Study On Optical Properties And Molecular Dynamics Of Ge₂Sb₂Te₅ Materials Via A First Principles

Semiconductor memory is the primary tool for modern information storage.According to the characteristics of the information storage,the semiconductor memory can be divided into a volatile memory and a nonvolatile memory.The nonvolatile memory is further divided into Ferroelectric Memory(FeRAM),Magnetic Random Access Memory(MRAM),Resistive Random Access Memory(RRAM)and Phase Change Random Access Memory(PCRAM).Phase change random access memory is considered as the next generation of the most promising memory due to its remarkable characteristics.The sulfur-based compound material GeSbTe is the most popular research material due to its potential application as record media in phase change memory.In this paper,based on the first-principles density functional theory,the molecular dynamics and optical properties of phase-change GeSbTe materials are simulated.The Si was doped into the supercell Ge2Sb2Te5(GST)structure by substitutional doping based on above structure,the molecular dynamics,thermodynamic stability and optical properties of the defective GST material were studied,which laid a theoretical foundation for further study of phase change memory.The research idea of this paper is that the molecular dynamics stability of phase change materials with different Ge-Sb-Te stoichiometry was firstly calculated.In the following,based on analysis the thermodynamic stability,the results show that the structure with chemical ratio of 2:2:5 has the best thermodynamic stability.The molecular dynamics and thermal stability of the Ge2Sb2Te5 structure with this stoichiometric ratio were discussed.In general,the conductive properties of intrinsic semiconductor materials are not ideal.We calculate the electrochemical properties of Si,which is the theoretical basis for doping Si atoms to improve the electrical conductivity of GST structures.In order to further explore the influence of the doping concentration of Si atoms on the structure and related properties of GST,we simulated the mean square displacement(MSD)of the GST structure at different Si doping concentrations.The results can be concluded that the doping of Si atoms can improve the thermal stability of the GST structure.Furthermore,by studying the partial correlation function(PCF),it can show that the doping of Si has an effect on the Ge-Te and Sb-Te bond lengths of the structure.Finally,by integrating the data of the PCF,the coordination number of each atom can be obtained.The coordination number information reflects that the Si-doped GST structure maintains a small amount of crystalline state at temperatures up to 1100k.Finally,the degree of disorder of GST materials with different Si doping concentrations is calculated,which proves that the GST structure is the most stable at 20%Si-doping concentration.The results show that the structure with the concentration of Si at 20%is the most stable,and the MSD in the Molecular dynamics and the degree of disorder are the lowest.The structure and optical properties of Si-doped GST materials were investigated by first-principles theory,and the optical properties of pure GST structure and the GST structure with 20%Si-doped concentrations were calculated.The optical properties of the main research include elastic modulus,complex dielectric function,absorptivity,reflectivity and conductivity.Firstly,the application of six elastic constants demonstrates that the Si-doped GST system still satisfies the physical and mechanical stability,which indicates that the structure can be stably existed in nature.Secondly,the influence of doping Si on the structure and optical properties of GST materials is discussed.The dielectric function shows that doping Si will result in a faster response speed for GST devices.At the same time,the Si-doped 20%GST structure is slightly larger than the intrinsic GST structure in the amorphous state.This indicates that the lattice arrangement order of the GST structure doped with Si is more aligned than the amorphous intrinsic GST structure.This makes the phase change time of the memory shorter,which means that the efficiency of reading and writing data is greatly improved.Finally,the conductivity of the materials was explored.It is obvious from the conductivity curve that the conductivity is significantly improved.It means that the conductivity of the GST structure is enhanced,and the macroscopic performance is that the memory has a lower turn-on voltage.In summary,it can be concluded that the GST material with Si doping concentration of 20%not only has good thermodynamic stability,but also its optical properties are improved.This is beneficial to the improved conductivity of GST materials,which makes it better suited for the semiconductor industry.The above conclusions can provide a theoretical basis for the application of GST materials in optical storage devices.