The formation process of buried K-fulleride layers by high dose implantation of 30 keV K+ at an implant temperature of 300°C has been studied as a function of dose. Raman scattering showed a transformation of the fullerene molecules to amorphous carbon (a-C) within the range of the implanted ions. At the elevated implant temperatures used most of the K diffuses into the depth where a doped layer is formed. The buried K-fulleride layer is stable on air due to a passivation by the a-C. The formation process is discussed on the basis of the K-C60 phase diagram and various thermodynamic processes like segregation, phase formation and diffusion.