INTERPRETATION OF THE OPTICAL-ABSORPTION BY INDIUM IONS IN ZINC-OXIDE

The total optical reflectance, R(infinity) of indium-doped zinc oxide powders was measured over the wavelength range from 350 to 750 nm. The indium contents of the powders were from 119 to 255 ppm. Indium, which in solid solution forms a shallow, donor-like state in ZnO, produces a uniform decrease in the optical reflectance of ZnO powder over the visible spectrum. From the behavior of the remission function f(R(infinity)) with respect to the In ion density N the optical absorption coefficient k is shown to be nonlinear with N, exhibiting an exponential dependence. The data can be interpreted to indicate that the donor electrons on the In are interacting among In ions, as both pair-wise In interactions and interactions of higher orders. It is demonstrated by experiment that the dependence of the In optical cross section upon ion density can be represented by an effective optical cross section for In given by sigma(eff) = sigma exp((4/3)pia3N) where sigma is the photoionization cross section for the In ion, and a is the effective Bohr radius of the donor electron about the defect.