It is demonstrated that persistent hole burning in the zero-phonon line of Eu2+ in SLs of CaF2:Eu/CdF2 with uniformly doped CaF2 layers is mainly due to tunneling of electrons from the 4f65d excited state of Eu2 to the conduction band of adjacent CdF2 layer. However in selectively doped suoerlattices (SLs), the two-step photoionization process dominates in the formation of spectral holes. Two mechanisms of spectral hole broadening were discussed. These are the Stark-shifting of the Eu2 f-d transition energy induced by internal electric field and phonon assisted transitions.