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Study of the superconductor-normal metal interface in hot-electron bolometer mixers

Ю.Д. Третьяков, Matvey I. Finkel, P. Larionov, Anna Maslennikova, Рябчун Сергей Александрович, N. Kaurova, B. Voronov, Gregory N Gol'tsman

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

During the last few years significant progress has been made in the field of hot-electron bolometer (HEB) mixers. It has been possible to combine a low noise temperature and high stability of phonon-cooled HEBs with an ultrawide gain bandwidth of the diffusion-cooled HEBs in a single device and thus create HEB mixers which offer a noise temperature of 5hν/kB across a 7-GHz band. The essential ingredient of this success is the in situ fabrication technology, where the deposition of the gold contacts follows the deposition of the superconducting film without vacuum breaking. Despite the evident technological breakthrough, the details of certain physical processes occurring both at the superconductor-normal metal (SN) interface and in the superconducting bridge are not completely understood in terms of their influence on the mixer performance. In particular, it is still unclear to what extent the properties of the SN interface are affected by the deposition temperatures of the superconductor and the normal metal, and also by the time interval elapsed between the deposition processes. We will gain some insight into the matter by studying the behaviour of the superconducting transition as a function of the deposition temperatures and time delay. A better understanding of this issue will allow us to move forward the HEB mixer technology towards new applications.

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During the last few years significant progress has been made in the field of hot-electron bolometer (HEB) mixers. It has been possible to combine a low noise temperature and high stability of phonon-cooled HEBs with an ultrawide gain bandwidth of the diffusion-cooled HEBs in a single device and thus create HEB mixers which offer a noise temperature of 5hν/kB across a 7-GHz band. The essential ingredient of this success is the in situ fabrication technology, where the deposition of the gold contacts follows the deposition of the superconducting film without vacuum breaking. Despite the evident technological breakthrough, the details of certain physical processes occurring both at the superconductor-normal metal (SN) interface and in the superconducting bridge are not completely understood in terms of their influence on the mixer performance. In particular, it is still unclear to what extent the properties of the SN interface are affected by the deposition temperatures of the superconductor and the normal metal, and also by the time interval elapsed between the deposition processes. We will gain some insight into the matter by studying the behaviour of the superconducting transition as a function of the deposition temperatures and time delay. A better understanding of this issue will allow us to move forward the HEB mixer technology towards new applications.
We report a noise bandwidth of 7 GHz in the new generation of NbN hot-electron bolometer (HEB) mixers that are being developed for the space observatory Millimetron. The HEB receiver driven by a 2.5-THz local oscillator offered a noise temperature of 600 K in a 50-MHz final detection bandwidth. As the filter center frequency was swept this value remained nearly constant up to the cutoff frequency of the cryogenic amplifier at 7 GHz. We believe that such a low value of the noise temperature is due to reduced radio frequency (RF) loss at the interface between the superconducting film and the gold contacts. We have also performed gain bandwidth measurements at the superconducting transition on HEB mixers with various lengths and found them to be in excellent agreement with the results of the analytical and numerical models developed for the HEB mixer with both diffusion and phonon cooling of hot electrons.
This work is devoted to the investigation of the applicability of NbZr as a new material for wideband hot- electron bolometer (HEB) mixer, which is a device of choice for most of low noise heterodyne receivers operating above 1 THz in astronomy applications. Although currently 10-nm NbZr films offer a maximum gain bandwidth of only 450 MHz at Tc on sapphire substrates, there is every reason to believe that the above result can be improved by decreasing the film thickness and using other substrate material which will provide better acoustic match with NbZr. operation can be described by relatively simple equations for the heat balance, and the mixer's roll-off frequency is completely determined by several time constants of the film material ((10),(11)). The values of these characteristic times can be obtained from the dependence of the gain bandwidth on temperature and film thickness. For the electron temperature to be uniform throughout the bridge it is necessary that either the LO frequency be greater than the superconductor gap frequency, or that the ambient temperature be about Tc (in the latter case the LO frequency need not exceed the gap frequency of the unheated superconductor). Also the bolometer bias should be small and the power levels of the oscillators should not influence the operating point of the bolometer. In this paper we study deposition process of ultrathin NbZr films on sapphire substrates, gain bandwidth of NbZr bolometers and the possibly of using this material as a new material for HEB mixers.
NbN phonon-cooled hot electron bolometer mixers (HEBs) have been realized with negligible contact resistance between the bolometer itself and the contact structure. Using a combination of in situ cleaning of the NbN film and the use of an additional superconducting interlayer of a 10 nm NbTiN layer between the Au of the contact structure and the NbN film superior noise temperatures have been obtained as low as 950 K at 2.5 THz and 750 K at 1.9 THz. Here we address in detail the DC characterization of these devices, the interface transparencies between the bolometers and the contacts and the consequences of these factors on the mixer performance.
We present our recent results of the fabrication and testing of NbN hot-electron bolometer mixers with in situ gold contacts. An intermediate frequency bandwidth of about 6 GHz has been measured for the mixers made of a 3.5-nm NbN film on a plane Si substrate with in situ gold contacts, compared to 3.5 GHz for devices made of the same film with ex situ gold contacts. The increase in the intermediate frequency bandwidth is attributed to additional diffusion cooling through the improved contacts, which is further supported by the its dependence on the bridge length: intermediate frequency bandwidths of 3.5 GHz and 6 GHz have been measured for devices with lengths of 0.35 μm and 0.16 μm respectively at a local oscillator frequency of 300 GHz near the superconducting transition. At a local oscillator frequency of 2.5 THz the receiver has offered a DSB noise temperature of 950 K. When compared to the previous result of 1300 K obtained at the same local oscillator frequency for devices fabricated with an ex situ route, such a low value of the noise temperature may also be attributed to the improved gold contacts.
Results of an experimental study of the noise temperature (T n ) and noise bandwidth (NBW) of the superconductor NbN hot-electron bolometer (HEB) mixer as a function of its temperature (T b ) are presented. It was determined that the NBW of the mixer is significantly wider at temperatures close to the critical ones (T c ) than are values measured at 4.2 K. The NBW of the mixer measured at the heterodyne frequency of 2.5 THz at temperature T b close to T c was ~13 GHz, as compared with 6 GHz at Tb = 4.2 K. This experiment clearly demonstrates the limitation of the thermal flow from the NbN bridge at T b ≪ T c for mixers manufactured by the in situ technique. This limitation is close in its nature to the Andreev reflection on the superconductor/ metal boundary. In this case, the noise temperature of the studied mixer increased from 1100 to 3800 K.
Superconducting hot-electron bolometers are presently the best performing mixing devices for the frequency range beyond 1.2 THz, where good-quality superconductor-insulator-superconductor devices do not exist. Their physical appearance is very simple: an antenna consisting of a normal metal, sometimes a normal-metal-superconductor bilayer, connected to a thin film of a narrow short superconductor with a high resistivity in the normal state. The device is brought into an optimal operating regime by applying a dc current and a certain amount of local-oscillator power. Despite this technological simplicity, its operation has found to be controlled by many different aspects of superconductivity, all occurring simultaneously. A core ingredient is the understanding that there are two sources of resistance in a superconductor: a charge-conversion resistance occurring at a normal-metal-superconductor interface and a resistance due to time-dependent changes of the superconducting phase. The latter is responsible for the actual mixing process in a nonuniform superconducting environment set up by the bias conditions and the geometry. The present understanding indicates that further improvement needs to be found in the use of other materials with a faster energy relaxation rate. Meanwhile, several empirical parameters have become physically meaningful indicators of the devices, which will facilitate the technological developments.