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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Development of phonon cooled hot-electron-bolometers (HEB) for applications in THz radioastronomy

Theo Andreas Scherer, Michael Frommberger, Matthias Schicke, Karl-Friedrich Schuster

Published Oct 4, 2006• 0 citations
Publisher

Abstract

Heterodyne detection of weak THz signals can be obtained with ultrathin superconducting microbridge bolometers where frequency mixing is achieved by rapid relaxation of hot electrons in resistive hot spots. These devices do not suffer from the superconducting band gap limit of conventional SIS mixers. Phonon cooled HEBs from NbN allow mixing with IF bandwidths up to several GHz. We discuss NbN thin film optimization and present characterization methods which allow to investigate the phonon cooling path.

DOI
10.1109/icimw.2004.1422156

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Nov 13, 1998•1 citation
Heterodyne detection of weak THz signals can be obtained with ultrathin superconducting microbridge bolometers where frequency mixing is achieved by rapid relaxation of hot electrons in resistive hot spots. These devices do not suffer from the superconducting band gap limit of conventional SIS mixers. Phonon cooled HEBs from NbN allow mixing with IF bandwidths up to several GHz. We discuss NbN thin film optimization and present characterization methods which allow to investigate the phonon cooling path.
Superconducting Hot Electron Bolometers (HEBs) are good candidates for detecting weak signals in the submillimeter or terahertz range. In this thesis work, a novel fabrication method was developed to make two types of niobium HEBs for different applications. HEBs were designed, fabricated, and then characterized at dc, microwave, and THz frequencies. The first type is a diffusion-cooled HEB, made with a short bridge that determines its cooling time. In this thesis, bridges were typically 400 nm long with bandwidths of about 1 GHz. These diffusion-cooled HEBs were developed as part of a collaboration with the University of Arizona (UA), to develop a proof-of-concept heterodyne array submillimeter camera. Devices were fabricated on thin fused quartz and silica substrates for waveguide coupling in the UA system for the astrophysically interesting 345 and 810 GHz atmospheric windows. The goal of this collaboration is to provide a basis of comparison between Nb diffusion-cooled HEB mixers and superconductorinsulator-superconductor mixers at these frequencies. The second type is a phonon-cooled HEB, made with a ∼3 µm long bridge. Its thermal response is dictated by the electron-phonon relaxation time. These devices were developed in collaboration with Prof. C. Schmuttenmaer’s lab in the Yale Chemistry department, Prof. G. Blake at Caltech,
We have investigated a phonon-cooled NbN hot electron bolometric (HEB) mixer in the frequency range from 0.7 THz to 5.2 THz. The device was a 3.5 nm thin film with an in- plane dimension of 1.7 X 0.2 micrometers 2 integrated in a complementary logarithmic spiral antenna. The measured DSB receiver noise temperatures are 1500 K, 2200 K, 2600 K, 2900 K, 4000 K, 5600 K and 8800 K. The sensitivity fluctuation, the long term stability, and the antenna pattern were measured and the suitability of the mixer for a practical heterodyne receiver is discussed.
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.
The performance of a NbN based phonon-cooled Hot Electron Bolometric (HEB) quasioptical mixer is investigated in the 0.65-3.12 THz frequency range. The device is made from a 3 nm thick NbN film on high resistivity Si and integrated with a planar spiral antenna on the same substrate. The in-plane dimensions of the bolometer strip are 0.2/spl times/2 /spl mu/m. The best results of the DSB noise temperature at 1.5 GHz IF frequency obtained with one device are: 1300 K at 650 GHz, 4700 K at 2.5 THz and 10000 K at 3.12 THz. The measurements were performed at 4.5 K ambient temperature. The amount of local oscillator (LO) power absorbed in the bolometer is about 100 nW. The mixer is linear to within 1 dB compression up to the signal level 10 dB below that of the LO. The intrinsic single sideband conversion gain measured at 650 GHz is -9 dB, the total conversion gain is -14 dB.
We have developed a low-noise waveguide heterodyne receiver for operation near 1 THz using phonon-cooled NbN hot-electron bolometers. The mixer elements are submicron-sized microbridges of 4 nm-thick NbN film fabricated on a quartz substrate. Operating at a bath temperature of 4.2 K, the double-sideband receiver noise temperature is 760 K at 1.02 THz and 1100 K at 1.26 THz. The local oscillator is provided by solid-state sources, and power measured at the source is less than 1 /spl mu/W. The intermediate frequency bandwidth exceeds 2 GHz. The receiver was used to make the first ground-based heterodyne detection of a celestial spectroscopic line above 1 THz.
Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range from 1.1 THz to 2.5 THz. The mixer device is a 0.15 - 0.3 micrometer microbridge made from a 10 nm thick Nb film. This device employs diffusion as a cooling mechanism for hot electrons. The double sideband noise temperature was measured to be less than or equal to 3000 K at 2.5 THz and the mixer IF bandwidth is expected to be at least 10 GHz for a 0.1 micrometer long device. The local oscillator (LO) power dissipated in the HEB microbridge was 20 - 100 nW. Further improvement of the mixer characteristics can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated. The HEB mixer is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies. HEB receivers are planned for use on the NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) and the ESA Far Infrared and Submillimeter Space Telescope (FIRST). The prospects of a submicron-size YBa2Cu3O7-(delta ) (YBCO) HEB are discussed. The expected LO power of 1 - 10 (mu) W and SSB noise temperature of approximately equals 2000 K may make this mixer attractive for various remote sensing applications.