First-principles Study On The Transformation Mechanism Of Crystal Ge₂Sb₂Te₅

Semiconductor memory is widely used in modern electronic products with its small size,storage speed.To meet the increasing market demands,improve memory performance and reduce memory power consumption,varieties of new memories have been developed,including Ferroelectric Memory(FeRAM),Magnetic Random Access Memory(MRAM),Resistive Random Access Memory(RRAM)and Phase Change Random Access Memory(PCRAM).It is PCRAM that has attracted seriously concern from social for its unique storage,low power consumption,high number of rewritable,well compatible with CMOS technology.The chalcogenide phase change storage material GeSbTe(GST)has become the main research object of phase change storage material because of its excellent performance and outstanding storage characteristics.In this thesis,the first principles pseudopotential method based on density functional theory and the generalized gradient approximation(GGA)of PBE functional method are applied to simulate and analyze the phase change materials of crystal GST.By doping impurity and vacancy,the influence of defects on the electrical properties of GST phase change materials is introduced,which provides theoretical guidance for the optimization of phase change memory.Intrinsic semiconductors are pure semiconductors that are completely without impurities and lattice defects.However,there are defects in the materials normally,thus the actual semiconductors are not absolutely pure.Generally,there are three kinds of defects in the crystal:doping,intrinsic defects and reverse defects.In this thesis,doping and intrinsic vacancy defects are investigated,it is mainly studied the defect effect of crystal GST doped with Si element and intrinsic Ge vacancy respectively.(1)The effect of doping:it can be divided into the gap doping and interstitial doping by the different doping location.Interstitial doping refers to the doping between atoms in the crystal,and the substitutional doping refers to the doping atoms replace some atoms in some positions of the intrinsic semiconductor material.In this thesis,comparing the formation energy of the crystal structure,to select the stability of the crystal structure and then simulate the density of states,band structure and charge density of the doped structure.Comparing the properties of the doped system with the intrinsic GST system,it can be found that the doped system exhibits more obvious P-type semiconductor properties and reduces the power consumption as well,and improves the ability of maintaining data.When considering the effect of doping on the electrical properties of the whole structure,the concentration and location of the doping are essential factors.Therefore,the specific work is that to further analyze the electronic structure and the effect of covalent bond by the different doping concentration and doping sites.The results show that when the doping concentration maintains at 20%,the maximum doping effect can be obtained with the lowest doping concentration,and the doping position has a slight effect on the electronic structure and the valence bond of the GST system.At the same time,this thesis analyzes the influence of atomic orbits on the density of states.It is found that the effects of p and d orbit are more obvious than s orbit.(2)The effect of vacancy:comparing three intrinsic vacancy defects in the formation of energy of GST structure,the structure of Ge vacancy is the easiest to form.The density of states and band structure of vacancy structure are studied.Comparing the electronic properties of the system with the vacancy concentration of 10%-30%,it can conclude that P-type semiconductor properties is more obvious of the GST system,the conduction band and band gap significantly widened as well.There will be the maximum band gap and the strongest date retention capacity when the vacancy concentration maintains at 20%.At the same time,the electrical properties of the system were not significantly changed by the vacancy simulation in different crystal directions.The effects of Ge vacancy on the orbital of atoms in the structure and the covalent bond between atoms are discussed.The results show that the contribution of the p and d orbits of the vacancy system is much larger than the s orbit,and the vacancy has no obvious effect on the total energy of the covalent bond in the structure.The above results may contribute to improve the electrical properties of PCRAM in theoretical.In this thesis,a series of studies on the defect effects in phase change memory based on crystal GST materials are studied.In Chapter 1,the development background and research status of semiconductor memory and phase change memory are summarized.The second chapter mainly introduces the basis theoretical and the related software.The third chapter describes the defects of semiconductor materials,the doping effect of GST material,the influence of different Si doping concentrations and positions on the electrical properties of the system.The fourth chapter discusses the effect of Ge vacancy on the electronic structure of the system.Finally,the.fifth chapter summarizes and prospects of the thesis.