相变存储材料Ge_xTe_(1-x)非晶结构的解析
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摘要
自从S. R. Ovshinsky在硫系薄膜中发现有序-无序排列的记忆效应后，相变材料就引起了世界各国的广泛重视。随后，人们利用相变材料的这个特性发展出相变存储器，它是依靠相变材料在晶态和非晶态之间快速相互转化时所表现出来的导电性(光反射率)差异来存储数据。由于其优良的性能，相变存储器被认为是最具有发展潜力的下一代非挥发性存储设备。然而，人们目前对于相变存储材料的理解，尤其是从原子层次上理解相变机制，还不够深入。这严重阻碍了新材料的发展与应用。
结构问题，尤其是非晶结构，是深入理解相变机制的前提和关键。然而，由于非晶结构的多样性和复杂性，当前对于非晶结构的研究还存在着许多问题。本论文综合利用透射电子显微学分析方法(明场像、选区电子衍射、X射线能量色散谱和电子能量损失谱等)，结合逆蒙特卡洛模拟，对二元相变存储材料Ge_xTe_(1-x)非晶体系的原子结构，结构随成分的演化，以及相变机制进行了系统的研究，具体内容如下：
首先，研究了剂量比GeTe薄膜的非晶结构。短程序：研究表明四面体Ge和八面体Ge构型共同存在于非晶结构中。其中19%的Ge原子采取四面体构型，其构型中的四个键基本上都是短键；而其余的Ge原子为八面体构型(缺陷八面体)，可以用3+n来表示，其中包括3个短键和n个长键。中程序：研究表明Ge-(GekTen)配位多面体通过Ge-Ge同极键形成以Ge原子为核心，Te原子为壳层的“团簇”结构。偏对分布函数表明，这些“团簇”结构正是GeTe非晶结构的中程序起源。它们彼此之间通过共享Te原子连接形成非晶网络，并且留下了大量的(9.0%)“空位”，这些“空位”在相变过程中起了非常重要的作用。
然后，研究了非剂量比GexTe1-x薄膜的非晶结构。研究表明Ge原子含量对非晶结构具有很强的调制作用，这些结构的变化直接影响到材料的结晶行为，对我们寻找合适的相变材料有重要的指导意义。1、它能够调制非晶的局部构型。随着Ge原子含量的减少，Ge-Ge同极键越来越少，而Te-Te同极键越来越多，且非晶中四面体Ge构型越来越多。当Ge原子含量从50%减少到30%时，四面体Ge构型的比例从19%增加到38%。2、它能够调制非晶的中程序结构，即“团簇”结构。随着Ge原子含量的减少，各类“团簇”的比例和连接方式都发生了变化，即第一类“团簇”越来越多，第四类“团簇”越来越少，而其它两类“团簇”变化相对较小。它们之间的连接方式也逐渐从单纯的共享外层Te原子连接变成共享外层Te原子和/或Te-Te键连接。3、它能够调制Te原子的分布。随着Ge原子含量的减少，只与Te原子成键的Te原子越来越多，且当Ge原子含量从40%减少到30%时，它们从弥散分布变成扎堆聚集分布，而剂量比GeTe没有这种Te原子。4、它能够调制“空位”的大小和体积。随着Ge原子含量的减少，“总的空位体积”和“大空位(体积大于353)”的数量相应增加，当Ge原子含量从50%减少到30%时，“总的空位体积”从9.0%增加到17.5%。
最后，基于非晶态和晶态的结构比较，我们探讨了以“同极键”和“空位”为主导的相变机制。结晶过程：当相变材料被加热到结晶温度时，非晶中的“同极键”被打断，同时，“空位”给它们的重新排列提供了活动空间。非晶化过程：当相变材料被加热到熔化状态时，晶体的长程有序结构被打乱，同时产生很多“同极键”；在紧接下来的淬火过程中，这些“同极键”被保存下来，并产生一定数量的“空位”。从这个机制我们可以发现，非晶结构中“同极键”和“空位”的数量直接影响它的相变过程。
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-x)thin 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.
结构问题，尤其是非晶结构，是深入理解相变机制的前提和关键。然而，由于非晶结构的多样性和复杂性，当前对于非晶结构的研究还存在着许多问题。本论文综合利用透射电子显微学分析方法(明场像、选区电子衍射、X射线能量色散谱和电子能量损失谱等)，结合逆蒙特卡洛模拟，对二元相变存储材料Ge_xTe_(1-x)非晶体系的原子结构，结构随成分的演化，以及相变机制进行了系统的研究，具体内容如下：
首先，研究了剂量比GeTe薄膜的非晶结构。短程序：研究表明四面体Ge和八面体Ge构型共同存在于非晶结构中。其中19%的Ge原子采取四面体构型，其构型中的四个键基本上都是短键；而其余的Ge原子为八面体构型(缺陷八面体)，可以用3+n来表示，其中包括3个短键和n个长键。中程序：研究表明Ge-(GekTen)配位多面体通过Ge-Ge同极键形成以Ge原子为核心，Te原子为壳层的“团簇”结构。偏对分布函数表明，这些“团簇”结构正是GeTe非晶结构的中程序起源。它们彼此之间通过共享Te原子连接形成非晶网络，并且留下了大量的(9.0%)“空位”，这些“空位”在相变过程中起了非常重要的作用。
然后，研究了非剂量比GexTe1-x薄膜的非晶结构。研究表明Ge原子含量对非晶结构具有很强的调制作用，这些结构的变化直接影响到材料的结晶行为，对我们寻找合适的相变材料有重要的指导意义。1、它能够调制非晶的局部构型。随着Ge原子含量的减少，Ge-Ge同极键越来越少，而Te-Te同极键越来越多，且非晶中四面体Ge构型越来越多。当Ge原子含量从50%减少到30%时，四面体Ge构型的比例从19%增加到38%。2、它能够调制非晶的中程序结构，即“团簇”结构。随着Ge原子含量的减少，各类“团簇”的比例和连接方式都发生了变化，即第一类“团簇”越来越多，第四类“团簇”越来越少，而其它两类“团簇”变化相对较小。它们之间的连接方式也逐渐从单纯的共享外层Te原子连接变成共享外层Te原子和/或Te-Te键连接。3、它能够调制Te原子的分布。随着Ge原子含量的减少，只与Te原子成键的Te原子越来越多，且当Ge原子含量从40%减少到30%时，它们从弥散分布变成扎堆聚集分布，而剂量比GeTe没有这种Te原子。4、它能够调制“空位”的大小和体积。随着Ge原子含量的减少，“总的空位体积”和“大空位(体积大于353)”的数量相应增加，当Ge原子含量从50%减少到30%时，“总的空位体积”从9.0%增加到17.5%。
最后，基于非晶态和晶态的结构比较，我们探讨了以“同极键”和“空位”为主导的相变机制。结晶过程：当相变材料被加热到结晶温度时，非晶中的“同极键”被打断，同时，“空位”给它们的重新排列提供了活动空间。非晶化过程：当相变材料被加热到熔化状态时，晶体的长程有序结构被打乱，同时产生很多“同极键”；在紧接下来的淬火过程中，这些“同极键”被保存下来，并产生一定数量的“空位”。从这个机制我们可以发现，非晶结构中“同极键”和“空位”的数量直接影响它的相变过程。
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-x)thin 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.
引文
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