Tantalum STJ for Photon Counting Detectors

TL;DR

This paper presents advancements in tantalum superconducting tunnel junctions (STJs) for photon counting, focusing on improved fabrication processes and demonstrating photon detection at near-infrared wavelengths for astronomical applications.

Contribution

The paper introduces a new fabrication process for Ta/Al-AlOx-Al/Ta STJs with aligned electrodes and vias, enhancing photon counting capabilities for low-light astronomical observations.

Findings

Successful photon counting at 0.78 μm wavelength.

Enhanced fabrication process with aligned electrodes and vias.

Use of digital filtering to reduce noise in photon detection.

Abstract

Superconducting Tunnel Junctions (STJ's) are currently being developed as photon detectors for a wide range of applications. Interest comes from their ability to cumulate photon counting with chromaticity (i.e. energy resolution) from the near infrared (2 $\mu$m) to the X-rays wavelengths and good quantum efficiency up to 80%. Resolving power can exceed 10 in the visible wavelength range. Our main goal is to use STJ's for astronomical observations at low light level in the near infrared. This paper put the emphasis on two main points: the improvement of the tantalum absorber epitaxy and the development of a new version of the fabrication process for making Ta/Al-AlOx-Al/Ta photon counting STJ's. The main features of this process are that pixels have aligned electrodes and vias patterned through a protecting SiO2 layer. These vias are then used to contact the top electrode layer. We use a double thin aluminum trapping layer on top of a 150 nm thick Ta absorber grown epitaxially. Photon counting experiments with Ta junction array are presented at \lambda = 0.78 $\mu$m. Digital filtering methods are used to compute the photon counting data in order to minimize the effects of noise.