Theoretical Investigation On Amorphous Structures Of Phase-change Materials With First-principles Simulations

In the 21 st century,based on the hardware support of high-performance memory,big data technology is changing our daily life.Nowadays,the demand for high-performance memory is increasing,while the traditional memory is facing the performance bottleneck.Obviously,the new memory technology is the mainstream development trend in the future.Among the new memory technologies,non-volatile phase change memory is the most promising one.Because it has speed close to DRAM,and storage capacity calose to three-dimensional flash memory.The performance of phase change memory is mainly determined by phase change materials(PCMs),so it is necessary to develop new materials with higher performance.However,the mechanism of amorphous PCMs has not been thoroughly studied so far.It restricts the efficiency of developing new PCMs.Therefore,in this work the first-principles calculations were used to study the amorphous structures of various PCMs.We revealed the microscopic mechanism and expanded the material genome engineering database,which facilitates the material design.The main content of this paper includes following three parts:Firstly,to reveal the effects of chalcogenide elements on the amorphous structures and properties,five traditional phase change materials(Sb2Te3,Sb2Se3,Sb2S3,Ge15Sb85 and Ge2Sb2Te5)were studied through DFT calculations.We found Se and S could strengthen the chemical bonds and make the local structure more distorted.Thus it improved the thermal stability and changing the optical properties.However,Se and S could generate more void,then increase the density difference between amorphous and crystalline states.Thus the circulation ability was weaken.On this basis,next we investigated Te-free Ge15Sb85 material.According to calculation results,amorphous Ge15Sb85 has less lone pairs and void than Ge2Sb2Te5.Then the local structures of amorphous Ge2Sb2Te5 and Ge15Sb85 were studied,it shows Te-free material has formed more tetrahedra and 5-fold rings.The difference of crystallization mechanism between them was further discussed,to analyze the mechanism of high thermal stability and super-fast crystallization rate of growth-controlled Ge15Sb85.Secondly,to improve the material performance,the doping strategy of PCMs was investigated through DFT calculations.The local structure of amorphous Sb was srudied and crystallization process was simulated.It shows amorphous Sb only defective octahedral structure in the,thus it crystallizes fastly at room temperature.On this basis,covalent elements C,Si and Ge(10%)were doped to Sb,to introduce more tetrahedral clusters.Both experiments and DFT calculatioans were performed to investigate the crystallization process.It confirms that the dopants increase the crystallization temperature but slow down the crystallization speed.According to the classical crystallization theory,the activation energy and driving force were calculated to reveal the mechanism of crystallization behavior.Furthermore,the influences of different metal dopants on the amorphous structure and properties of Sb were studied.On this basis,the properties of Cd,Sc,Y and Zr doped materials were predicted.These metal elements can be divided into three types according to local structure: Al,Zn and Ga tend to form tetrahedral structure,then Cd,In and Sn tend to form octahedral structures,while Sc,Ti,Y and Zr tend to form polyhedral structure,different from tetrahedra and octahedra.Then the ionicity and bonding strength were analyzed by calculating Bader charge and crystal orbital Hamilton population.It confirmed Cd doping might maintain the super-fast crystallization speed of Sb,similar to In and Sn.Meanwhile Sc,Y and Zr might dramatically improve the thermal stability of Sb,similar to Ti.Finally,to guide material design and meet various demands,two unconventional PCMs i.e.Cr Ge Te3 and K2Sb8Se13 were studied through DFT calculations.In amorphous Cr Ge Te3,there forms some stable Cr-Cr bonds and metal-like Cr clusters.This close-packed cluater not only increases the mass density,but also promotes more delocalized carriers.Thus it leads to abnormal density and resistivity changes.Subsequently,the amorphous-amorphous transition of K2Sb8Se13 was studied,it shows the coordination number increased significantly.Meanhwile the short-range cluster structure becomes more regular,and the Perierls-like distortion was weakened.Then the void concentration also decreases,indicating that the highdensity amorphous state is more likely to densed and ordered crystal.Finally the charge density distribution,electron localization function and density of states were calculated to reveal the mechanism of resistivity difference between two amorphous states.It provides guidance for the design of new multi-level PCMs.