Dielectric Dispersion and Phonon Line Shape in Gallium Phosphide

The infrared and Raman spectra of GaP near the transverse and longitudinal optic phonon modes are examined in detail and fitted with an oscillator with frequency-dependent damping. Some new low-frequency combination band assignments and an arsenic impurity-mode measurement are presented. A combination band near 360 ${\mathrm{cm}}^{-1\mathrm{}}$ causes the large frequency dependence of the damping and gives an asymmetric shape to the transverse optic mode. The new fits to this asymmetric mode revise upwards the previously accepted values of the index of refraction for frequencies below 2000 ${\mathrm{cm}}^{-1\mathrm{}}$.