Tuning YSZ- and SiN_x-Based Granular Metal Conductivity by Controlling Island Morphology and Interface Interactions

Granular metals (GMs) consist of metal islands dispersed within an insulating matrix. Depending on the metal concentration, GMs exhibit either metallic or insulating behavior. The metal concentration at which this transition occurs is the percolation limit and depends on the dimensionality of the sample and the geometry of the metal islands. Below the percolation limit, GM conduction is due to thermally activated tunneling and capacitive transport. Tunneling is strongly dependent on both island size and the tunneling barrier height. Consequently, GMs have been a focus of fundamental research on the conductivity in discontinuous films. Despite being studied for over 50 years, there remain significant gaps within the GM literature. For example, studies on GMs with high melting point metals and/or non-oxide insulators and studies on the interfacial metal-insulator interactions are almost entirely lacking. We synthesize thin GM films by co-deposition of Co or Mo with YSZ or SiN_x using radio frequency co-sputtering. The structural and electronic properties are studied as a function of volumetric metal concentrations, from 20% to 80%. The island morphologies of both as-deposited and post-annealed samples are examined using transmission electron microscopy (TEM). X-ray photoemission spectroscopy (XPS) is used to probe the metal-insulator interfaces. We confirmed that, for metal fractions below the percolation limit, conductivity was dominated by thermally activated charge carriers, by low-temperature (~77K) conductivity measurements. These low-temperature measurements showed a several orders-of-magnitude decrease in conductivity versus the room-temperature measurements due to the decreased tunneling decay lengths at low temperatures. High-temperature annealing offers a second means to control the tunneling probability since annealing results in increased island separation distances. Additionally, the YSZ-based granular metals show metal-induced oxygen vacancies as well as downward band bending at the metal-insulator interface which serve to increase the conductivity. For the SiN_x-based GMs, nitrogen vacancies result in strong metal-Si interactions which are dependent on the chosen metal. These metal-insulator interactions have been largely neglected in the literature and along with controlling the island separations via annealing provide a path for tuning the electrical transport in future granular metals.<br/>Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy National Nuclear Security Administration under contract DE-NA0003525.