Atomic oxygen effect on the in situ growth of stoichiometric YBa2Cu3O7−δ epitaxial films by facing targets 90° off-axis radiofrequency magnetron sputtering

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Abstract
(110)‐ and (103)‐oriented almost stoichiometric YBa2Cu3O7−δ(YBCO)films have been grown epitaxially on hot SrTiO3 (110) substrates using a 90° off‐axis rf magnetron sputtering technique, for fabrication of vertical sandwich‐type YBCO/insulator/YBCO or YBCO/normal metal/YBCO Josephson junctions utilizing the high‐quality YBCOfilms. The YBCOepitaxialfilms with high transition temperatures T c of ∼90 K have been depositedin situ only under the conditions of substrate temperatures T s of ∼650–∼700 °C and oxygen partial pressure P O2 of ∼5×10−3–∼10×10−3 Torr, which are in close proximity to the critical stability/decomposition line for YBa2Cu3O6 in the ordinary Y–Ba–Cu–O phase diagram. Using a quadrupole mass spectrometer, a high density of atomic oxygen has directly been observed to be efficiently produced in the sputter glow discharge under the above optimum conditions of P O2 . This atomic oxygen has played a key role in promoting the formation of the perovskite structure and the epitaxialgrowth of the YBCOfilms. Furthermore, Shapiro steps have successfully been observed for a Nb–YBCO point‐contact junction, which is made by pressing a Nb needle on a surface‐etched YBCOepitaxialfilm, under 525.4 GHz submillimeter‐wave irradiation.