SUBSTRATE DEPENDENCE OF THE CRYSTALLINITY AND MOSAIC TEXTURE OF YBA2CU3O7-DELTA ULTRATHIN FILMS DEPOSITED BY LASER ABLATION

C-axis oriented ultrathin films with good crystallinity were deposited by laser ablation on both SrTiO3 and NdGaO3 substrates. The full width at the half maximum of the rocking curves of the (005) peak DELTAomega of our ultrathin films with thickness d less-than-or-equal-to 150 angstrom is less than 0.11-degrees, and the chi(min) of the 150 angstrom films on SrTiO3 determined by RBS is equal to 8.5%. SrTiO3 and NdGaO3 have different degrees of lattice mismatches of 2% and 0.2%, respectively, with YBa2Cu3O7-delta at the deposition temperature of 700-degrees-C. To study the impact on the growth of YBa2Cu3O7-delta thin films by the lattice mismatch, the thickness dependence of the crystallinity of the films on SrTiO3 and NdGaO3 substrates was compared. DELTAomega of the films deposited on SrTiO3 increases slightly from 0.085-degrees to 0.11-degrees as d increases from 50 angstrom to 150 angstrom, but DELTAomega increases rapidly from d almost-equal-to 150 angstrom and saturates at 0.35-degrees. In contrast, films on NdGaO3 showed a rather flat thickness dependence of DELTAomega which is below 0.1-degrees. We interpreted the rapid increase in DELTAomega of the films on SrTiO3 in terms of strain relaxation. Lattice matching is one of the key factors to obtain good crystallinity. The grains of the films on SrTiO3 and MgO are not flat and smooth but have ''mosaic texture.'' In contrast, the films on NdGaO3 have no mosaic texture. The possible origins of the mosaic texture are also discussed.