First-principles Study Of Sn-Ge₂Sb₂Te₅ Ultra-fast Phase Transition Induced By Gaussian Picosecond Laser

With the advent of the“big data”era,massive data storage and analysis pose challenges to computer performance.The rapid advancement in the field of artificial intelligence has also promoted the development of data storage and processing in the direction of“integration of storage and computing.”Under this development trend,traditional semiconductor memory is incapable of current demand.It is urgently needed to find a new type of memory that has the advantages of non-volatility,fast speed,low energy consumption,and high integration.Under this demand,a variety of new non-volatile semiconductor memories represented by phase change memories have emerged.In addition,the outstanding potential of phase change memory in the field of non-von Neumann computing is considered to be a strong competitor in the field of"big data"and artificial intelligence in the future.The performance of phase change memory essentially depends on the nature of the material.The prototype material commonly used in phase change memory is Ge₂Sb₂Te₅(GST).GST can achieve rapid reversible transformation of crystalline and amorphous states under the irradiation of laser.Although it can meet some application requirements,it still has shortcomings,especially for the crystallization speed.Sn doping is an effective method to improve material properties,but the microstructure,bonding characteristics,and interaction with laser after doping remain to be studied.On the other hand,although GST can achieve gradual crystallization under multiple optical/electrical pulses and is potentially applied to neuromorphic computing in the direction of“integration of storage and computing”.However,its crystallization kinetics mechanism under complex pulses is still unclear.Therefore,in order to solve the above problems,experiments and first-principles simulation calculation methods are used to conduct research from two aspects:the structural properties of the material and the response of the material to the light field.In terms of the structural properties of the material,the local atomic structure,bonding characteristics and crystallization kinetics of Sn and Ge in the amorphous state were studied and compared.In terms of material response to the field.The phase transition mechanism of the material induced by single-pulse and multi-pulse lasers is successively studied.It focused on the evolution trend of the material microstructure under the ultra-high heating/cooling rate.Four innovative results are obtained.1.By using a single pulse to induce the crystallization of phase change materials,the doping of Sn does not change the behavior of GST crystallization,and its crystallization behavior is still solid phase transition under low laser fluence and liquid-solid phase transition crystallization under high laser fluence.However,Sn can effectively reduce the crystallization threshold,melting threshold and ablation threshold of the material,and increase the nucleation rate to refine the crystal grains.In addition,the doping of Sn with a larger atomic radius will expand the lattice constant of the material,thereby introducing mechanical stress.After coupling with the thermal stress introduced by the ultrafast laser,some distortion will appear in the Sn-doped crystalline material.When Sn is doped 10%,small-angle grain boundaries and local atomic shear appear in the crystal.When doped 30%,there are disorderly arranged Ge atoms that are replaced by Sn into the crystal lattice.2.In the amorphous state,the coordination number of Sn atoms is higher than that of Ge atoms.As the content of Sn increases,the proportion of"wrong bonds"for Sn increases while the proportion of"wrong bonds"for Ge decreases.This trend improve the ratio of Ge-Te and contribution of p orbital to the valence band of Ge,which reduces the sp~3hybrid structure of Ge atoms.In addition,although both are the fourth main group elements,the bonding character of Sn and Ge with the surrounding atoms are significantly different.The electrons around Ge atoms have a high degree of locality.Even in the low coordination number of Ge,there are lone pairs of electrons.When Ge bonds with surrounding atoms,the ELF is higher than 0.58,which is a significant covalent bond bonding character.The electrons around Sn are significantly delocalized,making the bonding character between Sn and the nearest neighbor atom similar to a metal bond,namely the electron-sharing bonding method.3.The crystallization process of materials is a process in which electrons tend to delocalize.Sn doping can significantly reduce the energy fluctuation and structure fluctuation during the incubation period of the material,and increase the crystallization speed.This is mainly due to the strong ionicity of the Sn-Te bond formed in the amorphous state after Sn doping.both the bonding strength and the bonding character are similar to the crystalline Sn-Te.Therefore,the Te atoms bonded with Sn have little fluctuation during the crystallization process,and the equilibrium position can be found fast.4.When the phase change material is gradually crystallized by the multi-pulse picosecond laser,there is also an incubation period similar to the isothermal annealing.During the incubation period,the number of 4-fold rings fluctuates more obviously.After the incubation period,the number of 4-fold rings will rise steadily until the material crystallization.After that,the number of 4-fold rings will fluctuate in a higher range of values.During the crystallization process,Sn fluctuates less than Ge so that conducive to crystallization by doping.The ultra-high heating/cooling rate has a significant effect on the crystallization kinetics of the material.During the heating process,the number of 4-fold rings in the system gradually decreases,and the total energy in the system gradually increases,which is not conducive to the crystallization of the material.In the cooling process,the number of 4-fold rings gradually increases as the total energy of the system decreases.However,when the cooling rate is42.8K/ps,even if the energy in the system decreases,the number of square rings also decreases.In addition,the reflectivity of the material will gradually increase as the pulse number increases,and there is a threshold effect.This is closely related to the number of linear atomic chains inside the material.