Edward A. Supple, Christopher J. K. Richardson, Brian P. Gorman
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引用次数: 0
摘要
报告了在 c 平面蓝宝石上通过等离子体辅助分子束外延生长的 NbTiN/AlN/NbTiN 器件堆栈的纳米级晶体结构和取向。利用相关的四维扫描透射电子显微镜和原子探针断层扫描(APT)对结构、取向、界面粗糙度和厚度进行了研究。这项研究发现,铌钛氮具有岩盐结构,并高度定向于⟨111⟩,围绕该轴的旋转与 c 平面蓝宝石中的阶梯边缘相对应,其孪晶边界具有无数的纳米级间距。在(111) NbTiN 上生长的(0001) 晶格氮化铝薄膜在厚度上呈现出纳米级的变化,从而在势垒结上形成针孔短路。生长在氮化铌上的氮化钛覆盖层是多晶、随机取向和高度应变的。APT 还用于确定超导体和电介质内部的局部化学变化。观察到阳离子:阳离子和阳离子:阴离子的比例均偏离预期。来自传统和纳米尺度计量学的结果凸显了在具有复杂和未充分研究的相空间的材料系统中设计氮化物三层异质结构所面临的挑战。
Atomic structure of a NbTiN/AlN/NbTiN Josephson junction grown by molecular-beam epitaxy
Nanometer-scale crystallographic structure and orientation of a NbTiN/AlN/NbTiN device stack grown via plasma-assisted molecular beam epitaxy on c-plane sapphire are reported. Structure, orientation, interface roughness, and thickness are investigated using correlative four-dimensional scanning transmission electron microscopy and atom probe tomography (APT). This work finds NbTiN that is rock salt structured and highly oriented toward ⟨111⟩ with rotations about that axis corresponding to step edges in the c-plane sapphire with a myriad of twin boundaries that exhibit nanoscale spacing. The wurtzite (0001) AlN film grown on (111) NbTiN exhibits nm-scale changes in the thickness resulting in pinhole shorts across the barrier junction. The NbTiN overlayer grown on AlN is polycrystalline, randomly oriented, and highly strained. APT was also used to determine local changes in chemistry within the superconductor and dielectric. Deviation from both intended cation:cation and cation:anion ratios are observed. The results from conventional and nanoscale metrology highlight the challenges of engineering nitride trilayer heterostructures in material systems with complicated and understudied phase space.