Aggregation of interstitial copper atoms in silicon

The formation energy of metastable Cu aggregates consisting of one to four interstitial Cu atoms is examined using ab initio electronic structure theory. Localized vibration energies are calculated for two aggregates. These are a substitutional and interstitial Cu pair and three interstitial Cu atoms. In the latter aggregate there is extensive atomic relaxation with a Si atom moving into a nearby interstitial position and leaving a Si vacancy and Si interstitial. There is no bond breaking. The relaxation stabilizes the complex to have trigonal symmetry with an almost exact tetrahedral geometry. The results highlight that several Cu slightly metastable aggregates may be possible in silicon having strong localized modes at similar energies.