The electronic properties of epitaxial semiconductors are strongly modified by the inclusion of semi-metallic nanoparticles grown in the semiconductor. For example, epitaxial semi-metallic nanoparticles can donate carriers, pin Fermi levels, shorten electron-hole recombination times, enhance electron tunneling and increase scattering of phonons in semiconductor host layers. There are dozens of cubic semi-metallic group III-V rare-earth compounds that can potentially be grown as nanoparticles in the III-V semiconductors. The largest number of studies to date have involved nanocomposites of erbium-based compounds in arsenide and antimonide semiconductors. Their molecular beam epitaxy growth, properties and device applications will be reviewed. (c) 2007 Elsevier B.V. All rights reserved.