Study Of The Phase Change Mechanisms In Ge₂Sb₂Te₅Based Phase-Change Memory

Phase-Change Random Access Memory (PRAM) is regarded as the most promising candidates for the next generation nonvolatile memory which can replace the existing memory technology. The characteristics of Ge2Sb2Te5(GST) are still under groping though it is a developed material. Especially, for GST plays an important role in PRAM, it needs to be optimized to meet the performance requirements. Thus the conduction and the phase change mechanisms of GST need to be clarified, though it has long been controversial. The objective of this study is to explore and clarify these mechanisms, and then to search some methods to improve the characteristics of GST material and the performance of PRAM.This thesis can be mainly divided into three parts, including introduction of PRAM, the first-principles calculations, as well as the foundations of the conduction model.(1) The characteristics of the next generation nonvolatile memory as well as their working principles were introduced. Especially, the development of PRAM, its working principle and research status were also introduced. The mechanisms of leakage current in the dielectric layer were discussed to provide a theoretical basis for the foundation of the conduction model in later chapter.(2) In order to find out the factors that affect material’s conduction characteristics, first-principles calculation was used to obtain the physical properties of the Hexagonal structure GST like the lattice constant, band structure, density of states, effective mass, etc. Feasible suggestions were proposed to improve the characteristics of GST, and the physical mechanism of the rapid phase change of GST was clarified.(3) Based on the different working states of PRAM, a complete conduction model for GST based phase-change memory contains temperature effect was established. According to this model, the conduction mechanisms and the change of I-V slope in subthreshold region were interpreted. The physical mechanism of the formation of S-shaped negative differential conductance was also clarified. The threshold voltage model was established, and useful suggestions to reduce the power consumption of PRAM were given as well.The calculation results contain following:(1) First-principles calculation results showed that the calculated bandgap was smaller than the optical measurement value. Density of states results showed the existence of a large carrier concentration on both sides of the Fermi surface, which would induce large leakage current at ON state. Si doping can be used to increase the resistance. The physical mechanism of the rapid phase change was caused by the slide of the whole Te atomic layer along the [210] orientation, left a vacancy, finally formed the metastable FCC structure.(2) The conduction mechanism in subthreshold region was considered to be hopping conduction. The S-shaped negative differential conductance was formed due to the high electric field induced carrier’s multiplication. The threshold voltage was only related to the thickness of phase change layer, thus the threshold voltage can be decreased by shrinking the cell size of the PRAM or reducing the GST layer thickness, the purpose of lowering the power consumption of PRAM would be achieved.The conclusions of the study can be summarized as:The conduction model proposed in this thesis provides a clarification of the phase change mechanisms in PRAM, and it can meet the requirements of the simulation of nanoscale PRAM such as electrical properties.