# THz Range Low-Noise SIS Receivers for Space and Ground-Based Radio Astronomy +
## 2 Materials and Methods
### Nb/AlO\({}_{x}\)/Nb and Nb/AlN/NbN SIS Mixers
The normal resistance of the SIS junction is \(R_{n}=22\)\(\Omega\); the quality parameter characterized by the ratio of the resistances under and above the gap \(R_{sg}\)/\(R_{n}=39\); the gap voltage \(V_{g}=2.8\) mV, and the energy gap spreading \(\delta V\) is approximately 0.1 mV. To realize a quantum-limited performance of the SIS mixers at sub-THz frequencies, several important issues should be considered:  
(1) For operation at sub-THz frequencies, tunnel junctions with very high tunnel barrier transparencies are required. Unfortunately, there is a limit to the increasing of the AlO\({}_{\text{x}}\) barrier transparency (current density of approximately 10-15 kA/cm\({}^{2}\)); at a higher current density, abrupt degradation of the junction's quality takes place. This problem was overcome by the development of Nb/Al-AIN/Nb tunnel junctions that demonstrate reasonably good \(R_{\text{sg}}\)/\(R_{n}\) > 10 at very high current densities of up to 100 kA/cm\({}^{2}\)[32; 33; 34; 35]. The procedure for fabrication of the Nb/AlN/Nb junction follows the well-known recipe for Nb/AlO\({}_{\text{x}}\)/Nb circuits that was described above; the only difference is the replacement of the oxidation with the nitridization process. To achieve a good matching between such high current density junctions and the antenna, submicron SIS junctions are required;  
(2) The operating frequency of the Nb SIS receivers is constrained by the gap frequency of Nb (approximately 700 GHz). The possible solution for this problem may be found in the fabrication and utilization of the devices with microstrip lines based on compounds of Nb with the higher gap frequencies, particularly NbTiN; the top electrode of the line is usually made from normal metal to avoid SIS junction overheating [36; 37; 38]. We developed an SIS mixer based on high critical current density Nb/AlN/NbN tunnel junctions embedded in a microstrip line consisting of a 320 nm NbTiN lower electrode (ground plane) and a 500 nm Al upper electrode [39]. The microstrip electrodes are separated by a 250 nm thick SiO\({}_{2}\) insulating layer. The SIS junction is placed on the NbTiN film and the top NbN layer is in contact with the upper Al electrode (see inset in Figure 2). A detailed procedure for making the circuit is described in [39]. Figure 1: IVC of the Nb-Al-Nb/Al-AlO\({}_{\text{x}}\)/Nb mixer element fabricated on quartz substrate (area of tunnel junction \(A=1\)\(\upmu\)m\({}^{2}\), \(V_{g}=2.8\) mV, \(R_{n}A=22\)\(\Omega\)\(\upmu\)m\({}^{2}\)); the Josephson supercurrent is suppressed by the magnetic field (black line). The IVCs of the SIS mixer pumped by LO at 241 GHz at two different LO powers are shown by the blue and red lines. A cross-section of the three-layer structure is presented in the inset. (3) The frequency of 950 GHz corresponds to a quasiparticle step size of 3.9 mV, which exceeds the \(V_{g}\) of conventional SIS junctions with Nb electrodes (\(V_{g}\) < 2.8 mV for Nb/AlN/Nb junctions with high current density). Further, the quasiparticle step from the negative IVC branch suppresses the positive step and decreases the voltage range available for the SIS mixer biasing, considering the presence of the unsuppressed Shapiro step feature [40] right on the quasiparticle step. The use of an AlN tunnel barrier in combination with an NbN superconducting top electrode provides a significant improvement in the performance of the SIS THz mixer since much higher gap voltages can be realized. To investigate this idea, we developed the Nb/AlN/NbN tunnel junction [30; 41]. The NbN film was deposited by DC-reactive magnetron sputtering at ambient temperature, with 1.8 W/cm\({}^{2}\) power density using an Ar + 9% N\({}_{2}\) gas mixture. Otherwise, the fabrication procedure was the same as for Nb/AlN/Nb junctions [30; 41]. The IV characteristic of such junctions fabricated on Si substrates demonstrated quality factor \(R_{gg}/R_{n}>\) 30 at the current density up to 10 kA/cm\({}^{2}\) and a gap voltage as high as 3.7 mV [41; 42].