ATOMIC-FORCE MICROSCOPY STUDY OF THE GROWTH-MECHANISM OF ULTRATHIN YBA2CU3O7 (YBCO) FILMS

An atomic force microscope (AFM) was used to study the growth mechanis m of ultrathin YBa2Cu3O7 (YBCO) films with thicknesses of 2.5 nm, 7.5 nm, 12.5 nm and 25 nm. The YBCO films were grown on SrTiO3 (100) cryst al substrates with and without a 40 nm thick buffer layer of PrBa2Cu3O 7 (PBCO) respectively. The AFM data indicated that the films grew with spiral and layer by layer growth modes, with screw dislocations being the typical spiral growth modes. Three kinds of screw dislocation wer e found on the 7.5 nm thick films with buffer layer. Two of them showe d constant terrace heights, which were 0.6 nm and 1.1 nm respectively, and the third kind of screw dislocation showed a combination of three different terrace heights of 1.6 nm, 1.0 nm and 0.6 nm. All of the sc rew dislocations had a common feature: the top of the screw dislocatio n had a columnar core and the height was four times greater than the l ength of the unit cell along the c-axis. Moreover, the heights of the terraces in all three kinds of screw dislocation were 0.6 nm or multip les of this value. This is approximately half the c-axis lattice param eter of YBCO, suggesting a two-dimensional growth mode along the film plane.