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Electronics and Communications in Japan (Part II Electronics)

Fabrication and IF bandwidth measurements of NbN hot‐electron bolometers

Shigehito Miki, Yoshinori Uzawa, Akira Kawakami, Zhen Wang

Published Jun 18, 2002• 16 citations
Publisher

Abstract

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

DOI
10.1002/ecjb.1111

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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
We report the fabrication and testing of hot electron bolometric mixers with an ultrathin NbN film as heterodyne receivers operating at terahertz frequencies. We found that the quality of the NbN strip is usually degraded by damage resulting from the fabrication process, and we developed a process for reducing such damage. We also fabricated HEB mixers with a new structure in order to study the IF bandwidth determined by the original quality of the NbN thin films. Investigations at 100 GHz revealed that the widest IF bandwidth of 2.0 GHz was obtained by a mixer based on a 2.8 nm-thick NbN film. An HEB mixer based on the 2.8 nm-thick NbN film was also fabricated and evaluated. The receiver noise temperature at 900 GHz was 780 K, the absorbed LO power was about 400 nW, and the conversion gain was -13 dB.
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.
Superconducting Hot-electron Bolometric (HE.B) mixers employing thin films ofNbN have become the devices of choice for heterodyne detection at tera,hertz frequen- cies. HEB mixers are inherently insensitive to the bandgap frequency of the superconducting material used in the device, in contrast to SIS mixers,. We have developed a fabrication pro- cess for thin NbN films at the National Institute of Standards and Technology in Boulder, CO. A two-fold approach can be taken. The first is to maximize the critical temperature of the superconducting device by growing thicker films at the expense of IF bandwidth, whereas the second approach focuses on maximizing the IF bandwidth at the expense of the critical tem- perature. So far, we have developed a film-deposition process utilizing our DC reactive mag- netron sputtering chamber. By current-biasing the RF plasma in a mixture of Ar and N2, while using a Nb target, we can control the film stoichiometry and produce films with thicknesses of 5 nm. The films are deposited on MgO substrates that are heated to about 800 °C during depo- sition. A typical critical temperature (TO is about 10 K and the transition width is very small (0.5 K). The films are evaluated by measuring their superconducting characteristics as well as their thickness and surface roughness by means of AFM analysis. Phonon-cooled HEB devices are fabricated from the films to study their performance as HEB mixers. The device fabrication process at UMass/Amherst involves lift-off lithography of the antenna (gold), and Reactive ion etching (R1E) or wet etching of the NbN. The I-V char- acteristics and noise temperatures of the devices were measured in order to determine their quality for PHEB applications.
The bandwidth of NbN phonon-cooled hot electron bolometer mixers has been systematically investigated with respect to the film thickness and film quality variation. The films, 2.5 to 10 nm thick, were fabricated on sapphire substrates using DC reactive magnetron sputtering. All devices consisted of several parallel strips, each 1 μ wide and 2 μ long, placed between Ti-Au contact pads. To measure the gain bandwidth we used two identical BWOs operating in the 120-140 GHz frequency range, one functioning as a local oscillator and the other as a signal source. The majority of the measurements were made at an ambient temperature of 4.2 K with optimal LO and DC bias. The maximum 3 dB bandwidth (about 4 GHz) was achieved for the devices made of films which were 2.5-3.5 nm thick, had a high critical temperature, and high critical current density. A theoretical analysis of bandwidth for these mixers based on the two-temperature model gives a good description of the experimental results if one assumes that the electron temperature is equal to the critical temperature.
We present the results of our studies of NbN phonon-cooled HEB mixers at terahertz frequencies. The mixers were fabricated from NbN film deposited on a high-resistivity Si substrate with an MgO buffer layer. The mixer element was integrated with a log-periodic spiral antenna. The noise temperature measurements were performed at 2.5 THz and at 3.8 THz local oscillator frequencies for the 3 x 0.2 μm2 active area devices. The best uncorrected receiver noise temperatures found for these frequencies are 1300 K and 3100 K, respectively. A water vapour discharge laser was used as the LO source. The largest gain bandwidth of 5.2 GHz was achieved for a mixer based on 2 nm thick NbN film deposited on MgO layer over Si substrate. The gain bandwidth of the mixer based on 3.5 nm NbN film deposited on Si with MgO is 4.2 GHz and the noise bandwidth for the same device amounts to 5 GHz. We also present the results of our research into decrease of the direct detection contribution to the measured Y-factor and a possible error of noise temperature calculation. The use of a square nickel cell mesh as an IR-filter enabled us to avoid the effect of direct detection and measure apparent value of the noise temperature which was 16% less than that obtained using conventional black polyethylene IR-filter.