Fine structure of the E1+Δ1 critical point in GaAsN

The E 1 and E 1 +Δ 1 transitions of thin, pseudomorphic GaAs 1 - x N x layers (0<x<2%) on GaAs substrates are investigated by spectroscopic ellipsometry. The complex dielectric function is determined at room temperature, for the spectral range from 2.4 to 3.6 eV. The optical transitions in the spectral region around 3 eV are analyzed by fitting analytical critical point line shapes to the second derivative of the dielectric function. In addition to the two GaAs-like critical points E 1 and E 1 +Δ 1 , we found evidence for another weak transition This can be interpreted as N induced splitting of E 1 +Δ 1 into two closely spaced transitions. For low N incorporation, the experimentally observed splitting is small and the subtransitions are nearly degenerate, whereas for N incorporations above 0.9%, the splitting becomes pronounced.