  We study the surface dynamics of silica films grown by low pressure chemical
vapor deposition. Atomic force microscopy measurements show that the surface
reaches a scale invariant stationary state compatible with the
Kardar-Parisi-Zhang (KPZ) equation in three dimensions. At intermediate times
the surface undergoes an unstable transient due to shadowing effects. By
varying growth conditions and using spectroscopic techniques, we determine the
physical origin of KPZ scaling to be a low value of the surface sticking
probability, related to the surface concentration of reactive groups. We
propose a stochastic equation that describes the qualitative behavior of our
experimental system.
