[Paper Review] Tunneling Spectroscopy of Atomically-Thin Al2O3 Films for Tunnel Junctions

Jamie Wilt, Youpin Gong+9 more|arXiv (Cornell University)|Jan 27, 2017

Semiconductor materials and devices20 references16 citations

TL;DR

This study demonstrates the fabrication of atomically-thin, high-quality Al2O3 tunnel barriers via atomic layer deposition (ALD) with in situ scanning tunneling spectroscopy (STS) and molecular dynamics simulations. By using a controlled H2O pulse to hydroxylate the Al surface, the researchers achieved a dense, leak-free barrier with a significantly enhanced barrier height and low defect density, enabling superior performance in Josephson junctions and advancing next-generation metal-insulator-metal tunnel junctions.

ABSTRACT

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.

Motivation & Objective

• To address the limitations of conventional thermal AlOx barriers, which suffer from defects and pinholes, by developing atomically-thin, high-quality tunnel barriers.
• To improve the metal-insulator (M-I) interface quality and barrier height in tunnel junctions for enhanced quantum coherence.
• To explore the role of surface hydroxylation via a controlled H2O pulse in enabling uniform, dense Al2O3 growth via ALD.
• To correlate in situ STS measurements with molecular dynamics simulations to understand nucleation and growth mechanisms.
• To validate the performance of ALD Al2O3 in functional Josephson junctions for quantum device applications.

Proposed method

• Employed in situ scanning tunneling spectroscopy (STS) to measure electronic properties of Al2O3 films during ALD growth.
• Used molecular dynamics simulations to model the initial nucleation and growth of Al2O3 on Al surfaces with and without hydroxylation.
• Applied a precisely tuned initial H2O pulse to hydroxylate the Al surface before ALD cycle initiation.
• Controlled ALD parameters to achieve atomic-layer precision in Al2O3 thickness and uniformity.
• Fabricated and characterized Josephson junctions using ALD Al2O3 as the tunnel barrier to validate performance.
• Compared barrier properties (height, defect density, leakage) between ALD Al2O3 and conventional thermal AlOx.

Experimental results

Research questions

• RQ1 How does surface hydroxylation via an initial H2O pulse affect the nucleation and growth of Al2O3 films during ALD?
• RQ2 What is the impact of ALD-grown Al2O3 on the tunnel barrier height and interface quality compared to thermal AlOx?
• RQ3 Can in situ STS detect and quantify defect density and electronic inhomogeneities in atomically-thin Al2O3 films?
• RQ4 To what extent does the ALD process reduce leakage currents and pinholes in tunnel junctions?
• RQ5 How do the electronic properties of ALD Al2O3 barriers correlate with their performance in Josephson junctions?

Key findings

• A controlled H2O pulse hydroxylated the Al surface, enabling the formation of a uniform, atomically-thin Al2O3 layer with superior nucleation.
• The ALD-grown Al2O3 barrier exhibited a significantly enhanced tunnel barrier height compared to thermal AlOx, as confirmed by in situ STS.
• The M-I interface quality was markedly improved, with reduced electronic inhomogeneities and defect density.
• The ALD Al2O3 film showed no measurable leakage, indicating a dense, pinhole-free structure.
• Josephson junctions fabricated with ALD Al2O3 demonstrated high coherence and performance, validating its suitability for quantum devices.
• Molecular dynamics simulations confirmed that hydroxylation promotes uniform, layer-by-layer growth, preventing island formation.