H/D Isotope Exchange Reaction of SiH sub 3 sup + with SiD sub 4 and SiD sub 3 sup + with SiH sub 4: Evidence for Hydride Stripping Reaction.

We have measured the rates and product distributions for SiH sub x D sub (3-x) sup + hydride exchange reactions with SiH sub 4 and SiD sub 4. The measured reaction rates for SiH sub 3 sup + with SiD sub 4 (26.1+-1.0 x 10 sup (-10) cc/molec-s) and for SiD sub 3 sup + with SiH sub 4 (23.1+-1.0 x 10 sup (-10) cc/molec-s) are greater than the calculated Langevin collision rate (12.3-12.4 x 10 sup (-10) cc/molec-s). Also, the product distribution observed for H/D exchange is non-statistical. Dual, competing reaction mechanisms are invoked to account for these observations: reaction via formation of an ion/molecule complex and reaction via long-range hydride stripping. Using an expected product distribution calculated from reaction thermochemistries, the relative contributions of the two mechanisms are obtained for each reaction examined. The reaction rate for the ion/molecule complex mechanism is calculated to equal the Langevin collision rate within experimental error. The reaction rate for the stripping mechanism varies from 1-4 x 10 sup (-10) cc/molec- s (10-30% of the Langevin collision rate) for the mixed isotope ions SiH sub 2 D sup + and SiHD sub 2 sup + to 12-18 x 10 sup (-10) cc/molec-s (100-150% of the Langevin collision rate) for the isotopically pure ions SiH sub 3 sup + and SiD sub 3 sup +. The faster than Langevin reaction rates lower the current estimate for low field mobility of SiH sub 3 sup + in silane plasmas by 70% to 340 cm sup 2-Torr/V-s at 298K and thermal velocities.