The motivation of this work is the tailored growth of Ge nanocrystals for photovoltaic applications. The use of superlattices provides a reliable method to control the Ge nanocrystal size after phase separation. In this paper, we report on the deposition of ( GeO _x – SiO ₂) superlattices via reactive dc magnetron sputtering and the self-ordered Ge nanocrystal formation during subsequent annealing. Attention is directed mainly to define proper deposition conditions for tuning the GeO _x composition between elemental Ge ( x =0) and GeO ₂ ( x =2) by the variation in the deposition temperature and the oxygen partial pressure. A convenient process window has been found which allows sequential GeO _x – SiO ₂ deposition without changing the oxygen partial pressure during deposition. The phase separation and Ge nanocrystal formation after subsequent annealing were investigated with in situ x-ray scattering, Raman spectroscopy, and electron microscopy. By these methods the existence of 2–5 nm Ge nanocrystals at annealing temperatures of 600–750 ° C has been confirmed which is within the superlattice stability range. The technique used allows the fabrication of superlattice stacks with very smooth interfaces ( roughness ≪1 nm ) ; thus the Ge nanocrystal layers could be separated by very thin SiO ₂ films ( d ≪3 nm ) which offers interesting possibilities for charge transport via direct tunneling.