Magnesium-diboride (MgB2) has been synthesized for the first time in the 1950’s, but its superconducting properties were discovered almost 5 decades later. It has a superconducting transition temperature of TC = 39 K, which enables electronic circuits based on this material to operate at a much higher temperature (~ 25 K) than low-temperature superconductors, using compact cryocoolers. MgB2 is simpler, cheaper and more stable over time as compared to high-temperature superconductors, with less anisotropy allowing efficient vortex pinning. This, together with the advantageous properties, like a relatively long coherence length and a high critical current density makes this material attractive for a number of applications as well as for fundamental studies, like the two-band superconductivity.
Superconducting MgB2 thin films are of great interest for such basic studies and electronic application. Ideally, epitaxial MgB2 thin films with a bulk-like value of TC are desired, which implies that deposition at high temperatures (usually above 600°C) is needed. The high Mg vapor pressure, the low Mg sticking coefficient at elevated temperatures as well as the high Mg and B sensitivity to oxygen are challenging factors in the thin film deposition. These need to be overcome, especially in obtaining smooth and single-phase films suitable for multilayer structures used in superconducting electronics.
Superconducting MgB2 thin films, presented in this thesis, were prepared in two ways: by pulsed-laser deposition (PLD) and by Hybrid Physical-Chemical Vapor Deposition (HPCVD).
The films deposited by PLD became superconducting after a two-step deposition process: deposition at low temperature (at room temperature or 200°C) followed by a high-temperature (~ 600°C) annealing step. These films were deposited from an Mg-enriched MgB2 target and as multilayers alternatingly from Mg and B targets. The extra Mg and a high laser repetition rate were used to compensate for Mg loss, which is a consequence of the Mg volatility. The films showed TC,0-values up to 28 K. The reduced transition temperature as compared to the bulk value is attributed to the small MgB2 grain-sizes (smaller than 5 nm), impurities present in the starting material and MgO inclusions formed in the films. The electron mean-free path value determined for those films confirmed that they are in the dirty limit. The films were polycrystalline with very small grain sizes and therefore the choice of the substrate was not significant for the films prepared by this method.
In the HPCVD method evaporation of solid Mg and a high ambient pressure (~ 100 Torr) assured higher Mg fluxes than in the PLD method, which enables the fabrication of the superconducting films in one-step at elevated temperatures (~ 720°C), resulting in bulk-like values for TC. X-ray diffraction data showed that the films grew epitaxially on both SiC and Al2O3 substrates. The films grown on Al2O3 rotated by 30° to reduce the lattice misfit, and literature reports indicate that MgO regions were formed at the MgB2/Al2O3 interface. On the other hand, MgB2 grew without any rotation on SiC due to the low misfit and no reaction was observed. The films were rather smooth (r.m.s. roughness was ~ 2.5-11 nm) depending on the substrate used, which makes them suitable for further structuring. The electron mean-free path value of those films showed that they are in the clean limit.
The PLD and HPCVD films were used for the fabrication of weak links in the form of Josephson junctions and nanobridges, which presents an important step for further implementation of MgB2 into electronics and superconducting sensors.
Ramp-type Josephson junctions based on the PLD films showed a modulation of the junction’s critical current in applied magnetic field and the appearance of Shapiro steps by applied microwave irradiation. These modulations on the polycrystalline films illustrate that the grain boundaries act as strong links, not critically affecting the superconducting phase coherence of the film area. Further improvement in the junction’s properties can be achieved using epitaxial films with bulk-like values of TC.
Weak links in form of nanobridges (down to 100 nm width) were made in the HPCVD films as a preliminary experiment to demonstrate the properties of a weak link made in epitaxial films. Critical current densities of those nanobridges were in order of mid-107 A/cm2 at 4.2 K. This reveals a very good quality of the thin films and nanostructures.
Superconducting Quantum Interference Devices (SQUIDs) based on nanobridges were realized in the PLD and HPCVD films. The nanobridges prepared in the PLD films have critical current densities of 7x106 A/cm2 at 4.2 K indicating good superconducting properties of the films and nanostructures. Voltage modulation was observed up to 20 K, which makes them already operable at temperatures attainable with relatively small cryocoolers. The SQUIDs made on the HPCVD films had also a critical current density of 5x107 A/cm2 at 4.2 K. They showed an outstanding voltage modulation in applied magnetic field, observed until 38.8 K. This allows the operation of the SQUIDs at about 30 K.
The white noise level measured on an inductively shunted MgB2 magnetometer was 76 mf0/ÖHz in the frequency range from 1 Hz to 1 kHz and the effective flux noise was 1 pT/ÖHz at 34.5 K, which is sensitive enough for recording an adult magnetocardiogram MCG. This preliminary result is very promising for the use of MgB2 thin film based devices in bio-magnetic measurements.