R&D: Evolution of Local Structure Surrounding Nitrogen Atoms Upon Amorphous to Crystalline Phase Transition in Nitrogen-Doped Cr2Ge2Te6 Phase-Change Material

Applied Surface Science has published an article written by Yi Shuan, Department of Materials Science, School of Engineering, Tohoku University, 6-6-11, Aobayama, Aoba-ku, Sendai 980-8579, Japan, Shogo Hatayama, Department of Materials Science, School of Engineering, Tohoku University, 6-6-11, Aobayama, Aoba-ku, Sendai 980-8579, Japan, and Device Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, Higashi 1-1-1, Tsukuba 305-8565, Japan, Yuta Saito, Device Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, Higashi 1-1-1, Tsukuba 305-8565, Japan, Paul Fons, Device Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, Higashi 1-1-1, Tsukuba 305-8565, Japan, and Department of Electronics and Electrical Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan, Alexander V.Kolobov, Device Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, Higashi 1-1-1, Tsukuba 305-8565, Japan, and Department of Physical Electronics, Faculty of Physics, Herzen State Pedagogical University of Russia, 48 Moika Embankment, St Petersburg 191186, Russia, Daisuke Ando, and Yuji Sutou, Department of Materials Science, School of Engineering, Tohoku University, 6-6-11, Aobayama, Aoba-ku, Sendai 980-8579, Japan.

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). Cr-N and Ge-N 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.“