Nanoscale Effects Of Local Motifs And Phase Change Properties Of Amorphous Phase Change Materials

With the continuous development of modern information technology in the field of intelligence,people have put forward higher requirements on the depth and breadth of data storage.They hope that the storage medium can be used to manufacture all kinds of memory with low cost and large capacity,and also shows long-term development potential in the field of display and neuromorphic computing.However,after decades of development,the existing memory seems to have come to an end in terms of storage capacity and manufacture cost,not to mention the potential of new technology applications.Therefore,a new type of storage medium with low cost and high performance is urgently needed.Phase change memory is one of the most promising next generation new memory.It has the advantages of faster switching,scaling property,simplified microelectronic process and easily integrated CMOS process.It has been received widespread attention from the industry,and taken the fastest development on the commercial road.There are not only high-capacity demonstration chips,but they are already used in a small number of commercial devices.In recent years,the application potential of phase change memory（PCM）has been further expanded by multi-level storage and neuromorphic computing.This makes phase change memory a big step towards the goal of“universal memory”in storage system.This paper deals with the relationship between the microscopic local structure and macroscopic phase change properties,as well as the device application of nanoscale phase change memory.Local motifs of nanomaterials,the influences of temperature and interface,and the performance of integrated devices is deeply studied,and they are summarized as follow:Local motifs of nano phase change materials.The preparation and optimization method of nanoscale thin films are introduced.From the microscopic point of view,the material interface is clear and flat,and the phase transition process can be realized smoothly.According to the study of X-ray absorption spectrum,the Ge-Ge bond is relatively common in the nanoscale samples of amorphous GeTe.The Ge-Te bond becomes shorter and the coordination number significantly decreases.This is consistent with the conjecture of a defect Ge₂Te₃ coordination configuration on the local motif.The local structure of this configuration remains intact in the bulk samples,which conforms to the experimental data of bulk GeTe materials.When the thickness is reduced to the nanometer scale,the Ge-Te bond breaks due to the small binding energy,forming a defective coordination structure,and Te atoms leave the lattice point to form a vacancy.Based on the bond angle distribution of the amorphous models formed by ab initio molecular dynamics simulation,it can be found that due to the increase of sp³ hybridization system energy,the Ge-Te bond breaks,thus forming a low-coordination configuration.The aperiodic condition of the nanoscale film destroys the integrity of the Te sublattices,reducing the local order required for phase transition,and thus increases the energy barrier of phase change,which is consistent with the exponential rise of crystallization temperatures of ultrathin films.The effect of temperature on the local structure of nano phase change materials.The method to study the transformation of octahedral and tetrahedral clusters by Raman spectroscopy is presented.As the thickness of phase change materials increases from several nanometers,the octahedron content increases and the tetrahedron one decreases.When the thickness increases to a certain extent,the nanoscale effect weakens and no longer affects the internal local structure.When the annealing temperature is lower than the phase change temperature point,the tetrahedral structures are locally stable,which can inhibit the influence of external temperature and the spontaneous relaxation.There are many elastic bonding clusters in the bulk models and spontaneous structure relaxation is easy to occur.When the annealing temperature is higher than the phase change temperature point,in ultrathin films,a large number of tetrahedral structures are in the excited state,which accelerates the transformation into octahedral structures,showing an accelerated crystallization.The effect of interface on the local structure of nano phase change materials.Oxides represented by SiO₂ will increase the amorphous content and the structural disorder.On the contrary,non-oxides represented by SiC promote crystallization and increase the structural disorder.The both effects are amplified by nano size effect.Metal represented by Ti₃W₇ can significantly accelerate crystallization,but the correlation between such promotion and size is weak.Performance control of nano phase change integrated devices.The horizontal and vertical electrode schemes were designed to suppress the resistance drift caused by nano size effect.The horizontal electrode configuration can reduce the high current of amorphization process and reduce the overall power consumption.The vertical electrode configuration can avoid the tunneling of large current and prolong the life of the device.Through the resistance drift test,it is found that the nanometer size effect inhibits the spontaneous relaxation process of the material,thus inhibiting the resistance drift.The conductance activation energy has a monotonic relation with the drift factor,so it can be used to quickly detect the resistance drift of phase change memory units,as well as the lifetime.According to the influence of dielectric or metal interface on the local motifs,the interface can significantly change the phase change characteristics of materials in nano scale,so it can be used to regulate the crystallization threshold of nano size phase change memory units.