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Register a new userProbing the Nucleation of Al2O3 in Atomic Layer Deposition on Aluminum for Ultrathin Tunneling Barriers in Josephson Junctions
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Ultrathin dielectric tunneling barriers are critical to Josephson junction (JJ) based superconducting quantum bits (qubits). However, the prevailing technique of thermally oxidizing aluminum via oxygen diffusion produces problematic defects, such as oxygen vacancies, which are believed to be a primary source of the two-level fluctuators and contribute to the decoherence of the qubits. Development of alternative approaches for improved tunneling barriers becomes urgent and imperative. Atomic Layer Deposition (ALD) of aluminum oxide (Al2O3) is a promising alternative to resolve the issue of oxygen vacancies in the Al2O3 tunneling barrier, and its self-limiting growth mechanism provides atomic-scale precision in tunneling barrier thickness control. A critical issue in ALD of Al2O3 on metals is the lack of hydroxyl groups on metal surface, which prevents nucleation of the trimethylaluminum (TMA). In this work, we explore modifications of the aluminum surface with water pulse exposures followed by TMA pulse exposures to assess the feasibility of ALD as a viable technique for JJ qubits. ALD Al2O3 films from 40 angstroms to 100 angstoms were grown on 1.4 angstroms to 500 angstroms of Al and were characterized with ellipsometry and atomic force microscopy. A growth rate of 1.2 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 tunneling barriers.
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The integration of two-dimensional (2D) materials with functional non-2D materials such as metal oxides is of key importance for many applications, but underlying mechanisms for such non-2D/2D interfacing remain largely elusive at the atomic scale. To address this, we here investigate the nucleation stage in atomic layer deposition (ALD) of the important metal oxide HfO2 on chemical vapor deposited graphene using atomically resolved and element specific scanning transmission electron microscopy (STEM). To avoid any deleterious influence of polymer residues from pre-ALD graphene transfers we employ a substrate-assisted ALD process directly on the as grown graphene still remaining on its Cu growth catalyst support. Thereby we resolve at the atomic scale key factors governing the integration of non-2D metal oxides with 2D materials by ALD: Particular to our substrate-assisted ALD process we find a graphene-layer-dependent catalytic participation of the supporting Cu catalyst in the ALD process. We further confirm at high resolution the role of surface irregularities such as steps between graphene layers on oxide nucleation. Employing the energy transfer from the scanning electron beam to in situ crystallize the initially amorphous ALD HfO2 on graphene, we observe HfO2 crystallization to non-equilibrium HfO2 polymorphs (cubic/tetragonal). Finally our data indicates a critical role of the graphenes atmospheric adventitious carbon contamination on the ALD process whereby this contamination acts as an unintentional seeding layer for metal oxide ALD nucleation on graphene under our conditions. As atmospheric adventitious carbon contamination is hard to avoid in any scalable 2D materials processing, this is a critical factor in ALD recipe development for 2D materials coating. Combined our work highlights several key mechanisms underlying scalable ALD oxide growth on 2D materials.
Atomic Layer Deposition (ALD) is a promising technique for producing Josephson junctions (JJs) with lower defect densities for qubit applications. A key problem with using ALD for JJs is the interfacial layer (IL) that develops underneath the tunnel barrier. An IL up to 2 nm forms between ALD Al2O3 and Al. However, the IL thickness is unknown for ALD films less 1 nm. In this work, Nb-Al-ALD-Al2O3-Nb trilayers with tunnel barriers from 0.6 - 1.6 nm were grown in situ. Nb-Al-AlOx-Nb JJs with thermally oxidized tunnel barrier were produced for reference. RN was obtained using a four-point method at 300 K. JC, and its dependence on barrier thickness, was calculated from the Ambegaokar-Baratoff formula. The Al surface was modeled using ab initio molecular dynamics to study the nucleation of Al2O3 on Al. Current voltage characteristics were taken at 4 K to corroborate the room temperature measurements. Together, these results suggest that ALD may be used to grow an ultrathin, uniform tunnel barrier with controllable tunnel resistance and JC, but a thin IL develops during the nucleation stage of ALD growth that may disqualify Al as a suitable wetting layer for ALD JJ based qubits.
Atomic layer deposition (ALD) is an essential tool in semiconductor device fabrication that allows the growth of ultrathin and conformal films to precisely form heterostructures and tune interface properties. The self-limiting nature of the chemical reactions during ALD provides excellent control over the layer thickness. However, in contrast to idealized growth models, it is experimentally challenging to create continuous monolayers by ALD because surface inhomogeneities and precursor steric interactions result in island growth during film nucleation. Thus, the ability to create pin-hole free monolayers by ALD would offer new opportunities for controlling interfacial charge and mass transport in semiconductor devices, as well as for tailoring surface chemistry. Here, we report full encapsulation of c-plane gallium nitride (GaN) with an ultimately thin (~3 {AA}) aluminum oxide (AlOx) monolayer, which is enabled by the partial conversion of the GaN surface oxide into AlOx using a combination of trimethylaluminum deposition and hydrogen plasma exposure. Introduction of monolayer AlOx significantly modifies the physical and chemical properties of the surface, decreasing the work function and introducing new chemical reactivity to the GaN surface. This tunable interfacial chemistry is highlighted by the reactivity of the modified surface with phosphonic acids under standard conditions, which results in self-assembled monolayers with densities approaching the theoretical limit. More broadly, the presented monolayer AlOx deposition scheme can be extended to other dielectrics and III-V-based semiconductors, with significant relevance for applications in optoelectronics, chemical sensing, and (photo)electrocatalysis.
Metal-Insulator-Metal tunnel junctions (MIMTJ) are common throughout the microelectronics industry. The industry standard AlOx tunnel barrier, formed through oxygen diffusion into an Al wetting layer, is plagued by internal defects and pinholes which prevent the realization of atomically-thin barriers demanded for enhanced quantum coherence. In this work, we employed in situ scanning tunneling spectroscopy (STS) along with molecular dynamics simulations to understand and control the growth of atomically thin Al2O3 tunnel barriers using atomic layer deposition (ALD). We found that a carefully tuned initial H2O pulse hydroxylated the Al surface and enabled the creation of an atomically-thin Al2O3 tunnel barrier with a high quality M-I interface and a significantly enhanced barrier height compared to thermal AlOx. These properties, corroborated by fabricated Josephson Junctions, show that ALD Al2O3 is a dense, leak-free tunnel barrier with a low defect density which can be a key component for the next-generation of MIMTJs.
In this paper, a method is presented to create and characterize mechanically robust, free standing, ultrathin, oxide films with controlled, nanometer-scale thickness using Atomic Layer Deposition (ALD) on graphene. Aluminum oxide films were deposited onto suspended graphene membranes using ALD. Subsequent etching of the graphene left pure aluminum oxide films only a few atoms in thickness. A pressurized blister test was used to determine that these ultrathin films have a Youngs modulus of 154 pm 13 GPa. This Youngs modulus is comparable to much thicker alumina ALD films. This behavior indicates that these ultrathin two-dimensional films have excellent mechanical integrity. The films are also impermeable to standard gases suggesting they are pinhole-free. These continuous ultrathin films are expected to enable new applications in fields such as thin film coatings, membranes and flexible electronics.
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