  We present a theoretical analysis of cooling the spins of lattice nuclei in
semiconductor structures by dynamic polarization with optically oriented
electrons. Our consideration is not restricted to short spin correlation times
of electrons. Therefore, it applies not only to electrons weakly bound to
shallow donors, but also to electrons localized in quantum dots or deep
impurity centers. The main result of the theory is that, in order to polarize
nuclear spins efficiently with strongly localized electrons, one should apply
external magnetic fields by far exceeding the local field of nuclear spin-spin
interaction.
