Ph.D. Thesis Work:

Growth and Investigations on YBa₂Cu₃O_7-δ Thin Films Sputtered at Low Pressures

[Deptt. Of Physics, IIT Delhi. (Jan 1992- Feb 1997)]

Thesis Abstract:

We have been successful in depositing in situ superconducting c-axis oriented YBa₂Cu₃O_{7- δ} thin films by rf-magnetron sputtering at a relatively low partial pressures of oxygen, ranging from 6.5x10^-4 to 1.6x10^-3 Torr. It has been demonstrated that there is sufficient active oxygen available even at these low oxygen pressures resulting in a single phasic YBCO (tetragonal in as-grown state). The presence of sufficient surface/volume diffusion of oxygen species, during the growth and oxygen cooling step is adequate for the stabilization of the films into c-axis oriented epitaxial orthorhombic phase. Both, ac-susceptibility and R-T data, indicated that the best results were obtained for the films at a substrate temperature of 775°C when the partial oxygen pressure and total pressure of 9.5x10^-4 Torr and 1.9x10^-3 Torr, respectively were maintained in the growth chamber (Ind. Vac. Soc. Bull., 25 (1994) pp33-36). This pressure is nearly one order of magnitude lower than that predicted by thermodynamic stability criterion for YBCO system. The films deposited on SrTiO₃, MgO and YSZ substrates exhibited zero-resistance temperature of 82-85K and a 'c'-value in the range of 11.725-11.755Å. The J_c^mag values, reported first time, for these low pressure sputtered films on SrTiO₃ were 2.3x10⁷ and 6.2x10⁴ A/cm² at 4.2 and 77K, respectively. These films also showed an excellent stability against ambient degradation (Thin Solid Films, 303 (1997) 128)

We have also shown that by reducing oxygen-supersaturation during the sputtering process to a limit, a Stranski-Krastanov type layer-by-layer growth becomes feasible (Proc. of the Intnl. Workshop on HTSCs: 10 years after its discovery, Narosa Publications, pp420-423 (Dec’96)). Layers with atomically flat finish grow one above other in a two dimensional manner forming 2c steps (or its multiple) and large flat terraces. Epitaxial YBCO films with an average surface roughness ~100Å could be easily deposited on SrTiO₃ substrate, whereas lesser surface-smoothness results on YSZ and MgO. However, as the film thickness increases beyond ~2000Å, the growth starts exhibiting 3D-like surface morphological features with large surface roughness.

Rutherford backscattering analysis of these c-axis elongated films have revealed a gradient in the composition across the top (nearly 20%) of the film thickness. It has been proposed that the composition gradient is adjusted by Y for Ba site disorder initiated by the preferential re-evaporation of Ba from the film surface. This disorder largely accounted for an extension of the c-axis lattice parameter and the lowering of T_c. The Ba deficiency caused during the growth of a layer is made-up by the Ba in-diffusion during the subsequent growth of the other over layers (Physica C, 280 (1997) 37).

Study of fluctuation induced excess conductivity in these films indicated a 2D-layered conduction of the carriers. An estimate of zero temperature c-axis coherence length of approx. half the interatomic separation, and effective superconducting layer thickness of nearly four unit cell lengths (d~4'c') has been made. The vortices in these films behave like quasi-bidimensional system with K-T like vortex-antivortex unbinding near T_c. The presence of weak interlayer coupling of Cu-O₂ layers and quasi bi-dimensional behavior in YBCO appears to be prevalent (Physica C, 235-240 (1994) 2687).

Irradiation of YBCO films grown on SrTiO₃ substrates with 100MeV ⁵⁸Ni ions exhibited that the critical current density of the films is enhancable at an optimum value of fluence near 8x10¹¹ ions/cm². Substrate induced defect structure seems to be an influential factor in determining the enhancement in Jc. The paramagnetic Meissner effect has been observed in the irradiated films for the first time at a fluence of 2x10¹² ions/cm². The nature of defects introduced in the films by ion-irradiation at this fluence appear mainly to be linked with the partial removal of oxygen from the chain-sites, which form the π-junctions and are responsible for the observed paramagnetic Meissner effect. The vacant sites thus created are reoccupied by oxygen on subsequent aging of the film. It can be concluded that the paramagnetic Meissner effect in cuprate superconductors is mainly extrinsic in nature (Philos. Mag. B, 77 (1998) 1669).