Bi2212 intrinsic Josephson junctions, fabrication and high frequency properties

Licentiatavhandling, 2008

The objective for this work was to investigate the high-frequency properties of Bi2212 intrinsic Josephson junctions (IJJ’s). Particularly, the possibility to reach the THz range with a SIS mixer is theoretically promising due to the high energy gap of Bi2212. Additionally the layered structure of Bi-based single crystals enable unique possibilities for compact voltage standards using Shapiro steps of large series array of IJJ’s. In the effort to reach these goals the fabrication technique used in our fabrication of Bi2212 IJJ’s resulted in studies of other effects then planed. The main results are the following: We have developed a flip-chip fabrication technique where the controlled etching rate allows fabrication of a specific number of IJJ’s in a well defined stack. We have this far detected Shapiro steps at both 300 and 600 GHz. We have also noted that the voltage gap that exists when an underdamped junction retraps to superconducting state can be large enough to prevent detection of Shapiro steps. This limits the lower frequencies that can be used for detection of Shapiro steps. Therefore frequencies of >600 GHz is recommended for detection of Shapiro steps in Bi2212 IJJ’s. We have developed a method to measure the in-plane critical current of a single Cu-O plane. Using a method where an additional junction is added to an array of IJJ’s as the topmost surrounding Cu-O layer reaches the critical current. A in-plane critical current for the crystal was estimated to about 2 MA cm-2. It is among the highest values ever reported. We have a method to reliably estimate the stack temperature of a stack containing several IJJ’s has been developed. It is based on the change in critical current (IC) and voltage (V) of the junctions due to temperature. When some of the junctions are in resistive state internal Joule heating will make the stack warmer. That affects IC and V and the temperature can be estimated due to the change. This method has been used to understand overheating problems in stacks of IJJ’s. The thermal increase of a single IJJ at 4.2 K can be higher then 100 K(mW)-1.