Growth distinctions of GdBa2Cu3O7−δ films on (101̄2) sapphire

Abstract The growth of epitaxial superconducting GdBa 2 Cu 3 O 7− δ films on (1012) sapphire (without buffer layer) has been investigated. The films have been grown in situ by pulsed laser deposition. The critical current density of 100-nm-thick films was above 10 6 A/cm 2 (77 K) and the surface resistance was 40 mΩ (77 K) at the frequency 75.2 GHz. The 300-nm-thick films detached from the substrate have been investigated by high resolution electron microscopy. It has been shown that the film consists of c -oriented single crystal grains about 0.5 μ m in size. In the a , b plane each grain is oriented in one of the two directions on the substrate surface (along the [2021] or [0221] sapphire axes). The differently oriented grains are connected via small angle boundaries of 3–4°. Such a film structure is caused by the lattice mismatch between GdBa 2 Cu 3 O 7− δ and (1012) sapphire. With increasing film thickness above 120 nm, the development of cracks along the small angle boundaries and degradation of superconducting properties takes place.