Anisotropy of the dielectric function for wurtzite InN

A ( 11 2 ̄ 0 ) a -plane InN film grown by molecular beam epitaxy on ( 1 1 ̄ 02 ) r -plane sapphire substrate with an AlN nucleation layer and a GaN buffer was studied by spectroscopic ellipsometry. The data analysis yields both the ordinary and the extraordinary dielectric tensor components perpendicular and parallel to the optical axis, respectively. Strong optical anisotropy is demonstrated over the whole energy range from 0.72 up to 9.5 eV. The line shapes of the tensor components and the polarisation behaviour are in very good agreement with the results of recently published band structure and dielectric function calculations. Above the band gap, five van Hove singularities are evidenced from the ordinary component, while three are resolved from the extraordinary part. The polarisation dependence below 1 eV can be interpreted in terms of optical selection rules for three energetically split valence bands around the Γ -point of the Brillouin zone, similar to the well known behaviour of wurtzite GaN. This emphasises a band gap of hexagonal InN of about 0.7 eV.