MgB2 hot-electron bolometer mixers for sub-mm wave astronomy

Doktorsavhandling, 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).