Microstructure And In-Situ Electrical Testing Of Phase-Change Heterostructure(PCH)Memory Materials

With the development of data intensive industries such as artificial intelligence,high requirements are put forward for data storage and processing devices.PCRAM?phase change random access memory?,which uses the electrical resistance contrast between a high-conductive crystalline phase and a low-conductive amorphous phase of the phase-change materials to store data,is expected to be applied in the field of non von Neumann computing due to its good performance.In this computing paradigm,PCRAM not only needs to store data,but also needs to process data and perform some computational tasks,which puts forward higher requirements for its performance.However,the current PCRAM devices have large noise and drift in electrical resistance during phase-change cycles,which is inconvenient to practical application.Recently,a new type of phase change heterostructure?PCH?material has appeared,which is deposited by sputtering TiTe₂?TT?and Sb₂Te₃?ST?targets alternately and is expected to solve this problem.Because of the different rate of amorphization/crystallization,the resistance of the material has multi-level state in the process of phase transition,so that it can be used in neural computing,machine learning et al.PCH memory materials have a series of excellent properties,such as fast phase transition speed,ultra-low noise,long cycle endurance,and the ability to realize multiple resistance intermediate states.However,in order to further improve its performance and ensure its large-scale application,there are still some problems to be solved.For example,the failure mechanism closely related to its cycle endurance,the transition mechanism closely related to its transition speed and low noise,and the reason why it has multiple intermediate states,etc.The exploration of these microstructure problems can not be realized by means of macro testing,but rely on in-situ technology.At present,there is no solution for in-situ observation of specific areas of device cell or thin film materials.Therefore,we have developed a scheme to prepare non-destructive in-situ electrical specimens for PCH devices and thin film materials with complex structures.By using advanced structural characterization methods and in-situ technology,and combining with two-dimensional finite element simulation method,we have conducted in-situ electrical research on PCH memory materials,and explored the evolution process,failure mechanism and cycle of electrical driven microstructure of PCH memory materials and inhomogeneity of phase transition.The specific results and conclusions are as follows:1.We have developed a scheme to prepare non-destructive in-situ electrical specimensfor PCH devices and thin films with complex structures,which can not only be conductive in specific areas,but also be observed at the atomic scale.It provides a guarantee for revealing in situ mechanism of electrical driven phase transition.2.The failure mechanism of PCH memory material is closely related to its cycle enduernce.In view of the structural changes and the dynamic migration of elements in the failure process of PCH memory materials,we applied the DC current with gradually increasing voltage combined advanced characterization tools.In order to explore the time and voltage related thermodynamic transition mechanism in PCH memory material's phase transition process and to provide guidance for the optimization of the next experimental scheme and the experimental parameters,we use the two-dimensional finite element simulation method to analyze the change of PCH device temperature with the time and voltage.The results can explain the reason why it is difficult to realize the RESET operation with DC current.At the same time,through the simulation,we have a further understanding of the change of the time related thermal field distribution and the change of the local temperature,which is of great significance to optimize the in-situ experimental scheme and select the more appropriate experimental parameters.3.Based on the simulation results,we applied electrical pulse with appropriate parameters to realize the electric drive controllable phase transition of PCH memory materials in in situ TEM,including the RESET and SET processes,and observed the structural changes in the dynamic process.The microstructure evolution,failure mechanism,cycle endurance and phase transition heterogeneity of PCH structure are revealed.The above results are helpful to further explore the evolution process of multiple intermediate states,the formation and growth mechanism of crystal nuclei in the process of phase transition,and the changes of the interface between TT layers and ST layers in the process of phase transition.At the same time,it provides possible support for further detailed structure analysis,material optimization and device upgrading.