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

The optical properties of GaAs/GaNxAs1-x superlattice structures grown by m etal-organic vapor-phase epitaxy are studied by variable angle-of-incidence spectroscopic ellipsometry for photon energies from 0.75 to 1.55 eV. We em ploy Adachi's critical-point composite model, and we report the direct-band -gap energy E-0 and the complex index of refraction of the GaNxAs1-x sublay ers for x less than or equal to 3.3% from analysis of the ellipsometry data . We observe a strong redshift of E-0 with increase in x, and a strong decr ease of the E-0 transition amplitude. The E-0 values obtained for the super lattice structures are in good agreement with photoluminescence results, an d also with previous reports from single epilayers. Structure, composition, layer thickness, and parallel and perpendicular lattice mismatch of the sa mples are studied by transmission electron microscopy and high-resolution x -ray diffraction investigations. (C) 2000 American Institute of Physics. [S 0003-6951(00)02420-7].