Sub-micron Structures in Laser Ablated YBa2Cu3O7-x Thin Films
Doctoral thesis, 1999
This thesis treats the epitaxial growth of high-Tc superconductors and other perovskite oxides as well as the patterning of these into sub-micron structures for superconducting electronic devices.
A computer controlled laser ablation system, based on a KrF excimer laser (248 nm) to induce ablation, was developed to allow complex thin film growth processes. The laser ablation deposition technique was optimised to grow thin films of the ceramic superconductor YBa2Cu3O7-y (YBCO), the ferroelectric material BaxSr1-xTiO3 and other relevant materials of superconducting electronics.
A number of deposited thin film systems were investigated. Homo-epitaxial buffer layers on YSZ bi-crystals improved the quality of YBCO deposited on top with respect to high frequency properties of Josephson junctions. YBCO thin films with a low doping level of cobalt were investigated by x-ray diffraction, critical temperature measurements and Raman spectroscopy. A cobalt substitution for copper in the YBCO unit cell was indicated for a relatively large range of cobalt concentration. The computerised system was also used to deposit ferroelectric multilayer films of BaxSr1-xTiO3 and YBa2Cu3O7-y. Both the crystalline quality and the microwave properties were optimised.
A patterning technology for ceramic materials based on a hard carbon mask for ion beam etching was developed. Nanostructures in YBCO thin films were fabricated and transport properties of especially bridges and trenches were investigated. Superconducting bridges having cross-section dimensions down to 50 nm width and thickness were evaluated. The critical current densities (jc) of the nanobridges were in the range of 4-9x105 A/cm2 at 77K and 1.6-2x107 A/cm2 at 4.2 K. The IV characteristics and the magnitudes of jc indicated a flux-flow limited current in the bridges.
Sub-micron YBCO step edge junctions of 400-1000 nm widths were processed on LaAlO3 substrates and their local transport properties were studied. There was a large spread in the characteristic parameters of the fabricated junctions with respect to the critical current density and to the response of external magnetic fields compared to wider junctions. Three kinds of junction characteristics were seen: (1) junctions with only a Josephson current, (2) junctions with a Josephson current and an excess current, and (3) tunnel junctions. The transport characteristics could be correlated to the microstructure of the junctions.