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首页> 外文期刊>Physical review >Influence of stress and strain on the kinetic stability and phase transitions of cubic and pseudocubic Ge-Sb-Te materials
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Influence of stress and strain on the kinetic stability and phase transitions of cubic and pseudocubic Ge-Sb-Te materials

机译：应力和应变对立方和假立方Ge-Sb-Te材料动力学稳定性和相变的影响

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Rewritable data-storage media and promising nonvolatile random-access memory are mainly based on phase-change materials (PCMs) which allow reversible switching between two metastable (amorphous and crystalline) modifications accompanied by a change in physical properties. Although the phase-change process has been extensively studied, it has not been elucidated how and why the metastable crystalline state is kinetically stabilized against the formation of thermodynamically stable phases. In contrast to thin-film investigations, the present study on bulk material allows to demonstrate how the cubic high-temperature phase of GeTe-rich germanium antimony tellurides (GST materials) is partially retained in metastable states obtained by quenching of bulk samples. We focus on compositions such as Ge_(0.7)Sb_(0.2)Te and Ge_(0.8)Sb_(0.13)Te, which are important materials for Blu-ray disks. Bulk samples allow a detailed structural characterization. The structure of a multiply twinned crystal isolated from such material has been determined from x-ray diffraction data (Ge_(0.7)Sb_(0.2)Te, Rim, α=4.237 A, c= 10.29 A). Although the metrics is close to cubic, the crystal structure is rhombohedral and approximates a layered GeTe-type atom arrangement. High-resolution transmission electron microscopy (HRTEM) on quenched samples of Ge_(0.8)Sb_(0.13)Te reveal nanoscale twin domains. Cation defects form planar domain boundaries. The metastability of the samples was proved by in situ temperature-dependent powder diffraction experiments, which upon heating show a slow phase transition to a trigonal layered structure at ca. 325 ℃. HRTEM of samples annealed at 400 ℃ shows extended defect layers that lead to larger domains of one orientation which can be described as a one-dimensionally disordered long-periodical-layered structure. The stable cubic high-temperature modification is formed at about 475 ℃. Powder diffraction on samples of Ge_(0.8)Sb_(0.13)Te with defined particle sizes reveal that the formation of the stable superstructure phase is influenced by stress and strain induced by the twinning and volume change due to the cubic → rhombohedral phase transition upon quenching. The associated peak broadening is larger for small crystallites that allow relaxation more readily. Consequently, the degree of rhombohedral distortion as well as the appearance of superstructure reflections upon annealing is more pronounced for small crystallites. The same is true for samples which were slowly cooled from 500 ℃. Hence, the lattice distortion accompanying the phase transition toward a stable trigonal superstructure is, to a certain degree, inhibited in larger crystallites. This kinetic stabilization of metastable states by stress effects is probably relevant for GST phase-change materials.
机译：可重写数据存储介质和有前途的非易失性随机存取存储器主要基于相变材料（PCM），相变材料允许在两个亚稳（非晶和晶体）修饰之间进行可逆切换，并伴随物理特性的改变。尽管已经对相变过程进行了广泛的研究，但尚未阐明如何以及为什么亚稳态晶态在动力学上稳定化，从而防止形成热力学稳定的相。与薄膜研究相反，对散装材料的当前研究可以证明富含GeTe的锗碲化锗（GST材料）的立方高温相如何通过散装样品的淬火而部分保持在亚稳态。我们着重于Ge_（0.7）Sb_（0.2）Te和Ge_（0.8）Sb_（0.13）Te等成分，它们是蓝光光盘的重要材料。大量样品可以进行详细的结构表征。从x射线衍射数据（Ge_（0.7）Sb_（0.2）Te，Rim，α＝ 4.237A，c ＝ 10.29A）已经确定了从这种材料分离的多晶孪晶的结构。尽管量度接近立方，但是晶体结构是菱面体的并且近似于层状GeTe型原子排列。 Ge_（0.8）Sb_（0.13）Te淬火样品的高分辨率透射电子显微镜（HRTEM）显示了纳米级双畴。阳离子缺陷形成平面畴边界。样品的亚稳定性通过原位温度相关的粉末衍射实验证明，该实验在加热时显示出缓慢的相变，在约200℃转变为三角形层状结构。 325℃。在400℃退火的样品的HRTEM显示出缺陷层的扩展，导致了一个取向的较大区域，可以描述为一维无序的长周期层状结构。在约475℃形成稳定的立方高温变质。在具有确定粒径的Ge_（0.8）Sb_（0.13）Te样品上的粉末衍射显示，稳定的上层结构相的形成受孪晶和淬火后立方→菱面体相变引起的孪生和体积变化引起的应力和应变的影响。对于较小的微晶而言，相关的峰展宽更大，从而使弛豫更容易。因此，对于小晶粒，菱形的变形程度以及退火时上层结构反射的出现更为明显。从500℃缓慢冷却的样品也是如此。因此，在较大的微晶中，在一定程度上抑制了伴随着相向稳定的三角超结构相变的晶格畸变。通过应力效应实现的亚稳态的动力学稳定可能与GST相变材料有关。

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