Electron Microscopy Study And Amorphous Structural Research On Phase Change Material Ge₂Sb₂Te₅

Phase change storage materials of amorphous Ge₂Sb₂Te₅(a-GST) films havebeen investigated by means of bright-field Transmission Electron Microscopy(BF-TEM), Selected-area Electron Diffraction (SAED), High Resolution ElectronMicroscopy (HREM), X-ray Energy Disperse Spectroscopy (EDS), Energy-FilteredTransmission Electron Microscopy (EFTEM) and in-situ Transmission ElectronMicroscopy (in-situ TEM). The methods of radial distribution function (RDF) andreverse Monte Carlo (RMC) have been studied and mastered for the structuralinvestigation on a-GST films. The sputtered a-GST (S-a GST) films were depositedon TEM grid. The method of preparing laser-induced a-GST (L-a GST) was studied.The physical properties of S-a GST films have been investigated. The amorphousstructures of S/L-a GST films have been decoded. Details are as follow:a) The method of electron-diffraction RDF has been studied and mastered.Furthermore, the RMC method was studied and mastered for simulatingamorphous structure. The two methods are powerful for decoding amorphousstructure.b) The S-a GST films were deposited with different sputtering power. It is foundthat the sputtering power can influence the microstructure and crystallizationtemperature. The scale of microstructure increases with the increase ofsputtering power. Temperature dependence of resistance (R-T) measurementsreveal that a change in the crystallization temperature occurs with a increaseof sputtering power. The sputtering power increases, the crystallizationtemperature decreases. Therefore, a low sputtering power is beneficial toimprove the quality of S-a GST films.c) The in-situ annealing process of the GST films at a series of temperatures wasinvestigated using TEM with a heating holder. The RDFs of amorphousphases and crystalline phases were calculated with each annealingtemperature. Combined the characterization of BF-TEM and HREM, it wasfound that the GST films preferentially form uniform nanosized grains, andthis preference is an initial characteristic of crystallization. The third nearestneighbour ordering, indicated by the appearance of a characteristic peak (0.52nm) in the RDF, initializes the crystallization process. The RDF analyses of series of the amorphous phases reveal that there is no significant change in theSRO with the annealing temperature.d) The method of preparing L-a GST was investigated. Through design newtypes of TEM grids and try suitable parameters of pulsed laser, the submicrondots of L-a GST were successfully prepared, which can be used to take SAEDpatterns and decode the amorphous structure with RDF/RMC methods. Thispreparation method is powerful for preparing amorphous sample and TEMsample.e) The amorphous structure of S-a GST was investigated by RDF/RMC methods.It is found that the local atomic configurations of Ge(Ge_xSbyTe_z) andSb(Ge_xTe_z) were connected by wrong bond of Ge-Ge/Sb bond intoTe-shell-like atomic cluster. The local atomic configuration defined the localstructure, and the atomic cluster controls the medium-range order. The spaceamong the stacked atomic clusters formed voids whose boarder e_xists Te-Tebond. These findings improved the understanding of amorphous structure ofS-a GST films.f) The amorphous structure of L-a GST and its comparison with S-a GST werestudied. The reasons of faster phase change of L-a GST than that of S-a GSTwere found. The types of Ge(Ge_xSbyTe_z) and Sb(Ge_xTe_z) local atomicconfigurations decreased, and the number of GeTe_zand SbTe_ztype withoutwrong bond increased. The two types of atomic configurations were atomiccluster of type I which was dominated cluster and e_xceeded in ratio than thatof S-a GST. The ratio of Ge tetrahedron in L-a GST also decreased. Therefore,L-a GST possesses much faster phase change than that of S-a GST.