Surface diffusion kinetics on amorphous silicon

A two-dimensional continuum model based on adatom-vacancy pair formation is developed to describe surface diffusion on amorphous surfaces. We report the first reliable values for D on an amorphous surface for temperatures between 505–590°C, focusing on the particular case of self-diffusion of amorphous silicon (a-Si). We obtain these parameters by reanalyzing molecular beam epitaxy (MBE) kinetic data published for the growth of hemispherical grained silicon (HSG). The mathematical model permits extraction of intrinsic diffusivities (that describe site-to-site hopping) and mass transfer diffusivities (that also contain the temperature-dependent coverage of mobile adatoms). The activation energies for both kinds of diffusion are lower than those previously measured for crystalline Si(111) at these temperatures.; In addition, the parameters for self-diffusion on amorphous silicon at processing temperatures (above 635°C) are determined by analyzing our own series of HSG growth experiments using chemical vapor deposition. The results of this study are compared to HSG growth data obtained from MBE growth at temperatures below 590°C. The parameters for intrinsic diffusion for both studies are reasonably close. Conversely, the mass transfer diffusion slows down significantly for T above 635°C during CVD growth. The activation energy increases by 0.2 eV while the pre-exponential factor decreases by a factor of two. The increase in EM for the high T regime is somewhat explainable, especially considering similarities between the diffusion mechanisms on a-Si and c-Si. The differences in the magnitude of the mass transfer diffusion parameters could be attributed in part to changes in the structural properties of the amorphous silicon film and/or hydrogen incorporation during the a-Si deposition.