First-principles Study On The Reversible Phase Transformation Process Of Ge-Sb-Te Alloy

It is advancing by leaps and bounds,and it’s not appropriate to use anything to describe the information storage.The current storage technology is facing an unprecedented challenge as the amount of information storage increased.Thenwe need to design new type memory with excellent performance in terms of storage speed,power consumption and integration.PCM because of its excellent performance,was considered as a promising candidate for the next generation of nonvolatile memory candidates with the efforts scientific researchers.The chalcogenide compound represented by Ge Sb Te can quickly and reversibly transform between the crystalline phase and the amorphous phase under certain conditions.At the same time,the two phases have obvious differences in resistance and reflectivity,are widely used for PCM storage medium.Among them,Ge₂Sb₂Te₅ is relatively stable in the phase transformation process and has obvious differences in various properties.Therefore,it is the most studied in GST alloys with different proportions.The liquid phase structures of three GST materials with different chemical ratios were simulated by MD method,and their structural properties and optical properties were analyzed.Then the phase transition rate of liquid phase and amorphous structure was compared,and the crystallization process of liquid Ge₂Sb₂Te₅was simulated and analyzed.First,we obtained（Ge Te）_x（x=1,2,3）/（Sb₂Te₃）liquids by MD simulation in VASP.By compared the MSD of three liquids,we found that Ge₂Sb₂Te₅ liquid has the best consistency.And it was verified that the composition of（Ge Te）promotes the melting process.At the same time,the optical properties of the three structures were analyzed,it was found that the atomic arrangement of Ge₂Sb₂Te₅is the most neat and the structure is the most stable;through the absorption and conductivity,it can be seen that Ge₂Sb₂Te₅ has lower power consumption.Secondly,we obtained the liquid and amorphous Ge₂Sb₂Te₅by controlling the length of melting time,and then compared the disorder degree of the two structures in the same environment,we found that the transition rate of the liquid phase was faster than the latter,which provides new ideas for improving device performance.Finally,we simulated the crystallization process of liquid Ge₂Sb₂Te₅,and calculated properties such as PCF,CDN and bond angle,etc.In the whole process,normal bonds increased and error bonds decreased;CDN of structure was gradually decreased.From the results of electron localization function can be observed that the bonding ability between atoms increased and the system tends to stable.The binding capacity between Ge atoms was greater than that of Sb atoms,indicated that Ge was more active in the crystallization process.The reflectivity tends to increase throughout the process,and the surface of structure was gradually neat and orderly.We had discussed the relevant properties of the liquid during the reversible phase transitions,liquid state was regarded as state between crystalline and amorphous.From perspective of the two phase transition processes,whether it was to obtained the liquid during the amorphization process or the crystallization process of liquid,the time and energy required for its transformation are less than that of the amorphous phase transition.Based on the three states,new directions for improving the storage rate and power consumption of the device was provided.In short,we used MD to simulate the phase transition process of Ge Sb Te compounds,so as to achieve the purpose of studying the microscopic structure,and analyze the trajectory of microscopic particle movement to discuss the mechanism of phase transition.The calculated results can provide research foundation for the experiment,which greatly reduces the cost of experiment.On the basis of the theories,the liquid is potentially powerful solution that improve storage speed and writing,and reduce the power consumption,which points out the research direction for our later work.