First-principles Study Of Phase-change Memory Semiconductors And Their Phase Transition Process For Lower Power Consumption

With the advent of the“big data”era,storage and analyis of such a large amount of date challenges the performance of computers and memories.Besides,the rapid advancement of artificial-intelligence?AI?also promotes the development of data storage and processing towarding the universal memory and in-memory calculation.Therefore,the performance of traditional semiconductor memory has become inadequate.The demand for new memory devices with non-volatile,high speed,low power consumption,and high density becomes more and more urgent.For this purpose,many new non-volatile memory devices,such as phase-change memory?PCM?,have been proposed.The PCM storages by the PCM materials.Typically,the PCM materials could reversibly transform between their crystalline and amorphous phases by electric pluses.The two phases have contrasts in resistance or reflectivity,which could provide two logic states of“0”and“1”.The PCM has drawn much attention and been commercialized for its good data retention and high speed.However,compared with other new non-volatile memories,such as resistive random access memory and spin torque transfer random access memory,PCM has a much higher power consumption.Especially,in highly integrated devices,high power consumption will cause many problems.Many efforts have been made to lower the power consumption of PCM.For example,scaling the phase-change area,designing new PCM materials with low power consumption,inducing phase transition by other methods which could avoid melting.In this thesis,we mainly investigate the mechanism of the amorphization process which determines the power consumption by first-principles calculations.Furthermore,we also propose feasible solutions to lower the power consumption of PCM materials.The detailed contents are presented as follows:1.Amorphization and possible applications of the vacancy-ordered cubic Ge₂Sb₂Te₅.We find a new cubic phase of Ge₂Sb₂Te₅ with high-degree vacancy ordering.By first-principles molecular dynamics simulations,we find melting of the new cubic phase starts around the vacancy ordered layer.Besides,Ge atoms diffuse into the vacancy layer which triggers the amorphization process.By controlling the melting time,the vacancy-order cubic Ge₂Sb₂Te₅ exhibits a quasi-two-dimensional amorphization process.And a partial amorphous phase is obtained,which can act as an intermediate state between the pure amorphous and pure crystalline phases for possible ternary-state data storage.Besides,we can realize a low power consumption phase transition process between the partial amorphous and pure crystalline phases.2.Phase change process of the thinnest PCM material:monolayer Sb₂Te₃.Scaling of PCM materials could reduce their power consumption significantly.However,the most widely studied Ge₂Sb₂Te₅ fails to transform back to crystalline phase when its thickness is less than 2nm.Here,we choose the layered Sb₂Te₃ which is easier to crystallize.We can get monolayer Sb₂Te₃ from the layered bulk phase.By first-principles molecular dynamics simulations,we realize the reversible switching between the crystalline and amorphous monolayer Sb₂Te₃.During its crystallization process,a Te-Sb-Te triple layer,which acts as nuclei,will form preferentially and then expand into a quintuple-layered crystalline phase?we call this process as“3+2”crystallization?.In addition,we also find the substrate will affect phase change process of monolayer Sb₂Te₃.For example,the unpassivated substrate will block the crystallization process.Therefore,based on the monolayer Sb₂Te₃,we propose a two dimensional PCM which holds both low power consumption and high storage density.3.Excitation induced ultrafast amorphization process of Sc-Sb-Te PCM material.Sc-Sb-Te is new kind of PCM material.It has drown much attentions for its ultrafast crystallization speed.Because the stable ScTe₆ motifs could act as nucleus which accelerate crystallization.In fact,Sc-Sb-Te also holds a low power consumption for its amorphization process.However,works about its amorphization process are still rare.Here,we investigate the amorphization process of Sc-Sb-Te upon excitation by time-dependent density functional theory molecular dynamic simulations.Upon excitation,elections will selectively occupy the t_2g states of Sc.This induces the bonding structure of the central Sc in ScTe₆ motifs transform into the t_2g bonding configuration.Such transformation also leads to the distortion of the atoms around the motifs,which makes the structure easier disordered and thus lower power consumption.Therefore,the ScTe₆ motifs could not only accelerate crystallization but also favor amorphization process.4.Structure and property evolution of monolayer InSe under external electric fields.Electric fields could lead to dislocation migration in a GeTe nanowire.And finally induce amorphization of the nanowire without melting which usually needs more energy.Generally believed,carrier–lattice coupling plays an important role in a electric field induced amorphization.While,without this coupling,the electric field also has athermal Coulomb force with nuclear and electrons.Here,we want to investigate the role of athermal Coulomb force on phase change process.However,when applying an electric field in the VASP calculation,the calculation model must have a slab or two dimensional structure.Therefore,we take the monolayer chalcogenide InSe as an example and study its structure and property evolution under external electric fields.We find the electric field has little impact on the structure and band gap of monolayer InSe.But the effective mass of hole at the valance band maximum can be decreased by fields.Therefore the athermal Coulomb force only could not lead to a phase change.Besides,electric fields could alter the absorption intensity for gas molecules on monolayer InSe.5.Doping mechanism of the host and guest molecules in organic single-crystalline semiconductors.Doping technology is the key role in controlling properties of the semiconductors.However,works about the basic stacking modes between host and guest molecules in organic single-crystalline semiconductors are still rare.Here,we take pentacene as the guest molecule and the crystal BSB-Me as the host.By first-principles calculation,we find the guset molecule exists in the“perfect”substitutional position inside the host.This is further confirmed by the angle-resolved polarized PL experiments.Furthermore,by changing the doping concentration,we also fabricate high-performance and color-tunable OLEDs.In summary,by first-principles calculations,we investigate the atomic structure evolution and phase transition mechanism of the PCM materials.We also propose feasible solutions for the high power consumption problem of the PCM.Our work offers possible directions and theoretical basis for experimential efforts.