Vacuum-ultraviolet-excited silicon humoepitaxy from Si2H6 has been investigated by in-situ spectroscopic ollipsometry and the crystallinity of the resultant films has been found to depend critically on the photon flux. At photon fluxes greater than 1 x 10(16) s(-1)mm(-2), epitaxial growth proceeds with the surface being covered by a crystalline Si network with avoid precent of 23%. When the Si2H6 gas supply is stopped while irradiation is continued, the photon flux triggers densification and recrystallization of the overlayer. At photon fluxes below 8 x 10(15) s(-1) mm(-2) on the Ether hand, the epitaxy is terminated halfway by the deposition of polycrystalline Si islands embedded in microcrystalline Si. Reflectivity can be used as complementary tool for monitoring the roughness evolution of the deposited film.