Optical phonons and free-carrier effects in MOVPE grown AlxGa1−xN measured by Infrared Ellipsometry

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Abstract
We report on the application of infrared spectroscopic ellipsometry (IR-SE) for wavenumbers from 333cm−1to 1200cm−1as a novel approach to non-destructive optical characterization of free-carrier and optical phonon properties of group III-nitride heterostructures. Undopedα-GaN,α-AlN,α-AlxGa1−xN (x= 0.17, 0.28, 0.5), andn-type silicon (Si) dopedα-GaN layers were grown by metal-organic vapor phase epitaxy (MOVPE) onc-plane sapphire (α-Al2O3). The four-parameter semi-quantum (FPSQ) dielectric lattice-dispersion model and the Drude model for free-carrier response are employed for analysis of the IR-SE data. Model calculations for the ordinary (∈⊥) and extraordinary (∈||) dielectric functions of the heterostructure components provide sensitivity to IR-active phonon frequencies and free-carrier parameters. We observe that theα-AlxGa1−xN layers are unintentionally doped with a back ground free-carrier concentration of 1–4 1018cm−3. The ternary compounds reveal a two-mode behavior in ∈⊥, whereas a one-mode behavior is sufficient to explain the optical response for ∈||. We further provide a precise set of model parameters for calculation of the sapphire infrared dielectric functions which are prerequisites for analysis of infrared spectra of III-nitride heterostructures grown onα-Al2O3.