Modulation Of Structure And Properties Of Chalcogenide Phase-change Materials By Pressure And Stress

The explosive growth of information and data from by the rapid development of science and technology has put forward higher requirements for storage devices.On the one hand,the way of increasing integration density through transistor size scaling cannot be maintained due to the limitation of physical mechanisms such as quantum tunneling effect;On the other hand,in current computer storage system,the distinct of access time and density between DRAM and 3D flash memory are huge,which is difficult to meet the high-bandwidth and high-speed data exchange scenarios.Non-volatile phase-change memory based on chalcogenide phasechange materials(PCM)has caught people’s eyes because of fast operation speed,good scalability,high endurance and well processing compatibility.However,some factors such as the density change during phase-change switching and the multi-layer structure in confined hole may subject the memory cell under pressure or stress.Proper modulation of pressure and stress can improve the performance of the phase-change cell and extend its applications.Moreover,due to the various properties of chalcogenide PCM origins to diverse structures,the pressure or stress,as a “clean” structural regulation tool is playing a key role for understanding the mechanism of chalcogenide PCM.In this paper,the two compressive environments of hydrostatic pressure and biaxial stress are applied,by combinating experiments with firstpeinciples calculations,research work on regulating the structure and properties of traditional PCM Ge2Sb2Te5(GST),new-type PCM Cr2Ge2Te6(CrGT)and Cr2Si2Te6(CrST)using pressure and stress are studied.In the traditional PCM GST,after a general summary of the regulation of hydrostatic pressure on its structure and properties,based on first-principles calculations and ab initio molecular dynamics(AIMD)simulations,the phase transition behaviors of crystal ine and amorphous GST in memory devices under biaxial stress are investigated.It is found when the cubic phase GST under biaxial stress,the increase of Sb and Te antisite hopping leads to the wrong bonds increased greatly.And amorphization process occurs when the compressive strain reachs 10%,and the Pierles distortion decreases under stress.In a-GST formed by introducing different biaxial stresses during melt quenching,the average coordination number of atoms is higher with more wrong bonds.The 5-and 6-membered rings containing wrong bonds increase,and the amount of octahedral-coordinated Ge and Sb atoms decreases.The introduction of biaxial stress affects the crystal ization process of amorphous GST.In particular,it is found that more Sb-Sb homopolar bonds in a-GST induced by the introduction of biaxial stress resulting in the formation of more AAATe rings(A = Sb)with longer atomic migration and slower growth of 4-membered rings during crystal ization,thereby exhibiting a longer practical crystal ization time.In the hydrostatic pressure experiments of CrGT,it is found that the reconstructed structural phase transition occurred at pressures between 16.5 GPa and 18.3 GPa,and mixed with the amorphization process at higher pressures.Based on first-principles calculations and AIMD simulations,it is found that the phase transition pressure is about 17.5 GPa,and the occurrence of the mixed structural phase transition is due to the R-3 phase intralayer Ge atoms flipping into the interlayer induced by pressure.The semiconductor-metal transition of CrGT at low pressure is elucidated based on the resistance change under pressure and electronic structure calculations.Through first-principles calculations,it is found that the flipping of Ge atom to interlayer can easily take place under biaxial stress,which proves the atomic mechanism of structural transition under pressure.Although CrST has the same atom arrangement as CrGT,it exhibits a different structural phase transition path than CrGT in hydrostatic pressure experiments.Based on first-principles calculations,it is found that CrST transforms into a quasi-3D phase with isotropic compression when the pressure reaches 20 GPa due to the elimination of the van der Waals gaps;Based on AIMD simulations and atom bonding analysis,it is found that the amorphization of CrST at pressures greater than 25 GPa is induced by the rotation of intralayer Si-Si atom pairs,which is different from the Ge atom flipping in CrGT.The origin is the strong covalency and stability of Si-Si bonds under pressure,while the metallic-like Ge-Ge bonds are more easily stretched and thus exhibit different atomic motions.The semiconductor-metal transition and spin reorientation of CrST at low pressure are predicted.Under biaxial stress,the stiff Si-Si bonds ensure the stability of the CrST lattice,while a certain degree of compressive strain will lead to the closure of band-gap and trigger the semiconductor-metal transition.