Thin superconducting NbTiN and NbN films with a few nm thickness are used for various device applications including hot electron bolometer mixers. Such thin films have lower critical temperature (T c) and higher resistivity than corresponding bulk materials. To improve them, we have investigated an effect of the AlN buffer layer between the film and the substrate (quartz or soda lime glass). The AlN film is deposited by DC magnetron sputtering, and the process condition is optimized so as that the X-ray diffraction intensity from the 002 surface of Wurtzite AlN becomes highest. By use of this well-characterized buffer layer, T c and resistivity of the NbTiN film with a few nm thickness are remarkably increased and decreased, respectively, in comparison with those without the buffer layer. More importantly, the AlN buffer layer is found to be effective for NbN. With the AlN buffer layer, T c is increased from 7.3 K to 10.5 K for the 8 nm NbN film. The improvement of T c and resistivity originates Improvement of T c of NbTiN and NbN Thin Films Using the AlN Buffer Layer 2 from the good lattice matching between the 002 surface of AlN and the 111 surface of NbTiN or NbN, which makes better crystallization of the NbTiN or NbN film. This is further confirmed by the X-ray diffraction measurement.

We present a characterization of a 1.5 THz waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixer which can be pumped by a commercial solid state tunable local oscillator (LO) source. The NbTiN HEB mixer is made from a 12 nm thick NbTiN thin film deposited on a quartz substrate at room temperature. A gold electrode is formed in situ on the NbTiN thin film without breaking vacuum to ensure good contact. The uncorrected DSB receiver noise temperature is measured to be 1700 K at 1.5 THz, whereas the mixer noise temperature is derived to be 1000 K after corrections for losses of the input optics and the intermediate frequency (IF) amplifier chain. The required LO power absorbed in the HEB mixer is evaluated to be 340 nW by using an isothermal technique. The IF gain bandwidth is supposed to be about 1.3 GHz or higher. The present results show that good performance can be obtained at 1.5 THz even with a relatively thick NbTiN film (12 nm), as in the case of 0.8 THz. In order to investigate the cooling mechanism of our HEB mixers, we have conducted performance measurements for a few HEB mixers with different microbridge sizes both at 1.5 and 0.8 THz. The noise performance of the NbTiN HEB mixers is found to depend on the length of the NbTiN microbridge. The shorter the microbridge is, the lower the receiver noise temperature is. This may imply a contribution of the diffusion cooling in addition to the phonon cooling.

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