Optical phonons and free-carrier effects in MOVPE grown Al _xGa_1-xN measured by infrared ellipsometry

We report on the application of infrared spectroscopic ellipsometry (IR-SE) for wavenumbers from 333cm^-1 to 1200cm^-1 as a novel approach to non-destructive optical characterization of free-carrier and optical phonon properties of group III-nitride heterostructures. Undoped α-GaN, α-AIM, αAl_xGa_1-xN (x = 0.17, 0.28, 0.5), and n-type silicon (Si) doped α-GaN layers were grown by metalorganic vapor phase epitaxy (MOVPE) on c-plane sapphire (α-Al₂O₃). The four-parameter semiquantum (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 the α-Al_xGa _1-x 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 on α-Al₂O₃.