Modelling oxygen vacancies at the Si(100)-SiO₂ interface

Using a structural model proposed by Ourmazd et al. as a starting point, we have determined some properties of the Si(100)-SiO2 interface. To obtain this information, we have performed a Monte Carlo simulation of the interface, using semiempirical interaction potentials. In a first step, we have just relaxed the system. While on the one hand the relaxation allows us to reach the usual values of bond lengths, on the other hand the silica at the interface retains several features of the initial configurations. There is no tendency for the free volume to collapse in the interfacial region, and the density is somewhat larger than that of tridymite. Two varieties of Si-Si bonds have been found. Owing to constraints imposed by the very low oxygen pressure, the real system accommodates a number of oxygen vacancies. We have shown that these vacancies are much more easily accommodated at the interface, because of electrostatic interactions.