Communications Research Laboratory (CRL)|
Kansai Advanced Research Center (KARC)
Ministry of Posts and Telecommunications (MPT)
Kobe 651-24, Japan
Tel: 81-78-969-2190; Fax: 2199
|Date Visited:||January 31, 1997|
|WTEC Attendees:||M. Beasley (report author), G. Gamota, H. Morishita, M. Nisenoff, F. Patten, R. Ralston, J. Rowell|
|Hosts:||Dr. Zhen Wang, Chief, Superconducting Radio Physics Section, Materials Science and Laser Technology Division|
The Kansai Advanced Research Center (KARC) is a branch of the Central Research Laboratory (CRL) of the Ministry of Posts and Telecommunications (MPT). KARC is located in the Kansai District of Japan and part of the new research capability being built up in the Kyoto/Osaka/Kobe area. KARC is the most basic research lab of MPT and has activities in Information Science and Technology, Materials Science and Laser Technology, and Biological Information Science.
Dr. Wang's group is interested in basic research on Josephson junctions (JJs) and on their very high frequency (THz) applications. There is both low and high temperature superconductivity (LTS and HTS) work. This group started in 1988 just after the discovery of HTS and had no prior experience in superconductivity. It moved to Kansai in 1991. It is the only group in MPT studying superconductivity. Also, MPT is the only government agency working in telecommunications. There is no government lab working on microwave components.
As Dr. Wang described, CRL's LTS work includes Nb and NbN tunnel junctions for SIS mixers. CRL researchers have developed AlN as the barrier material in their NbN junctions. They also have studied JJ series array local oscillators. In HTS they use YBCO step edge junctions in JJ mixers.
Dr. Wang reviewed the need for high gaps and low capacitance (and hence high Jc) in SIS mixers. He described the group's NbN/AlN/NbN process. The lower electrode is put down at room temperature and the 2 nm AlN barrier is deposited by reactive sputtering of Al. The AlN is not epitaxial. The junction I-Vs showed gap voltages of 5 meV but a rise in subgap conductance above half the gap voltage. Critical current densities of 54 kA/cm2 have been achieved in 1 x 1 micron junctions. Photon-induced steps out to 1 THz radiation have been observed.
The NbN junctions have been integrated with quasi-optical antennas on a single chip and used as heterodyne mixers at 345 GHz. Noise temperatures of 150 to 200 K have been achieved. Dr. Wang claimed that this was the first data on NbN SIS mixers. He also indicated that CRL had built a JJ series array that delivered 3 microwatts at 325 GHz to an on-chip resistive load. The YBCO step edge junctions have been used in JJ mixers with an external 300 GHz local oscillator. Dr. Wang acknowledged that there are greater noise problems with JJ mixers than with SIS mixers.
In response to questions from the WTEC team, Dr. Wang stated that it was not yet clear what would be the ultimate applications of this THz technology in telecommunications but noted some work at the MPT CRL in Tokyo doing ozone spectroscopy studies with one of his group's SIS mixers.
The 800 m2 cleanroom facilities of the group are outstanding. They include both film deposition and device fabrication capabilities. There is laser ablation for YBCO, reactive sputtering for NbN, and MBE for Nb with AES surface analytical capability. There is also a new reactive sputtering system that will permit deposition of NbCN. The group has its own lithography capability, including an e-beam mask maker. It also has its own scanning electron microscope and XRD. The turnaround time for full circuits is 4 days.
According to Dr. Wang, there are at present no linkages between his group and other groups working on superconductivity in Japan. He does get some students from Kobe University. The only other group working on very high frequency superconducting devices is an astronomy group near Tokyo, which has its own Nb junction fabrication capabilities.