Parallel twin boundaries arranged at similar intervals of nanometer length, that emitted an intense monochromatic light polarized parallel to the boundaries, were self-organized in an indirect-gap AlGaAs epilayer; the epilayer was grown on a rough As-deficient surface of an AlGaAs(0 0 1) substrate with any lattice constant, by conventional metal-organic chemical-vapor deposition. Most boundaries were of Sigma 3-type on {111}B, and they extended from the interface between the epilayer and the substrate. There existed no compositional fluctuation around the boundaries. The formation mechanism was discussed. (c) 2008 Elsevier B.V. All rights reserved.