Theoretical studies of H 2 desorption processes in chemical vapor deposition of boron-doped silicon surfaces

Abstract

The desorption of hydrogen in the chemical vapor deposition (CVD) of boron-doped silicon (100) surfaces is investigated using density functional theory and cluster models. The cluster models examine reactions of hydrogen atoms bound to adjacent BSi surface sites that would arise during the decomposition of B 2H 6 and SiH 4 on a B-doped polysilicon film that is being formed by CVD. Comparisons are made with the analogous processes involving desorption of H 2 from Si dimers, as well as with previous studies in the literature. Calculations show a barrier of approximately 25 kcal/mol for desorption of H 2 from Si 8BH 14 (‘HBSiH’) and Si 8BH 15 (‘HBSiH 2’) clusters. These calculated barriers may be compared to values of (a) 75–85 kcal/mol typically obtained from other cluster calculations for ‘prepairing’ and ‘isomerization pathways’ from Si dimers on the surface; (b) 53–58 kcal/mol for desorption from SiH 2 ‘defects’; and (c) 55–60 kcal/mol from two-dimensional periodic slab calculations of hydrogen desorption from Si surfaces. These results indicate that barriers for desorption of H 2 from BSi-containing surface dimers are lower than desorption of H 2 from Si surface dimers. These lower barriers arise in part because the hydrogen bound to the B surface atom involves four-coordinate B species which have lower bond energies than classical BH or SiH bonds. These lower barriers could play a role in explaining the increased growth rate of Si observed when chemical vapor deposition is carried out using both SiH 4 (or SiCl 2H 2) and B 2H 6 precursors. Finally we examined briefly the role of subsurface B atoms in the desorption of hydrogen from Si surfaces, as occurs in desorption studies on annealed B-doped Si films. Small lowerings in the hydrogen desorption energy (from 54 to 52 kcal/mol) and activation barrier (from 76 to ∼70 kcal/mol) were found due to the effect of subsurface B atoms in Si clusters.