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

Abstract

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.