Free-carrier effects and optical phonons in GaNAs/GaAs superlattice heterostructures measured by infrared spectroscopic ellipsometry

The infrared-optical properties of GaAs/GaN_xAs _1-xsuperlattice (SL) heterostructures (0 <x< 3.3%) are studied by variable angle-of-incidence infrared spectroscopic ellipsometry (IRSE) for wavenumbers from 250 cm^-1 to 700 cm^-1. The undoped SL structures where grown on top of a 300 nm thick undoped GaAs buffer layer on Te-doped (001) GaAs substrates by metal-organic vapor phase epitaxy (MOVPE). We observe the well-known Berreman-polariton effect within the GaAs LO-phonon region. We further observe a strong polariton-like resonance near the coupled longitudinal-optical plasmon-phonon frequency of the Te-doped substrate at 306 cm^-1. For analysis of the IRSE data we employ the harmonic oscillator dielectric function model and the Drude model for free-carrier response. The additional resonance feature is explained by pseudo surface polariton (PSP) interface modes between the Te-doped GaAs and the undoped GaAs buffer layer / SL film. We find that the PSP modes are extremely sensitive to free-carrier properties within the SL structures, and we obtain a strong increase in free-carrier concentration within the GaNAs SL sublayers with increasing x from analysis of the IRSE data. We further observe the localized vibrational modes of nitrogen at 470 cm^-1 in the GaN_xAs_1-x SL sublayers with a polar strength that increases linearly with x, and which can be used to monitor the nitrogen concentration in GaN_xAs_1-x.