Ion implantation of germanium and carbon ions into thin films of Ge 2Sb 2Te 5 (GST) and GeTe was applied to modify the properties of these phase change materials. It was found that it is possible to amorphize crystalline GST and GeTe using ion implantation for optimized ion doses and energies which depend on the film thickness, ion species and capping layer. A relatively low minimum dose is required for complete amorphization as judged by the absence of diffraction peaks in x-ray diffraction (XRD) scans. It is 4-5×10 13 cm -2 for germanium implantation into GST, and slightly higher (10 14 cm -2) for germanium implantation into GeTe. The properties of the re-amorphized films depend on ion species, dose and energy. The re-crystallization temperature of re-amorphized GST by ion implantation is comparable or higher than asdeposited amorphous GST. Carbon implantation in particular leads to a large increase in the crystallization temperature T x. A carbon dose of 10 16 cm -2 implanted into 20 nm amorphous GST yielded a crystallization temperature of 300°C, much higher than the crystallization temperature of 160°C we recorded for as-deposited, amorphous GST. Similarly, high dose carbon implantation into amorphous GeTe leads to large increase in T x. We recorded a shift in T x from 195°C for as-deposited GeTe to 400°C for C-implanted GeTe. Crystalline GeTe re-amorphized by a low dose germanium ion implantation exhibits a re-crystallization temperature below T x of as-deposited amorphous GeTe and T x increased with the implanted Ge dose to a crystallization temperature above that of unimplanted GeTe. Ion implantation can be regarded an additional tool to create phase change materials with different and improved switching properties that cannot be achieved by conventional sputter deposition. © 2011 Materials Research Society.