Implications of annealing on the structure and chemistry of Al/AlO_x/Al Josephson junctions

We investigate the effect of thermal annealing on the structure, chemistry, and electrical conductance of Al/AlO_x/Al Josephson junctions using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Secondary ion mass spectroscopy (SIMS), and resistivity measurements. The changes of junction resistance show non-monotonic behavior depending on the annealing temperatures: annealing Al/AlO_x/Al Josephson junctions to 350 ◦C for 2 hours in ultra-high vacuum conditions results in a decrease in the junction resistance whereas annealing these Al/AlO_x/Al Josephson junctions to 450 ◦C for 2 hours leads to an increase in the junction resistance. Corroborative TEM, XPS, and SIMS analyses of AlO_x tunnel barriers indicate that this non-monotonic behavior of junction resistance can be rationalized by changes of AlO_x tunnel barriers in three parameters: (i) thickness; (ii) hydrogen impurities; and (iii) Al-O bond densities in AlO_x tunnel barriers at the given annealing temperatures. We also discuss the implications of the current observations to improve the control of qubit frequency and coherence times of superconducting qubits.