Epitaxial superconducting tunnel diodes for light detection applications

Abstract

We demonstrate epitaxially integrated nanoscale superconductor tunnel diodes, realized using NbN on GaN thin films. Tuning the growth conditions leads to reduced interface defect density and to the emergence of the superconducting coherence peaks in the interface tunneling characteristics. The degree of disorder in the superconductor is correlated with the variance in the order parameter value of different domains. Epitaxial integration of the nanoscale layers allowed precise control on the quality of the superconductor at the interface, and, by extension, the variance in the order parameter value. The numerical calculations taking a normal distribution of superconducting order parameter at the interface with a fixed variance in its order parameter values closely match the measured interface transport characteristics at different temperatures. Strong sub-gap nonlinearity observed in the differential conductivity measurements were subsequently shown to be sensitive to photon incidence, thereby acting as a photodetector. Usage of superconducting interfaces with semiconducting layers such as GaN permit sensitivity tunability and enable large scale device fabrication and integration.