Light-hole and heavy-hole excitons:: the right probe for the physics of low N content GaAsN

We have performed transmittance and piezomodulated transmittance measurements of as-grown GaAsN layers on GaAs substrates. The absorption shows energy an splitting of tile ground state transition and a simultaneous increase of the splitting with the increase of the N content. This indicates the presence of a strain, which lifts the light- and heavy-hole valence band degeneracy. Surprisingly the piezomodulated transmittance shows that the heavy-hole exciton is the ground state! This implies that the GaAsN layers have a lattice parameter larger than that of GaAs and are under compression. The origin of the lattice parameter increase is the incorporation of N atoms on interstitial sites.