Insulator-metal transition in nanostructured Ni-Al thin films

The electrical resistance of Ni-Al alloy thin films prepared by de magnetron sputtering process was found to be abnormally high at room temperature. However, when heated at elevated temperatures, the resistance dropped significantly, exhibiting a remarkable negative temperature coefficient of resistance (TCR). The phenomenon was found to be substrate-independent. Cross-sectional transmission electron microscopy revealed that the films were essentially nanocrystalline and porous in nature. Analysis of the current density-electric field characteristics yielded a satisfactory agreement with either the space charge limited or the Poole-Frenkel models for electrical conduction. The negative TCR effect diminishes and the usual metallic resistance is restored in thicker films, probably due to reduction in particle separation and further coalescence of neighbouring crystallites.