Rich chemistry of copper in crystalline silicon

The interstitial copper ion (Cuᵢ⁺) is a very fast-diffusing impurity in Si. While the isolated interstitial is a shallow donor, it reacts with impurities 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 precipitates are strong electron-hole recombination centers. In this paper, interstitial and substitutional copper, copper-acceptor pairs, and the precipitation of copper at a model nanodefect, the ring hexavacancy, are studied in clusters 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 Cuᵢ⁺. The interstitial ion is not in the 3d¹⁰4sp⁰ configuration as often assumed, but transfers some electrons into the $4sp$ shell, which ultimately is responsible for the various covalent interactions between copper and its host crystal. In addition to energy-optimized configurations and binding energies, a number of details of the chemical interactions shed light on the behavior of copper in silicon.