Transition metals are important impurities in Si as they possess deep levels in the bandgap and interfere with the properties of devices. For example, they reduce storage time in MOS dynamical memory cells and lead to the formation of dark current spikes in CCDs.
Gold as an isolated impurity in silicon is one of the most studied defects in semiconductors. This is mainly due to its technological important and to its pivotal role in the understanding of the electronic properties of transition metal impurities in semiconductors. Elements like gold, platinum, silver or even titanium, are commonly used to produce metallic contacts. Our understanding of the impurities at the far right of the periodic table is based on the ``vacancy model'' [1]. This maintains that their electronic activity is controlled by the levels of the vacancy. Since, the vacancy can be passivated [2], there is a possibility that the transition metal impurity can also be passivated and this has been found in the case of Au [3].