The effects of island morphology, insulator vacancies, and band bending on the electrical transport in YSZ- and SiN_x-based granular metals

Granular metals (GMs) are a class of disordered conductor-insulator composites consisting of nanoscale metallic islands dispersed within a dielectric matrix. Between 30-50% metal concentration, by volume, the metal islands begin forming electrical pathways, and the GMs transition from a dielectric to a metallic regime. In the dielectric regime, GMs exhibit both thermally activated tunneling and capacitive transport. As such, GMs are of great interest for fundamental studies on electrical transport through discontinuous films. Despite over 50 years of GM research, almost all GM studies utilize SiO₂ or Al₂O₃ combined with a low temperature stability metal. Additionally, the interfacial metal-insulator interactions have not been studied. Here, we report the synthesis of GMs through co-sputtering of Co or Mo with YSZ or SiN_x, with volumetric metal concentrations ranging from 20% to 80%. Transmission electron microscopy (TEM) and X-ray photoemission spectroscopy (XPS) examined the island morphology and metal-insulator interactions, respectively. The effects of the island size, insulator vacancies, and band bending on the electrical transport of these GMs are discussed. How these factors change with post-growth annealing is also studied.