Niobium titanium nitride-based superconductor-insulator-superconductor mixers for low-noise terahertz receivers

Abstract

Integrating NbTiN-based microstrip tuning circuits with traditional Nb superconductorinsulator-superconductor ͑SIS͒ junctions enables the low-noise operation regime of SIS mixers to be extended from below 0.7 to 1.15 THz. In particular, mixers incorporating a NbTiN / SiO 2 / NbTiN microstrip tuning circuit offer low-noise performance below 0.8-0.85 THz, although their sensitivities drop significantly at higher frequencies. Furthermore, a microstrip geometry in which NbTiN is used as the ground plane material only ͑NbTiN / SiO 2 /Al͒ yields significant improvements in the sensitivities of SIS mixers operating up to 1.15 THz, with an upper operating frequency that depends upon the quality of the NbTiN layer, and thus its deposition process. Films deposited at room temperature have T c = 14.4 K and n,20 K ϳ 60 ⍀ cm, and offer low-noise performance up to 1 THz, whereas films deposited at 400°C have T c = 16 K and n,20 K ϳ 110 ⍀ cm, and offer low-noise performance up to 1.15 THz. Taken together, these results demonstrate that the high-frequency surface resistance of a NbTiN layer depends upon the film's structural properties. Most significantly, the drop in performance that is seen at F Ͼ 1 THz in mixers incorporating NbTiN ground planes deposited at room temperature is attributed to nonhomogeneities in the structural and electrical properties of these films, as is the poor performance of mixers that incorporate NbTiN wiring layers at F Ͼ 0.85 THz. The development of these NbTiN-based microstrip tuning circuits will enable the production of low-noise SIS mixers for the 0.8-0.96-and 0.96-1.12-THz frequency bands of the Heterodyne Instrument for the Far Infrared on board the European Space Agency's Herschel Space Observatory.

Low-noise 0.8-0.96- and 0.96-1.12-THz superconductor-insulator-superconductor mixers for the herschel space observatory

“…These results demonstrated that a NbTiN-SiO -Al microstrip RF matching network can be integrated with "standard" 1-m Nb-Al-AlO -Nb SIS junctions to yield low receiver noise temperatures up to 1 THz (and [11] demonstrates that this region of low-noise operation may be extended to at least 1.12 THz by the use of an NbTiN ground plane that is deposited at 400 C). However, the fixed-tuned RF bandwidth of the 1-THz waveguide mixer in [10] was limited to 100 GHz, whereas the quasi-optical mixer in [10] offered fixed-tuned bandwidths of 200 GHz.…”

“…As is discussed in [11], this "resist recessing" step yields a stepped junction profile in which the edges of the active portion of the Al-AlO barrier are not exposed to the Ar sputter etch of the barrier, since the final size of the top electrode is reached at the completion of the bottom electrode etch. This is expected to improve the quality of a typical junction (i.e., to reduce its leakage current) by reducing the risk of damage to the tunnel barrier during the etch process.…”

“…Beyond this, Fig. 4(b) demonstrates that the coupling to the junctions in the 1.04-THz design is strongly dependent upon the properties of the NbTiN ground plane-the use of a film deposited at room temperature is expected to result in a strong drop in coupling above 1 THz (see [9] and [10]), whereas the use of a film deposited at 400 C should offer strong coupling over the full 0.96-1.12-THz band (see [11]). …”

“…However, previous work has shown that the use of NbTiN-based 3 microstrip RF tuning circuits allows the sensitivity of 0.75-1.0-THz SIS mixers to be significantly improved [8]- [10]. Furthermore, as is discussed in more detail in [11], the integration of Nb SIS junctions with a NbTiN-SiO -Al microstrip tuning circuit in which the NbTiN ground plane is deposited at 400 C (in place of the room-temperature-deposited films used previously) allows low-noise SIS mixer operation to be extended to at least 1.12 THz.…”

Improved Nb SIS devices for heterodyne mixers between 700 GHz and 1.3 THz with NbTiN transmission lines using a normal metal energy relaxation layer

“…Jackson et al 12,13 reported receiver noise temperatures of approximately 200 K at 850 GHz and 400 K at 1 THz for a Nb SIS mixer using a hybrid embedding circuit consisting of a niobium-titanium-nitride (NbTiN) bottom layer and an Al wiring layer. Bin a Nb SIS mixer using a double normal-conducting embedding structure made of Al, however, they obtained a higher receiver noise temperature of approximately 840 K at 1042 GHz due to the rf -signal loss in the Al.…”