Superconducting THz mixers based on MgB2 film

Licentiate thesis, 2013

Superconducting NbN hot electron bolometer (HEB) mixers are widely used in terahertz radio astronomy. Such mixers have superior performance compared to SIS and Schottky diode mixers at frequency above 1THz. However, their drawback is a limited IF bandwidth. Therefore, as radio astronomy advances towards higher frequencies, mixers with even wider gain bandwidth are required. The gain bandwidth of the HEB mixers is determined by two consequent processes in the electron energy relaxation: the electron phonon interaction and the phonon escape into the substrate with corresponding time constants for each process. The electron-phonon interaction time is inversely dependent of the electron temperature of the film which is close to the critical temperature of the superconductor. The escape time is dependent of the film thickness. Materials with higher critical temperature and shorter electron relaxation time are needed to improve the IF bandwidth. The discovery of the superconductivity in the intermetallic compound magnesium diboride (MgB2) has generated a great interest in this research field. The high critical temperature and the short electron phonon interaction time make the MgB2 very attractive for HEB mixers fabrication aiming for better HEB mixers performances. In this thesis, novel terahertz HEB mixers based on magnesium diboride thin films are presented. MgB2 HEBs integrated with spiral antenna were fabricated, characterized and studied. The gain bandwidth was investigated with respect to the thickness and the critical temperature of the film. A gain bandwidth of 1.3GHz, 2.3GHz and 3.4GHz corresponding to a mixer time constant of 130 ps, 70 ps and 47 ps was measured in 30 nm, 15 nm and 10 nm MgB2 films, respectively. Another important figure of merit for receivers is the noise temperature which is influenced by several factors such as the dimension of the HEB and the critical current. For HEB mixers made from 10 nm MgB2 film the lowest mixer noise temperature was 600K measured at 2K bath temperature and 600GHz local oscillator (LO) frequency. Finally, using the two temperature model the experimental data were analyzed and the electron phonon interaction time, τe-ph of 7 to 15 ps, the phonon escape time, τesc of 6 to 42 ps and the specific heat ratio, ce/cph of 1.35 to 9 ps were extracting giving the first model for HEB mixers made of MgB2 films. Based on this research a gain bandwidth as large as 8-10GHz has been predicted in very thin MgB2 films.