  Embedding rare-earth pnictide (RE-V) nanoparticles into III-V semiconductors
enables unique optical, electrical, and thermal properties, with applications
in THz photoconductive switches, tunnel junctions, and thermoelectric devices.
Despite the high structural quality and control over growth, particle size, and
density, the underlying electronic structure of these nanocomposite materials
has only been hypothesized. Basic questions about the metallic or
semiconducting nature of the nanoparticles (that are typically < 3 nm in
diameter) have remained unanswered. Using first-principles calculations, we
investigated the structural and electronic properties of ErAs nanoparticles in
AlAs, GaAs, InAs, and their alloys. Formation energies of the ErAs
nanoparticles with different shapes and sizes (i.e., from cubic to spherical,
with 1.14 nm, 1.71 nm, and 2.28 nm diameters) show that spherical nanoparticles
are the most energetically favorable. As the diameter increases, the Fermi
level is lowered from near the conduction band to the middle of the gap. For
the lowest energy nanoparticles, the Fermi level is pinned near the mid-gap, at
about 0.8 eV above the valence band in GaAs and about 1.2 eV in AlAs, and it is
resonant in the conduction band in InAs. Our results show that the Fermi level
is pinned on an absolute energy scale once the band alignment at AlAs/GaAs/InAs
interfaces is considered, offering insights into the rational design of these
nanocomposite materials.
