Metal organic chemical vapor deposition of superconducting YBa₂Cu₃O_7-x thin films

Abstract

The discovery of YBCO superconductors has stimulated a great deal of scientific and technological research into thin films of these materials. Because the MOCVD technique is known to produce high quality films in the III/V and II/VI material groups, our approach has been to apply the method to superconducting thin films. Thin films were grown in a vertical high speed (0–2000 rpm) rotating disk reactor. The source materials were metalβ-diketonates kept at temperatures in excess of 100° in order to obtain growth rates of 0.3 to 0.5μm/hr. The precursors were transported to the chamber with a nitrogen carrier and injected separately in order to avoid any gas phase reactions. The chamber pressure was maintained at 76 Torr with an oxygen partial pressure of 38 Torr. A resistance heater was used to keep the substrate temperature at 500° YBa₂Cu₃O_7-x films were deposited simultaneously on a variety of substrates such as (100) MgO, (1-102) sapphire, (100) SrTiO₃ and (100) YSZ. Full XPS spectra were collected for the binary oxides. The scans demonstrate the existence of Y₂O₃, BaO, and CuO with the correct valence state for the metallic species. Energy dispersive analysis of x-ray (EDAX) was used to determine film compositions by comparing EDAX spectral intensity to a known superconducting standard. Appropriate changes were made in the precursor flows to correct the stoichiometry. The as-grown films were dark brown and semi-transparent. Cross-sectional SEM photomicrographs revealed an ordered columnar structure. After annealing at 950–980° however, the films on (100) SrTiO₃ appeared dull black and opaque. The surface morphology exhibited smooth large plate-like grains. X-ray data clearly display an orthorhombic phase, with c-axis perpendicular to the substrate surface. Four point resistance measurements for films on (100) SrTiO₃ show the onset of superconductivity at 90 K with a complete loss of resistance at 88 K. This sharp (≤2K) transition shows the high quality of these MOCVD grown YBCO films and are the first reported results from a large area (2 × 50 mm substrates) commercial reactor.