The nucleation and growth of silicon thin films on silicate glasses of variable composition using supersonic gas source molecular beam deposition

Abstract

Supersonic molecular beam techniques have been used to study the nucleation and growth of Si thin films on glass surfaces of variable composition using Si2H6 as the precursor to film growth. We have examined, in particular, the early stages of growth using scanning electron microscopy. Making use of molecular beam techniques to control accurately the precursor exposure we have examined trends in the evolution of the Si island density as a function of the composition of the glass, x, in (2⋅SiO2)1−x(Al2O3⋅CaO)x. The silica composition (1−x) for these samples was varied between 0.25 and 0.75, and comparisons were also made to the nucleation of Si on SiO2 thin films made by thermal oxidation and Corning 1737 display glass. We have found that the incubation time τinc varies only weakly with substrate composition, increasing by only a factor of 3 over the range 1−x=0.25–1.0. Examination of a later stage of nucleation and growth, the time for coalescence, τcoal, indicated a stronger dependence on composition, and this metric varied by a factor of 8 over the same range of composition. These results indicate that the intrinsic reactivity of the surface scales with the silica content of the surface. The maximum island density shows a much stronger, superlinear dependence on silica content, increasing by a factor of 15 as 1−x increased from 0.25 to 1.0. For the silica rich compositions, i.e., SiO2 and 1737, Nmax is essentially independent of substrate temperature and the results can be interpreted by a model for nucleation that is purely heterogeneous, and where surface diffusion plays a minimal role. In contrast, on the most silica dilute glass surface (1−x=0.25), Nmax exhibits an Arrhenius temperature dependence with an apparent activation energy of 1.1 eV. Coupled with the observation of a broader island size distribution on this surface, we conclude that surface diffusion plays a role in nucleation and growth on this silica dilute surface, possibly via Ostwald ripening.