GROWTH DISTINCTIONS OF GDBA2CU3O7-DELTA FILMS ON (10(1)OVER-BAR-2) SAPPHIRE

The growth of epitaxial superconducting GdBa2Cu3O7-delta films on (<10 (1)over bar 2>) sapphire (without buffer layer) has been investigated. The films have been grown in situ by pulsed laser deposition. The cri tical current density of 100-nm-thick films was above 10(6) A/cm(2) (7 7 K) and the surface resistance was 40 m Ohm (77 K) at the frequency 7 5.2 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 mu m in size. In the a,b plane each grain is oriented in one of the tw o directions on the substrate surface (along the [(2) over bar 021] or [<0(2)over bar 21>] sapphire axes). The differently oriented grains a re connected via small angle boundaries of 3-4 degrees. Such a film st ructure is caused by the lattice mismatch between GdBa2Cu3O7-delta and (<10(1)over bar 2>) sapphire. With increasing film thickness above 12 0 nm, the development of cracks along the small angle boundaries and d egradation of superconducting properties takes place. (C) 1998 Elsevie r Science B.V.