Fabrication of high tilt angle YBa${}_2$Cu${}_3$O${}_x$ thin films by pulsed laser deposition

YBa${}_2$Cu${}_3$O${}_x$ (YBCO) thin films were deposited on 14°-22° tilted-axes NdGaO${}_3$ (NGO) substrates with pulsed laser deposition. Two orientations of the film are possible due to the presence of symmetrically equivalent (110) and (1 - 1 0) facets on the substrate surface. At high deposition rates, the pseudo-a grains, (100)${}_{YBCO}$||(110)${}_{NGO}$, gradually overgrow the pseudo-c grains, (001)_{YBCO}||(110)_{NGO}, forming high-tilted (tilt angle of the c axis 69°-74°) YBCO films. The substrate tilt angles in the vicinity of 18.43° promote formation of high-tilted films as a result of graphoepitaxial seeding of the pseudo-a grains on the (110) facets on the substrate surface due to the matching of the c lattice constant of the film with the length of the (110) terraces on the substrate surface. The effect of graphoepitaxial seeding was observed for the substrate tilt angles in the range 16°-21°, probably due to undulating edges of steps on the substrate surface. An increase of the deposition rate above a threshold level of ∼ 3 nm/s results in a change of the growth mode with the formation of films with smooth surface morphology. The probable reason is a transition from vapor phase epitaxy to vapor-liquid-solid crystallization mechanism. The tilted-axes YBCO films demonstrate superconducting transition temperature superior to standard samples fabricated on (110) NGO substrates. The width of the superconducting transition for tilted-axes films increases due to the suppression of the critical current in the direction normal to the tilt axis.