Physica C 235-240 (1994)3339-3340 PHYSICA North-Holland Barriers formed by a plasma discharge process in all YBCO Josephson edge junctions E. Aharoni and G. Koren Physics Department and the Crown Center for Superconductivi~', Technion - Israel Institute of Technology, Haifa 32000, Israel All YBCO Josephson edge junctions were prepared with a barrier formed by an rf plasma discharge under different conditions. The structure of the barrier and the transport properties of these junctions, have been investigated. Under typical plasma conditions of 30-60 s plasma of 50-80 W at 100 mTorr of CF4 gas, and 500-600 °C, we found that the junctions behaved like weak links. The normal resistance was of the order of a few ~2, with a minimum at around 50 K, .typical to localization versus temperature. The temperature dependence of the critical current. I c, was found to be I c oc (T~ - T) 15. This reflects the asymmetric structure of these junctions, in which only the base electrode is exposed to the plasma. To learn about the thickness of the barriers a separate experiment was carried out on ultra thin YBCO films of various thicknesses that were exposed to the plasma. It was found that under the above plasma conditions films thinner than 100 A have lost their superconductivity. This determines an upper limit on the thickness of the barrier in our junctions. The composition of the barriers was investigated by Auger spectroscopy performed on thicker films that were also exposed to the same plasma. It was found that the surface layer down to about 50 A has lost its 1-2-3 stoichiometry, and contained a significant amount of carbon. This indicates a barrier thickness of about 50 A, in agreement with the result obtained from the ultra thin films. All YBCO Josephson edge junctions were prepared in-situ using laser ablation deposition and metallic masks, and the barrier was formed by applying plasma discharge in CF 4 gas to modify the edge of the base electrode [1,2]. The structure of the barrier and the transport properties of the junctions were characterized. Under typical plasma conditions of 30-60 s plasma of 50-80 W at 100 mTorr of CF 4 gas, and 500-600 °C, we found that the junctions behaved like weak links. The normal resistance was of the order of a few ~, with a minimum at 40-60 K, typical to localization versus temperature. Generally, the resistance normalized to the junction area RcvA, increased with increasing sample temperature, pressure, rf power, or discharge duration. The critical current density decreased correspondingly. Oxygen addition to the gas flowing through the cell during the discharge, caused stronger links. The normalized resistances of three junctions prepared with different plasma parameters are given in Fig. 1 as a fnnction of temperature. The normalized resistance of the first junction (a), created with the the above mentioned plasma parameters, is localized in temperature at about 60 K. The barrier in the second junction (b) has been created with 200 mTorr flowing CF 4 during the discharge. R~A, in this case increases with temperature. In the third exampl~e (c), the addition of 20% oxygen during the discharge created a strong link with normal resistance decreasing with temperature. The critical currents, below 40 K, were too high and prevented from measuring the normal resistance. The temperature dependence of the critical current, I c, was found to be I c oc (T c - T) 15. This is similar to the I c (T) dependence of SNIS junctions [3] and reflects the asymmetric structure of these junctions, in which only the base electrode is exposed to the plasma. To learn about the thickness of the barriers a separate experiment was carried out on ultra thin YBCO films of various thicknesses that were exposed to the plasma. The films were covered with an insulating YBCO layer deposited at 500 °C to protect from room environment. The resistivity vs. temperature of YBCO ultra thin films exposed to 50 W plasma in CF 4 gas at 600 °C for 60 s and annealed in oxygen at 750 °C, is given in Fig. 2. The thickness of the thinnest film 0921-4534/94/$07.00 © 1994- Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)02235-6