OPTICAL-PROPERTIES OF SILICON PIGMENTED ALUMINA FILMS

Plates of Al-Si alloy were anodized in a sulfuric acid solution. This treatment provides a Si-Al2O3 coating growing at a rate of 0.14 mu m/m in. The Si particles had sizes between 1 and 10 mu m, as seen by scann ing electron microscopy. Optical measurements showed a continuous decr ease of reflectance with increasing film thickness. The reflectance of the Si-Al2O3 coated aluminum could be understood from a four flux rad iative transfer theory, In order to explain our measurements, it was f ound necessary to include a free-carrier term in the dielectric permit tivity of Si. The free carriers are probably due to doping with Al. He nce, the relaxation time of the free carriers is determined by scatter ing from the charged Al impurities. (C) 1997 American Institute of Phy sics.