Study of In-Situ Laser-Deposition of Superconducting Thin Films by In-Situ Resistance Measurement
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STUDY OF IN-SITU LASER-DEPOSITION OF SUPERCONDUCTING THIN FILMS BY IN-SITU RESISTANCE MEASUREMENT Q.Y. YING, H.S. KIM, D.T SHAW AND H.S. KWOK
State University of New York at Buffalo, Institute on Superconductivity, Amherst, NY. 14260 ABSTRACT The electric resistance was measured in real time during laser evaporation deposition of superconducting thin films. It was found that different substrates led to different behaviors in the temporal change of the resistance. The results are consistent with the processes of nucleation, interface reaction and bulk-like growth. Structural transformation was also observed due to oxygen backfilling at the final stage of the deposition. INTRODUCTION Laser-deposition technique have been successfully used to make superconducting thin films [1-3]. Generally speaking, there are three major processes going on during the laser deposition: the laser-solid target interaction, the transport of the evaporated or ablated materials and the film formation on the substrate. There are reported studies on the first two subjects [4-9]. However, few studies have been reported on the film formation process itself. Yet this is an important issue for obtaining good films. YBCO superconducting thin films on various substrates have different qualities. The growth dynamics directly determines the film properties. High oxygen partial pressure is a key element in in-situ YBCO thin film growth. Therefore, an understanding of the incorporation of oxygen during thin film deposition would be very helpful in making high quality films. To monitor in real time the formation of YBCO thin film on a substrate, conventional techniques such as RHEED are limited by the deposition conditions, namely the high oxygen pressure. It turns out that the electrical properties of these ceramic materials makes in-situ resistance measurement a suitable monitoring technique [10-13]. Here we would like to present some results on the physics of YBCO film formation obtained by the in-situ resistance measurement method. EXPERIMENT The experimental setup is fairly simple [13]. To introduce electrical leads on the substrate, two silver pads were thermally evaporated on the substrate before deposition. Four wires then were introduced on the silver pads by silver paint. It should be mentioned that the four-probe configuration only cancels the contact resistance due to the silver paint. The contact resistance between the silver pad and the YBCO film is still present. But we found that this effect was negligibly small and does not affect our results. The YBCO thin films were deposited on four kinds of substrates, yttria stabilized ZrO2, SrTiO3, MgO and sapphire. In general, the substrate temperature was2held at about 620 0 C during the deposition. The laser intensity used was about 3 J/cm with a repetition rate of 10 Hz. The oxygen pressure inside the deposition chamber was 10 mTorr. The deposition rate under this condition was about 1.5 Aksec. Immediately after the deposition, oxygen gas was filled into the chamber to 500 torr, followed by a
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