VIBRATIONAL DISTRIBUTIONS OF AS2 IN THE CRACKING OF AS4 ON SI(100) AND SI(111)

The desorption dynamics of arsenic from Si (100) and Si (111) are studied by measuring the vibrational population distributions of desorbed AS2 using laser-induced fluorescence. In these measurements a steady state flux of desorbing AS2 is produced by continuously dosing a heated Si surface with a beam of AS4 from a conventional molecular beam epitaxy oven. Measurements of the fluxes of AS2 as a function of surface temperature suggest that the AS2 may be kinetically formed in two distinct steps: The AS4 first chemisorbs to form atoms on the surface which then recombine to desorb as dimers. However, there may also be direct dissociation of AS4 at the hot surface. The vibrational populations of AS2 (v" = 0-3) desorbed from Si(100) are Boltzmann and indicate a vibrational temperature T(v) that is nearly 350 K lower than the surface temperature T(s) (T(v) = 790 +/- 100 K vs T(s) = 1140 K). The vibrational populations of AS2 desorbed from Si (111) are not Boltzmann, but have an effective vibrational temperature 400 K colder than the surface (T(v) = 730 +/- 100 K vs T(s) = 1140 K). The observed lack of accommodation in the desorption from both surfaces is discussed in terms of the possible mechanisms.