Unveiling atomic structure and chemical composition of the Al/AlOx/Al Josephson junctions in qubits

Abstract

Qubits incorporating Josephson Junctions are essential for developing quantum computing devices, nevertheless, nanostructure and chemical information of the junction have yet been distinctly understood. To address it, comprehensive investigation is performed to decipher atomic structure and compositional features of the Josephson junction which basically consists of an Al/AlOx/Al tri-layer materials system. It is revealed that the top-Al and the bottom-Al layers are with the thickness of 25.8 ± 0.8 nm and 20.4 ± 1.4 nm, respectively, slightly smaller than the intended thickness of 30 nm, which may be due to the inherent error associated to the measuring method based on cross sectional transmission electron microscopy micrographs. In addition, both the top and bottom crystalline Al layers comprise columnar grains with the size of approximately 20 × 30 nm. The AlOx tunnel layer is roughly 2∼4 nm thick and is composed of amorphous materials. The interfaces between AlOx and Al electrodes are jagged rather than flat, implying thickness inhomogeneity which can be possibly ascribed to the mechanisms of atomic steps and grain boundary grooving. Moreover, EELS spectrum imaging reveals a 0.5 nm-thick interfacial layer at boundary of AlOx barrier and Al electrodes. Unique ELNES features at the interfacial layer, including a −2 eV onset-shift of the O-K edge, are correlated with reduction of electronic properties of the barrier layer. Atom probe tomography reveals that composition of the bottom-Al/Si substrate interface primarily comprises AlO, O, C, Al2O, SiO, SiO2, P, O2 and Al2O2 and small quantities of SiC2, B and AlO2. Furthermore, significant strain, e.g., ∼3% normal and shear strain, can be detected in the bottom-Al layer grains, which probably stems from the uneven Si surface during the qubit fabrication process.