Hybrid Physical-chemical Vapor Deposition Research Articles

Fabrication and Characterization of Ultrathin MgB<sub>2</sub> Films for Hot-Electron Bolometer Applications

Hot-electron bolometer mixers employing thin films of conventional superconducting materials have already been successfully fabricated in the past. Magnesium diboride (MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is a promising alternative to conventional superconductors, and we report the fabrication and study of ultrathin MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films of down to 10 nm deposited by hybrid physical-chemical vapor deposition technique. The MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films showed T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of above 36 K, while residual resistivities of up to 26 μΩ · cm were achieved. Critical currents of more than 6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> A · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> at 20 K have been measured for the films with thicknesses ranging from 10 to 100 nm. Fishtail structures have been observed in the magnetic field dependence of the critical current density for the thinnest of these films, indicating the presence of defects, which act as vortex pinning centers. From the magnetic field dependence, an average distance between adjacent pinning centers of 35 nm has been obtained for the thinnest films.

Enhancement of lower critical field by reducing the thickness of epitaxial and polycrystalline MgB2 thin films

For potential applications in superconducting RF cavities, we have investigated the properties of polycrystalline MgB2 films, including the thickness dependence of the lower critical field Hc1. MgB2 thin films were fabricated by hybrid physical-chemical vapor deposition on (0001) SiC substrate either directly (for epitaxial films) or with a MgO buffer layer (for polycrystalline films). When the film thickness decreased from 300 nm to 100 nm, Hc1 at 5 K increased from around 600 Oe to 1880 Oe in epitaxial films and to 1520 Oe in polycrystalline films. The result is promising for using MgB2/MgO multilayers to enhance the vortex penetration field.

Magnetotransport phenomena in Bi2Se3 thin film topological insulators grown by hybrid physical chemical vapor deposition

Intrinsic defects in Bi2Se3 topological insulators tend to produce a high carrier concentration and current leakage through the bulk material. Bi2Se3 thin films were grown by hybrid physical chemical vapor deposition on (0001) Al2O3 substrates with high Se vapor pressure to reduce the occurrence of Se vacancies as the main type of defect. Consequently, the carrier concentration was reduced to ∼5.75 × 1018 cm−3 comparable to reported carrier concentration in Bi2Se3 thin films. Magnetotransport measurements were performed on the films and the data were analyzed for weak anti-localization using the Hikami-Larkin-Nagaoka model. The estimated α and lϕ values showed good agreement with the symplectic case of 2-D transport of topological surface states in the quantum diffusion regime. The temperature and angular dependence of magnetoresistance indicate a large contribution of the 2-D surface carriers to overall transport properties of Bi2Se3 thin film.

Fabrication of superconducting nanowires from ultrathin MgB2 films via focused ion beam milling

High quality superconducting nanowires were fabricated from ultrathin MgB2 films by a focused ion beam milling technique. The precursor MgB2 films in 10 nm thick were grown on MgO substrates by using a hybrid physical-chemical vapor deposition method. The nanowires, in widths of about 300-600 nm and lengths of 1 or 10 μm, showed high superconducting critical temperatures (Tc’s) above 34 K and narrow superconducting transition widths (ΔTc’s) of 1-3 K. The superconducting critical current density Jc of the nanowires was above 5 × 107 A/cm2 at 20 K. The high Tc, narrow ΔTc, and high Jc of the nanowires offered the possibility of making MgB2-based nano-devices such as hot-electron bolometers and superconducting nanowire single-photon detectors with high operating temperatures at 15-20 K.

High-performance perovskite solar cells fabricated by vapor deposition with optimized PbI2 precursor films

High-performance CH3NH3PbI3 solar cells were achieved by hybrid physical–chemical vapor deposition (HPCVD) method with optimized spin coating process for PbI2 precursor films.

High Crystalline Quality Perovskite Thin Films Prepared by a Novel Hybrid Evaporation/CVD Technique

ABSTRACTPerformance of a perovskite based solar cell is highly determined by the crystalline qualities of the perovskite thin film sandwiched between an electron and a hole transport layer, such as grain size and uniformity of the film. Here, we demonstrated a new hybrid physical-chemical vapor deposition (HPCVD) technique to synthesis high quality perovskite films. First, a PbI2 precursor film was spin-coated on a mesoporous TiO2 (m-TiO2)/compact TiO2 (c-TiO2)/FTO substrate in ambient environment. Then, purified CH3NH3I crystal material was evaporated and the vapor reacted with the PbI2 precursor film in a vacuum pressure/temperature accurately controlled quartz tube furnace. In this technique, high vacuum (2mTorr) and low temperature (100°C) were applied to decrease perovskite film growth rate and reduce perovskite film defects. After vapor reaction, the perovskite film was annealed at 100°C for 10min in 20mTorr vacuum to recrystallize and remove CH3NH3I residue in order to further improve crystal quality of the thin film. Crystal quality of this perovskite thin film was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). SEM and AFM results illustrate perovskite thin films synthesized by this technique have larger grain sizes and more uniformity (RMS 11.6nm/Ra 9.3nm) superior to most existing methods. Strong peaks shown in the XRD chart at 14.18°, 28.52°, 31.96°, which were assigned to (110), (220), (330) miller indices of CH3NH3PbI3 perovskite crystal, indicate the complete reaction between CH3NH3I vapor and PbI2 precursor layer. High power conversion efficiency (PCE) up to 12.3% and stable efficiencies under four hours illumination of AM1.5 standard were achieved by these solar cells. This vacuum/vapor based technique is compatible with conventional semiconductor fabrication techniques and high quality perovskite film could be achieved through delicate process control. Eventually, perovskite based solar cells could be mass produced in low cost for large scale applications by this novel technique.

Properties of $\hbox{MgB}_{2}$ Ultrathin Films Fabricated on MgO (111) Substrate by Hybrid Physical–Chemical Vapor Deposition

We fabricate MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ultrathin films via the hybrid physical-chemical vapor deposition (HPCVD) technique. With the proper background pressure and B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> flow rate, we fabricate a series of ultrathin films ranging from 10 to 40 nm. These films grow on the MgO (111) substrate and are all c-axis epitaxial. The ultrathin films show good connectivity, very high T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (0) ≈ 35-38 K, and very low residual resistivity ρ(42 K) ≈ 1. 8-20. 3 μΩ·cm. With the increase in the film thickness, the critical transition temperature increases, and the residual resistivity decreases. In addition, the 10-, 20-, and 40-nm films show extremely high critical current density, i.e., J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ≈ 2. 0 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> , 2. 3 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> , and 2. 8×10 <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 5 K, respectively, which indicate that the films fabricated by HPCVD are well qualified for device applications.

Single-Crystal like MgB2thin films grown on c-cut sapphire substrates

Abstract Single-crystal like MgB 2 thin film was grown on (000) Al 2 O l3 substrate by using hybrid physical-chemical vapor deposition (HPCVD) system. Single crystal were properties studied by X-ray diffraction (XRD)and the full width half maximum at (FWHM) of the (0001) MgB 2 peak is 0.15°, which is very close to that has been reported for MgB 2 single-crystal. It indicates that the crystalline quality of thin film is good. Temperature dependence on resistivity was investigated by physical property measurement system (PPMS) in various applied fields from 0 to 9 T. The upper critical field ( H c2 ) and irreversibility field ( H irr ) were determined from PPMS data, the estimated values arand e comparable with that of MgB 2 single-crystals. The thin film shows a high critical temperature ( T c ) of 40.4 K with a sharp superconducting transition width of 0.2 K, and a high residual resistivity ratio (RRR=21), it reflects that MgB 2 thin film has a pure phase structure. Keywords

Crystal orientation dependence of the critical current density in a-axis- and c-axis-oriented MgB2 films

We have investigated the critical current density for MgB2 films having various crystal orientations prepared by using a hybrid physical-chemical vapor deposition system. An enhancement of the critical current density is clearly presented in MgB2 films with an a-axis or a b-axis orientation rather than a c-axis orientation. X-ray diffraction patterns reveal a suppression of c-axis orientations while a (100) orientation becomes dominant, and the surface morphology of the a-axis-oriented film shows that the orientation of the c-axis-oriented MgB2 grains parallel to the plane of the substrate. As the a-axis orientation becomes more dominant in the MgB2 films, the field performance of the critical current density clearly becomes better. These results suggest that the synthesis of MgB2 with high ab-plane orientations is one of the keys to enhancing the critical current density in MgB2.

A possibility of enhancing Jcin MgB2film grown on metallic hastelloy tape with the use of SiC buffer layer

We have grown <TEX>$MgB_2$</TEX> on SiC buffer layer by using metallic Hastelloy tape as the substrate. Hastelloy tape was chosen for its potential practical applications, mainly in the power cable industry. SiC buffer layers were deposited on Hastelloy tapes at 400, 500, and <TEX>$600^{\circ}C$</TEX> by using a pulsed laser deposition method, and then by using a hybrid physical-chemical vapor deposition technique, <TEX>$MgB_2$</TEX> films were grown on the three different SiC buffer layers. An enhancement of critical current density values were noticed in the <TEX>$MgB_2$</TEX> films on SiC/Hastelloy deposited at 500 and <TEX>$600^{\circ}C$</TEX>. From the surface analysis, smaller and denser grains of <TEX>$MgB_2$</TEX> tapes are likely to cause this enhancement. This result infers that the addition of SiC buffer layers may contribute to the improvement of superconducting properties of <TEX>$MgB_2$</TEX> tapes.

Effect of Different Thickness Crystalline SiC-Buffer Layers on Superconducting Properties and Flux Pinning Mechanism of &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;${\rm MgB}_{2}$ &lt;/tex-math&gt;&lt;/inline-formula&gt; Films

We have fabricated the superconducting ${\rm MgB}_{2}$ films grown on three different crystalline SiC-buffer layers of different thickness by means of the hybrid physical–chemical vapor deposition technique and pulsed laser deposition method. Significant changes in the microstructural and superconducting properties of ${\rm MgB}_{2}$ films with addition of crystalline SiC-buffer layers were observed. The microstructural analyses of ${\rm MgB}_{2}$ films revealed that columnar grains were formed perpendicularly to the substrates, thus, enhancing all critical current density values at 5 and 20 K for all SiC-buffered- ${\rm MgB}_{2}$ films. The scaling behavior of the flux pinning force shows a magnetic field-dependent feature with different pinning mechanisms, from which one can infer that there are additional pinning centers that exist in the samples and cannot be interpreted by a simple superposition of different types of elementary pinning sources.

Sandwich-type MgB2/TiB2/MgB2 Josephson junctions

We present results of sandwich-type MgB2/TiB2/MgB2 superconductor–normal metal–superconductor Josephson junctions with MgB2 films grown by hybrid physical–chemical vapor deposition and TiB2 barriers, between 5 and 10 nm in thickness, grown by DC magnetron sputtering. Most junctions with an 8 nm or thicker barrier of TiB2 showed little excess current and no subgap features and can be described by the resistively-shunted-junction model. Meanwhile prominent multiple Andreev reflection characteristics such as excess currents and subgap peaks were observed in some junctions with thinner barriers. DC and AC Josephson effects were observed in agreement with the theory. The results demonstrate the possibility of leakage-free sandwich-type all-MgB2 junctions using conducting barrier materials

Effect of different thickness crystalline SiC buffer layers on the ordering of MgB2 films probed by extended x-ray absorption fine structure

Extended X-ray absorption fine structure (EXAFS) spectroscopy is a powerful method to investigate the local structure of thin films. Here, we have studied EXAFS of MgB2 films grown on SiC buffer layers. Crystalline SiC buffer layers with different thickness of 70, 100, and 130 nm were deposited on the Al2O3 (0001) substrates by using a pulsed laser deposition method, and then MgB2 films were grown on the SiC buffer layer by using a hybrid physical-chemical vapor deposition technique. Transition temperature of MgB2 film decreased with increasing thickness of SiC buffer layer. However, the Tc dropping went no farther than 100 nm-thick-SiC. This uncommon behavior of transition temperature is likely to be created from electron-phonon interaction in MgB2 films, which is believed to be related to the ordering of MgB2 atomic bonds, especially in the ordering of Mg–Mg bonds. Analysis from Mg K-edge EXAFS measurements showed interesting ordering behavior of MgB2 films. It is noticeable that the ordering of Mg–B bonds is found to decrease monotonically with the increase in SiC thickness of the MgB2 films, while the opposite happens with the ordering in Mg–Mg bonds. Based on these results, crystalline SiC buffer layers in MgB2 films seemingly have evident effects on the alteration of the local structure of the MgB2 film.

The role of the substrate surface morphology in enhancing the MgB2 superconducting temperature

We hereby report on the role of the surface morphology of various substrates in the enhancement of the superconducting critical temperature of MgB2. MgB2 thin layers were grown by hybrid physical–chemical vapour deposition on silicon carbide SiC substrates/fibers and several other substrates, characterized by diverse surface morphologies. By investigating the structural, morphological and transport properties of MgB2 thin layers, the presented data show that the superconducting critical temperature Tc exceeds the bulk value only when the MgB2 films are grown on atomically flat (0001) SiC single crystals and on MgB2 bottom layers. These results further confirm the interpretation of the coalescence-driven tensile strain mechanism behind the enhancement of superconducting properties in MgB2 thin films.

Superconducting magnesium diboride coatings for radio frequency cavities fabricated by hybrid physical-chemical vapor deposition

We have investigated the coating of an inner surface of superconducting radio frequency cavities with a magnesium diboride thin film by hybrid physical-chemical vapor deposition (HPCVD). To simulate a 6 GHz rf cavity, a straight stainless steel tube of 1.5-inch inner diameter and a dummy stainless steel cavity were employed, on which small sapphire and metal substrates were mounted at different locations. The ${\mathrm{MgB}}_{2}$ films on these substrates showed uniformly good superconducting properties including ${T}_{c}$ of 37--40 K, residual resistivity ratio of up to 14, and root-mean-square roughness ${R}_{q}$ of 20--30 nm. This work demonstrates the feasibility of coating the interior of cylindrical and curved objects with ${\mathrm{MgB}}_{2}$ by the HPCVD technique, an important step towards superconducting rf cavities with ${\mathrm{MgB}}_{2}$ coating.

Study on single crystal MgB2 nanosheets grown by hybrid physical-chemical vapor deposition

This report is focused on the MgB2 nanosheets which have a hexagonal single crystal structure with variable thickness and different radial dimensions. The nanosheets are fabricated by hybird physical-chemical vapor deposition for the first time, as far as we know, and meanwhile they can be site-specific transferred so as to make physical properties measurement. Results of electrical and magnetic measurements indicate that the nanosheets is superconductive with a Tconset=38 K, Tc (0)=33 K. The images of scanning electrical microscope show that the nanosheets have a nanoscale thickness and have not only a large scale in wildth from several microns to hundreds of microns but also a flat cleaning surface. The selected area electrical diffraction data is consistent with the early report of MgB2 diffraction. According to those results, the single crystal nanosheets with high quality can be surely indentified as MgB2. It suggests a new technique for MgB2 single crystal fabrication, and a zero electrical behavior is observed in nanoscale single crystal MgB2. This could be a new opportunity to make a right material for the afterward research such as flux vortices, nanoscale mechanical properties.

Enhancement in High-Field J c Properties and the Flux Pinning Mechanism of MgB2 Thin Films on Crystalline SiC Buffer Layers

We have studied the influence of crystalline SiC buffer layers on the critical current density and on the flux pinning mechanism in MgB2 thin films. Crystalline SiC buffer layers were deposited on the Al2O3 (0001) substrates by using a pulsed laser deposition method, and then MgB2 thin films were grown on the SiC-buffered layer by using a hybrid physical-chemical vapor deposition technique. MgB2 thin films with crystalline SiC-buffered layers showed a significant critical current density’s enhancement in the high magnetic field region. An uncommon plateau-like behavior was also observed when the normalized flux pinning force density was scaled with the reduced magnetic field. Based on the analyses of the scaling behavior of the flux pinning force, grain boundary pinning is likely to be a dominant pinning mechanism in the SiC-buffered MgB2 thin films.

MgB2 coated conductors directly grown on flexible metallic Hastelloy tapes by hybrid physical–chemical vapor deposition

MgB2 coated conductors (CCs), which can avoid the low packing density problem of powder-in-tube (PIT) processed wires, can be a realistic solution for practical engineering applications. Here we report on the superior superconducting properties of MgB2 CCs grown directly on the flexible metallic Hastelloy tapes without any buffer layer at various deposition temperatures from 520 to 600 °C by using hybrid physical–chemical vapor deposition (HPCVD) technique. The superconducting transition temperatures (Tc) are in the range of 38.5–39.4 K, comparable to bulk samples and high quality thin films. Clear (101) and (002) reflection peaks of MgB2 are observed in the X-ray diffraction patterns without any indication of chemical reaction between MgB2 and Hastelloy tapes. From scanning electron microscopy, it was found that connection between MgB2 grains and voids strongly depend on the growth temperature. A systematic increase in the flux pinning force density and thereby the critical current density with decreasing growth temperature was observed for the MgB2 CCs. The critical current density (Jc) of Jc(5 K, 0 T) ∼107 A/cm2 and Jc(5 K, 2.5 T) ∼105 A/cm2 has been obtained for the sample fabricated at a low growth temperature of 520 °C. The enhanced Jc (H) behavior can be understood on the basis of the variation in the microstructure of MgB2 CCs with growth temperature.

Suppression of superconductivity in epitaxial MgB2 ultrathin films

MgB2 ultrathin films have potential to make sensitive superconducting devices such as superconducting single-photon detectors working at relatively high temperatures. We have grown epitaxial MgB2 films in thicknesses ranging from about 40 nm to 6 nm by using the hybrid physical-chemical vapor deposition method and performed electrical transport measurements to study the thickness dependence of the superconducting critical temperature Tc. With reducing film thickness d, although a weak depression of the Tc has been observed, which could be attributed to an increase of disorder (interband impurity scattering) in the film, the Tc retains close to the bulk value of MgB2 (39 K), being about 35 K in the film of 6 nm thick. We show that this result, beneficial to the application of MgB2 ultrathin films and in accordance with recent theoretical calculations, is in contrast to previous findings in MgB2 films prepared by other methods such as co-evaporation and molecular-beam epitaxy, where a severe Tc suppression has been observed with Tc about one third of the bulk value in films of ∼5 nm thick. We discuss this apparent discrepancy in experiments and suggest that, towards the ultrathin limit, the different degrees of Tc suppression displayed in currently obtained MgB2 films by various techniques may arise from the different levels of disorder present in the film or different extents of proximity effect at the film surface or film-substrate interface.

Thickness dependence of grain growth orientation in MgB2films fabricated by hybrid physical-chemical vapor deposition

We have investigated the effect of thickness of the MgB2 film on the grain growth direction as well as on their superconducting properties. films of various thicknesses were fabricated on c-cut substrates at a temperature of by using hybrid physical-chemical vapor deposition (HPCVD) technique. The superconducting transition temperature () was found to increase with increase in the thickness of the film. X-ray diffraction analysis revealed that the orientation of grains changed from c-axis to a-axis upon increasing the thickness of the film from 0.6 to 2.0 . grains of various orientations were observed in the microstructures of the films examined by scanning electron microscopy. It is observed that at high magnetic fields the 2.0--thick film exhibit considerably larger critical current density () as compared to 0.6--thick film. The results are discussed in terms of an intrinsic-pinning in similarly as intrinsic-pinning occurring in high-Tc cuprate superconductors with layered structure.