Characterization of Superconducting Tunnel Junction X-ray Detectors by Low Temperature Scanning Electron Microscopy

Abstract

One problem in superconducting tunnel junction x-ray detectors is that the two electrodes respond differently to monochromatic x-rays, and produce double peaks in energy spectra. To study the double peak phenomenon, Low Temperature Scanning Electron Microscope (LTSEM) is a versatile tool, since it is possible to analyze two electrodes separately. From the LTSEM analyses in combination with x-ray detection experiments, it has been revealed that the intrinsic charge output of the base electrode is higher than that of the counter electrode over a wide range of bias voltages in our Nb/Al polycrystalline junctions. The voltage dependence of charge output is considerably modified by flux quanta trapped in a terrestrial magnetic field.

Double peak phenomenon in superconducting tunnel junction x-ray detectors

“…1 to the base and counterevents was performed by employing low temperature scanning electron microscopy ͑LTSEM͒. 6 The base and counterelectrodes of another junction on the same wafer were separately stimulated by short electron beam pulses, and the decay times of the junction response were measured at a bias voltage of 1.3 mV and at 2.1 K. The details of LTSEM can be found in other publications. 7,8 The decay time constants were 0.21 and 0.15 s for the base and counterelectrodes, respectively.…”

“…7,8 The decay time constants were 0.21 and 0.15 s for the base and counterelectrodes, respectively. On the other hand, in the scatter plots of the signal heights against the charge sensitive preamplifier rise times, the x-ray events were classified into two groups having mean rise times ͑20%-80%͒ of 0.15 and 0.11 s. 6 When we take a qp loss time and a tunneling time, which are described in Sec. IV, a tunneling probability should be as small as less than 35% at this high bias voltage.…”

X-ray fluorescence spectrometry

“…In-depth characterization of STJs was also reported by Cristiano et al 81 and Shimizu and co-workers. 82 Low-temperature SEM was used by Ohkubo and colleagues 83 to investigate the double peaks arising from the different response of the base and counter electrode in the STJ.…”