Preparation And Physical Properties Studies Of Phase Change Material Ge₂Sb₂Te₅ Film And Superlattice

Phase change memory is considered to be the most promising candidate for next-generation non-volatile memory technology.Ge₂Sb₂Te₅ has been considered as the most mature phase change material with wide practical applicaitions.However,there are still some emergency problems available,such as the low crysallline temperature and resistance,slow crystalline rate and unclear pahse transiton mechanism etc.,which greatly hinder the industrialization of phase change memory devices based on Ge₂Sb₂Te₅.In this thesis,Ge₂Sb₂Te₅ was systematically studied and the author devoted his efforts to study the microstructural changes of Ge₂Sb₂Te₅ thin film during crystallization process,the phase change properties of C-doped Ge₂Sb₂Te₅ films as well as the optical properties of Ge₂Sb₂Te₅/Si superlattice films.A series of interesting results have been obtained from the corresponding studies.?1?By combining the use of temperature-dependent resistance measurement,X-ray diffraction,Raman spectroscopy and X-ray reflectivity techniques,the characteristics of microstructure changes of Ge₂Sb₂Te₅ during crystallization are extracted.The presence of the Ge₂Sb₂Te₅ surface/interface layer was proposed based on the X-ray reflectance measurement method.Although the total film thickness is reduced,the thickness of the surface/interface layer after crystallization increases due to the phase transition from the amorphous state to the metastable crystal cubic phase to the stable hexagonal phase.In addition,an increase in the average grain size,density,and surface roughness of GST was confirmed during the thermal annealing.?2?In the study,carbon was used as a doping element to dope GST.Two different experimental samples were used in the study.One was a GSTC film directly doped with carbon,and the other was a GSTC/SiO₂ film with a SiO₂ cap layer added.The role of the SiO₂ cap layer is to inhibit the oxidation of carbon.In the temperature resistance,X-ray diffraction,X-ray photoelectron spectroscopy,Raman spectroscopy,large differences between GSTC and GSTC/SiO₂ films were observed.Studies have found that the oxidation effect of carbon is detrimental to the phase transition of GST.?3?In order to study the optical properties of Ge₂Sb₂Te₅/Si superlattice films,spectral ellipsometry was used.In the amorphous reflectance measurement,it was found that the reflection spectrum can be significantly changed by changing the film thickness,the polarization direction of light,and the incident angle?.Since the Ge atoms in the ultrathin film are more often in the form of tetrahedral coordination,rather than in the presence of vacant octahedral coordination,this leads to the evolution of optical reflectivity.