Study On Ti Gradient Doped Sb₂Te₃ Phase Change Memory Materials And Devices

Phase change memory is a new type of nonvolatile memory with great development potential.The phase change memory based on Sb₂Te₃ has very fast SET speed,but the thermal stability of the device is poor.Doping Ti in Sb₂Te₃ can significantly enhance its amorphous stability,but it will inevitably slow down the SET speed of the corresponding memory cell.To solve this problem,Ti gradient doped Sb₂Te₃ phase change memory cells were prepared by magnetron sputtering from the point of view of material design.This gradient doping method compensates for the effect of Ti doping on the crystallization speed of Sb₂Te₃,so that Ti gradient doped Sb₂Te₃ phase change memory cell has both good thermal stability and fast SET speed.The formation energy of Ti at different doping sites in Sb₂Te₃ and the distribution of electron localization function around Ti atom are calculated by VASP software.The results show that the most reasonable doping site of Ti is to replace the position of Sb atom and form a strong ionic bond with Te atom（Ti-Te）.Due to the stronger chemical bonding,Ti atom tends to combine with the surrounding atoms to form stable local clusters,On the one hand,the stability of amorphous Ti-Sb₂Te₃ is enhanced,but on the other hand,the stable Ti-Te bond will reduce the movement ability of atoms and reduce the growth rate in the crystallization process of Ti-Sb₂Te₃.Several groups of Sb₂Te₃ films with different Ti doping concentrations were prepared and characterized by XRD、R-T and XPS.The experimental results show that with the increase of Ti doping concentration,the amorphous stability of film gradually increases,And the lattice mismatch between Ti-Sb₂Te₃ and pure Sb₂Te₃ is very small,that is,pure Sb₂Te₃film can crystallize first and then epitaxially induce the crystallization of Ti-Sb₂Te₃ film.Ti gradient doped Sb₂Te₃ phase change memory cells were prepared and compared with pure Sb₂Te₃ phase change memory cells and uniformly Ti doped Sb₂Te₃ phase change memory cells.The electrical properties of the above phase change memory cells were tested.The results show that Ti gradient doped Sb₂Te₃ phase change memory cells can have both good thermal stability and fast SET speed.Through the microscopic analysis of Ti gradient doped Sb₂Te₃phase change memory cell by TEM,the results show that in Ti gradient doped Sb₂Te₃ phase change memory cell,the pure Sb₂Te₃ layer near the bottom electrode can first crystallize under the stimulation of electric pulse and thus provide crystal template for adjacent Ti-Sb₂Te₃guiding its crystallization,Then the Ti-Sb₂Te₃ film with high doping concentration is crystallized.Compared with uniformly Ti doped Sb₂Te₃ devices,the above gradient Ti doped Sb₂Te₃ devices no longer need the process of Ti-Sb₂Te₃ film forming crystal nucleus first and then gradually crystallizing,so as to improve the overall crystallization speed.