Properties Of Interface Between Amorphous Si And Amorphous Sb₂Te₃for Phase-change Memory Application: Insights From First-principles Study

Phase-change memory is a new kind of no-volatile semiconductor memory, it usesphase-change materials to store information. Under certain external conditions,phase-change materials can undergo fast and reversible transitions between theamorphous and crystalline phases, while a significant electrical/optical contrast stillexists. Ge2Sb2Te5is generally studied for its good performance, but Ge2Sb2Te5is poorin amorphous phase stability and has relative high power consumption. In order toovercome these problems, many new kinds of phase-change material componentshave been developed, one of them is Si-Sb-Te materials.The SixSb2Te3with3<x<3.5has been proved to be the optimal compositions forSi-Sb-Te materials. Sb2Te3is very poor in amorphous phase stability, by dopingSb2Te3with Si, SixSb2Te3shows good amorphous phase stability. Phase separationphenomenon occurs and Si exists in amorphous state during switch progress,amorphous Si and Sb2Te3are enwrapped each other in form of reticular structure. Lotsof interfaces between amorphous Si and Sb2Te3can be found in the reticular structure.Generally speaking, interface always shows different properties from bulk material. Inour opinion, amorphous Si and Sb2Te3interface will play an important role inenhancing amorphous phase stability of Sb2Te3. In this paper, we obtained interfacestructure of amorphous Si and amorphous Sb2Te3by first-principles moleculardynamics simulation, deep insight into the mechanism of Si enhance Sb2Te3amorphous phase stability is illustrated by analyzing their interface properties in termsof atomic and electronic structures.There are four sections in this thesis. The first part presents the background ofthe phase-change memory and the phase-change material, and also the researchprogress for them.The second part presents our theoretical basics on first-principles calculations,density function theory and molecular dynamics simulation.In section three, we obtained interface structure of amorphous Si and amorphousSb2Te3by first-principles molecular dynamics simulation. Both bond geometry andelectronic structure analyses demonstrate that the local environment of Te in Sb2Te3has been evidently modulated with a-Si through their interfaces. Te bond structurechanges from p-bonding to the case of coexistence of sp3hybrid bonding p-bondingcharacteristic. At the same time, electrons in Sb2Te3also become more localized. Bothof these two changes not only destroy the similarity of bond structure betweenamorphous and crystalline phase of Sb2Te3, but also make it difficult to form resonantbonding in crystalline Sb2Te3. This is the mechanism of Si enhance Sb2Te3amorphousphase stability.In section four, we focus on phase-change characteristic of the three phase-change materials: Ge15Sb85, Sb2Te and Sb2Te3. In amorphous Ge15Sb85, we findthat Ge atoms tend to get together, and that’s why phase separation will easily occurin this kind of phase change material. The final structure of Sb2Te and Sb2Te3aftercrystallization shows ordered in structure but disordered in chemical composition.This particular structure may be really exist in phase-change process, and may also beone of the reasons why rapid switch can occur in phase-change materials.