Electrical and optical properties of discontinuous Al films near the percolation threshold

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Abstract
The electrical and optical properties of discontinuous Al films on NaCl substrates have been investigated. The temperature dependence of the resistance and the TCR were determined in situ under vacuum conditions for films with various volume fractions q. The reflectivity was measured in the spectral range from 220 to 700 nm. It has been found that the electrical and optical properties of the film are subject to significant changes near the percolation threshold, which is characterised by the critical range of the volume fraction ( Delta qc). This behaviour is to be attributed to the change in the type of conduction and in the character of optical properties. Films having volume fractions q< Delta qc exhibit optical anomalies, which take the form of a maximum in the wavelength dependence of reflectivity, the TCR of these films being smaller than zero. For q> Delta qc the optical anomalies disappear and the TCR is greater than zero.