Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits

Abstract

This work is concerned with Al/Al-oxide(AlOx)/Al-layer systems which are important for Josephson-junction-based superconducting devices such as quantum bits. The device performance is limited by noise, which has been to a large degree assigned to the presence and properties of two-level tunneling systems in the amorphous AlOx tunnel barrier. The study is focused on the correlation of the fabrication conditions, nanostructural and nanochemical properties and the occurrence of two-level tunneling systems with particular emphasis on the AlOx-layer. Electron-beam evaporation with two different processes and sputter deposition were used for structure fabrication, and the effect of illumination by ultraviolet light during Al-oxide formation is elucidated. Characterization was performed by analytical transmission electron microscopy and low-temperature dielectric measurements. We show that the fabrication conditions have a strong impact on the nanostructural and nanochemical properties of the layer systems and the properties of two-level tunneling systems. Based on the understanding of the observed structural characteristics, routes are suggested towards the fabrication of Al/AlOx/Al-layers systems with improved properties.

Structural and nanochemical properties ofAlOxlayers inAl/AlOx/Al-layer systems

“…Figure 5 shows the oxygen content x of the different AlOx layers with x = 1.5 corresponding to stoichiometric Al2O3. All amorphous layers are oxygen deficient as expected from previous experimental work [34,38,58] and simulations [54]. Dynamic oxidation with a low oxygen pressure of 15 µbar (EBPlas) shows an oxygen content of only x = 0.5.…”

“…Transmission electron microscopy (TEM) was performed with an FEI Titan³ 80-300 (Thermo EELS was performed in the scanning (S)TEM mode using a self-written acquisition script including binned-gain averaging [41] to enhance the signal-to-noise ratio. Chemical composition quantification is based on the k-factor method [38,42,43] using crystalline -and -Al2O3 as reference materials.…”

“…EELS acquisition conditions and the subsequent analysis process are in detail described in ref. [38] and in the Supplementary Information.…”

Optimization of Al/AlOx/Al-layer systems for Josephson junctions from a microstructure point of view

“…This is disadvantageous with respect to performance and necessitates optimization of the thickness homogeneity of the tunnel barrier. AlOx-thickness variations are predominantly caused by grain boundary grooving in the lower Al-electrode layer as shown by Nik et al [8] and our group [9]. Hence, microstructure and homogeneity of the lower Al layer determine to a large degree the properties of the whole Al/AlOx/Al-layer system and have to be optimized to provide the best possible surface for the formation of an AlOx-tunnel barrier with homogeneous thickness.…”

Simulating the fabrication of aluminium oxide tunnel junctions

“…Fritz et al report stoichiometries for oxides grown using four different techniques: thermal oxidation with and without UV illumination, plasma oxidation, and physical vapour deposition achieved by heating Al 2 O 3 -pellets with an electron beam 55 . The stoichiometries were determined using STEM electron energy loss spectroscopy (EELS) and are in the range 1.1-1.3 in the amorphous oxide regions except for the thermally oxidised sample without UV illumination which has a reported stoichimetry of 0.5.…”