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Applied Physics Letters

Resistive transition of niobium superconducting hot-electron bolometer mixers

D. Wilms Floet, J. J. A. Baselmans, T. M. Klapwijk, J. R. Gao

Published Nov 9, 1998• 31 citations
Publisher

Abstract

We present a model for the description of the resistive transition in hot-electron bolometer mixers. We show that the transition is a property of a superconducting microbridge connected to normal conducting cooling pads. Using the concepts of the superconducting proximity effect, charge-imbalance generation, and Andreev reflection, we have calculated the resistance versus temperature of the device and demonstrate its dependence on the length of the microbridge, both theoretically and experimentally. The analysis reopens the question of the relationship between the resistive transition and the situation in which the device is optimally operated as a heterodyne mixer.

DOI
10.1063/1.122603

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We present a model for the description of the resistive transition in hot-electron bolometer mixers. We show that the transition is a property of a superconducting microbridge connected to normal conducting cooling pads. Using the concepts of the superconducting proximity effect, charge-imbalance generation, and Andreev reflection, we have calculated the resistance versus temperature of the device and demonstrate its dependence on the length of the microbridge, both theoretically and experimentally. The analysis reopens the question of the relationship between the resistive transition and the situation in which the device is optimally operated as a heterodyne mixer.
We present a study of the resistive behavior of hot-electron bolometers in two different regimes. At bath temperatures near the critical temperature and under the condition of a low current density, the behavior can be understood in terms of charge-imbalance, Andreev reflection, and the superconducting proximity effect. At lower temperatures, a high current density and the application of LO power result in the formation of a resistive electronic hotspot. We propose to interpret heterodyne downconversion as the result of the length oscillation of the hotspot at the intermediate frequency.
We perform a theoretical analysis based on the superconducting proximity-effect model for the resistive transition of an aluminium superconducting hot electron bolometer mixer when the cooling banks are in the normal state. We compare our calculated result with a recently reported measurement. Our calculation qualitatively reproduces the observed feature in which the resistance drops quickly around Tc with decreasing temperature, but saturates at a value roughly being one half of the normal state resistance. It also opens the question about which type of cooling banks (normal or superconductor) is favourably for low noise performance.
We report two typical experimental results of waveguide diffusion-cooled Nb hotelectron bolometer (HEB) mixers. The device is a thin (10 nm) Nb bridge with a length of 0.3 Am and a width of 0.9 pm and is defined using photolithography. In the first experiment, the resistance of a HEB is studied as a function of temperature. We observe two critical temperatures, which correspond to the 71. of the Nb bridge and the T, of the Nb under the Au pads, respectively. These two 71. define a superconducting transition width AT of a HEB. The reduced T. of the Nb under the Au pads is explained by the superconducting proximity effect. In the second experiment, we measure relative conversion efficiency vs intermediate frequency (IF) by using two coherent sources, a carcinotron and a Gunn oscillator, and determine an IF roll-off (/F, 011 ,ff ) between 0.6 and 1.2 GHz, in agreement with a value estimated in terms of the diffusion-cooled model.
We present microwave (30 GHz) measurements on aluminum superconducting hot-electron bolometer (HEB) mixers. Aluminum HEB mixers have a lower superconducting transition temperature than niobium and niobium nitride devices, and are predicted to have improved sensitivity and require less local oscillator power. The devices studied consist of a narrow superconducting aluminum microbridge with contacts comprised of thick aluminum, titanium, and gold. A perpendicular magnetic field is used to suppress superconductivity in the contacts. The device length is much shorter than the electron–phonon inelastic length [L≪(Dτe–ph)1/2] and thus the intermediate frequency (IF) bandwidth is set by the diffusion time of hot electrons from the microbridge into the contacts. We discuss limitations on the IF bandwidth in very short HEB mixers with fully normal contacts. Overall, the microwave heterodyne mixing results show good performance with a mixer noise temperature inferred from the conversion efficiency and output noise of TM⩾4 K, double side band (DSB). Measurements at 618 GHz yield a conversion efficiency approximately 10 dB lower than observed at 30 GHz. Output saturation by background radiation is postulated to contribute significantly to this difference. A similar decrease in conversion efficiency in the microwave mixing measurements is observed when broadband noise is added to the rf input signal. We discuss the tradeoff between increased sensitivity and the occurrence of saturation effects in low transition temperature HEBs.
This paper presents the modeling and measurement of a quasioptical niobium nitride superconducting hot electron bolometer mixer at submillimeter wavelengths. The modeling is performed with a distributed hot spot model which is based on solving a heat balance equation for electron temperature along the superconducting microbridge. Particular care has been taken during the modeling concerning the temperature-dependent resistance and the bias current dependence of the critical temperature of the device. The dc and mixing characteristics of this mixer have been computed and we have observed a quite good match between the predicted and the measured results for both dc characteristics and mixing performances at submillimeter wavelengths.
We interpret the experimental observation of a frequency-dependence of superconducting hot electron bolometer (HEB) mixers by taking into account the non-uniform absorption of the terahertz radiation on the superconducting HEB microbridge. The radiation absorption is assumed to be proportional to the local surface resistance of the HEB microbridge, which is computed using the Mattis-Bardeen theory. With this assumption the dc and mixing characteristics of a superconducting niobium-nitride (NbN) HEB device have been modeled at frequencies below and above the equilibrium gap frequency of the NbN film.