Strain and annealing temperature effects on the optical properties of GaNAs layers grown by molecular beam epitaxy

Abstract

The ex-situ characterization of GaNAs/GaAs system grown by Molecular Beam Epitaxy with two different GaNAs layer thicknesses and similar nitrogen concentration (%N) is presented to explore the residual strain effect in the optimization of the inclusion of nitrogen on the GaAs host matrix by a rapid thermal annealing (RTA). Raman spectroscopy was used to study the effects of post-growth RTA on the alloy´s crystallinity and on %N by analyzing mode´s intensities and frequencies where both an increase of the local vibration mode and lattice disorder with the temperature were noticed. Band structure of the epilayers in the range of 1 to 1.5 eV and their modification with RTA were analyzed by photoreflectance. The E- transition redshifts, indicating increase of nitrogen in the alloy together with a modification of the spectral signature of GaAs by residual strain. Critical points above 1.5 eV were studied by ellipsometry where clear changes in the line shape of the second derivative of the pseudo-dielectric function imaginary part are produced with RTA, making more evident the presence of E+. The E1, E1+Δ1 and E΄0 transitions were also explored, E1 blue-shifts when the annealing has been successfully applied. With this study we show that in the optimization of GaNAs layers properties through RTA processes not only %N should be considered, but also additional parameters as thickness and strain.