First-principles Study Of The Electronic Rule And Storage Process Of Phase-change Memory Semiconductor

With the development of electronic-information technology and the universalization of internet of things,human society steps in the big-data era.A large amount of complex information needs to be recorded,transmitted and processed.So,the development of electronic-memory device,which is the carrier of electronic information,faces a big challenge.The demand for non-volatile memory devices with high density,high speed and low-power consumption becomes more and more urgent.On the other hand,the breakthrough of the artificial-intelligent?AI?technology,which will play a leading role in the technology revolution in the future,draws a lot of attentions.Both the big-data and AI applications bring up a new demand for the hardwares of processor and memory,namely,breaking the Von Neumann bottleneck.A typically non-Von Neumann architecture is the integration of computing and memory,such as in-memory computing which needs a new type of memory devices.So,electronic memory is a key technology to deal with the big-data applications and will play a central role in the development of information technology.As an emerging non-volatile memory technology,the phase-change memory?PCM?is possible to substitute both the dynamic random access memory and flash memory,and thus serve as a universal memory.Moreover,PCM also shows great potentials in non-Von Neumann computing.In short,PCM is a promising candidate for big-data and AI applications in the future.After decades of development,the design and the preparation process of PCM devices are quite mature.Now,the PCM devices show excellent performances and are commercialized already.However,despite the fact that the performances of PCM have met the demand of some applications,there are still some insufficiencies in PCM's performances,such as the high power consumption and the poor data retention at high temperatures.The recording speed,the recording density and the cyclability of PCM have a lot of room for improvement,as well.An important factor that hinders the optimization of devices'performance is the insufficient understanding of the phase-transition mechanism.In this thesis,the microscopic mechanisms for ultrafast phase transitions in PCM materials are investigated by first-principles calculations based on density functional theory and time-dependent density functional theory.The investigations are carried out from two aspects:First,the essential structures and properties of PCM materials;Second,the response of PCM materials to external fields.For the first aspect,the micro structure,electronic bonding,optoelectronic property and the phase-transition dynamics of both the conventional Ge₂Sb₂Te₅ alloy and the new type of GeTe/Sb2Te3 superlattice are studied and compared.For the second aspect,the phase-transition dynamics under the traditional thermal effect,under the thermal+electronic excitation effects and under the thermal+electronic excitation+nonadiabatic electron-phonon coupling effects are studied,respectively.The detailed innovative conclusions are presented as follows:1.The strong electron-polarized structural motifs in amorphous state of the conventional phase-change material Ge₂Sb₂Te₅ alloy.Due to the complexity of the structural and bonding characteristics of amorphous states and the lack of effective research methods,the relationship between micro structures and optoelectronic properties is still not fully understood.A general understanding of the amorphous state is that the electrons are localized.This work reveals that there are many linear atom chains in amorphous Ge₂Sb₂Te₅.About 60%atoms are participated in these atom-chain motifs.Insightful investigations reveal that the electrons in these chains are strong polarized,which updates the understanding of the electronic property of amorphous Ge₂Sb₂Te₅.Moreover,compared with the previous proposed motifs,such as the tetrahedral and pyramidal configurations,the advantage of the atom chain is that it establishes the relationship between micro structure and optoelectronic property.In other words,the property of phase-change materials can be controlled by the change of the amount of atom chains.2.The stacking-fault motions induced insulator-metal transition in the new type of phase-change material GeTe/Sb2Te3 superlattice.The phase-change memory devices based on GeTe/Sb₂Te₃ superlattice show ultralow power consumption.However,its switching mechanism is still unclear with intensive debates.This work studies the possible mechanism of phase change and the accompanying property transition in this material.GeTe/Sb₂Te₃ superlattice is separated into many layers blocks by van der Waals gaps.We find that the break of the local chemical stoichiometry of individual layer blocks without changing the stoichiometry of the whole material can turn the material into a metallic state.According to this fundament,we propose a new way to control the electrical property,i.e.,the change of the local stoichiometry.We also find that the break of the local stoichiometry can be realized by the stacking-fault motions in GeTe/Sb2Te3 superlattice.Then the stacking-fault motions induced insulator-metal transition is demonstrated by further calculations.This transition provides a significant change of electrical property without melting,which can serve as an explanation for the ultralow power consumption.The present investigations reveal a picture of electrical transition in GeTe/Sb₂Te₃ superlattice and may guide us to improve its device performances.3.Ultrafast laser induced mechanical effects in semiconductors.Some experimental and theoretical studies indicated that not only the thermal effect but also the electronic excitation effect play significant roles in ultrafast laser induced phase transitions.However,the influence of the non-thermal effect on material is still not fully understood.By first-principles calculations,this work reveals two general mechanical effects in some semiconductors including phase-change alloy Ge₂Sb₂Te₅:The quantum electronic stress and atomic force.Firstly,the excitation can induce a negative pressure on the lattice which is in proportion to the excitation intensity and effective deformation potential of the conductors.We find that the stress can cause a giant expansion in superhard diamond.Secondly,the excitation induced forces on atoms depend on the element type and locally structural symmetry.In the distorted crystalline Ge₂Sb₂Te₅,electronic excitation induces inhomogeneous local forces on the atoms which causes a simultaneous amorphization.This phase-transition is a solid-to-solid transition which enables faster data writing and better cyclability.This work could be an important step in advancing femtosecond laser techniques for the atomic-level control of structures,rather than relying on traditional melting or ablation approaches which often apply to much larger and non-atomic scales.Moreover,according to these conclusions we propose new possible laser-processing schemes,namely,cool-lattice processing and material-structure-guided processing.4.Ultrafast laser induced ultrafast rhombohedral-to-cubic transition in phase-change material GeTe.Ultrafast laser induced order-to-order transition is rarely seen due to,perhaps,the momentless feature of photos.Compared with the conventional crystalline-to-amorphous transition,the order-to-order transition means a finer way to control material's structure.However,the micro mechanism in the ultrafast phase transition is still under debates.In this work,we use the state-of-the-art excitation-state calculation method,namely,time-dependent density-functional-theory molecular dynamics that contains the nonadiabatic electron-phonon coupling effects,to reveal the mechanism of optical excitation induced rhombohedral-to-cubic transition in GeTe.We find that the transition can be completed in 1 ps with a5%-valence-electron excitation.The excitation induces coherent forces along[001]which may be attributed to the unique energy landscape of Peierls-distorted solids.The forces drive the A_1g optical phonon mode in which Ge and Te move out of phase.Upon damping of the A1g mode,phase transition takes place,which involves no atomic diffusion,defect formation,or the nucleation and growth of the cubic phase.Also,insightful analysis verified that it is really a non-thermal effect induced transition.In summary,by first-principles calculations,we investigated the structure,the property and the phase-transition mechanism of both conventional and new phase-change materials.Moreover,the excitation effects are considered in different levels.The results of these investigations provide information of atomic structure,electronic property and phase-transition dynamics in an ultrashort time scale.According to these information and mechanisms,the performance of phase-change memory may be further improved.