Optical evidence of the direct-to-indirect-gap transition in GaAs-AlAs short-period superlattices

We have studied electronic properties of GaAs-AlAs short-period superlattices grown by molecular-beam epitaxy. A direct-gap to ‘‘indirect’’-gap transition has been evidenced through optical experiments on a large number of samples. It also corresponds to a type-I to type-II superlattice transition. Our results are in good agreement with an envelope-function description applied to each extremum of the host-material band structure. As a result of spatial transfer of electrons in AlAs, we have obtained an accurate value of the offset parameter Δ$E_c^{\Gamma }$/Δ$E_g^{\Gamma }$=0.67. .AE