CXIX. A study of transparent, highly conducting gold films

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Abstract
Thin gold films deposited on a substrate of bismuth oxide or, better, between two films of bismuth oxide, are found to have exceptionally high electrical conductivity and optical transmission. Heating to several hundred degrees centigrade further enhances these properties, irreversibly. Typical results, for gold thickness 100 å and heat-treatment at 450°c, are film-resistance 3 1/2 ohms ‘ per square ’ and white-light transmission 75% (or 82% using controlled interference effects). The measured electrical and optical constants are remarkably close to those of gold in bulk. This appears to be due to a very fine-grain, continuous structure. Electron microscopy and diffraction show that even at 60 Å thickness no holes of resolvable size exist in the film, that the lateral dimension of the gold particles is some ten times the film thickness, and that the enclosing bismuth oxide films are amorphous, at least initially. The effect of the heating appears to include a considerable reduction in internal surface and also a marked reduction in surface-roughness. Surface reflection of conduction-electrons is found to change from fully diffuse to almost fully specular, as a result of the heating. The function of the bismuth oxide seems to be to preserve, up to quite high temperatures, surface energy conditions controlling mobility of grains, and opposing increase in gold surface area. This would account for the absence of aggregation during deposition, and of grain-growth perpendicular to the surface during heating which would otherwise disrupt the films at much lower temperatures. In this respect, and possibly also in promoting a smooth gold surface, the oxide enclosure seems to simulate one of gold itself. The films have a possible use as transparent conducting coatings, for the purpose of de-icing aircraft windows by direct electrical heating.

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