$YBa_2Cu_3O_x(YBCO)$ high temperature superconducting thin films were grown on $SrTiO_3$ and Si substrates by PLD(Pulsed laser deposition).
The effects of the deposition parameters on film qualities have been investigated by using $SrTiO_3$ substrate. To achieve the stoichiometric deposition, sufficient laser energy density was required. Lower energy density makes the films with excess Ba and Cu, which have lower melting temperature than that of Y, especially at low oxygen pressure. The optimum oxygen pressure was a function of deposition temperature. That is, higher depostion temperature needs higher oxygen pressure to obtain high quality films and vice versa. The best films were deposited at 780℃ depostion temperature, 200mtorr oxygen pressure and 2J/㎠ laser energy density. The films show over 90K of critical temperature and $10^6A/㎠$ of critical current density.
YBCO films have also grown on silicon substrates with YSZ single buffer layer and $CeO_2$/YSZ double buffer layers. The YSZ and YBCO films show epitaxial growing with c-axis normal to the substrate. There are several types of defects including thermally induced cracks, grainboundary and outgrowths. Among them, the grainboundary have the critical effects on the film quaility, especially, electron transport properties. HRTEM image showed two types of in-plane growth orientations-o° rotation growth (YBCO<110>//Si<110> and 45° rotation growth(YBCO<110>//Si<100>)-in YBCO film on YSZ buffered Si. The 45° rotation growth have lower interface energy due to the direct matching of oxygen sublattices between Cu-O plane of YBCO and (001) plane of YSZ. However the lower lattice misfit of o° rotation growth geometry permits the films to grow with two types of in-plane growth orientation.
The introduction of $CeO_2$ layer between YSZ and YBCO can reduce o° rotation growth to 3% because the 45° rotation have smaller lattice misfits between YBCO and YSZ as well as lower interfacial energy than o° rotation. It is much smaller than the case of YSZ single buffer layer, 30%. The YBCO films on $CeO_2$/YSZ double buffer layer shows clean surface with out grainboundary and outgrowths. So it is believed that the outgrowths have to do with in-plane growth orientation. The film shows 84K of critical temperature and $10^3A/㎠$ at 77K and $10^4A/㎠$ at 10K of critical current densities. The reason why the films on Si have lower value of critical current density is believed that the stress from the difference of thermal expansion coefficients remains in the film yet.
