Annihilation of thermal double donors in silicon

We performed systematic experiments on the annihilation of six species of thermal double donors, or TDDs (TDD1-TDD6) under various conditions in both carbon-lean and carbon-rich Si crystals, by means of low-temperature infrared spectroscopy. We found that two kinds of TDD annihilation occurred in two different time regions. The first annihilation occurred typically within 1000 min at 500°C, and is ascribed to the dissociation of oxygen clusters responsible for TDDs. We analyzed TDD annihilation on the basis of the model of successive dissociation of oxygen clusters, and obtained an activation energy of about 4 eV and a pre-exponential factor of the order of 10²²s^-1, regardless of TDD species and carbon density. We discuss the origin of such a high activation energy and a large pre-exponential factor. The second annihilation occurred typically after 1000 min at 500°C, and is ascribed to the neutralization of TDDs. The neutralization behavior strongly depends on the amount of carbon in the silicon crystals. The C-rich crystal shows the usual thermal activation process with an activation energy of 1.7 eV and a pre-exponential factor of about 10⁶s^-1, independently of TDD species. On the other hand, the C-lean crystal shows no distinct temperature dependence on neutralization rate, suggesting an athermal process. These results strongly suggest two different mechanisms of TDD neutralization, one of which is related to carbon and the other independent of carbon. We propose models to explain the features of these two kinds of TDD neutralization.