Evolution of the local structure surrounding nitrogen atoms upon the amorphous to crystalline phase transition in nitrogen-doped Cr2Ge2Te6 phase-change material

Abstract

Phase-change memory based upon resistive switching due to phase transitions between the amorphous and crystalline phases of chalcogenide-based phase-change materials (PCMs) has been widely studied for the next generation non-volatile memory (NVM). Unlike traditional PCMs such as Ge2Sb2Te5, nitrogen doped Cr2Ge2Te6 (NCrGT) was reported to not exhibit a bulk resistivity change upon the amorphous to crystalline phase transition, instead the contact resistivity contrast in the presence of metal electrodes was found to play an essential role. In the current study, a distinct difference in the local structure around N atoms in amorphous and crystalline NCrGT films was found using X-ray absorption near-edge spectroscopy (XANES) and hard X-ray photoelectron spectroscopy (HAXPES). CrN and GeN bonds and N2 molecular vibrational modes were found to be present in an amorphous NCrGT film, while N atoms were found to be present at Te substitutional positions in the crystalline phase. A previous study revealed that Cr nanoclusters are present in the undoped amorphous phase, whose concentration was found to decrease as crystallization progressed leading to an increase in the resistivity. In the present study, it was determined that N doping inhibits Cr nanocluster formation in the amorphous NCrGT phase and leads to a constant Cr nanocluster volume fraction even after crystallization, ensuring negligible resistance contrast between the amorphous and crystalline phases. These results will prove useful in understanding the role of dopant elements in the promising PCM CrGT.