Nanoscale Phase Separation and Building Blocks of Ge₂Sb₂Te₅N and Ge₂Sb₂Te₅N₂ Thin Films

Abstract

The atomic structures of thin films of nitrogen-doped Ge2Sb2Te5 (GST) rapid phase-change memory materials with the general compositions Ge2Sb2Te5N (10% N-GST) and Ge2Sb2Te5N2 (18% N-GST) have been investigated by reverse Monte Carlo refinement based on experimental electron diffraction reduced density functions and density functional theory molecular dynamics simulations. It was found that the nitrogen dopant forms predominantly Ge−N bonds, resulting from nanoscale germanium nitride phase separation during cooling. As in the case of pure GST, the main building blocks of the structure are squares of Ge(Sb)−Te−Sb(Ge)−Te atoms with the contribution from rings containing homopolar Sb−Sb, Te−Te, and Ge−Sb bonds increasing with the nitrogen doping level. These squares are related to the elementary building blocks of the corresponding crystalline structures of the metastable cubic phase of GST. The persistent fragments of the germanium nitride phase are azadigermiridine type N−Ge−Ge−N four-membered rings, with central Ge−Ge bonds. There is an indication from theoretical simulations that two amorphous phases may be present in 18% N-GST, with different bonding types.