  In an effort to build a stronger microscopic foundation for radiation damage
models in gallium arsenide (GaAs), the electronic properties of
radiation-induced damage clusters are studied with atomistic simulations.
Molecular dynamics simulations are used to access the time and length scales
required for direct simulation of a collision cascade, and density functional
theory simulations are used to calculate the electronic properties of isolated
damaged clusters that are extracted from these cascades. To study the physical
properties of clusters, we analyze the statistics of a randomly-generated
ensemble of damage clusters because no single cluster adequately represents
this class of defects. The electronic properties of damage clusters are
accurately described by a classical model of the electrical charging of a
semiconducting sphere embedded in an uniform dielectric. The effective band gap
of the cluster depends on the degree of internal structural damage, and the gap
closes to form a metal in the high-damage limit. We estimate the Fermi level of
this metallic state, which corresponds to high-energy amorphous GaAs, to be
0.46 +/- 0.07 eV above the valence band edge of crystalline GaAs.
