Cr-Triggered Local Structural Change in Cr2Ge2Te6 Phase Change Material
Cr-Triggered Local Structural Change in Cr2Ge2Te6 Phase Change Material
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DOI：
10.1021/acsami.9b11535
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发表时间：
2019-11-20
影响因子：
9.5
通讯作者：
Sutou, Yuji
中科院分区：
文献类型：
--
作者：
Hatayama, Shogo;Shuang, Yi;Sutou, Yuji
Cr2 Ge 2 Te 6（CrGT）是一种相变材料，其在晶相中的电阻率高于在非晶相中的电阻率。CrGT表现出超低的非晶化操作能量。在这项研究中，在晶体CrGT的电阻增加的起源相比，非晶CrGT和潜在的相变机制进行了研究，在本地的结构变化和相关的电子状态的变化。晶体CrGT中费米能级的态密度随着退火温度的升高而降低，并且在380 ℃退火后变得可以忽略不计。同时，费米能级从价带附近移动到带隙中心，导致电阻增加。从非晶到结晶CrGT的相变通过具有与非晶相类似的局部结构的亚稳结晶相发生。Cr纳米团簇被证实存在于非晶相和晶相中。Cr纳米团簇的存在诱导了晶相中的Cr空位。这些Cr空位产生空穴载流子，导致p型导电。光电子能谱的Cr 2s核心水平清楚地表明，在退火后的结晶CrGT中的Cr-Cr键的分数的减少和Cr-Te键的分数的增加。同时，随着Cr-Cr键的减少，Cr纳米团簇的配位数减少。总之，这些结果意味着，在晶体CrGT中的电阻增加的起源是由Cr原子从Cr纳米团簇扩散的Cr空位的填充。
Cr2Ge2Te6 (CrGT) is a phase change material with higher resistivity in the crystalline phase than in the amorphous phase. CrGT exhibits an ultralow operation energy for amorphization. In this study, the origin of the increased resistance in crystalline CrGT compared to amorphous CrGT and the underlying phase change mechanism were investigated in terms of both local structural change and associated change in electronic state. The density of states at the Fermi level in crystalline CrGT decreased with increasing annealing temperature and became negligible upon annealing at 380 degrees C. Simultaneously, the Fermi level shifted from the vicinity of the valence band to the band gap center, leading to an increase in resistance. The phase change from amorphous to crystalline CrGT occurred through a metastable crystalline phase with a local structure similar to that of the amorphous phase. Cr nanoclusters were confirmed to exist in both the amorphous and crystalline phases. The presence of Cr nanoclusters induced Cr vacancies in the crystalline phase. These Cr vacancies generated hole carriers, leading to p-type conduction. Photoelectron spectroscopy of the Cr 2s core level clearly indicated a decrease in the fraction of Cr-Cr bonds and an increase in the fraction of Cr-Te bonds in crystalline CrGT upon annealing. Meanwhile, the coordination number of the Cr nanoclusters decreased as the number of Cr-Cr bonds was reduced. Together, these results imply that the origin of the increased resistance in crystalline CrGT is the filling of Cr vacancies by Cr atoms diffusing from Cr nanoclusters.