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.

Towards quantitative treatment of electron pair distribution function

“…A mechanism for fast switching between the amorphous and crystalline phases of Ge-Sb-Te through plane rotations, similar to those in Rubik's cube, was proposed based on ePDF analysis of the amorphous structure (Borisenko et al, 2009b). The analysis of ePDFs of nitrogendoped Ge-Sb-Te glasses showed that nitrogen predominantly bonds to germanium, and that the increase of the nitrogen content also increases contribution of rings with homopolar bonds (Borisenko et al, 2009a). Carbon-doped Ge-Sb-Te glasses demonstrate the formation of atomic scale carbon clusters coordinated by germanium atoms (Borisenko et al, 2011).…”

Microscopic Mechanism of Doping‐Induced Kinetically Constrained Crystallization in Phase‐Change Materials

“…Therefore, the atomistic origins responsible for the altered crystallization kinetics upon N doping remain unanswered, in spite of its scientific, and practical, importance. In this report, we show how a minor level of N doping in GST can have a decisive effect on the crystal nucleation‐and‐growth processes from AIMD simulations with model sizes larger than those of previous doped GST models (see Supporting Information). A microscopic mechanism governing the crystallization kinetics in NGST is provided from the investigation of various aspects of doping effects on an atomic scale, from which most of the experimental observations reported so far can be comprehensively understood, presumably with implications for the physics of crystallization in other doped glasses.…”

Nitrogen-Doping Effect on Ge2Sb2Te5 Chalcogenide Alloy Films during Annealing

“…Notably, nitrogen has been used to increase the crystallization temperature and to obtain a stable transition between the cubic crystalline state and the amorphous state. [7][8][9][10][11][12][13][14] Privitera et al proposed two models to explain the increase in the crystallization temperature. 9) According to the first model, some of the chemical bonds are expected to be transformed into more covalent bonds, because nitrides are more covalent than Ge 2 Sb 2 Te 5 alloys.…”