Phonon modes and critical points of GaPN

Spectroscopic ellipsometry in the mid-infrared and near-infrared to vacuum-ultra violet spectral range is employed to study the phonon properties and critical points of a GaP_0.977N_0.023 layer grown on GaP. We observe a two-mode phonon behaviour, i.e., a GaN-like and a GaP-like phonon. We detect six critical-point structures, which we assign as E^dir₀, E′₀, E₁, E′₁, E¹₂ and E²₂ transitions. We observe a blueshift of the direct band gap E^dir₀ and of the E₁ transition in contrast to the redshift of the photoluminescence peak and the absorption tail, which we observed on the same sample. The critical-point energies E′₀ E′₁, and E₂ do not show any significant composition dependence.