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IEEE Transactions on Terahertz Science and Technology

Broadening the IF Band of a THz Hot-Electron Bolometer Mixer by Using a Magnetic Thin Film

Akira Kawakami, Yoshihisa Irimajiri, Taro Yamashita, Satoshi Ochiai, Yoshinori Uzawa

Published Oct 9, 2018• 11 citations
Publisher

Abstract

To expand the intermediate frequency (IF) band and improve the sensitivity of a hot-electron bolometer mixer (HEBM), we proposed and examined a new HEBM structure using a magnetic thin film. We found that it was possible to suppress the superconductivity of the 5-nm-thick niobium nitride (NbN) thin film by the addition of a 1.8-nm-thick nickel (Ni) thin film. It was also confirmed that the superconductivity disappeared in the Au (70 nm)/Ni (1.8 nm)/NbN (5 nm) trilayer for forming the electrodes of the HEBM. By using the magnetic thin film for the electrodes, we suggested that the superconductivity of the HEBM strip would be affected and that hot spots would be formed near the electrodes. This approach is effective for shortening the hot-electron drift length and will lead to the expansion of the IF bandwidth. We think that the new structure lowers the required local oscillator power and improves the HEBM sensitivity by suppressing the proximity effect under the electrode. The IF bandwidth of the fabricated Ni-HEBMs was evaluated at 1.9 THz. We confirmed that the IF bandwidth expands, and the evaluated bandwidths were in the range of 5.1-5.7 GHz at 4 K. Ni-HEBMs with 0.1-μm strip length were also fabricated and evaluated. The IF bandwidth was about 6.9 GHz at 4 K.

DOI
10.1109/tthz.2018.2874355

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Experimental work on the development of a quasi-optical heterodyne receiver for the THz frequency range is reported. The mixing device is a superconducting hot electron bolometer (HEB) made from ultrathin, d = 3 - 5 nm, niobium nitride (NbN) films. Work described in this thesis comprises: The design of a prototype receiver using quasi-optical coupling. Device fabrication involving reactive magnetron sputtering of NbN films and lithographic definition of the antenna integrated mixer. An investigation of the receiver performance at frequencies between 0.6 THz and 2.5 THz. The best performance is obtained with HEB mixers made from NbN films with a thickness d = 3-3.5 nm and a critical temperature T c = 9 - 9.5 K. Double-side band receiver noise temperatures are 530 K at 0.6 THz, 650 K at 1.6 THz and 1100 K at 2.5 THz. Corrected for losses in the receiver input optics, these values amount to only 10 times the theoretical quantum limit. MgO as the substrate material has been shown to provide improved acoustic coupling between NbN film and the substrate compared to other materials such as Si or sapphire. A maximum IF gain bandwidth of 4.5 GHz has been measured under the same operating conditions which yield lowest noise temperature. The local oscilltor power requirement is ~250 nW absorbed in the bolometer. Quasi-optically coupled HEB mixers have become the device-of-choice for radio astronomical applications in the frequency regime at and above 1 THz.

•Jan 1, 2001•4 citations
To expand the intermediate frequency (IF) band and improve the sensitivity of a hot-electron bolometer mixer (HEBM), we proposed and examined a new HEBM structure using a magnetic thin film. We found that it was possible to suppress the superconductivity of the 5-nm-thick niobium nitride (NbN) thin film by the addition of a 1.8-nm-thick nickel (Ni) thin film. It was also confirmed that the superconductivity disappeared in the Au (70 nm)/Ni (1.8 nm)/NbN (5 nm) trilayer for forming the electrodes of the HEBM. By using the magnetic thin film for the electrodes, we suggested that the superconductivity of the HEBM strip would be affected and that hot spots would be formed near the electrodes. This approach is effective for shortening the hot-electron drift length and will lead to the expansion of the IF bandwidth. We think that the new structure lowers the required local oscillator power and improves the HEBM sensitivity by suppressing the proximity effect under the electrode. The IF bandwidth of the fabricated Ni-HEBMs was evaluated at 1.9 THz. We confirmed that the IF bandwidth expands, and the evaluated bandwidths were in the range of 5.1-5.7 GHz at 4 K. Ni-HEBMs with 0.1-μm strip length were also fabricated and evaluated. The IF bandwidth was about 6.9 GHz at 4 K.
Hot-electron bolometer (HEB) mixers are a complex multi-layer thin film structure containing an ultra-thin superconducting film of NbN as a detecting element and a thick normal metal layer as an antenna structure. We have optimized the fabrication process starting with ultra-thin NbN films, Au films for antenna structures and their patterning using e-beam lithography and lift-off. The coupling between normal conducting antenna and NbN detector has been improved by introducing an intermediate NbN film to reduce proximity suppression of superconductivity in the detecting element. A critical temperature of about 9.5 K is reached for NbN films with a thickness between 5 nm and 6 nm. A twofold increase of the film thickness increases the critical temperature to 12 K. We have shown that a 20 nm thick buffer layer of NbN under a much thicker Au layer is sufficient to ensure a critical temperature of the bi-layer of 9 K. This value is close to the critical temperature of 5.5 nm thick HEB devices. The noise temperature of HEB mixer made using improved technology is about 800 K and was measured in a liquid cryogen free system with a quantum cascade laser as 2.5 THz local oscillator.
Abstract As part of an effort to develop heterodyne rf receivers operating in the terahertz band, we have fabricated hot‐electron bolometers (HEB) using NbN thin films, and investigated their intermediate frequency (IF) bandwidths. The NbN thin films we studied were grown on single‐crystal MgO substrates by dc reactive sputtering. Our 2.8‐nm‐thick films exhibited superior superconducting properties, including transition temperatures TC of 9.7 K and 20 K resistivities ρ20 of 185 µΩ·cm. Using electron beam diffraction, high‐resolution TEM images, and other techniques to study the crystallinity of these films, we determined that the NbN grows epitaxially on the MgO substrate as single‐crystal material. The HEB elements we made consisted of a NbN microstrip, a Au electrode section, and an Al antenna section. In the course of studying the IF bandwidths of these devices in the millimeter‐wave range, we found one device with an IF bandwidth of 2.5 GHz, the highest of any device we made. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 85(7): 77–83, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/ecjb.1111
Hot electron bolometer mixers (HEBM) are expected to be used as low-noise heterodyne mixers in the terahertz frequency range. We are developing quasi-optical HEBMs for applications such as atmospheric remote sensing and radio astronomy in the terahertz frequency range. Therefore, we developed a fabrication method using niobium nitride (NbN) ultrathin films deposited on a silicone (Si) substrate for terahertz applications. First, a magnesium oxide buffer layer 50 nm thick was deposited on a Si substrate at approximately 525 °C. After deposition of this layer, NbN ultrathin films were deposited at ambient temperature. Using the buffer layers, we found the transition temperature of the 3.5-nm-thick NbN thin films to be approximately 10 K. The same method was applied to fabricate NbN-HEBMs on a Si substrate. The fabricated quasi-optical mixers were designed to operate at 3.1 THz. The receiver noise temperature was found to be approximately 1930 K (DSB) at 3.1 THz.
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.
Experimental work on the development of a quasi-optical heterodyne receiver for the THz frequency range is reported. The mixing device is a superconducting hot electron bolometer (HEB) made from ultrathin, d = 3 - 5 nm, niobium nitride (NbN) films. Work described in this thesis comprises: The design of a prototype receiver using quasi-optical coupling. Device fabrication involving reactive magnetron sputtering of NbN films and lithographic definition of the antenna integrated mixer. An investigation of the receiver performance at frequencies between 0.6 THz and 2.5 THz. The best performance is obtained with HEB mixers made from NbN films with a thickness d = 3-3.5 nm and a critical temperature T c = 9 - 9.5 K. Double-side band receiver noise temperatures are 530 K at 0.6 THz, 650 K at 1.6 THz and 1100 K at 2.5 THz. Corrected for losses in the receiver input optics, these values amount to only 10 times the theoretical quantum limit. MgO as the substrate material has been shown to provide improved acoustic coupling between NbN film and the substrate compared to other materials such as Si or sapphire. A maximum IF gain bandwidth of 4.5 GHz has been measured under the same operating conditions which yield lowest noise temperature. The local oscilltor power requirement is ~250 nW absorbed in the bolometer. Quasi-optically coupled HEB mixers have become the device-of-choice for radio astronomical applications in the frequency regime at and above 1 THz.