Somjit Vrindaa, Yildiz Bilge
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
ACS Appl Mater Interfaces. 2022 Sep 21;14(37):42613-42627. doi: 10.1021/acsami.2c08706. Epub 2022 Sep 9.
Identifying the structure of the AlO/Al interface is important for advancing its performance in a wide range of applications, including microelectronics, corrosion barriers, and superconducting qubits. However, beyond the study of a few select terminations of the interface using computational methods, and top-down, laterally averaged spectroscopic and microscopic analyses, the explicit structure of the interface and the initial stages of propagation of the interface into the metal are largely unresolved. In this study, we utilize ab initio grand canonical Monte Carlo to perform a physically motivated, unbiased exploration of the interfacial composition and configuration space. We find that at equilibrium, the interface is atomically sharp with aluminum vacancies and propagates in a layer-by-layer fashion, with aluminum excess in the oxide layer at the interfacial plane. Oxygen incorporation, aluminum vacancy formation, and aluminum vacancy annihilation are the building blocks of AlO formation at the interface. The localized interfacial mid-gap states from under-coordinated aluminum atoms from the oxide and the immediate depletion of aluminum states near the Fermi level upon oxygen incorporation prevent oxygen dissolution ahead of the interface front and result in the layer-by-layer propagation of the interface. This is in sharp contrast to the ZrO/Zr system, which forms interfacial sub-oxides, and also explains the favorable self-healing nature of the AlO/Al system. The occupied interfacial mid-gap states also increase the calculated n-type Schottky barrier heights. Additionally, we identify that interfacial aluminum core-level shifts linearly depend on the aluminum coordination number, whereas interfacial oxygen core-level shifts depend on long-range ordering at the interface. The detailed geometric and electronic insights into the interface structure and evolution expand our understanding of this fundamental interface and have important implications for the engineering and design of AlO/Al-based corrosion coatings with enhanced barrier properties, controllable transistor technologies, and noise-free superconducting qubits.
确定AlO/Al界面的结构对于提升其在包括微电子、腐蚀阻挡层和超导量子比特等广泛应用中的性能至关重要。然而，除了使用计算方法对界面的一些特定终止情况进行研究，以及自上而下的横向平均光谱和微观分析外，界面的明确结构以及界面向金属中传播的初始阶段在很大程度上仍未得到解决。在本研究中，我们利用从头算巨正则蒙特卡罗方法对界面组成和构型空间进行基于物理的无偏探索。我们发现，在平衡状态下，界面原子级尖锐且存在铝空位，并以逐层方式传播，在界面平面的氧化层中有过量的铝。氧的掺入、铝空位的形成和铝空位的湮灭是界面处AlO形成的基本过程。来自氧化物中配位不足的铝原子的局域界面中间能隙态以及氧掺入后费米能级附近铝态的立即耗尽，阻止了氧在界面前沿之前溶解，并导致界面的逐层传播。这与形成界面亚氧化物的ZrO/Zr系统形成鲜明对比，也解释了AlO/Al系统有利的自愈特性。占据的界面中间能隙态还增加了计算得到的n型肖特基势垒高度。此外，我们发现界面铝芯能级的位移线性依赖于铝的配位数，而界面氧芯能级的位移则依赖于界面处的长程有序。对界面结构和演化的详细几何和电子见解扩展了我们对这个基本界面的理解，并对具有增强阻挡性能的AlO/Al基腐蚀涂层、可控晶体管技术和无噪声超导量子比特的工程设计具有重要意义。
