Study On High-pressure Phase Transformation Behavior Of Novel Sb-Te Binary Alloy Phase Change Memory Materials

Phase change memory?PCM?has the advantages of non-volatile,high-speed readability,long cycle life,lower power consumption,etc.It is currently considered to be the promising next-generation semiconductor memory technique that is most likely to replace mainstream memories such as SRAM,DRAM,FLASH.As the soul of PCM,phase change materials?PCMs?have long been in active research and development.Currently,Ge₂Sb₂Te₅has been widely used,which belongs to the first type of Ge-Sb-Te series alloy PCMs.The second type of Sb-Te binary alloy PCMs are being widely studied.Because of its relatively low crystallization temperature and crystalline resistivity,the long-term data storage ability is poor and the device consumes large power.Therefore,researchers try to improve the properties of materials through doping method,and have achieved preliminary results.Thereinto,the doping of Ti element and Sc element are taken as typical examples.Especially for doping Sc element,the write speed of the phase change memory enters the sub-nanosecond stage,and it is most likely to replace the Ge₂Sb₂Te₅PCMs.As we all know,PCM uses the current pulse to heat materials to switch between amorphous and crystalline states,and show the huge resistance difference to achieve the storage function.As another important thermodynamic parameter besides temperature,pressure can also induce the structural transformation of the material,accompanied by striking changes in physical properties.In this work,we use the magnetron sputtering method to prepare PCMs samples,and adjust the sputtering power to change the atomic ratio in the sample.Amorphous Sc_0.3Sb₂Te₃?SST?is prepared by co-sputtering of pure Sc target and Sb₂Te₃alloy target.Amorphous Ti_0.3Sb₂Te₃?TST?is prepared by co-sputtering of pure Sc target and Sb₂Te₃alloy target.Crystalline Sc_0.2Sb₂Te₃is sputtered by Sc-doped Sb₂Te₃alloy target.This thesis focuses on the in-situ high-pressure synchrotron X-ray diffraction to study the high-pressure phase transition behavior of PCMs mentioned above.The prepared amorphous SST is embedded with FCC nanocrystals.With the increase of pressure,the nanocrystals gradually disappear,“dissolve”in the amorphous matrix that thus forms a more disordered high-density amorphous state?HDA?.This phase transition process is termed as low density amorphous-high density amorphous?LDA-HDA?transition.Further increasing the pressure,HDA will crystallize into a body-centered cubic?BCC?phase.After the pressure is released,the BCC phase returns to the original amorphous phase,indicating that the structural transition of amorphous SST under high pressure is reversible,which is analogous to previously studied GST materials.We also found a similar structural transition process in amorphous TST,but the specific phase transition pressure points are different.This may be due to different doping elements,resulting in different chemical bonding strengths and different vacancy ratios,which thus lead to a little differences in their pressure sensitivity.For FCC-SST,there exists a distinct structural phase change under high pressure.When the pressure is increased to 13.9 GPa,the FCC phase changes to an amorphous state.Continuing to increase the pressure,it will crystallize into an unknown phase I at 23 GPa,and further transforms into a BCC phase at 31.9GPa.Upon unloading,the BCC phase first transformed into the crystalline I phase,and then became an amorphous phase at a lower pressure,and persisted to ambient pressure.During the whole phase transition,the obvious structural hysteresis can also be seen,and synchrotron XRD shows that the structural transition is irreversible.To understand the structural phase transition behavior of the three materials under high pressure,we used in-situ high-pressure Raman and in-situ high-pressure resistance measurements to verify the phase transition process mentioned above.Raman spectrum can reflect the chemical bonding changes of materials under high pressure,and resistance can suggest the changes in electronic transport properties accompanied by structural changes.High-pressure resistance tests show that the resistance of amorphous SST,amorphous TST,and FCC-SST will drop by about five orders of magnitude under pressure.This huge resistance difference meets the requirements for phase-change memory.The high-pressure studies of these three PCMs provide new insights into the phase-change mechanism of amorphous nanocomposites,and also provide more guidance for designing novel phase-change materials.