Assuming that the pre-exponential factor for surface diffusion is given by kBT/h: a) How low must the diffusion activation energy be to give a diffusion length, of roughly 100 nm between successive collisions with impinging vapor, for deposition of Si at 1 um/h and Ts = 400 C on Si (001)? b) How much of a decrease in Ts would correspond to a x10 decrease in the diffusion length?