In Situ Electron Microscopy Study Of Electron Beam Induced Phase Transformation Of Phase-Change Materials

The generally used phase change material is Ge2Sb2Te5 nano-film for blue light DVD. So Ge2Sb2Te5 is also used in the universal memory named PCRAM which uses electric pulse to induce phase transition between the amorphous and the crystalline at nanosecond scale, which is functionalized to conduct the data storage by measuring the localized electric resistance difference of the two phases. The Sb2Te3-Six(x =0,0.4,2)show more promising electric properties as a potential phase change material used for the universal memory. Unfortunately, the phase transition mechanisms of these materials keep uncertain and this drags the development and applications of these materials seriously. Clarifying the phase transformation mechanisms of these materials has great significance and impacts for the materials'applications in various fields. This study used transmission electron microscope as a principle means. The phase transition characters of Ge2Sb2Te5 and Sb2Te3-Six were studied by advanced in situ electron microscopy techniques.Using X-ray diffraction and advanced electron microscopy, the crystalline structure of the polycrystalline phases of Ge2Sb2Te5 and Sb2Te3-Six were determined as: (1) the face center cubic structure with the lattice constant of 6.02 and 6.07 angstrom for the metastable phase of Ge2Sb2Te5 and Sb2Te3-Six annealed at 200℃respectively; (2) the body center cubic structure with lattice constant of 4.45 angstrom for the mediate phase of Ge2Sb2Te5 annealed at 200℃; (3) the body center cubic structure with lattice constant of 4.37 angstrom for the mediate phase of Sb2Te3-Six (x =0.4,2)induced by electron beam.The fine procedure of the electron irradiation induced phase transformation of Ge2Sb2Te5 and Sb2Te3-Six was clarified based on the in situ recording the evolution characters of the selection area electron diffraction patterns and its radial intensity profile. The details are as following: (1) when the first and second halo rings were sharpened, the diffraction ring of (105) sites(2.48 ?), (106) sites(2.23 ?) and (110) sites(2.10 ?)of stable trigonal phase at the hexagonal cell emerge gradually but changed into only some more and stranger (110) sites and new (203) sites(1.73?)diffraction spots at last in the crystallization of continuous amorphous films of Ge2Sb22Te5; (2) the diffraction ring of (200) sites(3.04 ?)increased gradually a change to the diffraction ring of (015) sites(3.13 ?)of stable trigonal phase at the hexagonal cell whose discrete diffraction spots of (015) and (1010) sites(2.32 ?)simultaneously emerge and changed into more and stranger in the phase transition of meta-stable of Sb2Te3; (3) the first halo rings(3.12 ? for center) were sharpened to the diffraction ring of (110) sites (3.09 ?)of the body center cubic structure whose diffraction rings of (112) (1.78 ?)and (200) sites (2.19 ?)were split from the second halo rings (1.91 ? for center)simultaneously in the crystallization of continuous amorphous films of Sb2Te3-Si0.4;(4) discrete diffraction spots of (015) and (1010) sites of stable trigonal phase at the hexagonal cell emerge and changed into more and stranger in the phase transition of mediate phase of Sb2Te3-Si2.The electron beam induced phase change which is not achieved until it surpasses a threshold current density was confirmed by that the crystallization appeared is not induced for 46.82 G/cm2 for 25 pA/cm2 but 9.36 G/cm2 with 50 pA/cm2 by a 100 keV electron beam for Sb2Te3-Six. With the easer phase change induced by an higher electron beam not only in crystallization of the amorphous Ge2Sb2Te5和Sb2Te3-Si(xx = 0.4, 2)but also in phase transition Sb2Te3-Si2 induced by a same current density electron beam with 80,100 and 120 keV, the assumption that nether the temperature rise nor second electron induced scission plays a major role is not received since less contribution is gain from the calculation of interaction between the materials and electron beam with higher energy.The available properties of Ge2Sb2Te5和Sb2Te3-Six(x = 0, 0.4)nano-films with the resistances determined by four-point probe and phase change information determined by differential scanning calorimetry was also gained: compared with the nano-films of Sb2Te3,(1) the resistivity of the stable rhombus phase of Sb2Te3-Si0.4 rises from 0.236 to 281m?.cm as a results of the decreased carries consistence from 0.519 to 0.0885 nm-3and the decreased carrier mobility from 37.35 to 2.89 cm2V-1s-1; (2) the difference of resistivity between the amorphous and stable rhombus phase of Sb2Te3-Si0.4 rises from 3 to 4 scales where the resistivity of Sb2Te3-Si0.4 is higher than Ge2Sb2Te5 in amorphous phase; and (3) the crystallization temperature of Sb2Te3-Si0.4 rises from 100 to 200℃while the activity energy of crystallization rises from 2.0 to 2.5eV。...