Optical band-gap shrinkage in highly transparent and conducting ZnO thin films deposited by the Pyrosol process

An optical band-gap shrinkage was found in highly transparent and conducting thin films deposited by the Pyrosol process. The narrowing appears at carrier concentrations which are consistent with a metal-semiconductor transition. The gap-shrinkage dependence on carrier concentration is consistent with the model proposed by P. E. Schmid [Phys. Rev. B 23, 5531 (1981)] for Si:As and Si:B.