We present a novel method to grow sharply defined self-organized nanostructures into UHV-compatible Al50Ga50As-GaAs shadow masks. The lateral control of epitaxial growth is achieved by self-organized as well as the masks' geometry. We show that we obtain prism-shaped wire structures with triangular cross sections. These wires exhibit well-defined (1 1 1) side faces due to the effect to self-organized growth. Furthermore, we demonstrate the achievement of lateral carrier confinement with this technique.