High T_c YBa₂Cu₃O_7−x and Tl_mCa_nBa₂Cu_n+1O_x Thin Films by Sputter Deposition from Stoicthiometric Oxide Targets

Abstract

Thin films of YBa₂Cu₃O_7-x and Tl_mCa_nBa₂Cu_n+1O_x (m= 1, 2; n= 1, 2) are sputter-deposited from stoichiometric oxide targets using an off-centered and a facing target sputtering configurations, respectively. These configurations are used to minimize the preferential resputtering effect due to energetic particle bombardments of the growing films, encountered during sputter deposition from a Ba containing oxide target. Examination of plasma during sputtering of various targets reveals a smaller dark space and more intense plasma for the targets having higher Ba concentration. These together with the decreasing negative DC voltage of the target with increasing Ba content indicate a high secondary electron yield for Ba, resulting in the presence of a substantial amount of negatively charged particles (e.g., O) during sputter deposition. These particles are accelerated with full cathode (target) potential toward the substrate, causing the preferential resputtering effect observed in the conventional parallel plate sputtering configuration. The compositions of the YBa₂Cu₃O_7−X and Tl_mCa_nBa₂Cu_n+1O_x films sputter-deposited using the off centered and the facing target configurations are almost identical (>95%) to that of the target because this effect is greatly minimized in these sputtering configurations. The absence of energetic particle bombardments in the facing target sputtering is consistent with the results that a near-zero optical emission intensity at the substrate was observed with a spatially resolved optical spectrometer. Using these sputtering configurations, we have succeeded in the in-situ growth of YBa₂Cu₃O_7−x thin films at ≃650 °C on Si substrates directly (no buffer layers) with a zero resistance T_c as high as 79 K and the the preparation of various superconducting Tl_mCa_nBa₂Cu_n+1Ox thin films with a zero resistance T_c as high as 123 K. The effects of film thickness, substrate temperature, and cooling down conditions on the superconducting properties of YBa₂Cu₃O_7−x thin films on Si are reported. In addition, the post-annealing conditions and effects of substrate temperature and heating rate during post annealing on the nucleation and growth morphologies of the various Tl_mCa_nBa₂Cu_n+1O_x thin films are presented and discussed.