Anomalous electric conductivity for insoluble SiO2/Pb films prepared by sputtering

Abstract

Dual phase sputtered films of an insoluble SiO2/Pb system were prepared by sputtering and their electric conductivity mechanisms were investigated. The microstructure of the sputtered films consisted of an amorphous SiO2 phase and a crystalline fcc Pb phase. The homogeneous granular structure, where Pb particles of 5-20 nm in diameter were embedded homogeneously in a SiO2 matrix, was observed in the Pb content of 64~71 vol.%, and the inhomogeneous granular structure, where fine and coarse Pb particles coexisted, was observed in the Pb content of 78-92 vol.%. Dissolution of Si or O into the Pb phase was not detected. The sputtered films showed a great diversity in TCR signs and inclinations according to Pb content. As Pb content decreased, the positive TCR slope decreased and changed into a negative TCR from Pb 64 vol.%. This is because as Pb content decreased, the percolation cluster composed of Pb particles, which makes metallic electric conductivity possible, diminished gradually, and electric channels disappeared finally, changing into other electric conductivity mechanisms induced from the tunneling effect of electrons. Sputtered films were conductors with high electric resistivity when they had homogeneous granular structures. This is probably due to the fabrication method, which makes atomic level control during film preparation possible. It seems that because the movement of electrons was obstructed by fine Pb particles dispersed in an insulator SiO2 matrix and lattice defects introduced during sputtering, the mean free path length of electron was reduced drastically. Also the significantly reduced cross section area in the links of channel where the contact regions of particles that form percolation clusters also generates high resistivity. The percolation limit for this system was Pb 67 vol.%. There was an inverse relationship between 9-,93 and TCR, which corresponds to Mooji's empirical rule, but TCR was still positive when 9293>2 ~f2m at certain compositions. It seems that this result is also due to the unique hybrid microstructure of these films. The electric conductivity mechanism of sputtered films was investigated using the power law of percolation electric conductivity and a result of p=4.1 was obtained. A composite cell effective-medium theory was applied to explain this result, and it was found that the sputtered films consisted of composite cells (A and B type). This coincided well with the microstructures observed by TEM.