Hybrid Physical-chemical Vapor Deposition Research Articles

MgB[formula omitted] Josephson Junctions fabricated by Focused Helium Ion Beam with different irradiation doses on SiC and MgO substrates

Recently, high quality MgB2 single junctions and junction arrays fabricated by Focused Helium Ion Beam irradiation (He-FIB) have been reported. However, the effects of irradiation doses and substrates on junction properties have not been analysed systematically. In this work, 30 nm MgB2 thin films were deposited on SiC (0001) and MgO (111) substrates using Hybrid Physical–Chemical Vapour Deposition (HPCVD) technique and then etched into 2–4 μm wide microbridges. 2000–6000 ions/nm doses of He-FIB were irradiated across these microbridges, creating narrow junction barriers. R–T curves exhibit a clear foot-like structure that enlarges as irradiation dose increases. Temperature dependence of junction properties (critical current Ic, junction normal state resistance Rn, characteristic voltage Vc) shows that all junctions in our experiment exhibit superconductor–normal–superconductor (SNS) like behaviour and higher irradiation doses create thicker junction barriers. Furthermore, we analyse junction properties dependence on irradiation doses which yields an exponential change law. Finally, Fraunhofer diffraction-like patterns and Shapiro steps of junctions are also measured. Our results show MgB2 Josephson Junctions with adjustable properties can be fabricated by He-FIB on SiC and MgO substrates, which provides a solid foundation for future application devices research.

Roles of Fe-ion irradiation on MgB2 thin films: Structural, superconducting, and optical properties

The effects of Fe-ion irradiation on the crystal structure and superconducting properties of MgB2 thin films were investigated. Pristine samples were prepared using hybrid physical-chemical vapor deposition (HPCVD), and ion irradiation was performed at three different doses of 5 × 1013, 1 × 1014, and 2 × 1014 ions/cm2. The measured temperature-dependent resistivity showed that as the irradiation dose increased from pristine to most irradiated, the superconducting critical temperature, Tc, significantly decreased from 38.33 to 3.02 K. The crystal structures of the films were investigated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements. The results showed that the higher the dose, the greater the change in crystal structure, such as the lattice constant and bond length. This suggests that the destruction of the crystal structure at higher doses leads to the degradation of superconductivity in the irradiated MgB2 thin films. Raman spectroscopy showed that the electron-phonon coupling constant decreased with increasing irradiation dose, which was directly related to the reduction of Tc in the samples. The optical conductivity indicates that the charge-carrier density of the σ-band plays an important role in the superconductivity of ion-irradiated MgB2.

Properties of superconducting MgB2 spherical shells deposited on 2 mm and 1 mm diameter Si3N4 spheres

Superconducting spherical shells may not only provide an appealing platform for exploring superconductivity in three-dimensional (3D) geometries with curved surfaces, but also be practically applied in important fields, such as in gyroscopes and gravimeters. In inertial confinement fusion (ICF), the perturbation on the target capsule caused by traditional contact support methods is recognized as one of the major contributors to performance degradation in ICF implosions. It was proposed recently that coating the capsule with a thin superconducting MgB2 spherical shell to realize non-contact support by means of magnetic levitation at the required temperatures K might offer a promising solution to this longstanding problem. To simulate the coating of ICF capsules, we deposited MgB2 spherical shells onto polycrystalline Si3N4 spheres (diameter d = 2 mm and 1 mm) via a hybrid physical–chemical vapour deposition technique. For both diameters, the spherical shells, of about 1 µm in thickness and covering the whole spheres completely, were polycrystalline with thin plate-shaped MgB2 crystallites oriented randomly and closely connected. The spherical shells exhibited superconducting transition at a zero resistance temperature of 38.5–39.4 K in four-probe resistance measurements and ideal diamagnetism at low temperatures in magnetization measurements, suggesting the good crystallinity and homogeneity of the shells. The upper critical field , the irreversibility field and the lower critical field were characterized and showed similar magnitudes and temperature dependencies for the d = 2 mm and 1 mm shells, yielding of 7–8 T and of 7–12 mT at 20 K. The superconducting critical current density values, evaluated from the magnetization hysteresis loops, were A cm−2 and A cm−2 at 20 K under zero and 2 T applied field, respectively, for the d = 1 mm shell. The occurrence of small flux jumps was observed at low temperatures up to 12 K and in low fields below 0.3 T. The results demonstrate the feasibility of fabricating MgB2 spherical shells with properties applicable to potential areas employing magnetic levitation such as in ICF and that MgB2 spherical shells may be exploited as an experimental system to study superconductivity in 3D curvilinear geometry.

Ultrathin epitaxial MgB2 on SiC: Substrate surface-polarity-dependent properties

High-quality, ultrathin superconducting films are required for advanced devices such as hot-electron bolometers, superconducting nanowire single-photon detectors, and quantum applications. Using hybrid physical-chemical vapor deposition, we show that ${\mathrm{MgB}}_{2}$ films as thin as 4 nm can be fabricated on the carbon-terminated $6H$-SiC (0001) surface with a superconducting transition temperature above 33 K and a rms roughness of 0.7 nm. Remarkably, the film quality is a function of the SiC surface termination, with the C-terminated surface preferred to the Si-terminated surface. To understand the ${\mathrm{MgB}}_{2}$ thin film/SiC substrate interactions giving rise to this difference, we characterized the interfacial structures using Rutherford backscattering spectroscopy/channeling, electron-energy-loss spectroscopy, and x-ray photoemission spectroscopy. The ${\mathrm{MgB}}_{2}$/SiC interface structure is complex and different for the two terminations. Both terminations incorporate substantial unintentional oxide layers influencing ${\mathrm{MgB}}_{2}$ growth and morphology, but with a different extent, intermixing, and interface chemistry. In this paper, we report measurements of transport, resistivity, and the critical superconducting temperature of ${\mathrm{MgB}}_{2}$/SiC that are different for the two terminations, and they link interfacial structure variations to observed differences. The result shows that the C face of SiC is a preferred substrate for the deposition of ultrathin superconducting ${\mathrm{MgB}}_{2}$ films.

Hall effects of c-axis-oriented Mg1−xAlxB2 thin film

We synthesized c-axis-oriented Mg1-xAlxB2 thin film on an Al buffered layer of c-cut Al2O3 substrate via hybrid physical–chemical vapor deposition (HPCVD) and measured the longitudinal and Hall resistivities in the ab-plane. X-ray diffraction (XRD) showed the shift of the (000l) MgB2 peaks to higher 2θ values, indicating that Al was substituted for Mg. The 185-nm Mg1-xAlxB2 showed a critical temperature (Tc) of 26.8 K with a broadened transition width of approximately 11 K. In the normal state, the Hall coefficient was positive and decreased as the temperature increased. At T=100K, Hall coefficient (RH) is 18.97×10-11 m3/C, from which the hole charge carrier density (nhole) was determined to be 3.29×1022 holes/cm3. The suppression of nhole compared to pure MgB2 of approximately 2×1023 holes/cm3 supported the hypothesis that Al3+ substitutes to Mg2+. In the superconducting state, universal Hall scaling behavior with a constant β of 1.7±0.2 was observed due to the appearance of Al impurities.

Control of Competing Thermodynamics and Kinetics in Vapor Phase Thin-Film Growth of Nitrides and Borides.

Thin-film growth is a platform technique that allows the preparation of various undeveloped materials and enables the development of novel energy generation devices. Preferred phase formation, control of crystalline orientation and quality, defect concentration, and stoichiometry in thin films are important for obtaining thin films exhibiting desired physical and chemical properties. In particular, the control of crystalline phase formation by utilizing thin-film technology favors the preparation of undeveloped materials. In this study, thin-film growth of transition metal nitride and rare-earth metal boride was performed using remote plasma–assisted molecular beam epitaxy and hybrid physical–chemical vapor deposition techniques, and was successfully achieved by tuning the competition between thermodynamics and kinetics during vapor-phase thin-film growth. Growth conditions of high crystalline quality titanium nitride thin films and high phase purity ytterbium boride thin films were not thermodynamically favorable. Appropriate control of the contribution degree of thermodynamics and kinetics during vapor-phase thin-film growth is crucial for fabricating high phase purity and high crystalline quality thin films.

Low kinetic inductance superconducting MgB2 nanowires with a 130 ps relaxation time for single-photon detection applications

Properties of superconducting nanowires set the performance level for superconducting nanowire single photon detectors (SNSPDs). Reset time in commonly employed large area SNSPDs, 1–10 ns, is known to be limited by the nanowire’s kinetic inductance to the load impedance ratio. On the other hand, reduction of the kinetic inductance in small area (waveguide integrated) SNSPDs prevents biasing them close to the critical current due to latching into a permanent resistive state. In order to reduce the reset time in SNSPDs, superconducting nanowires with both low kinetic inductance and fast electron energy relaxation are required. In this paper, we report on a study of kinetic inductance in narrow (15–100 nm) and long (up to 120 μm) superconducting MgB2 nanowires made from 5 nm thick films, offering such combination of properties. Such films were grown using hybrid physical chemical vapor deposition, resulting in a critical temperature of ∼32 K, and a switch current density of 5 × 107 A cm−2 (at 4.8 K). Using microwave reflectometry, we measured a kinetic inductance of L k0(4.8 K) = 1.3–1.6 pH/□ regardless of the nanowire width, which results in a magnetic field penetration depth of ∼90 nm. These values are very close to those in pristine MgB2. We showed that after excitations by a 50 fs pulsed laser the reset time in 35 nm × 120 μm MgB2 nanowires is 130 ps, which is more than a factor of 10 shorter than in NbN nanowires of similar length-to-width ratios. Depending on the bias current, such MgB2 nanowires function as single-, double, or triple-photon detectors for both visible (λ = 630 nm) and infrared (λ = 1550 nm) photons, with a dark count rate of <10 cps. Although the apparent photon detection efficiency seems so far to be low, further technological advances (uniform nanowire width, smaller thickness, increasing the switching current closer to the pair-breaking current) may improve this figure of merit.

High-Performance Perovskite Solar Cells Fabricated by a Hybrid Physical–Chemical Vapor Deposition

Abstract For the first time, we used a hybrid physical–chemical vapor deposition (HPCVD) method to fabricate perovskite solar cells (PSCs) based on perovskite films with both organic cations and halogen anions. A high power conversion efficiency (PCE) of 18.1% was achieved based on a mixed perovskite film of MAxFA1−xPb (IyBr1−y)3 and the efficiency of the PSCs with MAPbI3 and MAxFA1−xPbI3 films were 14.5% and 16.4%, respectively. Perovskite material components and bandgaps were precisely tuned to achieve high photoelectric conversion performance. Three different types of perovskite films employed include MAPbI3, MAxFA1−xPbI3, and MAxFA1−xPb (IyBr1−y)3 (which are also designated as MAPbI3, MA0.89FA0.11PbI3, and MA0.54FA0.46Pb (I0.94Br0.06)3 with the respective bandgaps of 1.60 eV, 1.58 eV, and 1.61 eV. The experimental results demonstrate the ability to fabricate both organic cation and halogen anion mixed perovskite films by the HPCVD method and achieve easily adjustable bandgaps. In addition, the perovskite films fabricated by HPCVD have superior surface morphology, large crystal size, and low surface roughness. Eventually, this vapor-based method will have great potential in the fabrication of large-area and flexible PSCs to promote commercial application and industrialization of future PSCs.

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 Radio Frequency Properties of 3-GHz SRF Cavities Coated with MgB2

Magnesium diboride (MgB2) is considered a potential material for superconducting radio frequency cavities. MgB2-coated Cu cavity will allow for a higher operational temperature than a bulk Nb cavity because of the higher transition temperature of MgB2 and the high thermal conductivity of Cu. Using the hybrid physical chemical vapor deposition technique, MgB2 coatings were successfully achieved on the inner wall of 3-GHz Cu cavities. The surface and superconducting properties of the coatings were characterized using small samples on Cu plugs mounted at different locations of mock cavities. RF measurement of a MgB2-coated single-cell 3-GHz test cavity was carried out, and it showed superconducting transition at 36 K. The quality factor of this test cavity was lower than expected due to poor connectivity and inclusion of Mg–Cu alloy in the MgB2 coating.

Analysis and mitigation of defective circular structures in HPCVD-grown MgB2 thin films

In MgB2 films synthesized via hybrid physical-chemical vapor deposition, circular structures containing Mg microparticles were detected, with diameters ranging from 17 to 25 μm. The thinning of MgB2 film was observed inside these circular structures, which formation was found to be induced by rapid heating process and intense evaporation of Mg slugs. Tiny Mg droplets sputtering onto the substrates during rapid heating process influence the growth of MgB2 film and tend to form circular structures around Mg particles, which problem can be mitigated by slowing down the heating process. As a result, MgB2 films with smooth surfaces were obtained, which is instrumental in the fabrication of MgB2 film electronic devices.

Growth and anisotropic transport properties of off-axis MgB2 thin films

Off-axis MgB2 thin films with c-axis tilted away from the film normal have been fabricated on MgO (211) substrate via a hybrid physical-chemical vapor deposition method. Microscopy images show that the film surface is characterized by terrace steps, which, in line with the x-ray diffraction experiment, demonstrates the tilted growth of the film. To probe the resistivity anisotropy resulted from this tilted growth, resistance measurements in Van der Pauw configuration have been performed. By refining an equivalent-circuit model proposed previously to simulate the measurement, the anisotropy ratio ρT/ρL is estimated, where ρT or ρL is the resistivity transverse or parallel to the terrace steps respectively. The validity of the estimation has been checked by direct measurements of ρT and ρL in standard linear-four probe configuration, which are performed by patterning microbridges from the film along or across the terrace steps. The patterned microbridges also enable us to measure the anisotropy of the superconducting critical current density Jc, which shows that Jc,L parallel to the terrace steps is higher than Jc,T transverse to the terrace steps and the anisotropy ratio Jc,L/Jc,T decreases as temperature increases, following a simple [1+(T/Tc)2]−1/2 dependence with Tc the superconducting transition temperature. The results are discussed under the framework of multiband transport of MgB2 which exhibits anisotropy between the crystallographic c-axis and ab-plane because of the anisotropic contribution of the σ bands.

HPCVD Thin Film MgB2 for Superconducting Magnetic Shielding

Superconducting magnetic shielding (SMS) is crucial for superconducting electronic devices such as superconducting quantum interference devices (SQUIDs). Current magnetic shielding devices suffer from high construction price, poor mobility, and flexibility. The MgB2 SMS with cryocooler could be the potential the solution. In this work, a hybrid physical–chemical vapor deposition system, which is capable of depositing on a long cylindrical surface, is demonstrated. A dense and robust MgB2 film has successfully been deposited on the inside wall of a Φ50 × 120 mm 304 stainless steel tube. A microscopic image indicated that the deposited MgB2 showed a dendritic structure with cracks on it. Sample with different B2H6 flow rates has been made, current result showed a better performance with higher B2H6 flow rate. However, safety issue should be taken into consideration as we further increase the B2H6 concentration. The sample deposited with 10 sccm 25% B2H6 showed the best performance so far with a TC of 34.4 K. A SQUID testing system specifically for high-temperature SQUID with MgB2 SMS with a liquid-helium-free cryogenic system has been design and under construction for the future researches.

MgB2 Josephson junctions produced by focused helium ion beam irradiation

Planar magnesium diboride Josephson junctions are fabricated using focused helium ion beam irradiation. A single track of ion irradiation with a 30 kV He+ beam with nominal beam diameter &amp;lt; 0.5 nm is used to create a normal-metal barrier on a MgB2 film deposited by hybrid physical-chemical vapor deposition. Josephson coupling is observed below the critical temperature of the electrodes for a He+ doses between 8x1015/cm2 to 4x1016/cm2. Analysis of the temperature dependence of the normal resistance and critical voltage of the junctions shows highly uniform barriers with nearly ideal resistively-shunted junction behavior for higher-dose junctions, while nonequilibrium effects dominate the properties of lower-dose junctions over most of the temperature range. These results demonstrate that focused helium ion beam irradiation can produce high-quality proximity-coupled MgB2 Josephson junctions with tailorable properties, promising for use in superconducting devices and circuits.

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.

Rapid deposition and thermoelectric properties of ytterbium boride thin films using hybrid physical chemical vapor deposition

Thin films of YbB4 and YbB6 have been deposited on sapphire substrates via hybrid physical chemical vapor deposition (HPCVD) using elemental ytterbium and decaborane, B10H14, as sources. The deposition condition dependence of the composition, purity, crystallinity and electrical properties has been investigated using x-ray diffraction (XRD), reflection high-energy diffraction (RHEED), profilometry and thermoelectric property measurements. A YbB6 thin film (∼2.5 µm) was produced with a power factor reaching ca. 200 µW m−1 K−2 at the highest investigated temperature, 563 K. Such performance slightly surpasses that of bulk material and offers a positive prospect for high-temperature thermoelectric application.

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.

Planar structured perovskite solar cells by hybrid physical chemical vapor deposition with optimized perovskite film thickness

The thickness of perovskite absorber layer is a critical parameter to determine a planar structured perovskite solar cell’s performance. By modifying the spin coating speed and PbI2/N,N-dimethylformamide (DMF) solution concentration, the thickness of perovskite absorber layer was optimized to obtain high-performance solar cells. Using a PbI2/DMF solution of 1.3 mol/L, maximum power conversion efficiency (PCE) of a perovskite solar cell is 15.5% with a perovskite film of 413 nm at 5000 rpm, and PCE of 14.3% was also obtained for a solar cell with a perovskite film of 182 nm thick. It is derived that higher concentration of PbI2/DMF will result in better perovskite solar cells. Additionally, these perovskite solar cells are highly uniform. In 14 sets of solar cells, standard deviations of 11 sets of solar cells were less than 0.50% and the smallest standard deviation was 0.25%, which demonstrates the reliability and effectiveness of hybrid physical chemical vapor deposition (HPCVD) method.

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.

Magnesium diboride on inner wall of copper tube: A test case for superconducting radio frequency cavities

Superconductor magnesium diboride is considered one of the viable materials to substitute bulk niobium for superconducting radio frequency cavities. Utilizing a ${\mathrm{MgB}}_{2}$ coating on the inner wall of a copper cavity will allow operation at higher temperatures (20--25 K) than Nb cavities due to the high transition temperature of ${\mathrm{MgB}}_{2}$ (39 K) and the high thermal conductivity of Cu. In this paper, we present results of ${\mathrm{MgB}}_{2}$ coating on Cu tubes with similar dimensions to a 3 GHz cavity, as the first step towards coating the actual cavity, using the hybrid physical chemical vapor deposition technique. The results show successful coating of a uniform ${\mathrm{MgB}}_{2}$ layer on the inner wall of the Cu tubes with ${T}_{c}$ as high as 37 K.