Hybrid Physical-chemical Vapor Deposition Method Research Articles

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.

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.

Low-noise THz MgB2 Josephson mixer

The potential applications for high frequency operation of the Josephson effect in MgB2 include THz mixers, direct detectors, and digital circuits. Here we report on MgB2 weak links which exhibit the Josephson behavior up to almost 2 THz and using them for low-noise heterodyne detection of THz radiation. The devices are made from epitaxial film grown in the c-axis direction by the hybrid physical-chemical vapor deposition method. The current in the junctions travels parallel to the surface of the film, thus making possible a large contribution of the quasi-two-dimensional σ-gap in transport across the weak link. These devices are connected to a planar spiral antenna with a dielectric substrate lens to facilitate coupling to free-space radiation for use as a detector. The IcRn product of the junction is 5.25 mV, giving confirmation of a large gap parameter. The sensitivity of the mixer was measured from 0.6 THz to 1.9 THz. At a bath temperature of over 20 K, a mixer noise temperature less than 2000 K (DSB) was measured near 0.6 THz.

The Magnetic Relaxation of MgB2Thin Films Grown by Hybrid Physical–Chemical Vapor Deposition

The magnetic relaxation of MgB 2 thin film deposited on Al 2 O 3 substrate by hybrid physical-chemical vapor deposition method was investigated. The rate of magnetic relaxation (the logarithmic decay rate) S, which was obtained by the time dependence of magnetic moment m(t) at fixed magnetic field H and temperature T, of the MgB 2 thin film was investigated. The decay of magnetic moment m(t) was carried out at temperatures between 5 and 37.5 K in the field ranges of 0.1-1.0 T, using PPMS-VSM (Quantum Design). The decrease of magnetic moment m(t) for 7200 s at T = 5 K was about 2%-3% and 0.2%-0.3% when the field was applied parallel to the c-axis and the ab-plane, respectively. The logarithmic decay rate S in the field parallel to the ab-plane was much smaller than that parallel to the c-axis over all temperature ranges. The time-independent behavior of logarithmic decay rate S at low temperatures below T = 10 K implies the presence of quantum creep effect.

Thickness dependence of Jc (0) in MgB2 films

MgB2 superconducting films, whose thicknesses range from 10nm to 8µm, have been fabricated on SiC substrates by hybrid physical–chemical vapor deposition (HPCVD) method. It is the first time that the Tc and the Jc of MgB2 films are studied on such a large scale. It is found that with the increasing of thickness, Tc elevates first and then keeps roughly stable except for some slight fluctuations, while Jc (5K, 0T) experiences a sharp increase followed by a relatively slow fall. The maximum Jc (5K, 0T)=2.3×108Acm−2 is obtained for 100nm films, which is the experimental evidence for preparing high-quality MgB2 films by HPCVD method. Thus, this work may provide guidance on choosing the suitable thickness for applications. Meanwhile, the films prepared by us cover ultrathin films, thin films and thick films, so the study on them will bring a comprehensive understanding of MgB2 films.

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.

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.

High Quality MgB<sub>2</sub>/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.

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.

Single-crystal MgB2 hexagonal microprisms via hybrid physical-chemical vapor deposition

Single-crystal MgB2 hexagonal microprisms of 30 μm have been produced in large scale by a facile hybrid physical-chemical vapor deposition method. These microprisms show perfect single-crystal structure and superconducting properties. The morphological variations corresponding to different growing stages are revealed and the possible growth mechanism is suggested.

Ultrathin MgB2 filmsfabricated on Al2O3 substrate by hybrid physical–chemical vapor deposition with highTc andJc

Ultrathin MgB2 superconducting films with a thickness down to 7.5 nm are epitaxially grown on (0001)Al2O3 substrate by a hybrid physical–chemical vapor deposition method. The filmsare phase-pure, oxidation-free and continuous. The 7.5 nm thin film shows aTc(0) of 34 K, which isso far the highest Tc(0) reported in MgB2 with the same thickness. The critical current density of ultrathinMgB2 films below 10 nm is demonstrated for the first time asJc ∼ 106 A cm − 2 for theabove 7.5 nm sample at 16 K. Our results reveal the excellent superconducting properties of ultrathinMgB2 films with thicknesses between 7.5 and 40 nm onAl2O3 substrate.

Structural, optical, and electronic properties of room temperature ferromagnetic GaCuN film grown by hybrid physical-chemical vapor deposition

Ferromagnetic Cu-doped GaN film was grown on a GaN-buffered sapphire (0001) substrate by a hybrid physical-chemical-vapor-deposition method (HPCVD). The GaCuN film (Cu: 3.6 at.%) has a highly c-axis-oriented hexagonal wurtzite crystal structure, which is similar to GaN buffer but without any secondary phases such as metallic Cu, CuxNy, and CuxGay compounds. Two weak near-band edge (NBE) emissions at 3.38 eV and donor-acceptor-pair (DAP) transition at 3.2 eV with a typical strong broad yellow emission were observed in photoluminescence spectra for GaN buffer. In contrast, the yellow emission was completely quenched in GaCuN film because Ga vacancies causing the observed yellow emission in undoped GaN were substituted by Cu atoms. In addition, GaCuN film exhibits a blue shift of NBE emission, which could be explained with the +2 oxidation state of Cu ions, replacing +3 Ga ions resulting in band gap increment. The valance sate of Cu in GaCuN film was also confirmed by x-ray photoelectron spectroscopy (XPS) analysis. The GaCuN film shows ferromagnetic ordering and possesses a residual magnetization of 0.12 emu/cm3 and a coercive field of 264 Oe at room temperature. The unpaired spins in Cu2+ ions (d9) are most likely to be responsible for the observed ferromagnetism in GaCuN.

Growth of MgB 2 thin films on alumina-buffered Si substrates by using hybrid physical–chemical vapor deposition method

By using hybrid physical-chemical vapor deposition (HPCVD) method, MgB 2 thin films were fabricated on silicon substrates with buffers of alumina grown by using atomic layer deposition method. The growth conditions were in a range of growth temperatures of 500–600 °C and under the reactor pressures of 25–50 Torr. There are some interfacial reactions in the as-grown films with impurities of mostly Mg 2Si, MgAl 2O 4, and other phases. The T c’s of MgB 2 films were observed to be as high as 39 K, but the transition widths were increased with growth temperatures. The magnetization was measured as a function of temperature down to the temperature of 5 K, but the complete Meissner effect was not observed, which shows that the granular nature of weak links is prevailing. We discuss the formation processes of mostly Mg 2Si impurity in HPCVD process, considering the diffusion and reaction of Mg vapor with silicon substrates.

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.

Double-side superconducting MgB2 films for microwave and electronic applications

Double-side superconducting MgB2 films have been grown on double-side polished sapphire substrates by using hybrid physical-chemical vapor deposition method. The zero resistance temperatures TC(0)>37K and the critical current densities JC(5K,0T)>3×106A∕cm2 have been achieved for both sides. In the first 8.73G MgB2 microstrip resonator fabricated by a double-side film, an unloaded Q value of 3400 at 11K was obtained corresponding to surface resistance of 1.2mΩ for the MgB2∕Al2O3 interface. By simply using a sapphire substrate with two polished holes, superconducting connection has been achieved for the double-side MgB2 film. The results show potential applications for the superconducting MgB2 films in microwave and electronic devices.

A Study on the Components of MgB2 Thick Film Prepared via HPCVD

Superconducting MgB2 thick film has been prepared via hybrid physical-chemical vapor deposition method on Al2O3 (0001) substrate by using B2H6 and magnesium ingot as raw materials reacted from 730 to 830°C for 40 min under 20 to 30 kPa. Its thickness is about 40 µm. The MgB2 thick film shows Tc (onset) = 39.0 K and Tc (0) = 37.2 K. X-ray diffraction pattern shows that the film grown along (101) direction has small amount of impurities of Mg and MgO. Scanning electron microscopy and energy dispersive X-ray spectroscopy indicated that these impurities existed indeed and were Mg rich. The MgO film was formed on the surface of the MgB2 thick film to further protect the sample from oxidation. We presented a new mechanism for the formation of the thick film.