Understanding atomic structures of amorphous C-doped Ge2Sb2Te5 phase-change memory materials

The atomic structure of thin films of the carbon-doped Ge2Sb2Te5 (GST) rapid phase-change memory material Ge2Sb2Te5C (10% C-GST) was investigated by reverse Monte Carlo refinement using experimental electron diffraction reduced scattering data accompanied by density functional theory (DFT) molecular dynamics (MD) simulations and energy optimizations. For comparison, the structure of amorphous Ge2Sb2Te5C2 (18% C-GST) was obtained by DFT MD simulation of cooling from the melt. The results suggest that the carbon dopant forms atomic scale carbon clusters coordinated predominantly by Ge atoms. This becomes more evident with increasing carbon concentration. For 10% C-GST the building blocks of the matrix can be identified as squares of Ge(Sb)–Te–Sb(Ge)–Te atoms, related to the elementary building blocks of the corresponding crystalline structure of the metastable cubic phase of pure GST. The increased contribution of homopolar Te–Te bonds and Sb(Te)–Te–Sb(Te)–Te square fragments is suggested with the higher dopant level in 18% C-GST.