Vertical inhomogeneities in superconducting tunnel junctions
Publication date: 16 April 2006
Authors: Martin, D.D.E., et al.
Journal: Nuclear Instruments and Methods in Physics Research Section A
In a quest to further improve the performance of superconducting tunnel junctions as photon detectors over the broad spectral range from optical to X-ray wavelengths, a fundamental understanding of their limits is required. Recently fabricated Ta/Al STJs have shown an exceptionally high responsivity (number of collected charge carriers versus absorbed photon energy) and spectral resolution (R = E/Delta_E > 22 at 2.5eV). This high spectral resolution has now revealed some unique features when plotted against photon wavelength. The experimental data indicate the important role of the photon absorption profile. We have shown that vertical inhomogeneity is a fundamental consequence of the quasiparticle generation process in the thin film such that pair breaking phonons emitted in the process of energy down conversion have a chance to escape depending on the absorption depth. This results in an inhomogeneous broadening of the detected signal. We also found that another, previously unknown fundamental noise source exists which is related to statistical fluctuations of the angular distribution of phonons emitted in the down-conversion process. We present the new experimental data and compare them to the predictions of the down-conversion theory. We show that, while the responsivity is rather constant in the optical wavelength range, the intrinsic resolution exhibits a number of features which can be explained by changing statistical variations of the phonon losses as function of absorption depth.Link to publication