Probing the Nucleation of Al2O3 in Atomic Layer Deposition on Aluminum for Ultrathin Tunneling Barriers in Josephson Junctions

摘要

Ultrathin dielectric tunneling barriers are critical to Josephson junction(JJ) based superconducting quantum bits (qubits). However, the prevailingtechnique of thermally oxidizing aluminum via oxygen diffusion producesproblematic defects, such as oxygen vacancies, which are believed to be aprimary source of the two-level fluctuators and contribute to the decoherenceof the qubits. Development of alternative approaches for improved tunnelingbarriers becomes urgent and imperative. Atomic Layer Deposition (ALD) ofaluminum oxide (Al2O3) is a promising alternative to resolve the issue ofoxygen vacancies in the Al2O3 tunneling barrier, and its self-limiting growthmechanism provides atomic-scale precision in tunneling barrier thicknesscontrol. A critical issue in ALD of Al2O3 on metals is the lack of hydroxylgroups on metal surface, which prevents nucleation of the trimethylaluminum(TMA). In this work, we explore modifications of the aluminum surface withwater pulse exposures followed by TMA pulse exposures to assess the feasibilityof ALD as a viable technique for JJ qubits. ALD Al2O3 films from 40 angstromsto 100 angstoms were grown on 1.4 angstroms to 500 angstroms of Al and werecharacterized with ellipsometry and atomic force microscopy. A growth rate of1.2 angstroms/cycle was measured, and an interfacial layer (IL) was observed.Since the IL thickness depends on the availability of Al and saturated at 2 nm,choosing ultrathin Al wetting layers may lead to ultrathin ALD Al2O3 tunnelingbarriers.