is put on this style of accelerating cavity, MgB2-thick-film/metal-cavity-body. Using Hybrid Physical-Chemical Vapor Deposition method, the micronmeter-thick MgB2 film on Nb substrate was prepared in the present investigation. The experimental results showed that Tc(0) reached 38.5 K and the Hc2 was about 20 T. In order to test the films fracture toughness the film as the inner surface was bended. It was found that at small bending angle the influence on the superconducting properties was little. When the bend radius increased to 5 mm, some cracks smaller than 1 micron occured on the film surface. However, the film still attached to the Nb substrate and the Tc was as high as ~33.5 K, showing that the MgB2 film fabricated by HPCVD had a good compact state with the Nb substrate and good mechanical toughness. These results indicate that the clean MgB2 thick film has a better feature in the application of superconducting accelerate cavity.

MgB2 Thin Films on Metal Substrates for Superconducting RF Cavity Applications

Magnesium diboride is a promising material for superconducting RF (SRF) cavity applications. Compared to the currently used superconductor for SRF cavities Nb, MgB2 has the potential to achieve lower RF loss and higher acceleration field due to its higher critical temperature and thermodynamic critical magnetic field. Since the RF field only penetrates a few penetration depths into a superconductor, a superconducting coating of several hundred nanometers on a metal cavity is sufficient for superb SRF cavity performances. In this work, we report the properties of MgB2 thin films deposited by the hybrid physical–chemical vapor deposition (HPCVD) technique on different metal substrates including Nb, Mo, Ta, and stainless steel. All the films were polycrystalline, as indicated by X-ray diffractometry and scanning electron microscopy, and showed Tc∼39 K, determined by resistance versus temperature, magnetic susceptibility, and dielectric resonator measurements. MgB2 films deposited on Nb substrates polished to various degrees of smoothness exhibit similar Tc. The result is a promising step in the investigation of using MgB2 as an alternative to Nb for SRF cavities.

Thickness dependence of critical current density in MgB2 films fabricated by hybrid physical-chemical vapor deposition

MgB2 superconducting films with a thickness of 10 nm to 8 μ have been prepared on SiC substrates by hybrid physical-chemical vapor deposition (HPCVD). The study on Tc and Jc shows that as the film grows thicker, Tc increases and then keeps stable, which Jc increases at first, and then drops dramatically. We get the maximum Tc at 41.4 K and Jc at 2.3× 108 A·cm-2. This also shows that we can use the method of HPCVD to prepare high-quality of clean MgB2 film. And its thickness can be from 10nm ultrathin films and 100 nm thin films up to 8 μm thick film. It is the first time so far as we know that Tc and Jc are studied in this range of thickness. This will lead to a complete and systematical understanding of the superconducting MgB2 films. And it is also important and practical to choose the thickness when preparing MgB2 films.

Templated one-step catalytic fabrication of uniform diameter MgxBy nanostructures

Direct fabrication of MgxBy nanostructures is achieved by employing Ni–Mg incorporated MCM-41 in the Hybrid Physical–Chemical Vapor Deposition (HPCVD) reaction. Different reaction conditions are tested to optimize the fabrication process. TEM analysis shows the fabrication of MgxBy nanostructures starting at a reaction temperature of 600 °C, and the yield of the nanostructures increases with the reaction temperature. The as-synthesized MgxBy nanostructures have the diameters in the range of 3–5 nm, which do not increase with the reaction temperature. EELS analysis of the template removed nanostructures confirms the existence of B and Mg with minimal contamination of Si and O. NEXAFS and Raman spectroscopy analyses suggested a concentric layered structure for our as-synthesized MgxBy nanotube/nanowire, which is in good agreement with the theoretical calculations. Ni K-edge XAS indicates that the formation of MgNi alloy particles is important for the Vapor–Liquid–Solid (VLS) growth of MgxBy nanostructures with fine diameters, and the presence of Mg vapor not just Mg in the catalyst is crucial for the formation of Ni–Mg clusters. Physical templating by MCM-41 might also help to confine the diameter of the nanostructures. DC magnetization measurements indicate possible superconductive behaviors in the as-synthesized sample.

Properties of MgB2 ultra-thin films fabricated on MgO(111) substrate by hybrid physical-chemical vapor deposition

We fabricate MgB2 ultra-thin films via hybrid physics-chemical vapor deposition technique (HPCVD). Under the same background pressure, the same H2 flow rate and the same deposition time, by changing the B2H6 flow rate, we fabricate a series of ultra-thin films with thickness values ranging from 10 nm to 40 nm. These films grow on MgO(111) substrate, and are all c-axis epitaxial. These films show the good connectivity, a very high Tc(0) ≈ 35-38 K and a very low residual resistivity ρ(42 K) ≈ 1.8-20.3 μΩ·cm-1. As the thickness increases, critical transition temperature also increases and the residual resistivity decreases. The 20 nm film also shows an extremely high critical current density Jc (0 T, 5 K) ≈ 2.3×107 A/cm2, which indicates that the films fabricated by HPCVD are well qualified for device applications.

Large-Area &lt;jats:formula formulatype="inline"&gt; &amp;lt;img src="/images/tex/812.gif" alt="\hbox {MgB}_{2}"&amp;gt; &lt;/jats:formula&gt; 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 and Properties of MgB2 Coated Superconducting Tapes

We present the formation of MgB2 coatings by simple and novel aerosol deposition technique which has a potential to escalate towards the fabrication of long superconducting tapes. The thin MgB2 coatings were produced by using pre-synthesized MgB2 powder. The ability of this technique to form a precursor powder in a thin film form has greatly reduced the intricacies involved in the synthesis of MgB2 by other techniques like hybrid physical chemical vapor deposition etc. The as-synthesized thin films were characterized by the x-ray diffraction technique to study the structural properties. The thin films were found to be x-ray amorphous in nature depicting the formation of frustrated structure which showed a superconducting transition onset at around 36 K.

High Critical Current Densities in MgB2 Films Grown on Hastelloy Tape by Hybrid Physical-Chemical Vapor Deposition

We report on the high critical current densities in MgB2 films directly grown on Hastelloy tapes without any buffer layer by using the hybrid physical-chemical vapor deposition method. MgB2 films were formed by reaction of Mg metal vapor with the incoming B2H6 gas on the heated substrates. In MgB2 films grown for 10 min at 500 °C in total working pressure 100 Torr with gas mixing ratio H2:B2H6=70:30, we observed the transport critical current density (J c) was approximately 106 A/cm2 at 4 T and 20 K in magnetic fields applied parallel to the substrate plane. This value is higher than those observed in epitaxial MgB2 films on sapphire substrates grown by using the same method. Magnetic field dependence of J c of this sample was well explained by the grain-boundary pinning model. Our result opens up a possibility that the coated conductors made of MgB2 films have a strong potential for high current applications.

Carbonaceous thin film coated on nanoparticle as fuel cell catalyst formed by one-pot hybrid physical–chemical vapor deposition of iron phthalocyanine

The development of new methods for the formation of noble-metal-free fuel cell catalysts is currently important in order to provide active catalysts and to realize fuel cell advancements. We attempted the one-pot physical–chemical vapor deposition of iron phthalocyanine (FePc) on the surface of high-surface-area carbon particles as the substrate. This method enabled loading of FePc on the carbon material and concurrent heat treatment, resulting in the formation of carbon–carbon composite nanoparticles with a catalytic activity for the cathodic oxygen reduction, which functioned in a cathode in the polymer electrolyte fuel cell. The increasing temperature rate during the heat treatment, arranging the FePc–substrate configuration and simply adjusting their ratio controlled the amount of the loading and the thickness of the carbonaceous film, which was demonstrated by the transmission electron microscope images and the pore structure analysis. The catalytic activity was dependent on these parameters. The partial retention of the center structure of the FePc (Fe–Nx moiety) in the carbonaceous thin film was indicated by the Fe-K edge extended X-ray absorption fine structure. The results of this study also showed the possibility of coating various kinds of nanoparticles with functional carbonaceous thin films.

MgB2nonlinear properties investigated under localized high rf magnetic field excitation

The high transition temperature and low surface resistance of MgB2 attracts interest in its potential application in superconducting radio frequency accelerating cavities. However, compared to traditional Nb cavities, the viability of MgB2 at high rf fields is still open to question. Our approach is to study the nonlinear electrodynamics of the material under localized rf magnetic fields. Because of the presence of the small superconducting gap in the $\pi$ band, the nonlinear response of MgB2 at low temperature is potentially complicated compared to a single-gap s-wave superconductor such as Nb. Understanding the mechanisms of nonlinearity coming from the two-band structure of MgB2, as well as extrinsic sources of nonlinearity, is an urgent requirement. A localized and strong rf magnetic field, created by a magnetic write head, is integrated into our nonlinear-Meissner-effect scanning microwave microscope [Tamin Tai, X. X. Xi, C. G. Zhuang, D. I. Mircea, and S. M. Anlage, IEEE Trans. Appl. Supercond. 21, 2615 (2011)]. MgB2 films with thickness 50 nm, fabricated by a hybrid physical-chemical vapor deposition technique on dielectric substrates, are measured at a fixed location and show a strongly temperature-dependent third harmonic response. We propose that several possible mechanisms are responsible for this nonlinear response.

Magnetization and electric transport properties of single-crystal MgB2 nanowires

High quality single-crystal magnesium diboride (MgB2) nanowires with lengths exceeding 10 μm were successfully synthesized by hybrid physical chemical vapor deposition. The magnetization and electrical transport properties of single-crystal MgB2 nanowires (NWs) were measured. The superconducting transition temperature of the NWs was 37 K, as confirmed by magnetization measurements. The disordered behavior of the nanowires was observed by four-terminal current–voltage characteristic measurements of an individual NW from T = 10 to 300 K. The temperature-dependent resistivity curves for seven NWs collapsed into a universal curve described by the variable range hopping model, showing intrinsic nonmetallic transport properties. This implies that the granular superconducting defect states are critical to the superconductivity of the individual MgB2 NWs.

The Study of SiC Substrate MgB&lt;sub&gt;2 &lt;/sub&gt;thick Films Growing along C Axis

We have fabricated MgB2 thick films on SiC substrates growing along c axis by using hybrid physical–chemical vapor deposition (HPCVD) technique. The thickness was 8μm. Electric measurement showed that the Tc (onset) was 41.4K, and the transition width was 0.5K, the residual resistance ratio (RRR) was near 7. Magnetic measurement showed that the critical current density was 1.7×106A/cm2 at 5K in a self field.

Surface impedance measurements of single crystal MgB2 films for radiofrequency superconductivity applications

We report microstructure analyses and superconducting radiofrequency (SRF) measurements of macroscopic scale epitaxial MgB2 films. MgB2 films on 5 cm diameter sapphire disks were fabricated by a hybrid physical chemical vapor deposition (HPCVD) technique. The electron-beam backscattering diffraction (EBSD) results suggest that the film is a single crystal complying with a MgB2(0001)∥ Al2O3(0001) epitaxial relationship. The SRF properties of different film thicknesses (200 and 350 nm) were evaluated under different temperatures and applied fields at 7.4 GHz. A surface resistance of 9 ± 2 μΩ has been observed at 2.2 K.

MgB2 thin films with high Jc fabricated on Al tape substrates by electron beam evaporation

MgB2 thin films on nontextured Al tape substrates were fabricated by electron beam evaporation. MgB2 thin film with a boron buffer layer of about 3nm thickness was also prepared. The thickness of MgB2 thin films is 250nm. The obtained MgB2 thin films on Al tape substrates were boron rich in composition and c-axis oriented. The self-field Jc of the MgB2 thin film with a boron buffer layer at 10K and 20K are 9.45×1010A/m2 and 4.85×1010A/m2, respectively. The magnetic field reduction of Jc in MgB2 thin films on Al tape substrates is smaller compared with MgB2 wires fabricated by a powder-in-tube method and MgB2 thin films fabricated by a hybrid physical chemical vapor deposition method. The field angular dependences of Jc of MgB2 thin films on Al tape substrates are similar to that of the MgB2 thin film on Si, which was reported previously. This result indicates that grain boundaries act as a dominant pinning center in MgB2 thin films on Al tape substrates.

The Effect of Nano-Particles in Superconducting MgB&lt;sub&gt;2&lt;/sub&gt; 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.

Structural and electrical properties of epitaxial Bi2Se3 thin films grown by hybrid physical-chemical vapor deposition

We report the epitaxial growth of Bi2Se3 thin films on (0001) Al2O3 substrates by hybrid physical-chemical vapor deposition (HPCVD). The HPCVD technique combines the thermal decomposition of trimethylbismuth with the thermal evaporation of Se and leads to a high Se partial pressure in the growth ambient. The Bi2Se3 films are highly c-axis oriented on sapphire but contain planar defects including stacking faults and twin boundaries. Variable-temperature Hall-effect measurements demonstrate a carrier concentration of 5.8 × 1018 cm−3 and a mobility of 900 cm2/Vs at 4.2 K. These results demonstrate the potential of HPCVD for producing high quality Bi2Se3 films for topological insulator studies.

MgB2 films fabricated on molybdenum substrate by hybrid physical–chemical vapor deposition for superconducting RF cavity applications

Magnesium diboride (MgB2) is a good candidate material for superconducting RF cavities because of its higher transition temperature of ∼39 K and because of the absence of weak links between grains which prevents other high-Tc superconducting materials, such as YBCO, from being used for SRF cavities. Differently from MgB2 films fabricated on Cu or stainless steel(SS), we have fabricated MgB2 films on Mo substrate by hybrid physical–chemical vapor deposition. The films are about 0.7 µm thick and the Tc (onset) is as high as 38 K, with a corresponding transition width of 0.9 K. The upper critical field at T = 0 K, HC2(0), is extrapolated as 13.5 T and the critical current density at 5 K, JC (5 K, 0 T), is 6.5 × 105 A cm−2. Analysis results indicate that MgB2 film on molybdenum substrate has suitable superconducting properties and does not show the problems which often occur for MgB2 films fabricated on other metal substrates, such as the large cracks on SS substrate and the reaction between the substrate and the Mg on Cu substrate.