The interstitial copper ion (Cu-i(+)) is a very fast-diffusing impurity in
Si. While the isolated interstitial is a shallow donor, it reacts with impu
rities and defects and these reactions affect the electrical properties of
the material. Copper passivates shallow acceptors, forms pairs with various
impurities, including itself, and precipitates at defects. Some Cu precipi
tates are strong electron-hole recombination centers. In this paper, inters
titial and substitutional copper, copper-acceptor pairs, and the precipitat
ion of copper at a model nanodefect, the ring hexavacancy, are studied in c
lusters at the ab initio Hartree-Fock level. The chemistry of copper in Si
is not as simple as commonly believed, even in the case of Cu-i(+). The int
erstitial ion is not in the 3d(10)4sp(0) configuration as often assumed, bu
t transfers some electrons into the 4sp shell, which ultimately is responsi
ble for the various covalent interactions between copper and its host cryst
al. In addition to energy-optimized configurations and binding energies, a
number of details of the chemical interactions shed light on the behavior o
f copper in silicon. [S0163-1829(99)00232-5].