Substrate dependence of the crystallinity and mosaic texture of YBa2Cu3O7−δ ultrathin films deposited by laser ablation

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Abstract
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 δω of our ultrathin films with thickness d ≤ 150 Å is less than 0.11°, and the χmin of the 150 Å 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−δ at the deposition temperature of 700 °C. To study the impact on the growth of YBa2Cu3O7−δ thin films by the lattice mismatch, the thickness dependence of the crystallinity of the films on SrTiO3 and NdGaO3 substrates was compared. δω of the films deposited on SrTiO3 increases slightly from 0.085° to 0.11° as d increases from 50 Å to 150 Å, but δω increases rapidly from d ≍ 150 Å and saturates at 0.35°. In contrast, films on NdGaO3 showed a rather flat thickness dependence of δω which is below 0.1°. We interpreted the rapid increase in δω 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.