Doping effects on optical properties of epitaxial ZnO layers determined byspectroscopic ellipsometry

Optical properties of Al- and Ga-doped ZnO layers have been studied in the spectral range from 1.5 to 5.4 eV using a four zone null spectroscopic elli psometer and in the spectral range from 0.5 to 6.5 eV using near-normal inc idence reflectivity measurements. The layers were prepared by RF magnetron sputtering onto (1120) oriented single-crystal sapphire substrates. Al- and Ga-doping gives rise to a shift of the fundamental absorption edge from 3. 4 to 3.7 eV. The model dielectric function (MDF) based on an excitonic stru cture derived by Tanguy [Phys. Rev. B 60 (1999) 10660] was completed by the Sellmeier and I)rude terms. The Drude term describes a free-electron contr ibution originating from presence of the dopant. Spectroscopic ellipsometry and reflectometry are very sensitive to a surface roughness. The surface r oughness was modeled by a surface layer of the Bruggeman effective medium a nd by diffraction theory. (C) 2001 Elsevier Science B.V. All rights reserve d.