Dynamic evolution of thermally induced element distribution in nitrogen modified phase change materials

In the semiconductor industry, doping modification is a common and effective method to regulate the properties of materials. For Ge2Sb2Te5 (GST) phase change materials, incorporation with nitrogen has been widely reported due to improvement in its thermal stability and reduction in its power consumption. However, whether in amorphous or crystalline structures, the distribution of N is still unclear since it is low in content and insensitive in the electron microscopy detection. Here, by employing advanced atom probe tomography technology, we directly reveal the three-dimensional element distribution in different N-doped GST (NGST) structures and systematically analyze the evolution of element distribution during phase change. Nitrogen is found to be homogenously distributed in the as-deposit amorphous NGST as nanoscale N-rich clusters most together with germanium. The N-rich clusters remains nearly the same during the phase transition from amorphous to rocksalt phase, but change in the cluster become increasingly significant as the heating temperature continues rising, where GeN begins to precipitate at the grain boundaries. Furthermore, it is found that precipitated GeN clusters are turning into more Ge-rich in the two step phase transitions of NGST, e.g., Ge/N ratio equals 1.2 in rocksalt structure and subsequently becomes 21 in the hexagonal phase.