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 GaP0.977N0.023 laver grown on Gap. We observe a two-mo de phonon behaviour, i.e., a GaN-like and a GaP-like phonon and El transiti on. We detect six critical-point structures, which we assign as E-0(dir), E -0', E-1, E-1', E-2(1) and E-2(2) transitions. We observe a blueshift of th e direct band gap E-0(dir) and of the E-1 transition in contrast to the red shift of the photoluminescence peak and the absorption tail, which we obser ved on the same sample. The critical-point energies E-0', E-1', and E-2 do not show any significant composition dependence.