TEM
investigations of
Al/AlOx/Al
J
osephson junctions
Authors: Fritz Stefan , Schneider Reinhard , Radtke Lucas , Weides Martin , Gerthsen Dagmar 
Published: December 2016

The development of fully‐operational quantum computers is one of the major goals in information science. Quantum computers rely on the quantum‐coherent evolution of their constituents, the quantum bits (qubits). For this purpose all sources of decoherence have to be identified, to eliminate them as far as possible, or to reduce their effect, e.g., by an optimized qubit design. Superconducting qubits based on Josephson junctions (JJs) provide the most advanced platform. Their coherence is limited by low‐frequency charge fluctuations, flux noise, and critical current fluctuations (frequently with a 1/f spectrum) [1,2]. Much progress has been made in the last 15 years reducing their effect, leading to an increase of the coherence times by five orders of magnitude. Still a major source of decoherence appears to be environmental two‐level systems (TLS) and other imperfections within the amorphous AlO
x
‐layer [3,4] of Al/AlO
x
/Al‐based JJs. Many properties of the TLS have been probed experimentally, however, identifying their true microscopic nature remains an open problem. Recent transmission electron microscopy (TEM) investigations showed the potential of these techniques for analyzing and improving the properties of JJs and qubits [5,6].

In this work Al/AlO
x
/Al‐layer systems for JJs were analyzed in an
FEI Titan³ 80‐300
. Different samples were fabricated by electron beam physical vapor deposition with varying oxidation parameters like, e.g., oxidation time

t
o

, oxygen pressure

p
o

, UV‐enhanced oxidation and thermally enhanced oxidation. The morphology of the samples was analyzed by high‐resolution TEM (HRTEM). Fig. 1 shows an AlO
x
‐layer fabricated at room temperature with

t
o

= 12.5 min and

p
o

= 0.0145 mbar, resulting in an oxide layer with an average thickness of 1.9 nm. Fig. 1a shows a smooth oxide layer with homogenous thickness at an Al/AlO
x
interface of a single Al‐grain. Fig. 1b shows the same layer at an Al‐grain boundary in the lower electrode layer, where the oxide layer is bent illustrating the negative influence of grain boundaries. Thickness variations due to grain boundaries can also be found. Furthermore, the bonding characteristics between Al‐ and O‐atoms were analyzed by electron energy loss spectroscopy (EELS). The electron loss near edge structure of the Al‐L
2,3
edge is shown in Fig. 2 for the AlO
x
‐layer (green spectrum) and crystalline a‐ and g‐Al
2
O
3
reference samples (black and red spectra). The first maximum of the absorption edges is located at 77.3 eV and 80.2 eV which is characteristic for tetrahedral‐ and octahedral‐coordinated Al‐sites. By analysing the peak intensities [7] the fraction of tetrahedral‐coordinated Al‐sites can be determined to 40 at%. Although the AlO
x
‐spectrum is similar to the expected spectra for amorphous Al
2
O
3
, there are some unexpected features which can be connected to medium range order (82 – 90 eV). By comparison of the data obtained from differently fabricated samples the influence of individual fabrication parameters can be analyzed to improve the qubit properties by optimization of the fabrication process.