Determination of the optical constants of zinc oxide thin films by spectroscopic ellipsometry

Spectroscopic ellipsometry (SE) has been used to determine the complex pseudo dielectric functions, ${\epsilon }_1{\mathrm{(E)+i\epsilon }}_2\mathrm{(E),}$ of ZnO films on (0001) ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ substrates over the spectral range of 1.33 and 4.96 eV at room temperature. The SE measurements are carried out with $\mathrm{E\perp c}$ at angles of incidence of 60° and 65° with respect to the surface normal. Below the band gap, the refractive index $n$ is found to follow the first order Sellmeir dispersion relationship $n^2{\text{(λ)=1+1.881λ}}^2{\mathrm{/(\lambda }}^2{\text{−0.0538}}^2\mathrm{).}$ A free excitonic structure located at the band edge of 3.32 eV is clearly observed in the pseudo absorption spectrum. Elliott expression with Lorentzian broadening is used to model the pseudo absorption coefficient above the band edge.