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

The desorption of hydrogen in the chemical vapor deposition (CVD) of boron- doped silicon (100) surfaces is investigated using density functional theor y and cluster models. The cluster models examine reactions of hydrogen atom s bound to adjacent B-Si surface sites that would arise during the decompos ition of B2H6 and SiH4 on a B-doped polysilicon film that is being formed b y CVD. Comparisons are made with the analogous processes involving desorpti on of H-2 from Si dimers, as well as with previous studies in the literatur e. Calculations show a barrier of approximately 25 kcal/mol for desorption of H-2 from Si8BH14 ('HB -SiH') and Si8BH15 ('HB-SiH2') clusters. These cal culated barriers may be compared to values of(a) 75-85 kcal/mol typically o btained from other cluster calculations for 'prepairing' and 'isomerization pathways' from Si dimers on the surface; (b) 53-58 kcal/mol for desorption from SiH2 'defects'; and (c) 55-60 kcal/mol from two-dimensional periodic slab calculations of hydrogen desorption from Si surfaces. These results in dicate that barriers for desorption of H-2 from B-Si-containing surface dim ers are lower than desorption of H-2 from Si surface dimers. These lower ba rriers arise in part because the hydrogen bound to the B surface atom invol ves 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 th e increased growth rate of Si observed when chemical vapor deposition is ca rried out using both SiH4 (or SiCl2H2) and B2H6 precursors. Finally we exam ined briefly the role of subsurface B atoms in the desorption of hydrogen f rom Si surfaces, as occurs in desorption studies on annealed B-doped Si fil ms. Small lowerings in the hydrogen desorption energy (from 54 to 52 kcal/m ol) and activation barrier (from 76 to similar to 70 kcal/mol) were found d ue to the effect of subsurface B atoms in Si clusters. (C) 1999 Elsevier Sc ience B.V. All rights reserved.