Analysis of Hydrogen Contamination in Al/AlOx/Al Josephson Junctions

Abstract

Hydrogen contamination in Josephson junctions is a potential source of device-to-device variability and two-level-system loss in superconducting qubits. In this work, we investigate hydrogen incorporation in oxidized aluminum barriers by combining molecular dynamics simulations with atomistic quantum transport calculations. The oxide growth simulations are performed using CHGNet for Al surfaces exposed to dense O$_{\text{2}}$ and H$_{\text{2}% }$O environments, yielding amorphous AlO$_{\text{x}}$ layers with hydrogen content comparable to experimentally relevant levels. From $400$ statistically independent samples, we find that the number of H atoms in the oxide is well described by a beta-binomial distribution, reflecting correlations induced by the self-limiting oxidation process. Structural analysis shows that most hydrogen atoms reside near the AlO$_{\text{x}}$ surface and predominantly form Al-OH and Al-OH-Al motifs. To assess the impact of hydrogen on transport, we construct Al/Al$_{\text{2}}$O$_{\text{3}} $/Al junction models and perform NEGF-DFT calculations with NanoDCAL, using a GGA+U scheme to calibrate the band gap and band alignment. H atoms are found to increase the transmission coefficient near the Fermi level and shift the electronic structure in a manner consistent with effective p-type doping. By combining the H atom number statistics from molecular dynamics with the transmission coefficients from quantum transport calculations, we obtain a probability distribution for the Josephson energy. For a Josephson junction with an average hydrogen content of $2.56$ at.\%, the resulting Josephson energy is predicted to be $% E_{J}/h=10.92\pm 0.26$ GHz. These results provide an atomistic picture of hydrogen contamination and an estimate of device variability in Josephson junctions.

Figures (5)

• Figure 1: (a) Initial configuration consisting of an Al surface, a gas of O$_{\text{2}}$ and H$_{\text{2}}$O molecules, and a vacuum region. (b) Atomic structure after $3$ ps of MD simulation, showing the formation of an amorphous AlO$_{\text{x}}$ layer on the Al surface. Because of periodic boundary conditions, the initial left-most Al layer appears as the right-most Al layer. (c) Top view of the AlO$_{\text{x}}$ surface at $t=3$ ps. Al, O, and H atoms are represented by gray, red, and white spheres, respectively.
• Figure 2: Distribution of H atom numbers in AlO$_{\text{x}}$.
• Figure 3: (a1) and (a2): Relaxed atomic structures of JJ and JJ-H. The H atom is highlighted by a blue circle. (b1) and (b2): Corresponding averaged potentials in the $xz$ plane obtained from NanoDCAL self-consistent calculations.
• Figure 4: Transmission coefficients for JJ (blue curve) and JJ-H (red curve), respectively. The Fermi energy is indicated by the vertical dotted line.
• Figure 5: Probability distribution of Josephson energy $E_{J}/h$.