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Kamil Postava, H. Sueki, Mitsuro Aoyama, Tomuo Yamaguchi, K. Murakami, Yasuhiro Igasaki

Doping effects on optical properties of epitaxial ZnO layers determined by spectroscopic ellipsometry

Applied Surface Science 175 (2001) 543–548

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 ellipsometer and in the spectral range from 0.5 to 6.5 eV using near-normal incidence 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 structure 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 contribution originating from presence of the dopant. Spectroscopic ellipsometry and reflectometry are very sensitive to a surface roughness. The surface roughness was modeled by a surface layer of the Bruggeman effective medium and by diffraction theory.

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