Nitrogen doping-induced local structure change in a Cr2Ge2Te6 inverse resistance phase-change material

Fast and reversible switching utilizing the resistance contrast upon phase transition of phase change materials (PCMs) has been widely studied for next generation nonvolatile memory (NVM). Cr2Ge2Te6 (CrGT) and N-doped CrGT (NCrGT) PCMs have been demonstrated to show enhanced memory performance compared to the traditional Ge2Sb2Te5 (GST) PCM. We investigated here the crystallization behavior of Cr2Ge2Te6 (CrGT) and the effect of nitrogen (N) doping on it. We revealed that the Ge- or Cr-centered defective octahedral structure dominated in the amorphous phase of CrGT and the evolution of the c-axis-aligned Ge-Ge dimer and Cr-centered octahedral structure marked the beginning of crystallization. The further formation of new Cr-Te bonds resulted in a dramatic decline of carrier density in the crystalline phase, which explained the inverse resistance change between the amorphous and crystalline phases of CrGT. The N atom can form stronger bonds with Ge or Cr than Cr-Te, restraining the shift of Cr to the center of the octahedron to form the new Cr-Te bonds, resulting in a constant carrier density change during crystallization. The thermal stability and data retention properties also show an improvement by N doping.