ETCH DEFECTS IN YBA2CU3O7-DELTA SINGLE-CRYSTALS GROWN FROM FLUX

The crystallographic defects in (001), (100)/(010) and (011) faces of YBa2Cu3O7-delta (YBCO) single crystals, revealed by chemical etching with acetic acid, as well as mixed solutions of acetic acid, nitric acid and methanol, have been studied, We have found clear evidence for growth hillocks and steps on these etched faces. Three types of etch pits are observed: Type I is a square pyramid with growth steps along the [100] and [010] directions; type II is an octagonal pattern of the above square truncated along the [110] direction; and type III is a shallow ellipse with long and short axes in the [100]/[010] and [001] direction, respectively. Due to their spiral growth features types I and II are designated screw dislocation etch pits and type III is thought to be of Hopper-like morphology, on the basis of the growth mechanism. Clustering and tangling of dislocation etch pits have also been observed. The etching rate is anisotropic with different speeds in the four principal directions being V(a[110]) > V(a[100]/[010]) > V(c[001]) > V(i[111]). The density of the dislocation etch pits is of the order of 10(6) cm-2 in the (001) and 10(5) cm-2 in the (100)/(010) surfaces.