Epitaxial Y1Ba2Cu3O7−δ thin films with (103)/(110)- and (100)/(010)-orientation on NdGaO3 and SrTiO3 substrates grown by ion-beam sputter deposition

Abstract

A fully oxygen-compatible ion-beam sputter deposition process (IBS) has been implemented for investigation of four film/substrate couples: (103)/(110)YBCO on (110)SrTiO3 (STO) and on (100)NdGaO3 (NGO), and (100)/(010)YBCO on (110)NGO and on (100)STO. For comparison, some (103)/(110)YBCO films have also been prepared by off-axis rf-magnetron sputtering. Below about 600 °C semiconducting, sub-nm flat, and perfectly single-crystalline YBCO films crystallize on these substrates with a crystallographic unit cell of about 1/3 of the Cu-O subcell of YBCO and perfect registration with the Ti4+-O and Ga3+-O sublattice of STO and NGO, respectively. At higher temperature superconducting YBCO films grow coherently epitaxially in the first ∼100 nm thickness; in thicker films the lattice-misfit strain relaxes to the “free” lattice constants. Above ∼680 °C a faceted (103)YBCO orientation grows on (110)STO and (100)NGO substrates with uniform in-plane orientation of the [010]YBCO direction parallel to [001]STO and [001]NGO. Along [010]YBCO the (103)YBCO films exhibit high crystalline perfection and intrinsic superconducting properties approaching those of (001)YBCO films in the plane; i.e., Tc,0 > 88 K, ΔTc,0 < 0.6 K, R300/R100 > 2.9, ρ100 > 250 μ°Cm, and jc (77 K) ⋚106 A/cm2. Practical use of (103)YBCO films is hampered by the large surface roughness. Above ∼680 °C a mixed (100)/(010)YBCO orientation grows on (110)NGO substrates, exhibiting a very smooth surface but less attractive superconducting properties; typically, Tc,0 ⋚ 80 K, ΔTc,0 ∼ 1 K, R300/R100 ∼ 1.2, ρ100 > 3 m°Cm, and jc (77 K) ⋚105 A/cm2. On (100)STO substrates the YBCO film orientation varies from pure (100)YBCO between ∼580 and ∼630 °C and mixed (100)/(010)YBCO below ∼660 °C to pure (001) YBCO above ∼670 °C. With rising temperature the surface roughness increases from <2 to ∼6 nm-rms, while the other parameters continuously improve to state-of-the-art values for c⊥-oriented films. Specifically, mixed (100)/(010)YBCO films reach Tc,0 > 86 K, δc,0 ∼ 1 K, R300/R100 > 2.8, ρ100 < 3 m°Cm, and jc (77 K) > 105 A/cm2. (100)/(010)YBCO films on (100)STO are a promising candidate for sandwich-type SIS-JJ.