Structural and optical properties of (AlAs)(m)/(GaAs)(m) type-I ultra short-period superlattices (USSLs) whose thickness ratio of AlAs and GaAs layers is 3:7 were studied. Especially, we paid attention to the superlattices (SLs) with the period length (Lambda) of fractional monolayer (ML) grown by molecular beam epitaxy (MBE). From the photoluminescence (PL) measurements of the USSL, we found that the PL energies increase as the As decrease. The observed PL energies dependence on the As well agrees with the calculation results from a simple model based on Kronig-Penny model even in the case of the As are quite small. (c) 2007 Elsevier B.V. All rights reserved.