MgB2 hot-electron bolometer mixers for sub-mm wave astronomy
Doctoral thesis, 2017

Spectroscopy and photometry in the terahertz (THz) range of remote space objects allows for a study of their chemical composition, because this range covers rotational lines from simple molecules and electron transition lines from atoms and ions. Due to high spectral resolution, THz heterodyne receivers al- low for studying dynamical properties of space objects manifested in doppler-shifted emission lines. Niobium nitride (NbN) hot-electron bolometer (HEB) mixers currently used at frequencies >1 THz, provide a typical gain bandwidth (GBW) of 3 GHz, and consequently, a noise bandwidth (NBW) of 4 GHz. This property severely limits the functionality of astronomical instruments. Moreover, the low critical temperature (Tc = 8–11 K) of NbN ultrathin films necessitates usage of liquid helium (LHe) for device cooling, which reduces lifetime of spaceborne missions.

In this thesis, a study of HEB mixers dedicated for sub-mm wave astronomy applications made from magnesium diboride (MgB2) ultrathin films is presented. It is shown that MgB2 HEB mixers reach a unique combination of low noise, wide noise bandwidth, and high operation temperature when 8 nm thick MgB2 films (Tc = 30 K) are used. The hybrid physical chemical vapour deposition (HPCVD) technique allows for reproducible deposition of such thin films. The high Tc of MgB2 (39 K), and consequently, short (3 ps) electron- phonon interaction time result in a GBW of up to 10 GHz and possibility of operation at temperatures >20 K, where compact cryocoolers are available. The GBW was observed to be almost independent on both bias voltage and bath temperature. A NBW of 11 GHz with a minimum double sideband (DSB) receiver noise temperature of 930 K is achieved at a 1.63 THz local oscillator (LO) and a 5 K bath temperature. At 15 K and 20 K, noise temperatures are 1100 K and 1600 K, respectively. From 0.69 THz to 1.63 THz noise increases by only 12%, and hence, low noise performance is expected even at higher frequencies. The minimum receiver noise temperature is achieved in a quite large range of both bias voltages (5–10 mV) and LO power. Compared to initial results, higher sensitivity and larger NBW are due to a larger HEB width (lower contact resistance), applied in-situ contact cleaning, and a smaller film thickness. The increase of noise temperature when operation temperature rises from 5 K to 20 K is due to a reduction of conversion gain by 2–4 dB caused be the reduced LO power absorbed in the HEB. The output noise of the HEB remains the same (120–220 K depending on the bias point).

hot-electron bolometer

gain bandwidth

noise bandwidth

thin film

electron-phonon interaction

noise temperature

THz detector

conversion gain

mixer

magnesium diboride

superconductor

Kollektorn, MC2, Chalmers University of Technology, Kemivägen 9, Göteborg
Opponent: Dr. Jian-Rong Gao, SRON Netherlands Institute for Space Research, Groningen/Utrecht, the Netherlands

Author

Evgenii Novoselov

Chalmers, Microtechnology and Nanoscience (MC2), Terahertz and Millimetre Wave Laboratory

MgB2 Hot-Electron Bolometer Mixers at Terahertz Frequencies

IEEE Transactions on Applied Superconductivity,; Vol. 25(2015)p. 2301104-

Journal article

Effect of the Critical and Operational Temperatures on the Sensitivity of MgB2 HEB Mixers

IEEE Transactions on Terahertz Science and Technology,; Vol. 6(2016)p. 238-277

Journal article

Study of MgB2 ultra-thin films in submicron size bridges

IEEE Transactions on Applied Superconductivity,; Vol. 27(2017)

Journal article

MgB2 hot electron bolometer mixers for THz heterodyne instruments

SPIE Proceedings,; Vol. 9914(2016)p. 9914N-

Paper in proceedings

Wideband THz HEB mixers using HPCVD MgB2 thin films

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz,; Vol. 2016-November(2016)p. Arti no 7758933-

Paper in proceedings

Broadband MgB2 Hot-Electron Bolometer THz Mixers operating up to 20K

IEEE Transactions on Applied Superconductivity,; Vol. 27(2017)

Journal article

Low noise terahertz MgB2 hot-electron bolometer mixers with an 11GHz bandwidth

Applied Physics Letters,; Vol. 110(2017)p. 032601-

Journal article

Gain and Noise in THz MgB2 Hot-Electron Bolometer Mixers With a 30-K Critical Temperature

IEEE Transactions on Terahertz Science and Technology,; Vol. 7(2017)p. 704-710

Journal article

Astronomical observations in the terahertz (THz) range provide important information about the origin of the universe, development of galaxies, stars formation, etc. Many THz astronomical instruments have been used on space-, airborne-, balloon- platforms, and on the ground. Currently used NbN superconducting phonon-cooled hot-electron bolometer (HEB) mixers provide high sensitivity, but have a limited intermediate (IF) frequency bandwidth (< 4 GHz) and require cooling down to liquid helium temperatures (< 6K). These disadvantages of NbN HEB mixers limit their possible applications, e.g. for observation of broad emission lines or in spaceborne missions.

In this work, MgB2 thin films as a novel material for utilization in THz HEB mixers are studied. High critical temperature (> 30 K vs 8-11 K in NbN), fast electron cooling, and possibility of ultrathin film deposition make MgB2 the best candidate to substitute NbN in HEB development. As a part of the research project, a hybrid physical chemical vapour deposition (HPCVD) technique providing high quality MgB2 ultrathin films was developed. Films as thin as 5 nm were grown and utilized for HEB fabrication without significant degradation of film quality. Achieved MgB2 HEB mixers demonstrate low noise performance comparable to that for NbN HEB mixers, but provide three times larger IF bandwidth and ability to work at temperatures of up to 20K with a minimal sensitivity degradation. The IF bandwidth was measured independently by two different methods, and the bolometric nature of THz detection in MgB2 HEBs is confirmed experimentally.

Infrastructure

Kollberg Laboratory

Chalmers Materials Analysis Laboratory

Nanofabrication Laboratory

Areas of Advance

Nanoscience and Nanotechnology (2010-2017)

Materials Science

Subject Categories

Atom and Molecular Physics and Optics

Other Physics Topics

Condensed Matter Physics

ISBN

978-91-7597-575-7

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4256

Publisher

Chalmers University of Technology

Kollektorn, MC2, Chalmers University of Technology, Kemivägen 9, Göteborg

Opponent: Dr. Jian-Rong Gao, SRON Netherlands Institute for Space Research, Groningen/Utrecht, the Netherlands

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Latest update

5/2/2018 2