We deposit the ZnO film by thermal evaporator and calculate the refractive index from ES data and found that it is less than one. is it possible? what should be the reasons?
No, it's not possible for ZnO to have a refractive index smaller one. In the visible region (400 to 800 nm) the Cauchy relation is a good approximation for ZnO.
You get negative results if the start values of the fit procedure are too far beyond the real values. Values smaller one for the index of refraction are only possible within an anormal region of the dispersion relation.
Another possible reason are bad values (n,k) for the substrate.
I think Not possible for any material to have refractive index one. because for this the velocity in that material should be greater than velocity in vacuum, which is not possible.