Mn5Ge3 compound, with its room-temperature ferromagnetism and possibility to epitaxially grow on Ge, acts as a potential spin injector into group-IV semiconductors. It is shown that the realization of Ge/Mn5Ge3 heterostructures is highly hampered by Mn segregation toward the Ge growing surface. The Mn segregation length can be estimated in-situ and in real time by means of reflection high-energy electron diffraction. We present here an approach allowing to greatly reduce or even to prevent the Mn segregation, whose principle is based on filling the Mn5Ge3 lattice with interstitial carbon atoms. In addition, we show that interstitial carbon in Mn5Ge3 allows to enhance not only the Curie temperature of Mn5Ge3Cx layers but also in the whole Ge/Mn5Ge3/Ge heterostructures. (c) 2011 Elsevier B.V. All rights reserved.