Field-assisted diffusion of silver in SnO2 thin films

Tin oxide films are known for their wide spread applications in the field of optoelectronics as transparent conductors and as atmosphere sensitive ceramic pellets for chemical sensor fabrication. Forming durable ohmic contacts to these films is vitally necessary for the effective performance and long life of almost all such devices. Silver electrodes are commonly considered as low resistance ohmic contacts to many metal-oxide semiconductors. Particularly in devices operating at elevated temperatures, silver migration can considerably decrease useful device lifetime. Here, we study the performance of silver electrodes paste printed on SnO2 thin films and show that the I-V characteristics of the silver/SnO2 contact changes with operating time at elevated temperatures. SnO2 films are grown on alumina substrates by ultrasonic spray pyrolysis technique. A pair of Ag electrodes are paste-printed on the surface of the grown films followed by a heat-treatment step to stabilize the deposits. The as-fabricated Ag/SnO2 contacts present ohmic characteristics. Continuous application of DC voltages to the samples operating at 350 degrees C results in a significant drop in the devices' resistance. The results originate from field-assisted migration of silver ions from the anode to the cathode electrode and formation of Ag micro-filaments in the polycrystalline SnO2 layer.