Temperature Dependence of Resistivity and Noise in Electrodeposited Bismuth Samples for X-ray Transition-Edge Sensors

A combination of gold and bismuth is commonly used as absorber for X-ray transition-edge sensors, due to high atomic numbers and, consequently large cross sections, and to their complementary thermal properties. Since Au conductivity dominates, especially at cryogenic temperatures, electrodeposited bismuth films have been grown over sputtered gold, so that bismuth forms a bridge on the gold fingers and, therefore, the conductivity contribution from the underlying Au layer can be removed. Resistance as a function of the temperature has been investigated for pristine and annealed samples, both with and without a dc magnetic field. A general behavior has been observed, thus evidencing three different zones. At high temperatures a metal regime, described in terms of a Fermi liquid model, is identified. In an intermediate temperature region, a hysteretic and irreversible insulating-type resistance is observed. While, at low temperatures a resistance plateau appears. To better understand the electrical conduction properties, noise measurements have been also performed. Two possible models have been considered to interpret experimental data in the intermediate temperature region: Fluctuation-Induced Tunneling and Quantum Interference Effects. The analysis here reported gives indication on the granular structure of the investigated films, which could be represented as random networks of tunneling junctions, where the nodes are the conducting pathways connecting different junctions. In these regions, temperature-induced intergranular processes could be effectively detected giving additional information on the prediction of the ultimate energy resolving capabilities of detectors with bismuth absorbers.