Appleton Robert J, McClure Zachary D, Adams David P, Strachan Alejandro
School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA.
Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185, USA.
J Chem Phys. 2024 May 7;160(17). doi: 10.1063/5.0203532.
Ge-Sb-Te (GST) alloys are leading phase-change materials for data storage due to the fast phase transition between amorphous and crystalline states. Ongoing research aims at improving the stability of the amorphous phase to improve retention. This can be accomplished by the introduction of carbon as a dopant to Ge2Sb2Te5, which is known to alter the short- and mid-range structure of the amorphous phase and form covalently bonded C clusters, both of which hinder crystallization. The relative importance of these processes as a function of C concentration is not known. We used molecular dynamics simulation based on density functional theory to study how carbon doping affects the atomic structure of GST-C. Carbon doping results in an increase in tetrahedral coordination, especially of Ge atoms, and this is known to stabilize the amorphous phase. We observe an unexpected, non-monotonous trend in the number of tetrahedral bonded Ge with the amount of carbon doping. Our simulations show an increase in the number of tetrahedral bonded Ge up to 5 at.% C, after which the number saturates and begins to decrease above 14 at.% C. The carbon atoms aggregate into clusters, mostly in the form of chains and graphene flakes, leaving less carbon to disrupt the GST matrix at higher carbon concentrations. Different degrees of carbon clustering can explain divergent experimental results for recrystallization temperature for carbon doped GST.
锗锑碲（GST）合金因其在非晶态和晶态之间的快速相变而成为数据存储领域的主要相变材料。正在进行的研究旨在提高非晶相的稳定性以改善数据保持能力。这可以通过向Ge2Sb2Te5中引入碳作为掺杂剂来实现，已知碳会改变非晶相的短程和中程结构并形成共价键合的C簇，这两者都会阻碍结晶。这些过程作为碳浓度函数的相对重要性尚不清楚。我们基于密度泛函理论使用分子动力学模拟来研究碳掺杂如何影响GST-C的原子结构。碳掺杂导致四面体配位增加，尤其是锗原子的四面体配位增加，已知这会稳定非晶相。我们观察到四面体键合的锗数量随碳掺杂量呈现出意想不到的非单调趋势。我们的模拟表明，四面体键合的锗数量在碳含量达到5 at.%之前增加，之后数量饱和并在碳含量高于14 at.%时开始减少。碳原子聚集成簇，主要呈链状和石墨烯薄片形式，在较高碳浓度下，较少的碳会破坏GST基体。不同程度的碳簇化可以解释碳掺杂GST再结晶温度的不同实验结果。
