Near-band-gap optical functions spectra and band-gap energies of GaNAs/GaAs superlattice heterostructures measured by spectroscopic ellipsometry

The optical properties of GaAs/GaN_xAS_1-x. superlattice structures grown by metal-organic vapor-phase epitaxy are studied by variable angle-of-incidence spectroscopic ellipsometry for photon energies from 0.75 to 1.55 eV. We employ Adachi's critical-point composite model, and we report the direct-band-gap energy E₀ and the complex index of refraction of the GaN_xAs_1-x sublayers for x≤3.3% from analysis of the ellipsometry data. We observe a strong redshift of E₀ with increase in x, and a strong decrease of the E₀ transition amplitude. The E₀ values obtained for the superlattice structures are in good agreement with photoluminescence results, and also with previous reports from single epilayers. Structure, composition, layer thickness, and parallel and perpendicular lattice mismatch of the samples are studied by transmission electron microscopy and high-resolution x-ray diffraction investigations.