Influence of the domain size on the band gap of ordered (GaIn)P

The domain size and the band-gap energy of ordered epitaxial (GaIn)P layers have been determined by means of transmission electron microscopy and photoluminescence measurements, respectively. With decreasing domain size the symmetry of the ordered layers is reduced from the trigonal space group R3m (CuPtB type) to the monoclinic Pm space group. Pseudopotential band-structure calculations reveal an increase of the band-gap energy of ordered (GaIn)P with diminishing domain size. This would affect the determination of the degree of order by methods, which rely on the band-gap energy of perfectly ordered (GaIn)P. By correlating the theoretical with the experimental band-gap energies we demonstrate that the order parameter h of layers with small domains is higher if the dependence of the band-gap energy on the domain size is considered. Hence, for the determination of the degree of order not only the band-gap energy, but also the domain size have to be taken into account.