| Research Abstract |
In this research project, we have intensively studied the atomistic process in SiO_2 and at Si/SiO_2 interfaces, by using first principles calculations, macroscopic simulations and secondary ion mass spectroscopy (SIMS) experiments. We have investigated the effect of Si/SiO_2 interface reactions to impurity diffusion and Si self-diffusion in and at Si/SiO_2 interfaces by first principles calculations and by simultaneous diffusion equations. As a result, we have obtained the following remarkable results.
(1) We investigated the diffusion mechanisms of boron in SiO_2. We find that a B atom forms various stable and meta-stable geometries in SiO_2 with point defects, depending on its charge state and surrounding environments. In the case of B in SiO_2 with an O interstitial, a B atom forms a very stable B-O complex in which the B atom is bound to the O interstitial. We also found that B atoms diffuse from Si substrates into SiO_2 layers under the existence of O species. Once the B-O complex
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is formed, the B atom diffuses via the SiO_2 network keeping this B-O unit with unexpectedly small activation energies of 2.1-2.3 eV that well reproduce experiments. Moreover, B-O complex can be regarded as the complex of B_<Si>-Si-O complex, since it is believed that B atoms substitute Si atoms in SiO_2. Accordingly, B diffusion are expected to be promoted by the existence of SiO. Since it has been reported that SiO content is high near Si/SiO_2 interfaces, our results imply that B diffusion will be promoted near Si/SiO_2 interfaces. These results are intimately related to the following SIMS experiments that high Si self-diffusivity is observed near Si/SiO_2 interfaces.
(2) Simultaneous diffusion of Si in thermally grown SiO_2 is modeled taking into account the effect of SiO molecules generated at the Si/SiO_2 interface and diffusing into SiO_2 to enhance both Si diffusion. Based on the model, we simulated experimental SIMS profiles of co-implanted ^<30>Si in ^<28>SiO_2, which show high diffusivities near the interface that is supposed to have high SiO content. The experimental results were simulated by simultaneous diffusion equations, assuming that the diffusivity of SiO, which enhances the diffusivities of Si. The present result indicates that Si atoms in SiO_2 diffuse correlatively via SiO generated at Si/SiO_2 interfaces. Less
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