Point defect: also induced

A number of the well-known y-induced centers in n-type Si are also neutralized by atomic hydrogen (Pearton, 1982). The A-center (oxygen-vacancy complex, Ec-0.18eV and divacancy level (Ec-0.23eV) are passivated, while the E-center (phosphorus-vacancy complex, Ec -0.44 eV) is thermally removed at relatively low temperatures and its susceptibility to hydrogenation could not be determined. Point defects... 

Previous sections of this chapter have shown that it is possible to introduce defects into a perfect crystal by adding an impurity. Such an addition causes point defects of one sort or another to form, but they no longer occur in complementary pairs. Impurity-induced defects are said to be extrinsic. We have also noted that when assessing what defects have been created in a crystal, it is important to remember that the overall charge on the crystal must always be zero. 

In this respect, the cluster size, from which on the defect-induced perturbation on the electronic structure disappears, is a key quantity. The calculations showed [178] that effect of the surface defects on the adhesion energy decreases rapidly with cluster size. Thus, for particles of nanometer size, differences in the bonding to the substrate tend to vanish as the larger polarizability of the particle screens the effect of the defect and the relative effect of defect-related bonds become less important due to the larger number of metal-oxide bonds at the interface. However, because point defects are the most likely sites for the initial steps of nucleation, one has to expect that also large metal particles are still located at these sites unless the temperature is sufficiently high to permit diffusion of particles. 

Since the details of these equations are explained elsewhere, only key ideas are briefly described here. One of these is to classify the solute atom clusters into irradiation-induced clusters and irradiation-enhanced clusters. Irradiation-induced clusters correspond to solute atom clusters with or without Cu atoms, whose formation mechanism is assumed to be the segregation of solute atoms based on point defect cluster or matrix damage (heterogeneous nucleation). On the other hand, the irradiation-enhanced clusters correspond to so-called CRPs (Cu-rich precipitates) or CELs (Cu-enriched clusters), and the formation mechanism is the clustering of Cu atoms above the solubility limit enhanced by the excess vacancies introduced by irradiation. This model also assumes that the formation of solute atom clusters and matrix damage is not independent to each other, which is a very different model from the conventional two-feature models as described in the previous sections. Another key idea is the introduction of a concept of a thermal vacancy contribution in the diffusivity model. This idea is essentially identical to that shown in Rg. 11.11. This is a direct modeling of the results of atomic-level computer simulations. ... 

The penetration of chlorine atoms into the passive films is suggested by close examination of the relaxed structures in Figures 7.10 and 7.11. It seems to occur independently of the O-enriched or O-deficient nature of the films and of the implemented defect site. However, this aspect was not addressed by the authors in their study and thus cannot be further discussed. Detailed studies relevant for testing the penetration-induced voiding mechanism of passivity breakdown would require implementing O vacancies as point defects not only at the surface but also in the bulk of the passive films of appropriate crystalline structure. Implementation of field-assisted transport in the passive film and at its interfaces would also be required. 

---