Amorphous Structure Study On Phase Change Material Ge_xTe_1-x

The phase cha nge materia ls have attracted world-wide interest s ince theStanford R. Ovshinsk y found the rapid a nd revers ib le e lectrica l s witc h phe no menain c ha lcoge nide materia ls, whic h can be applied in the modern informatio n me moryfie ld. The princ ip le o f p hase cha nge me mory （PCM） is s imp ly based on theremarkab le contrast o f the optica l/e lectrica l properties between the amorp hous （a-）and crysta lline phase （c-）. However, because of structura l p lura lity and co mp lexity,the structure of the amorpho us for PCM is still a mystery at present. Moreover, thestructure, especia l the amorpho us structure is critica l and important for theunderstand ing o f phase cha nge mecha nis m o f PCM.Here, the amorp hous struc tures of as-deposited Ge_xTe_1-xthin films ha ve beenstud ied by advanced trans miss ion e lectro n microscopy （TEM）, inc lud ing the brightfie ld ima ge, selected area electro n diffractio n, energy d ispers ive X-ray spectroscopyand Electro n Energy-Loss Spectroscopy techniq ue, comb ined with reverseMonte-Carlo s imulatio ns （RMC）.The ato mic struc ture o f stoic hio metric Ge Te thin films was stud ied. RMC mode lrevea led that tetrahedra l and octahedral Ge coexists in the a-GeTe. Tetra hedral Geaccounts about19%of total Ge, with four s hort bonds. The defective octahedral Gewith the3+n conifguration （three short bonds, n long bonds） accounts about81%.The Ge-centered polyhedrons are connected together by Ge-Ge bonds, formingGemTenclusters. Partia l pair d istrib utio n functio ns revea l that these clusters are theorigin of med ium range order in a-GeTe. The clusters rando mly d istrib utio n in thethree-dime ns ion a morpho us network by sharing Te, leaving9%free vo lumes asvo ids whic h is very importa nt in the p hase change process.The ato mic structure o f GexTe1-xthin films was studied. The res ults s how thatthe atomic structure of GexTe1-xthin films was tuned by the amo unt of Ge （x）. Firstly,it can tune the loca l co nfigura tion. As Ge co ntent decreases, the number o f Ge-Gebonds decreases while the Te-Te bonds increase, as well as the percentagetetrahedra l Ge. The percentage o f tetrhedra l Ge increases fro m19%to38%whe n xdecreases fro m0.5to0.3. Second ly, it can tune the a mo unt o f cluster a nd itsconnectio n. Reduc ing Ge conte nt increases the amo unt of first type cluster, butdecreases the fo urth type c luster. As Ge content decreases, the connectio n of c lusterschange fro m only s haring Te ato ms to sharing Te ato ms and/or Te-Te bonds. Thirdly, it ca n tune the d istrib utio n of Te ato ms. Red uc ing Ge content increased the free Teatoms tha t do not bond with Ge atoms. These Te atoms are absent in a-GeTe, anddiscrete ly mixed in a-Ge0.4Te0.6, but severe ly clustered in a-Ge0.3Te0.7. Fina lly, thevo ids also exhib it syste matic cha nge with the variation o f Ge conte nts. The vo idvo lume increases fro m9.0%to17.5%whe n x decreases fro m0.5to0.3. The fractio nof larger vo ids （vo lume larger tha n353） a lso increases with decreas ing Gecontents. All these cha nges are close ly tied to the dra matic c hange o f crysta llizatio nbehavio ur in a-GexTe1-xand ma y guide us to find new PCMs.The p hase-cha nge mec hanis m was d iscussed based on the ho mopo lar bonds andvo ids. In crys ta llizatio n process, the ho mopo lar bonds whic h are absent incrysta lline are broken when the te mperature increases to crysta llizatio n te mperature.In the meantime, the vo ids provide e nough space for the ato mic rearra nge me nt toform order struc ture. In a morphizatio n process, the crysta lline structure is disorderedand ho mopolar bonds are easily formed when the te mperature increase to me lt point.In the ne xt que nching process, the ho mopo lar bonds are preserved and vo ids areformed. The a mount of ho mopolar bonds and voids are very importa nt for phasechange process.